The 5-HT5A receptor regulates excitability in the auditory startle circuit: functional implications for sensorimotor gating

Participation of Hippocampal 5-HT5A, 5-HT6 and 5-HT7 Serotonin Receptors on the Consolidation of Social Recognition Memory

Social recognition is the ability of animals to identify and recognize a conspecific. The consolidation of social stimuli in long-term memory is crucial for the establishment and maintenance of social groups, reproduction and species survival. Despite its importance, little is known about the circuitry and molecular mechanisms involved in the social recognition memory (SRM). Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator, which plays a key role in learning and memory. Focusing on the more recently described 5-HT receptors, we investigated in the CA1 region of the dorsal hippocampus the participation of 5-HT_5A, 5-HT₆ and 5-HT₇ receptors in the consolidation of SRM. Male Wistar rats cannulated in CA1 were subjected to a social discrimination task. In the sample phase the animals were exposed to a juvenile conspecific for 1 h. Immediately after, they received different pharmacological treatments. Twenty-four hours later, they were submitted to a 5 min retention test in the presence of the previously presented juvenile (familiar) and a novel juvenile. The animals that received infusions of 5-HT_5A receptor antagonist SB-699551 (10 µg/µL), 5-HT₆ receptor agonist WAY-208466 (0.63 µg/µL) or 5-HT₇ receptor agonist AS-19 (5 µg/µL) intra-CA1 were unable to recognize the familiar juvenile. This effect was blocked by the coinfusion of WAY-208466 plus 5-HT₆ receptor antagonist SB-271046 (10 µg/µL) or AS-19 plus 5-HT₇ receptor antagonist SB-269970 (5 µg/µL). The present study helps to clarify the neurobiological functions of the 5-HT receptors more recently described and extends our knowledge about mechanisms underlying the SRM.

Structure-Based Design of a Chemical Probe Set for the 5-HT5A Serotonin Receptor

The 5-HT_5A receptor (5-HT_5AR), for which no selective agonists and a few antagonists exist, remains the least understood serotonin receptor. A single commercial antagonist, SB-699551, has been widely used to investigate the 5-HT_5AR function in neurological disorders, including pain, but this molecule has substantial liabilities as a chemical probe. Accordingly, we sought to develop an internally controlled probe set. Docking over 6 million molecules against a 5-HT_5AR homology model identified 5 mid-μM ligands, one of which was optimized to UCSF678, a 42 nM arrestin-biased partial agonist at the 5-HT_5AR with a more restricted off-target profile and decreased assay liabilities versus SB-699551. Site-directed mutagenesis supported the docked pose of UCSF678. Surprisingly, analogs of UCSF678 that lost the 5-HT_5AR activity revealed that 5-HT_5AR engagement is nonessential for alleviating pain, contrary to studies with less-selective ligands. UCSF678 and analogs constitute a selective probe set with which to study the function of the 5-HT_5AR.

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Serotonin type 5A receptor antagonists inhibit D-lysergic acid diethylamide discriminatory cue in rats

PURPOSE

Like other psychedelics, D-lysergic acid diethylamide (LSD) affects numerous serotonin receptors, and according to the current dogma, the 5-HT_2A receptors are considered the main target for its hallucinogenic effects. LSD, however, also displays agonistic activity at the 5-HT_5A receptors, which mediate some of LSD-induced behavioural effects.

METHODS

Using male Sprague Dawley rats, we examined the effects of 5-HT_2A and 5-HT_5A receptor antagonists on LSD-induced stimulus control in the two-lever drug discrimination test using a FR10 schedule of reinforcement.

RESULTS

In animals trained to discriminate 0.08 mg/kg LSD from vehicle 15 minutes after injection, LSD produced dose-related increases in response, with an ED50 (±95% confidence limits) of 0.0384 (± 0.025-0.051) mg/kg). LSD-like responses were observed when the training dose of LSD was given 5-30 but not 90 minutes before the test. Confirming earlier reports, the 5-HT antagonist ketanserin (2 mg/kg) attenuated the LSD response in 50% of rats, and due to pretreatment with 0.2 and 2 mg/kg MDL 100907, 63% and 67% of animals, respectively, failed to select the LSD lever. We then investigated the effects of two 5-HT_5A receptor antagonists, and we found that 56% and 60% of rats pretreated with 3 and 10 mg/kg SB 699551, respectively, failed to select the LSD lever. Due to pretreatment with 0.01 mg/kg ASP 5736, 58% of rats did not select the LSD lever. This dose also reduced the response rate but not the number of rats failing to complete the test.

CONCLUSIONS

The present results suggest that antagonists of the 5-HT_5A receptor may inhibit subjective effects of LSD in rats.

Social context influences sensorimotor gating in female African cichlid fish Astatotilapia burtoni

Disruption in prepulse inhibition (PPI), a sensorimotor gating phenomenon found in many species, has been associated with various psychiatric disorders in humans. Social defeat has been identified as a mediator of naturally evoked reductions of PPI in African cichlid fish Astatotilapia burtoni where males reversibly alter social status and their sensorimotor gating abilities. Here we investigated A. burtoni females, which establish a male-like social hierarchy with dominant (DOM) and subordinate (SUB) individuals when housed in communities without males. We asked if DOM and SUB females demonstrate socially induced PPI differences comparable to their male DOM and SUB counterparts. Results suggest that social defeat reduced PPI in SUB females as compared to DOM females (p = 0.033) and mixed-sex community female controls (p = 0.017). However, socially defeated females in same-sex communities remained proactive when engaging in antagonistic behaviors, which appears beneficial in avoiding substantial reductions in PPI as seen in reactive, socially defeated males. In open field swimming tests, SUB females exhibited increased anxiety-related behavior (thigmotaxis) as compared to females from mixed-sex communities (COM). Taken together, our results emphasize social defeat is a reliable modulator of PPI independent of sex, and anxiety related to social defeat might be a factor in mediating PPI plasticity.

Does Valproic Acid/Na Valproate Suppress Auditory Startle Reflex in Patients With Epilepsy?

BACKGROUND AND OBJECTIVE

Auditory startle response (ASR) was normal in juvenile myoclonic epilepsy whereas it was suppressed in progressive myoclonic epilepsy. However, both groups were using valproic acid/Na valproate (VPA) in different doses. Therefore, we aimed to analyze whether VPA has an impact on ASR in a cohort of epilepsy. For this purpose, we included patients with epilepsy and analyzed ASR in patients who were using VPA.

PATIENTS AND METHOD

We included 51 consecutive patients who had epilepsy and were using VPA between January 2014 and January 2016. Two control groups of 37 epilepsy patients using other antiepileptic drugs (AEDs) and of 25 healthy subjects were also constituted. All participants underwent investigations of ASR and startle response to somatosensory inputs (SSS) under similar conditions.

RESULTS

An analysis of patients using VPA, not using VPA and healthy subjects revealed significantly longer latency and lower probability of orbicularis oculi (O.oc) and sternocleidomastoid responses after auditory stimulation, decreased total ASR probability and longer latency of O.oc response after somatosensory stimulation in patient groups compared with healthy subjects. Multivariate analysis showed type of AED had a role in the generation of abnormalities. VPA, carbamazepine, and multiple AED use caused suppression of ASR. Total ASR probability was decreased or O.oc latency got longer with longer duration of VPA use whereas serum VPA level at the time of investigation did not correlate with total ASR probability.

DISCUSSION

Both ASR and SSS are suppressed by the effect of VPA, especially in patients using for a long period and in patients using other AEDs with VPA. Given the fact that VPA leads to long-standing synaptic changes of dopaminergic transmission, abnormalities of this network may be the more likely cause.

Differential processing in modality-specific Mauthner cell dendrites

KEY POINTS

The present study examines dendritic integrative processes that occur in many central neurons but have been challenging to study in vivo in the vertebrate brain. The Mauthner cell of goldfish receives auditory and visual information via two separate dendrites, providing a privileged scenario for in vivo examination of dendritic integration. The results show differential attenuation properties in the Mauthner cell dendrites arising at least partly from differences in cable properties and the nonlinear behaviour of the respective dendritic membranes. In addition to distinct modality-dependent membrane specialization in neighbouring dendrites of the Mauthner cell, we report cross-modal dendritic interactions via backpropagating postsynaptic potentials. Broadly, the results of the present study provide an exceptional example for the processing power of single neurons.

ABSTRACT

Animals process multimodal information for adaptive behavioural decisions. In fish, evasion of a diving bird that breaks the water surface depends on integrating visual and auditory stimuli with very different characteristics. How do neurons process such differential sensory inputs at the dendritic level? For that, we studied the Mauthner cells (M-cells) in the goldfish startle circuit, which receive visual and auditory inputs via two separate dendrites, both accessible for in vivo recordings. We investigated whether electrophysiological membrane properties and dendrite morphology, studied in vivo, play a role in selective sensory processing in the M-cell. The results obtained show that anatomical and electrophysiological differences between the dendrites combine to produce stronger attenuation of visually evoked postsynaptic potentials (PSPs) than to auditory evoked PSPs. Interestingly, our recordings showed also cross-modal dendritic interaction because auditory evoked PSPs invade the ventral dendrite (VD), as well as the opposite where visual PSPs invade the lateral dendrite (LD). However, these interactions were asymmetrical, with auditory PSPs being more prominent in the VD than visual PSPs in the LD. Modelling experiments imply that this asymmetry is caused by active conductances expressed in the proximal segments of the VD. The results obtained in the present study suggest modality-dependent membrane specialization in M-cell dendrites suited for processing stimuli of different time domains and, more broadly, provide a compelling example of information processing in single neurons.

Cellular Mechanisms of Cortisol-Induced Changes in Mauthner-Cell Excitability in the Startle Circuit of Goldfish

Predator pressure and olfactory cues (alarm substance) have been shown to modulate Mauthner cell (M-cell) initiated startle escape responses (C-starts) in teleost fish. The regulation of such adaptive responses to potential threats is thought to involve the release of steroid hormones such as cortisol. However, the mechanism by which cortisol may regulate M-cell excitability is not known. Here, we used intrasomatic, in vivo recordings to elucidate the acute effects of cortisol on M-cell membrane properties and sound evoked post-synaptic potentials (PSPs). Cortisol tonically decreased threshold current in the M-cell within 10 min before trending towards baseline excitability over an hour later, which may indicate the involvement of non-genomic mechanisms. Consistently, current ramp injection experiments showed that cortisol increased M-cell input resistance in the depolarizing membrane, i.e., by a voltage-dependent postsynaptic mechanism. Cortisol also increases the magnitude of sound-evoked M-cell PSPs by reducing the efficacy of local feedforward inhibition (FFI). Interestingly, another pre-synaptic inhibitory network mediating prepulse inhibition (PPI) remained unaffected. Together, our results suggest that cortisol rapidly increases M-cell excitability via a post-synaptic effector mechanism, likely a chloride conductance, which, in combination with its dampening effect on FFI, will modulate information processing to reach threshold. Given the central role of the M-cell in initiating startle, these results are consistent with a role of cortisol in mediating the expression of a vital behavior.

Neuromodulatory Regulation of Behavioral Individuality in Zebrafish

Inter-individual behavioral variation is thought to increase fitness and aid adaptation to environmental change, but the underlying mechanisms are poorly understood. We find that variation between individuals in neuromodulatory input contributes to individuality in short-term habituation of the zebrafish (Danio Rerio) acoustic startle response (ASR). ASR habituation varies greatly between individuals, but differences are stable over days and are heritable. Acoustic stimuli that activate ASR-command Mauthner cells also activate dorsal raphe nucleus (DRN) serotonergic neurons, which project to the vicinity of the Mauthner cells and their inputs. DRN neuron activity decreases during habituation in proportion to habituation and a genetic manipulation that reduces serotonin content in DRN neurons increases habituation, whereas serotonergic agonism or DRN activation with ChR2 reduces habituation. Finally, level of rundown of DRN activity co-segregates with extent of behavioral habituation across generations. Thus, variation between individuals in neuromodulatory input contributes to individuality in a core adaptive behavior. VIDEO ABSTRACT.

Differential effect of clomipramine on habituation and prepulse inhibition in dominant versus subordinate rats

Many patients with depression have comorbidities associated with an impairment of sensorimotor gating, such as e.g. schizophrenia, Parkinson Disease, or Alzheimer disease. Anti-depressants like clomipramine that modulate serotonergic or norepinephrinergic neurotransmission have been shown to impact sensorimotor gating, it is therefore important to study potential effects of clomipramine in order to rule out an exacerbation of sensorimotor gating impairment. Prior studies in animals and humans have been inconclusive. Since serotonin and norepinephrine levels are closely related to anxiety and stress levels and therefore to the social status of an animal, we tested the hypothesis that acute and chronic effects of clomipramine on sensorimotor gating are different in dominant versus subordinate rats, which might be responsible for conflicting results in past animal studies. We used habituation and prepulse inhibition (PPI) of the acoustic startle response as operational measures of sensorimotor gating. After establishing the dominant animal in pair-housed male rats, we injected clomipramine for two weeks and measured acute effects on baseline startle, habituation and PPI after the first injection and chronic effects at the end of the two weeks. Chronic treatment with clomipramine significantly increased habituation in subordinate rats, but had no effect on habituation in dominant animals. Furthermore, PPI was slightly enhanced in subordinate rats upon chronic treatment while no changes occurred in dominant animals. We conclude that the social status of an animal, and therefore the basic anxiety/stress level determines whether or not clomipramine has a beneficial effect on sensorimotor gating and discuss possible underlying mechanisms.

Glycine and GABAA receptors mediate tonic and phasic inhibitory processes that contribute to prepulse inhibition in the goldfish startle network

Prepulse inhibition (PPI) is understood as a sensorimotor gating process that attenuates sensory flow to the startle pathway during early stages (20–1000 ms) of information processing. Here, we applied in vivo electrophysiology and pharmacology to determine if PPI is mediated by glycine receptors (GlyRs) and/or GABA_A receptors (GABA_ARs) in the goldfish auditory startle circuit. Specifically, we used selective antagonists to dissect the contributions of target receptors on sound-evoked postsynaptic potentials (PSPs) recorded in the neurons that initiate startle, the Mauthner-cells (M-cell). We found that strychnine, a GlyR antagonist, disrupted a fast-activated (5 ms) and rapidly (<50 ms) decaying (feed-forward) inhibitory process that contributes to PPI at 20 ms prepulse/pulse inter-stimulus intervals (ISI). Additionally we observed increases of the evoked postsynaptic potential (PSP) peak amplitude (+87.43 ± 21.53%, N = 9) and duration (+204 ± 48.91%, N = 9). In contrast, treatment with bicuculline, a GABA_AR antagonist, caused a general reduction in PPI across all tested interstimulus intervals (ISIs) (20–500 ms). Bicuculline also increased PSP peak amplitude (+133.8 ± 10.3%, N = 5) and PSP duration (+284.95 ± 65.64%, N = 5). Treatment with either antagonist also tonically increased post-synaptic excitability in the M-cells, reflected by an increase in the magnitude of antidromically-evoked action potentials (APs) by 15.07 ± 3.21%, N = 7 and 16.23 ± 7.08%, N = 5 for strychnine and bicuculline, respectively. These results suggest that GABA_ARs and GlyRs are functionally segregated to short- and longer-lasting sound-evoked (phasic) inhibitory processes that contribute to PPI, with the mediation of tonic inhibition by both receptor systems being critical for gain control within the M-cell startle circuit.

Structure and function of serotonin G protein-coupled receptors

Serotonin receptors are prevalent throughout the nervous system and the periphery, and remain one of the most lucrative and promising drug discovery targets for disorders ranging from migraine headaches to neuropsychiatric disorders such as schizophrenia and depression. There are 14 distinct serotonin receptors, of which 13 are G protein-coupled receptors (GPCRs), which are targets for approximately 40% of the approved medicines. Recent crystallographic and biochemical evidence has provided a converging understanding of the basic structure and functional mechanics of GPCR activation. Currently, two GPCR crystal structures exist for the serotonin family, the 5-HT1B and 5-HT2B receptor, with the antimigraine and valvulopathic drug ergotamine bound. The first serotonin crystal structures not only provide the first evidence of serotonin receptor topography but also provide mechanistic explanations into functional selectivity or biased agonism. This review will detail the findings of these crystal structures from a molecular and mutagenesis perspective for driving rational drug design for novel therapeutics incorporating biased signaling.

The study of psychiatric disease genes and drugs in zebrafish

Mutations associated with psychiatric disease are being identified, but it remains unclear how the affected genes contribute to disease. Zebrafish is an emerging model to study psychiatric disease genes with a rich repertoire of phenotyping tools. Recent zebrafish research has uncovered potential developmental phenotypes for genes associated with psychiatric disorders, while drug screens have behaviorally characterized small molecules and identified new classes of drugs. Behavioral studies have led to promising models for endophenotypes of psychiatric diseases. While further research is needed to firmly link these models to psychiatric disorders, they are valuable tools for phenotyping genetic mutations and drugs. Recently developed tools in genome editing and in vivo imaging promise additional insights into the processes disrupted by mutations in psychiatric disease genes.

The Mauthner-cell circuit of fish as a model system for startle plasticity

The Mauthner-cell (M-cell) system of teleost fish has a long history as an experimental model for addressing a wide range of neurobiological questions. Principles derived from studies on this system have contributed significantly to our understanding at multiple levels, from mechanisms of synaptic transmission and synaptic plasticity to the concepts of a decision neuron that initiates key aspects of the startle behavior. Here we will review recent work that focuses on the neurophysiological and neuropharmacological basis for modifications in the M-cell circuit. After summarizing the main excitatory and inhibitory inputs to the M-cell, we review experiments showing startle response modulation by temperature, social status, and sensory filtering. Although very different in nature, actions of these three sources of modulation converge in the M-cell network. Mechanisms of modulation include altering the excitability of the M-cell itself as well as changes in excitatory and inhibitor drive, highlighting the role of balanced excitation and inhibition for escape decisions. One of the most extensively studied forms of startle plasticity in vertebrates is prepulse inhibition (PPI), a sensorimotor gating phenomenon, which is impaired in several information processing disorders. Finally, we review recent work in the M-cell system which focuses on the cellular mechanisms of PPI and its modulation by serotonin and dopamine.

Progressively increased M50 responses to repeated sounds in autism spectrum disorder with auditory hypersensitivity: a magnetoencephalographic study

The aim of this study was to investigate the differential time-course responses of the auditory cortex to repeated auditory stimuli in children with autism spectrum disorder (ASD) showing auditory hypersensitivity. Auditory-evoked field values were obtained from 21 boys with ASD (12 with and 9 without auditory hypersensitivity) and 15 age-matched typically developing controls. M50 dipole moments were significantly increased during the time-course study only in the ASD with auditory hypersensitivity compared with those for the other two groups. The boys having ASD with auditory hypersensitivity also showed more prolonged response duration than those in the other two groups. The response duration was significantly related to the severity of auditory hypersensitivity. We propose that auditory hypersensitivity is associated with decreased inhibitory processing, possibly resulting from an abnormal sensory gating system or dysfunction of inhibitory interneurons.

5-HT systems: emergent targets for memory formation and memory alterations

Drugs acting through 5-hydroxytryptamine (serotonin or 5-HT) systems modulate memory and its alterations, although the mechanisms involved are poorly understood. 5-HT drugs may present promnesic and/or antiamnesic (or even being amnesic) effects. Key questions regarding 5-HT markers include whether receptors directly or indirectly participate and/or contribute to the physiological and pharmacological basis of memory and its pathogenesis; hence, the major aim of this article was to examine recent advances in emergent targets of the 5-HT systems for memory formation and memory alterations. Recent reviews and findings are summarized, mainly in the context of the growing notion of memory deficits in brain disorders (e.g., posttraumatic stress disorder, mild cognitive impairment, consumption of drugs, poststroke cognitive dysfunctions, schizophrenia, Parkinson disease, and infection-induced memory impairments). Mainly, mammalian and (some) human data were the focus. At least agonists and antagonists for 5-HT1A/1B, 5-HT2A/2B/2C, 5-HT3, 5-HT4, 5-HT6, and 5-HT7 receptors as well as serotonin uptake inhibitors seem to have a promnesic and/or antiamnesic effect in different conditions and 5-HT markers seem to be associated to neural changes. Available evidence offers clues about the possibilities, but the exact mechanisms remain unclear. For instance, 5-HT transporter expression seems to be a reliable neural marker related to memory mechanisms and its alterations.