The role of the serotonin receptor subtypes 5-HT1A and 5-HT7 and its interaction in emotional learning and memory

1Serotonin (5-HT) is a biogenic amine acting as a neurotransmitter and neuromodulator. The distribution of serotonin-containing neurons in the CNS have been studied in different species and have been found to be localized exclusively in the brainstem. The majority of the serotonergic cell bodies reside in the dorsal and median raphe nuclei but send axons almost to the entire brain, including cortical, limbic, midbrain, and hindbrain regions. As expected from the wide projection pattern of the 5-HT neurons, serotonin modulates variable physiological functions, such as sleep, arousal, feeding, temperature regulation, pain, emotions, and cognition.

2The pleiotropic behavioral effects of 5-HT are mediated by a family of at least 14 5-HTR subtypes. These 5-HTR subtypes are distributed in a brain- and cell-specific manner and regulate distinct physiological processes, through different and sometimes opposing signaling pathways.

3The 5-HT1AR is one of the best-studied 5-HTR subtypes due to its implication in anxiety-like behaviors, in depression as well as in cognitive processes that are impaired in several psychiatric disorders (review by; ). Its potential role as a drug target has been also investigated. The most common antidepressants, the SSRIs, act by targeting the 5-HT1AR, supporting the key role of the 5-HT1AR in the pathophysiology of mood disorders.

4The 5-HT7Rs are implicated in depression and anxiety, and evidence has been provided for their role in learning and memory (reviewed by ). Interestingly, the 5-HT7R and 5-HT1AR exert opposing roles in the modulation of fear learning, pointing at the importance of both 5-HTR subtypes and their signaling interaction in the regulation of emotional learning.

5After a brief introduction about the characteristics of 5-HT1A and 5-HT7R (distribution, signaling, and ligands), this review will focus on the role of 5-HT1AR, 5-HT7R as well as its interplay in emotional learning processes. The interaction between the 5-HT1AR and 5-HT7R signaling will be discussed and results of studies using different available 5-HT1AR and 5-HT7R ligands on fear learning tasks are summarized. A considerable extent of this review will also be dedicated to describe the region-specific effects of 5-HT1AR and 5-HT7R, via local rather than systemic administration. Overall, the aim of this review is to draw general conclusions about the role of both 5-HT1AR and 5-HT7R in fear learning, which may contribute to our better understanding of the mechanisms underlying dysregulated learning and memory in affective disorders. The focus here is on fear learning because this one-trial learning task allows for exact timing of pharmacological manipulations to discriminate between different memory phases.

6All the 5-HTR subtypes belong to the G protein-coupled receptor superfamily, with the exception of the 5-HT3R as ionotropic receptor. The metabotropic 5-HTR subtypes consist of seven transmembrane domains and are classified into four groups based on the type of G proteins to which they are coupled. The 5-HT1Rs (5-HT1AR, 5-HT1BR, 5-HT1DR, 5-HT1ER, 5-HT1FR) couple to Gαi/Gαo proteins, whereas the 5-HT2Rs (5-HT2AR, 5-HT2BR, 5-HT2C) couple to Gαq proteins, and the 5-HT4R, 5-HT6R, and 5-HT7R couple to Gαs proteins. For the 5-HT5Rs (5-HT5AR and 5-HT5BR) G-protein coupling is not established yet.

75-HT1AR was the first 5-HTR subtype to be cloned and is characterized by its high affinity for 5-HT. 5-HT1ARs are widely distributed throughout the CNS and are present in both pre- and postsynaptic sites. Presynaptically, 5-HT1ARs are exclusively located on the cell bodies and dendrites of 5-HT neurons in the dorsal and median raphe nuclei and function as 5-HT1A autoreceptors which tightly regulate 5-HT neuronal activity.

8Postsynaptically, the highest level of 5-HT1AR is found in the limbic system based on receptor autoradiography and mRNA expression. Both techniques showed the distribution of the 5-HT1AR in the lateral septum, cingulate and entorhinal cortices, with particularly high expression in the hippocampus (reviewed by ). At the cellular level, the postsynaptic 5-HT1AR is expressed in cortical pyramidal neurons as well as pyramidal, GABAergic and granular cells of the hippocampus. At least in the hippocampal formation, the 5-HT1AR is located on somata and dendrites of pyramidal and granular neurons, as well as on the dendritic spines of pyramidal neurons. Moreover, 5-HT1AR immunoreactivity has been demonstrated in different subgroups of neurons in the septal complex with GABAergic septohippocampal parvalbumin-containing projection neurons, GABAergic calbindin D-28-containing neurons as well as cholinergic septohippocampal neurons. This indicates that systemic administration of 5-HT1AR ligands can modify hippocampal function through effects on septohippocampal neurons that are responsible for the theta rhythm which plays an important role in memory functions.

9Activation of 5-HT1AR leads to neuronal hyperpolarization, an effect mediated by pertussis-toxin-sensitive Gαi/o proteins. Gαi/o proteins are negatively coupled with the signaling pathway of adenylyl cyclase and thereby decrease the cAMP formation. Despite their high density in the dorsal raphe nucleus, 5-HT1A autoreceptors do not seem to inhibit AC, but mediate neuronal inhibition through different signaling pathways. Both post- and presynaptic 5-HT1ARs inhibit neuronal firing via the activation of G protein-coupled inwardly rectifying potassium channels as well as the inhibition of Ca2+ channels. A multitude of other signaling pathways and effectors has been also linked to the activation of the 5-HT1AR (reviewed by; ).

10The 5-HT7R was the last 5-HTR subtype to be cloned by using a targeted screening analysis of mammalian cDNA libraries and probes from already known receptors. Although 5-HT7Rs demonstrate a high interspecies homology (>90%; ), they share a low homology with the other 5-HTR subtypes (<50%; ). Northern blot analysis and in situ hybridization studies demonstrate high expression of 5-HT7R in the CNS and particularly in the hypothalamus (suprachiasmatic nucleus), thalamus, hippocampus, and cerebral cortex. Like 5-HT1AR, the 5-HT7R is also localized in the raphe nuclei in both rodent and human brain, which has raised questions about its role in the regulation of 5-HT levels. At the neuronal level, 5-HT7R is expressed in hippocampal CA pyramidal neurons with a higher density in CA3 than in CA1 and a differential expression, with selective localization on the cell bodies in CA1 pyramidal neurons. Little is known, however, about the expression patterns of 5-HT7R in cortical neurons, where it is suggested that 5-HT7R may have a role during the developing stages of cortical circuits.

115-HT7R activation activates adenylyl cyclase signaling and consequently the conversion of ATP to cAMP through coupling to Gαs. Although cAMP activation is commonly mediated by the PKA, it has been demonstrated that Epac, a member of the cAMP-regulated guanine nucleotide exchange family, has a crucial role in PKA-independent signaling. For instance, 5-HT7Rs activate the MAPK/ERK signaling pathway via the stimulation of the Epac factor. Binding of cAMP to Epac leads to the activation of several other signaling pathways (reviewed by ).

12The expression of 5-HT1AR and 5-HT7R in the limbic system support a role in the modulation of functions like mood, memory processing as well as emotional association with memory. The 5-HT1AR has been proposed to modulate anxiety based on studies with 5-HT1AR knockout mice and the response to antidepressant drugs. Several partial 5-HT1AR agonists, e.g., buspirone, have been used to treat anxiety and depression, whereas co-administration of pindolol (β-adrenergic and 5-HT1AR antagonist) with SSRIs enhances their therapeutic efficacy and shortens their onset of action (reviewed by ). A considerable body of literature demonstrates the 5-HT1AR involvement in various hippocampus-dependent learning and memory tasks (reviewed by ).

13In contrast, the available data on the function of 5-HT7R is relatively limited, mainly due to the lack of selective agonists specific for this 5-HTR subtype. The physiological role of 5-HT7R has been closely linked with the regulation of sleep, circadian rhythm, pain and also mood (reviewed by ). Accumulating data implicates the 5-HT7R in the action of antidepressant drugs, whereas the results from anxiety studies are contradictory. Interestingly, studies using 5-HT7R knockout mice revealed the crucial role of this receptor in hippocampus-dependent memory.

14Agents that act as receptor ligands may be agonists or antagonists. Agonists initiate physiological changes by activating downstream signaling pathways, whereas antagonists bind to receptors without producing any effect. Ligands can be divided in three categories based on their function:

15Full agonists produce a maximal response equivalent to the endogenous agonist (here 5-HT). These agonists have high efficacy (i.e., the ability to initiate changes which leads to effects) for the binding receptor.

16Partial agonists are not capable of producing the maximal functional response even when they occupy the entire receptor population. These agonists present intermediate efficacy. Respectively, we could refer to partial antagonists that bind to the active site (competitive antagonism) but do not completely abolish the receptor-mediated effects.

17Mixed profile ligands that (appear to) act both as agonists and as antagonist in distinct receptor populations. More likely, they have different agonist profiles at different receptor sites (e.g., pre-versus postsynaptic 5-HT1AR) and therefore appear to exert antagonist function in the presence of a full agonist, while acting as weak (partial) agonist thereby lowering the efficacy of the full agonist.

18The function of any ligand used to study the role of 5-HT1AR and 5-HT7R is essential for the correct interpretation of the behavioral outcome. It is also important to mention that the intrinsic efficacy of a ligand is equally depended on the characteristics of response system; in our case the different brain populations of 5-HT1AR and 5-HT7R and their downstream signaling pathways. Agonists acting on the same receptor can produce different effects depending on their physicochemical properties, brain distribution, full or partial agonism as well as the number of coupled receptors in a brain area. The specificity of the compounds used is another very important characteristic that should be always taken into consideration and is referred to the ligand’s specific binding to the targeted receptor. Ligands with low specificity cannot be used to clarify the functional role of 5-HT1AR and 5-HT7R, since the produced effects can be also mediated via the binding to other proteins than the receptor of interest.

19The physicochemical properties of compounds play an essential role for the drug uptake and diffusion with lipophilicity, solubility and molecular mass being among the most important properties. The lipophilic nature of ligands is particularly important when they are administered locally. Increasing lipophilicity leads to enhanced blood–brain barrier diffusion, prevents the drug restriction in the area of interest and consequently produces wider effects, despite local application. This is evident from dorsohippocampal infusion of the blood–brain barrier penetrating drug 8-OH-DPAT, a full 5-HT1AR agonist, which impairs tone-dependent memory, whereas this does not occur when the NMDAR antagonist APV and the GABAAR agonist muscimol are locally applied. The latter study is one of the few demonstrating the selective drug action in the dorsal hippocampus based on fluorescently labeled muscimol as bodipy conjugate. Besides the solubility of compounds and the applied dose, it is thus of high importance to consider other physico-chemical properties, such as half-life in vivo, to avoid misleading conclusions due to their wider spread (e.g., diffusion or potential active transport) in brain outside the target sites. The molecular weight of compounds can also provide valuable information about the diffusion capacity.

20The prototypic 5-HT1AR agonist 8-OH-DPAT was the first full agonist developed and is still the most widely used to study the functional role of 5-HT1AR in behavioral manipulations. Despite its high selectivity for the 5-HT1AR, 8-OH-DPAT also acts as a 5-HT7R agonist and observed effects can be the result of an interplay between the two receptor subtypes (see below).

21Additionally, several full and partial agonists have been synthesized (see Table 1), but only a few of them have been used in fear learning studies, such as the buspirone and tandospirone. Buspirone belongs to the arylpiperazine (partial) agonists and acts also as antagonist with high specificity for the dopamine D2 receptor. Tandospirone (SM-3997) is a 5-HT1AR partial agonist and was initially studied for its anxiolytic properties in rats and mice. Similar to buspirone, tandospirone also exhibits dopamine antagonist action with a potency that is considerably lower than the one for the 5-HT1AR. An overview of currently available 5-HT1AR agonists is provided in Table 1.

22WAY-100635 and NAD-299 are the most commonly used selective antagonists in the study of the 5-HT1AR. Both ligands have high potencies and penetrate easily into the brain. However, NAD-299 was found to have higher selectivity for the 5-HT1AR than WAY-100635.

23The last years novel compounds have been used to assess the role of 5-HT1AR in emotional learning, such as the potent and selective 5-HT1AR antagonists SRA-333 (lecozotan; ), MC18 fumarate and VP08/34 fumarate.

24The agents that were initially used as 5-HT1AR antagonist were 2-methoxyphenylpiperazine derivatives with structural similarity to buspirone, such as BMY-7378 and NAN-190. However, these ligands were characterized as partial 5-HT1AR antagonist with antagonist properties only at the postsynaptic HT1AR and lower affinity for the α-adrenergic receptors.

25Finally, S-15535 is reported to act as a postsynaptic 5-HT1AR antagonist while also behaving as an agonist on presynaptic 5-HT1A autoreceptors, and therefore, it is characterized as a mixed profile ligand. However, a more recent study indicates predominantly weaker agonist activity of S-15535 at postsynaptic 5-HT1ARs. An overview of currently available 5-HT1AR antagonists is provided in Table 2.

26The lack of selective and potent 5-HT7R agonists is one of the major limitations to study the role of 5-HT7R in learning and memory. Currently, only a few selective 5-HT7R agonists exist and even less has been used in learning and memory studies. AS-19 and LP-44 are highly selective but low efficacy (partial) HT7R agonists whose functional role in fear learning was recently assessed. LP-211 is a novel highly selective 5-HT7R agonists but it has so far only been tested in an autoshaping Pavlovian/instrumental learning task. An overview of currently available 5-HT7R agonists is provided in Table 3.

27SB-258719 is the first selective 5-HT7R antagonist described but has not yet been used to investigate the role of 5-HT7R in the modulation of emotional learning. Both SB-656104-A and SB-269970 possess high potency and selectivity for 5-HT7R. These are the most commonly used 5-HT7R antagonists in behavior studies. An overview of currently available 5-HT7R antagonists is provided in Table 3.

28The experimental studies on emotional learning and memory in animals are based originally on psychological analysis of conflict behavior involving approach and avoidance of conditioned stimuli. Traditionally, the assays used to investigate animal behavior are based on the association of pleasant (i.e., motivationally related reward like food) or aversive stimuli (i.e., conditions related to negative feelings like pain and danger) to environmental cues involving classical (Pavlovian) or instrumental conditioning.

29The FC and the PA tasks are the most commonly used associative learning paradigms based on contextual fear learning. This type of learning is dependent on the operation of neuronal circuits in the limbic system, such as hippocampus and amygdala as demonstrated by us in mice (e.g.,,; ). Unlike FC, PA also includes instrumental learning. In the step-through PA test, the animal needs to suppress its innate preference for the dark compartment (where it previously received a foot shock) and remain in the bright compartment. In the step-down PA paradigm, however, the retention is examined in the dark compartment, where the animal received the foot shock (unconditioned stimulus) after stepping down from an elevated platform. The PA test procedure can be modified to examine any facilitating effect of the treatment on PA retention. More specific information on the PA task is provided elsewhere. A refined version of this task may provide for better translational aspects to assess pathological fear states such as post-traumatic-like responses based on deliberate choice of mice.

30The single-trial learning design of FC and PA, which is sufficient to establish long-term and remote memory, allows the exact timing of the drug treatment in relation to training and retention test. Thereby, unlike multi-session tasks, one-trial tasks provide a unique advantage to study learning mechanisms as well as drug effects (here 5-HT1AR and 5-HT7R ligands) on the different phases of learning and memory, i.e., the acquisition phase that consists of encoding and early consolidation, consolidation, the recall (retrieval and expression) phase as well as the extinction phase and reconsolidation.

31An overview of the behavioral effects of various 5-HT1AR ligands is provided in Table 4.

32Despite the differences among the 5-HT1AR ligands in their chemical and pharmacological features (e.g., receptor selectivity and partial or full agonist properties; see Tables 1 and 2), there is strong evidence for the impairing effect of postsynaptic 5-HT1AR activation on fear memory. Systemic, pretraining administration of the full 5-HT1AR agonist 8-OH-DPAT shows a biphasic effect on PA performance, with the low dose range (0.01, 0.03 mg/kg) facilitating and the high dose range (0.1–1 mg/kg) impairing PA retention 24 h after training in both rats and mice. The impairing dose of 8-OH-DPAT (0.2 and 0.3 mg/kg) also induces signs of the serotonin syndrome linking the postsynaptic 5-HT1AR to the learning deficits. In line with these results, FC studies demonstrated that pretraining systemic injections of high doses (0.1–0.5 mg/kg) of 8-OH-DPAT impair fear learning. Pretreatment with the selective 5-HT1AR antagonist WAY-100635 (0.03–1 mg/kg) blocked the impairment in freezing (FC) and transfer latency (PA), confirming and extending the detrimental role of the postsynaptic 5-HT1AR activation on memory acquisition.

33The observed memory deficit was already present in short-term memory tests performed 1 h after training for FC retention and 5 min after PA training. Thus, postsynaptic 5-HT1AR activation specifically impairs memory encoding of the aversive experience and not memory consolidation. In agreement to that observation, immediate 8-OH-DPAT post-training administration did not alter PA or FC retention.

34Intracranial administration of 5-HT1AR agonists and/or antagonists was used to further elucidate the distinct function of pre- versus postsynaptic 5-HT1ARs in fear learning. Pre- but not post-training intra-hippocampal infusion of 8-OH-DPAT impairs contextual FC, pointing at the important role of the postsynaptic 5-HT1AR in acquisition processes as observed after systemic administration.

35Unlike the unambiguous implication of the postsynaptic 5-HT1AR in memory acquisition, its role in fear retrieval and expression is less clear. The systemic 5-HT1AR agonist NDO-008 (0.5 mg/kg) administered before the retention test to rats impairs slightly PA performance. In contrast, systemic administration of buspirone at the dose of 1 and 3 mg/kg had no effect on fear expression in mice. These different effects may partly depend on the readouts and the side effects elicited by higher 5-HT1AR dosages, such as the hypolocomotion induced together with the serotonin syndrome. The hypolocomotion confounds the interpretation of fear expression results in mice when based on freezing. Moreover, it also possible that differences exists between rats and mice, although our own data shows high similarity of results in these two species.

36Therefore, a recent study tried to clarify the role of the 5-HT1AR in fear recall, by assessing the effect of 8-OH-DPAT on fear-conditioned HR responses (reviewed by ) upon training and 24 h after training, in mice. Systemic pretest administration reduced the conditioned maximum HR as a consequence of the significantly reduced baseline HR before the presentation of the conditioned stimulus (tone). However, the tone-induced HR increase was preserved during the retention of auditory fear in mice with similar magnitude as compared to that in controls. Additionally, 8-OH-DPAT reduced the unconditioned tachycardia elicited by novelty exposure as a consequence of altered HR dynamics indicating autonomic dysregulation with enhanced parasympathetic function through postsynaptic 5-HT1AR activation. Thus, the claims of anxiolytic actions of pretest injection of 5-HT1AR agonists as initially reported in human studies and partly in animal models cannot be supported unambiguously at least in learned fear experiments.

37Local administration approaches tried to distinguish the role of the post- versus the presynaptic 5-HT1AR in the different aspects of fear expression. Bilateral microinjections of a selective 5-HT1AR agonist flesinoxan decreased the expression of conditioned contextual freezing when injected into the hippocampus or amygdala but not in the medial prefrontal cortex, as well as the fear-potentiated startle responses when infused into the central amygdala.

38The role of 5-HT1A autoreceptors in fear expression was also studied by pretest infusion of 8-OH-DPAT into the median raphe nuclei. This resulted in impaired contextual freezing responses, but not fear-potentiated startle suggesting the existence of raphe-dependent serotonergic regulation that appears to modulate the freezing response to the aversive context. In contrast, hippocampal 8-OH-DPAT impaired the expression of both contextual freezing and fear-potentiated startle. However, 8-OH-DPAT mediates hyperlocomotion in rats (but hypolocomotion in mice) leading to a similar problem of potentially confounded interpretation of freezing performance during the drug state as mentioned before for mice.

39In contrast to the well-studied implication of 5-HT1ARs on memory acquisition and recall, there is only one study with 5-HT1AR ligands on fear extinction. The systemic 5-HT1AR agonist buspirone abolishes the fear extinction in mice. Similarly, the systemic 5-HT1AR antagonist WAY-100635 before a second sampling trial impaired the extinction of object recognition memory in rats. Further studies are needed to determine the precise role of 5-HT1ARs in memory extinction and/or reconsolidation in emotional learning tasks. Furthermore, local rather than systemic approaches are necessary to identify the neurocircuitry involved in these processes. The roles of other 5-HTRs in fear learning and the consequences of altered 5-HT neurotransmission on fear extinction are reviewed by.

40The paucity of studies 5-HT7R functions on emotional learning is mainly due to the lack of selective ligands, especially agonists (;;; see Table 5 and text above). Recent data from an autoshaping task showing that the 5-HT7R agonist, LP-211, when administered systematically after the training session, reversed scolopamine-induced amnesia, in rats. The same group also shows a facilitating effect on memory formation by the 5-HT7R agonist AS-19 administered after an autoshaping training session. The enhancing effect of 5-HT7Rs on memory consolidation was blocked by pre-injection of the 5-HT7R antagonist SB-269970 indicating the specific involvement of the 5-HT7R.

41investigated the role of 5-HT7R on emotional learning in mice using a step-through PA paradigm. Pretraining systemic administration of the 5-HT7R antagonist SB-269970 enhanced the impairing effect of low doses of 8-OH-DPAT. This result supports the notion that 5-HT7R activation has a beneficial modulatory role in learning opposing the function of 5-HT1AR activation. Accordingly, pretraining 5-HT7R activation by the combined use of the 5-HT1AR antagonist NAD-299 with the 5-HT1AR and 5-HT7R agonist 8-OH-DPAT facilitated PA retention. This PA facilitation by NAD-299 together with 8-OH-DPAT was again blocked by the 5-HT7R antagonist SB-269970 indicating a procognitive effect of 5-HT7R activation by this drug combination. However, the 5-HT7R agonists LP-44 and AS-19 failed to mediate this PA facilitation, despite dose-dependent tests. Despite their high in vitro potency to stimulate intracellular signaling cascades, the 5-HT7R agonists LP-44 and AS-19 have moderate agonist efficacy in vivo. This finding is in agreement with previous pharmacological characterization in vivo and may explain why the facilitatory effect of NAD-299 with 8-OH-DPAT could not be mimicked by the putative agonists LP-44 and AS-19.

42To further address the role of 5-HT7Rs on emotional learning, performed hippocampal infusions with the 5-HT7R agonist AS-19 in mice. Since they failed to find clear facilitatory effects, as observed after systemic treatment, they concluded that “5-HT7Rs appear to facilitate memory processes in a broader cortico-limbic network and not the hippocampus alone.” The failure of the SB-269970 to enhance emotional memory, upon hippocampal infusions, may be the consequence of the low dose that can be locally infused due to the relatively poor solubility of SB-269970. However, systemic administration of this 5-HT7R antagonist fully blocked the PA facilitation observed after 5-HT1AR blockade. Hence, the hippocampus-dependent involvement of the 5-HT7Rs needs to be re-investigated with selective highly potent 5-HT7R agonists, because also the low potency of AS-19 may have contributed to the lack of effects by dorsohippocampal 5-HT7R agonist application on PA. Finally, although the role of 5-HT7R in memory consolidation has been suggested, there are currently insufficient data supporting this view. More work is also required to clarify the role of 5-HT7R in memory extinction and reconsolidation, which are both essentially unexplored.

43The interaction of the two 5-HTR subtypes in emotional learning has been studied by using 8-OH-DPAT, which exerts agonistic effects for both 5-HT1ARs and 5-HT7Rs. To dissect the function of these 5-HTRs, pre-treatment with selective 5-HT1AR antagonists is used to exclusively activate 5-HT7R. were the first to suggest the functional interplay between the two 5-HTRs on the behavioral level as the activation of 5-HT7R counteracted the 5-HT1AR-mediated impairments in PA performance. The interaction between the two 5-HTRs and their functional antagonism was then extended by experiments in mice, demonstrating that 5-HT7R activation and concomitant 5-HT1AR blockade leads to PA facilitation. The facilitatory effect on emotional memory by the 5-HT1A antagonist NAD-299 was related to stimulation of 5-HT7Rs under conditions with reduced 5-HT1AR transmission. These findings suggest that the states of 5-HT1ARs and 5-HT7Rs play a critical role for 5-HT effects on emotional memory. Consequently, the elevation of endogenous 5-HT via SSRIs will most likely result in differential cognitive/emotional effects depending on genetic and/or epigenetic regulation and occupancy of these two 5-HTRs in health and disease. This condition will affect the expression of the 5-HT1AR and change the relative balance between 5-HTR subtypes, which together will eventually determine the physiological actions of 5-HT and the clinical efficacy of SSRI treatment.

44As described above, 5-HT1ARs and 5-HT7Rs mediate opposing effects regarding the neuronal excitability. 5-HT1AR activation reduces the activity of adenyl cyclase, whereas 5-HT7R activation stimulates adenyl cyclase activity and thereby increases intracellular cAMP thereby increasing neuronal excitability. Accordingly, 5-HT7R stimulation in the hippocampus was found to activate pyramidal neurons, unlike 5-HT1AR activation which inhibited pyramidal neurons. Both 5-HTRs are expressed in glutamatergic hippocampal pyramidal neurons. Therefore, it is likely that 5-HT1AR and 5-HT7R stimulation decreases and increases glutamate release in the hippocampus, respectively. In line with these results, 5-HT7R activation enhances the AMPA receptor-mediated synaptic currents on CA1 pyramidal neurons, whereas 5-HT1AR activation inhibits the AMPA receptor-mediated transmission between CA3 and CA1 pyramidal neurons in both pre- and postsynaptic sites. However, the 5-HT1AR-mediated inhibitory effect on glutamatergic neurotransmission was stronger than the 5-HT7R-mediated facilitatory effect. One explanation for the increased effectiveness of 5-HT1AR in controlling the input from the Schaffer collaterals may stem from the different localization of the two receptors on the CA1 pyramidal neurons: 5-HT7Rs are found on the cell bodies, whereas the 5-HT1ARs appear to be mainly localized on dendrites.

45Differences in the expression of the receptors could also play an essential role in their distinct activation pattern from the endogenous 5-HT. The progressive reduction of post-synaptic 5-HT7R levels during postnatal development, together with the maintenance of the expression level of 5-HT1AR, could increase the ratio of membrane 5-HT1ARs over 5-HT7Rs. Consequently, a model has been proposed regarding the molecular mechanisms that underlie the regulation of the 5-HT1ARs and 5-HT7Rs. 5-HT1AR and 5-HT7R form heterodimers both in vitro and in vivo. This heterodimerization plays a functional role by decreasing Gi protein coupling of the 5-HT1AR and by reducing the ability of 5-HT1AR to activate potassium channels, without affecting the Gs protein coupling of the 5-HT7R. The heterodimerization additionally contributes to the desensitization of the 5-HT1AR through facilitated internalization.

465-HT1AR and 5-HT7R are co-localized in the cell membrane of hippocampal neurons, where their heterodimerization induces an inhibitory effect on the 5-HT1AR-mediated activation of potassium channels in hippocampal neurons. As mentioned above the post-synaptic levels of 5-HT7R are lower compared to the expression levels of post-synaptic 5-HT1AR, whereas this is not the case for the pre-synaptic 5-HT7R. These regional differences in the 5-HT7R levels and therefore in the concentration of the heterodimers, can explain the preferential desensitization of 5-HT1A autoreceptors by SSRIs and more generally the region- and cell- specific differences in the signaling pathway mediated by the 5-HT1AR activation (see ). In summary, the above data suggest that the positive or negative consequences of a drug on emotional memory and cognition depend on the relative level of 5-HTR expression and, its efficacy in activating different receptors with their downstream signaling pathways.

47Genetic and/or epigenetic effects regulate the receptor’s state and eventually define the physiological actions of endogenous 5-HT. A characteristic example is the Ala50Val variant of the 5-HT1AR, located in the transmembrane region 1, that leads to loss of response to 5-HT and consequently to the interruption of 5-HT signaling. Moreover, the human polymorphism Gly22Ser attenuates the downregulating effect induced by long-term 8-OH-DPAT stimulation in comparison to the Val28 variant and wild-type without effect on the ligand binding capacity. It is suggested that individuals with the Ser22 variant have higher sensitivity to SSRIs treatment since its serotonergic effect depends on the efficiency of 5-HT1AR transmission. Furthermore, carriers of the short (s) allele of the 5-HT transporter promotor region possess behavioral abnormalities, such as increased levels of anxiety and FC as well as stronger fear potentiated startle in comparison to long (l) allele carriers. Accordingly, the therapeutic efficacy of SSRIs is reduced in patients homozygous for the s-allele when compared with heterozygous or l-allele carriers.

48The epigenetic regulation of 5-HTR subtypes is also implicated in the differential emotional and cognitive modulation induced by the serotonergic signaling. It is widely accepted that 5-HT1AR binding is reduced in the brain of depressed humans (e.g., ) as well as in stressed rats (e.g., ) as indication of epigenetic modulation. 5-HT1AR activation in the basolateral amygdala and the prelimbic area of the prefrontal cortex in low-anxious rats reduced fear potentiated startle, whereas 5-HT1AR activation in the periaqueductal gray of high-anxious rats had the opposite effect. These findings highlight how environmental conditions can contribute to individual differences in 5-HT1AR-mediated response differences. In line with this, single-housed mice display a stronger hypothermic effect upon 5-HT1AR activation by 8-OH-DPAT, which is associated with an increased depressive-like state, in comparison to their group-housed counterparts. However, the mechanisms underlying the inter-individual differences in serotonergic signaling and consequently in cognitive and emotional modulation are not clear yet.

49A linkage disequilibrium study identified two polymorphisms (rs3808932 and rs12412496) in the human HTR7 suggesting that it is a schizophrenia susceptibility gene. However, to the best of our knowledge, there is no evidence for the effect of 5-HT7R polymorphisms on serotonergic signaling or the interaction between polymorphisms of 5-HT7 and 5-HT1ARs. Therefore, to elucidate the functional interaction between 5HT1AR and 5-HT7R, it is of high importance to understand which polymorphisms influence the expression of those 5-HTRs and how these changes affect emotional and cognitive functions. This knowledge could potentially reveal the polymorphisms that modulate the endophenotypes of different affective disorders, closely linked with the function of 5-HT1AR and 5-HT7R, such as anxiety and depression.

50In contrast to the above electrophysiological results, in vivo microdialysis in awake rats showed that the local blockade of 5-HT1AR increased extracellular acetylcholine (ACh) levels but failed to show changes in hippocampal glutamate release in the ventral hippocampus and the prefrontal cortex. The result with ACh is consistent with the pro-cognitive effect of (postsynaptic) 5-HT1AR blockade in PA. However, the expected glutamate increase may not be detectable because of the limited capacity of microdialysis to detect small transmitter changes restricted to the synaptic cleft. More sensitive techniques are required such as enzyme-based microelectrode amperometry, which is selective for the detection of extracellular glutamate with (1) spatial resolution in the μm level, (2) sub-second temporal resolution and (3) sensitivity in the μm range of glutamate. This novel technology is suited to provide evidence for the expected enhancement of glutamatergic transmission in the hippocampus by both 5-HT1AR inhibition and 5-HT7R activation.

51It is clear that the impairing effects of low dose NMDA receptor antagonists (e.g., MK-801) and cholinergic antagonist (e.g., scopolamine) can be prevented by serotonergic manipulations. Thus, these two pharmacological models of cognitive impairment relevant for Alzheimer’s disease are both alleviated by 5-HT1AR inhibition demonstrating a role for both enhanced glutamatergic and cholinergic transmission for improved cognitive function (e.g.,; ). An overview of these modulatory effects is provided in Figure 1.

52During the last three decades many studies have indicated important regulatory functions of 5-HT signaling for emotional and cognitive functions. However, the complexity of the serotonergic signaling due to the existence of at least 14 pre- and postsynaptic 5-HTRs subtypes with multiple transduction mechanisms makes it exceedingly difficult to assign unambiguously the physiological and behavioral role of a single 5-HTR subtype. However, the use of specific ligands in combination with systemic and intrahippocampal administration, receptor autoradiography and in vivo neurochemical measurements are powerful tools in identifying the action of specific ligands in local networks of the brain including subareas of the hippocampus. This approach, combined with in vivo electrophysiology and genetic tools, can also better define the functional role of 5-HT in the neuronal circuitry underlying cognitive function.

53Overall a number of open questions need to be answered to further improve our understanding of the role of serotonergic signaling via the different 5-HTRs in health and disease:

54How do 5-HTRs modulate hippocampal and cortical glutamatergic transmission with a focus on activation and inhibition of 5-HT1ARs and 5-HT7Rs? This needs to be determined with newly developed amperometry methods in in vivo recordings.

55What are the roles of 5-HT1ARs and 5-HT7Rs in defined hippocampal subregions for emotional and cognitive functions? This requires the development of new ligands with low lipophilicity for local actions tested in vivo. Alternatively, is should be possible to shut down the second messenger coupling of neurons selectively expressing 5-HT1ARs and 5-HT7Rs by Designer Receptors Exclusively Activated by Designer Drugs (DREADD) technology.

56What are the roles of 5-HT1ARs and 5-HT7Rs in different memory phases? As indicated there are considerable inconsistencies about the role of 5-HT1ARs and 5-HT7Rs in the consolidation process. In addition, extinction and reconsolidation are so far poorly explored.

57The regulation of 5-HTR expression has so far focused on the 5-HT1AR. This needs to be extended to other 5-HTRs including the 5-HT7R. Besides the use of radio-ligands in imaging studies, the subcellular immunohistochemical analyses of 5-HTR protein levels requires the development of specific antibodies.

58Finally, despite the evidence of the beneficial effects of 5-HT1AR antagonists in preclinical models, the therapeutic potential to facilitate cholingergic and/or glutamatergic neurotransmission for improved cognitive function in human neuropathology (e.g., Alzheimer’s disease) or in aging is so far not explored.

59The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.