Trevor Sharp

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Catechol-o-methyltransferase inhibition improves set-shifting performance and elevates stimulated dopamine release in the rat prefrontal cortex.

The Val158Met polymorphism of the human catechol-O-methyltransferase (COMT) gene affects activity of the enzyme and influences performance and efficiency of the prefrontal cortex (PFC); however, although catecholaminergic neurotransmission is implicated, the underlying mechanisms remain elusive because studies of the role of COMT in PFC function are sparse. This study investigated the effect of tolcapone, a brain-penetrant COMT inhibitor, on a rat model of attentional set shifting, which is dependent on catecholamines and the medial PFC (mPFC). Additionally, we investigated the effect of tolcapone on extracellular catecholamines in the mPFC using microdialysis in awake rats. Tolcapone significantly and specifically improved extradimensional (ED) set shifting. Tolcapone did not affect basal extracellular catecholamines, but significantly potentiated the increase in extracellular dopamine (DA) elicited by either local administration of the depolarizing agent potassium chloride or systemic administration of the antipsychotic agent clozapine. Although extracellular norepinephrine (NE) was also elevated by local depolarization and clozapine, the increase was not enhanced by tolcapone. We conclude that COMT activity specifically affects ED set shifting and is a significant modulator of mPFC DA but not NE under conditions of increased catecholaminergic transmission. These data suggest that the links between COMT activity and PFC function can be modeled in rats and may be specifically mediated by DA. The interaction between clozapine and tolcapone may have implications for the treatment of schizophrenia.

5-HT1 agonists reduce 5-hydroxytryptamine release in rat hippocampus in vivo as determined by brain microdialysis

1 An intracerebral perfusion method, brain microdialysis, was used to assess changes of 5‐hydroxytryptamine (5‐HT) release in the ventral hippocampus of the chloral hydrate‐anaesthetized rat in response to systemic administration of a variety of 5‐HT1 receptor agonists. 2 A stable output of reliably detectable endogenous 5‐HT was measured in dialysates collected from ventral hippocampus with the 5‐HT reuptake inhibitor, citalopram, present in the perfusion medium. 3 Under these conditions the putative 5‐HT1A agonist 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin (8‐OH‐DPAT) caused a dose‐dependent (5–250 μg kg−1, s.c.) reduction of 5‐HT in hippocampal dialysates. 4 Similarly, the putative 5‐HT1A agonists gepirone (5 mg kg−1, s.c.), ipsapirone (5 mg kg−1, s.c.) and buspirone (5 mg kg−1, s.c.) markedly reduced levels of 5‐HT in hippocampal perfusates whereas their common metabolite 1‐(2‐pyrimidinyl) piperazine (5 mg kg−1, s.c.), which does not bind to central 5‐HT1A recognition sites, had no effect. 5 5‐Methoxy‐3‐(1,2,3,6‐tetrahydro‐4‐pyridinyl)‐lH‐indole (RU 24969), a drug with reported high affinity for brain 5‐HT1B binding sites, also produced a dose‐dependent (0.25–5 mg kg−1, s.c.) decrease of hippocampal 5‐HT output. 6 These data are direct biochemical evidence that systemically administered putative 5‐HT1A and 5‐HT1B agonists markedly inhibit 5‐HT release in rat ventral hippocampus in vivo.

Interaction between a selective 5-HT1A receptor antagonist and an SSRI in vivo: effects on 5-HT cell firing and extracellular 5-HT.

1 The acute inhibitory effect of selective 5‐hydroxytryptamine (serotonin) reuptake inhibitors (SSRIs) on 5‐HT neuronal activity may offset their ability to increase synaptic 5‐HT in the forebrain. 2Here, we determined the effects of the SSRI, paroxetine, and a novel selective 5‐HT1A receptor antagonist, WAY 100635, on 5‐HT cell firing in the dorsal raphe nucleus (DRN), and on extracellular 5‐HT in both the DRN and the frontal cortex (FCx). Extracellular electrophysiological recording and brain microdialysis were used in parallel experiments, in anaesthetized rats. 3 Paroxetine dose‐dependently inhibited the firing of 5‐HT neurones in the DRN, with a maximally effective dose of approximately 0.8 mg kg−1, i.v. WAY 100635 (0.1 mg kg−1, i.v.) both reversed the inhibitory effect of paroxetine and, when used as a pretreatment, caused a pronounced shift to the right of the paroxetine dose‐response curve. 4 Paroxetine (0.8 mg kg−1, i.v.), doubled extracellular 5‐HT in the DRN, but did not alter extracellular 5‐HT in the FCx. A higher dose of paroxetine (2.4 mg kg−1, i.v.) did increase extracellular 5‐HT in the FCx, but to a lesser extent than in the DRN. Whereas 0.8 mg kg−1, i.v. paroxetine alone had no effect on extracellular 5‐HT in the FCx, in rats pretreated with WAY 100635 (0.1 mg kg−1), paroxetine (0.8 mg kg−1, i.v.) markedly increased extracellular 5‐HT in the FCx. 5 In conclusion, pretreatment with the selective 5‐HT1A receptor antagonist, WAY 100635, blocked the inhibitory effect of paroxetine on 5‐HT neuronal activity in the DRN and, at the same time, markedly enhanced the effect of paroxetine on extracellular 5‐HT in the FCx. These results may be relevant to recent clinical observations that 5‐HT1A receptor antagonists in combination with SSRIs have a rapid onset of antidepressant effect.

Induction of c-Fos expression in specific areas of the fear circuitry in rat forebrain by anxiogenic drugs.

BACKGROUND The fact that induction of anxiety- and panic-related symptoms is a property common to a range of drugs suggests that common neural substrates underlie their behavioral effects. METHODS We used Fos immunocytochemistry to test the effects of four anxiogenic drugs (FG-7142, yohimbine, m-chlorophenylpiperazine [mCPP], and caffeine) on anxiety-related circuitry in rat forebrain. RESULTS All four drugs commonly increased Fos-like immunoreactivity in 7 of 41 brain areas investigated, namely, central nucleus of the amygdala, bed nucleus of the stria terminalis, lateral septum, paraventricular nucleus of the hypothalamus, lateral hypothalamus, infralimbic and prelimbic cortex. All drugs but one (mCPP) also increased Fos expression in the basolateral and medial amygdala, the dorsomedial hypothalamus, cingulate cortex, and parts of the motor cortex. CONCLUSIONS The results suggest that the anxiogenic drugs selected activate a restricted set of forebrain areas. Most of these areas have previously been shown to be activated by environmentally evoked anxiety and to have anatomic connections with hindbrain regions that are activated by the same drugs and by environmentally evoked anxiety. Together, these data are consistent with the theory of an integrated forebrain and hindbrain neuronal system that is important for anxiety states evoked by both drug and environmental manipulations.

An electrophysiological and neuroanatomical study of the medial prefrontal cortical projection to the midbrain raphe nuclei in the rat.

In this study we utilized electrophysiological and pathway tracing methods to investigate the projections from the medial prefrontal cortex to the midbrain raphe nuclei of the rat. Initial pathway tracing experiments using retrograde (horseradish peroxidase conjugates with wheatgerm agglutinin or choleratoxin B subunit) and anterograde (Phaseolus vulgaris-leucoagglutinin) markers demonstrated a direct, bilateral projection to the dorsal raphe nucleus and median raphe nucleus from the medial prefrontal cortex, and the origin of this projection was localized predominantly in the ventral medial prefrontal cortex (infralimbic/dorsal penduncular cortices). Using chloral hydrate-anaesthetized rats, extracellular recordings were made mostly from 5-hydroxytryptamine neurons in the dorsal raphe nucleus, but non-5-hydroxytryptamine dorsal raphe neurons were also studied, as was a small number of 5-hydroxytryptamine neurons in the median raphe nucleus. In an initial study, electrical stimulation of the ventral medial prefrontal cortex caused a post-stimulus inhibition in the majority (49/56) of dorsal raphe 5-hydroxytryptamine neurons tested (mean duration of inhibition, 200+/-17 ms); in some cases (8/56) the inhibition was preceded by short-latency (26 +/-3 ms) orthodromic activation, and a small number of cells was antidromically activated (6/56). Both single spiking and burst-firing 5-hydroxytryptamine neurons in the dorsal raphe nucleus responded in the same way, and median raphe 5-hydroxytryptamine neurons were also inhibited (5/5). In contrast, few (2/12) of the non-5-hydroxytryptamine dorsal raphe neurons tested were inhibited by ventral medial prefrontal cortex stimulation. The effects of stimulation of the dorsal and ventral medial prefrontal cortex were compared on the same raphe 5-hydroxytryptamine neurons (n=17): ventral medial prefrontal cortex stimulation inhibited 16/17 of these neurons while only 8/17 were inhibited by dorsal medial prefrontal cortex stimulation. Finally, the inhibitory effect of ventral medial prefrontal cortex stimulation on 5-hydroxytryptamine cell-firing was not altered by 5-hydroxytryptamine depletion with p-chlorophenylalanine or by systemic administration of the selective 5-hydroxytryptamine1A receptor antagonist WAY 100635. The latter findings indicate that the inhibition is not due to release of raphe 5-hydroxytryptamine which could theoretically arise from anti- or orthodromically activated 5-hydroxytryptamine neurons. Our results show that stimulation of the ventral medial prefrontal cortex causes a marked post-stimulus inhibition in the vast majority of midbrain raphe 5-hydroxytryptamine neurons tested. It seems likely that the projection from ventral medial prefrontal cortex to the midbrain raphe nuclei mediates the responses of 5-hydroxytryptamine neurons to cortical stimulation. These data are relevant to recent discoveries of functional and structural abnormalities in the medial prefrontal cortex of patients with major depressive illness.

Differential involvement of serotonin and dopamine systems in cost-benefit decisions about delay or effort

RationaleAlthough tasks assessing the role of dopamine in effort-reward decisions are similar to those concerned with the role of serotonin in impulsive choice in that both require analysis of the costs and benefits of possible actions, they have never been directly compared.ObjectivesThis study investigated the involvement of serotonin and dopamine in two cost-benefit paradigms, one in which the cost was delay and the other in which it was physical effort.MethodsSixteen rats were trained on a T-maze task in which they chose between high and low reward arms. In one version, the high reward arm was obstructed by a barrier, in the other, delivery of the high reward was delayed by 15 s. Serotonin and dopamine function were manipulated using systemic pCPA and haloperidol injections, respectively.ResultsHaloperidol-treated rats were less inclined either to exert more effort or to countenance a delay for a higher reward. pCPA had no effect on the performance of the rats on the effortful task, but significantly increased the rats’ preference for an immediate but smaller reward. All animals (drug treated and controls) chose the high reward arm on the majority of trials when the delay or effort costs were matched in both high and low reward arms.ConclusionA dissociation was found between the neurotransmitter systems involved in different types of cost-benefit decision making. While dopaminergic systems were required for decisions about both effort and delay, serotonergic systems were only needed for the latter.

In Vivo Measurement of Extracellular 5‐Hydroxytryptamine in Hippocampus of the Anaesthetized Rat Using Microdialysis: Changes in Relation to 5‐Hydroxytryptaminergic Neuronal Activity

Abstract: The effect of manipulating the activity of central 5‐hydroxytryptamine (5‐HT) neurones on extracellular 5‐HT in ventral hippocampus of the chloral hydrate‐anaesthetized rat was studied using the brain perfusion method, microdialysis. Basal levels of 5‐HT in the dialysates were close to the detection limits of our assay using HPLC with electrochemical detection. However, addition of the selective 5‐HT reuptake inhibitor citalopram (10−6M) to the perfusion medium produced readily measurable amounts of dialysate 5‐HT. Citalopram, therefore, was used throughout our experiments. Hippocampal dialysate levels of 5‐HT sharply declined over the first hour after dialysis probe implantation, but then became constant. This stable output of 5‐HT was reduced by 57% in rats treated 14 days previously with intracerebroven‐tricular injections of the 5‐HT neurotoxin 5,7‐dihydroxy‐tryptamine. Electrical stimulation (1‐ms pulse width, 300 μA, 2–20 Hz) of the dorsal raphe nucleus for 20 min caused a rapid rise in hippocampal 5‐HT output, which immediately declined on cessation of the stimulus and was frequency‐dependent. Addition of tetrodotoxin (10−6M) to the perfusion medium reduced 5‐HT levels to 75% of predrug values. Injection of the 5‐HT1A agonist 8‐hydroxy‐2‐(di‐n‐propylami‐no)tetralin (0.5 and 2.5 μg) into the dorsal raphe nucleus caused a dose‐related fall in hippocampal output of 5‐HT compared to saline‐injected controls. We conclude from these data that the spontaneous output of endogenous 5‐HT into hippocampal dialysates, measured under our experimental conditions, predominantly originates from central 5‐HT neurones and changes in accordance with their electrical activity.

Role of the medial prefrontal cortex in 5‐HT1A receptor‐induced inhibition of 5‐HT neuronal activity in the rat

We examined the involvement of the frontal cortex in the 5‐HT1A receptor‐induced inhibition of 5‐HT neurones in the dorsal raphe nucleus (DRN) of the anaesthetized rat using single‐unit recordings complemented by Fos‐immunocytochemistry. Both transection of the frontal cortex as well as ablation of the medial region of the prefrontal cortex (mPFC) significantly attenuated the inhibition of 5‐HT neurones induced by systemic administration of the 5‐HT1A receptor agonist, 8‐OH‐DPAT (0.5–16 μg kg−1, i.v.). In comparison, the response to 8‐OH‐DPAT was not altered by ablation of the parietal cortex. The inhibitory effect of 8‐OH‐DPAT was reversed by the 5‐HT1A receptor antagonist, WAY 100635 (0.1 mg kg−1, i.v.) in all neurones tested. In contrast, cortical transection did not alter the sensitivity of 5‐HT neurones to iontophoretic application of 8‐OH‐DPAT into the DRN. Similarly, cortical transection did not alter the sensitivity of 5‐HT neurones to systemic administration of the selective 5‐HT reuptake inhibitor, paroxetine (0.1–0.8 mg kg−1, i.v.). 8‐OH‐DPAT evoked excitation of mPFC neurones at doses (0.5–32 μg kg−1, i.v.) in the range of those which inhibited 5‐HT cell firing. At higher doses (32–512 μg kg−1, i.v.) 8‐OH‐DPAT inhibited mPFC neurones. 8‐OH‐DPAT (0.1 mg kg−1, s.c.) also induced Fos expression in the mPFC. The neuronal excitation and inhibition, as well as the Fos expression, were antagonized by WAY 100635. These data add further support to the view that the inhibitory effect of 5‐HT1A receptor agonists on the firing activity of DRN 5‐HT neurones involves, in part, activation of a 5‐HT1A receptor‐mediated postsynaptic feedback loop centred on the mPFC.

Effect of the 5-HT1A receptor agonist 8-OH-DPAT on the release of 5-HT in dorsal and median raphe-innervated rat brain regions as measured by in vivo microdialysis

Recent electrophysiological studies, measurements of 5-HT synthesis and in vivo voltammetry recordings of 5-HT metabolism have suggested that serotoninergic neurones in the median raphe (MR) are less sensitive to 5-HT1A autoreceptor stimulation relative to those in the dorsal raphe (DR). To further study the putative differences in regulation between ascending 5-HT projections from the raphe nuclei we have used microdialysis to measure the release of 5-HT in ventral hippocampus, globus pallidus, dorsal hippocampus, frontal cortex, nucleus accumbens and medial septum, following systemic administration of the specific 5-HT1A agonist 8-OH-DPAT. The results show that the baseline output of 5-HT was similar in each of the areas studied. While 8-OH-DPAT decreased dialysate levels of 5-HT in all areas, the inhibition of 5-HT release seen in globus pallidus was significantly less marked compared to that observed in the other five regions. The results indicate that 5-HT1A autoreceptor-mediated control of 5-HT release is functional in all of the brain areas studied, including those receiving a preferential 5-HT innervation from the DR and MR. We find little evidence in support of the idea that brain 5-HT neuronal projections are heterogenous with respect to 5-HT1A autoreceptor regulation of 5-HT release; the globus pallidus, however representing a possible exception to this.