Evidence of the modulatory role of serotonin in acetylcholine release from striatal interneurons

Potential Therapeutic Application for Nicotinic Receptor Drugs in Movement Disorders

Emerging studies indicate that striatal cholinergic interneurons play an important role in synaptic plasticity and motor control under normal physiological conditions, while their disruption may lead to movement disorders. Here we discuss the involvement of the cholinergic system in motor dysfunction, with a focus on the role of the nicotinic cholinergic system in Parkinson's disease and drug-induced dyskinesias. Evidence for a role for the striatal nicotinic cholinergic system stems from studies showing that administration of nicotine or nicotinic receptor drugs protects against nigrostriatal degeneration and decreases L-dopa-induced dyskinesias. In addition, nicotinic receptor drugs may ameliorate tardive dyskinesia, Tourette's syndrome and ataxia, although further study is required to understand their full potential in the treatment of these disorders. A role for the striatal muscarinic cholinergic system in movement disorders stems from studies showing that muscarinic receptor drugs acutely improve Parkinson's disease motor symptoms, and may reduce dyskinesias and dystonia. Selective stimulation or lesioning of striatal cholinergic interneurons suggests they are primary players in this regulation, although multiple central nervous systems appear to be involved.

IMPLICATIONS

Accumulating data from preclinical studies and clinical trials suggest that drugs targeting CNS cholinergic systems may be useful for symptomatic treatment of movement disorders. Nicotinic cholinergic drugs, including nicotine and selective nAChR receptor agonists, reduce L-dopa-induced dyskinesias, as well as antipsychotic-induced tardive dyskinesia, and may be useful in Tourette's syndrome and ataxia. Subtype selective muscarinic cholinergic drugs may also provide effective therapies for Parkinson's disease, dyskinesias and dystonia. Continued studies/trials will help address this important issue.

A new outlook on cholinergic interneurons in Parkinson's disease and L-DOPA-induced dyskinesia

Traditionally, dopamine (DA) and acetylcholine (ACh) striatal systems were considered antagonistic and imbalances or aberrant signaling between these neurotransmitter systems could be detrimental to basal ganglia activity and pursuant motor function, such as in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). Herein, we discuss the involvement of cholinergic interneurons (ChIs) in striatally-mediated movement in a healthy, parkinsonian, and dyskinetic state. ChIs integrate numerous neurotransmitter signals using intrinsic glutamate, serotonin, and DA receptors and convey the appropriate transmission onto nearby muscarinic and nicotinic ACh receptors to produce movement. In PD, severe DA depletion causes abnormal rises in ChI activity which promote striatal signaling to attenuate normal movement. When treating PD with L-DOPA, hyperkinetic side effects, or LID, develop due to increased striatal DA; however, the role of ChIs and ACh transmission, until recently has been unclear. Fortunately, new technology and pharmacological agents have facilitated understanding of ChI function and ACh signaling in the context of LID, thus offering new opportunities to modify existing and discover future therapeutic strategies in movement disorders.

The pharmacology of the neurochemical transmission in the midbrain raphe nuclei of the rat

Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain, loaded with [(3)H]serotonin ([(3)H]5-HT), superfused and the release of [(3)H]5-HT was determined at rest and in response to electrical stimulation. Compartmental analysis of [(3)H]5-HT taken up by raphe tissue indicated various pools where the neurotransmitter release may originate from these stores differed both in size and rate constant. 5-HT release originates not only from vesicles but also from cytoplasmic stores via a transporter-dependent exchange process establishing synaptic and non-synaptic neurochemical transmission in the serotonergic somatodendritic area. Manipulation of 5-HT transporter function modulates extracellular 5-HT concentrations in the raphe nuclei: of the SSRIs, fluoxetine was found 5-HT releaser, whereas citalopram did not exhibit this effect. Serotonergic projection neurons in the raphe nuclei possess inhibitory 5-HT(1A) and 5-HT(1B/1D) receptors and facilitatory 5-HT(3) receptors, which regulate 5-HT release in an opposing fashion. This observation indicates that somatodendritic 5-HT release in the raphe nuclei is under the control of several 5-HT homoreceptors. 5-HT(7) receptors located on glutamatergic axon terminals indirectly inhibit 5-HT release by reducing glutamatergic facilitation of serotonergic projection neurons. An opposite regulation of glutamatergic axon terminals was also found by involvement of the inhibitory 5-HT(7) and the stimulatory 5-HT(2) receptors as these receptors inhibit and stimulate glutamate release in raphe slice preparation, respectively, Furthermore, postsynaptic 5-HT(1B/1D) heteroreceptors interact with release of GABA in inhibitory fashion in raphe GABAergic interneurons. Serotonergic projection neurons also possess glutamate and GABA heteroreceptors; NMDA and AMPA receptors release 5-HT, whereas both GABAA and GABAB receptors inhibit somatodendritic 5-HT release. Evidence was found for reciprocal interactions between serotonergic and glutamatergic as well as serotonergic and GABAergic innervations in the raphe nuclei. Serotonergic neurons in the raphe nuclei also receive noradrenergic innervation arising from the locus coeruleus and alpha-1 and alpha-2 adrenoceptors inhibited [(3)H]5-HT release in our experimental conditions. The close relation between 5-HT transporter and release-mediating 5-HT autoreceptors was also shown by addition of L-deprenyl, a drug possessing inhibition of type B monoamine oxidase and 5-HT reuptake. L-Deprenyl selectively desensitizes 5-HT(1B) but not 5-HT(1A) receptors and these effects are not related to inhibition of 5-HT metabolism but rather to inhibition of 5-HT transporter.

Atropine, a muscarinic cholinergic receptor antagonist increases serotonin, but not dopamine levels in discrete brain regions of mice

We investigated the effects of atropine, a muscarinic acetylcholine (ACh) receptor antagonist, on the level of serotonin in discrete brain regions, the nucleus raphe dorsalis (NRD), nucleus caudatus putamen (NCP), cerebral cortex and the cerebellum. Biogenic amines were assayed employing HPLC electrochemistry in these regions 30 min following different doses of atropine (5, 10, 25mg/kg; i.p.), and at various time points (15, 30, 60, 120 min) after 25mg/kg of the drug. The cholinergic receptor antagonist caused a dose-dependent alteration in the level of serotonin in NRD, but the increase was not dose-dependent for other regions studied. The metabolite of serotonin, 5-hydroxyindoleacetic acid was unaffected. Atropine did not affect the levels of dopamine or its metabolites dihydroxyphenyl acetic acid and homovanillic acid. The present study suggests significant effect of this antimuscarinic agent on the synthesis of serotonin in the central serotoninergic pathways, which may have clinical relevance.

Activation of 5-HT2 receptors enhances the release of acetylcholine in the prefrontal cortex and hippocampus of the rat

The role of 5-HT2 receptors in the regulation of acetylcholine (ACh) release was examined in the medial prefrontal cortex and dorsal hippocampus using in vivo microdialysis. The 5-HT(2A/2C) agonist +/-1-(2,5-dimethoxy-4-iodophenyl) -2- aminopropane hydrochloride (DOI) (1 and 2 mg/kg, i.p.) significantly increased the extracellular concentration of ACh in both brain regions, and this response was attenuated in rats treated with the 5-HT(2A/2B/2C) antagonist LY-53,857 (3 mg/kg, i.p.). Treatment with LY-53,857 alone did not significantly alter ACh release in either brain region The 5-HT(2C) agonist 6-chloro-2-(1-piperazinyl)-pyrazine) (MK-212) (5 mg/kg, i.p.) significantly enhanced the release of ACh in both the prefrontal cortex and hippocampus, whereas the 5-HT2 agonist mescaline (10 mg/kg, i.p.) produced a 2-fold increase in ACh release only in the prefrontal cortex. Intracortical, but not intrahippocampal, infusion of DOI (100 microM) significantly enhanced the release of ACh, and intracortical infusion of LY-53,857 (100 microM) significantly attenuated this response. These results suggest that the release of ACh in the prefrontal cortex and hippocampus is influenced by 5-HT2 receptor mechanisms. The increase in release of ACh induced by DOI in the prefrontal cortex, but not in the hippocampus, appears to be due to 5-HT2 receptor mechanisms localized within this brain region. Furthermore, it appears that the prefrontal cortex is more sensitive than the dorsal hippocampus to the stimulatory effect of 5-HT2 agonists on ACh release.

Differential serotonergic inhibition of in vitro striatal [3H]acetylcholine release in prenatally cocaine-exposed male and female rats

Previous research indicates that prenatal cocaine (pCOC)-exposure results in greater 5-HT3 agonist-induced inhibition of electrically evoked [3H]acetylcholine (ACh) overflow in rat striatal slices. The present study examines the effects of fluoxetine (FLU)-induced and exogenous serotonin (5-HT) on electrically evoked ACh release from striatal slices prepared from adult male and female (in periods of diestrus or proestrus) rats exposed to saline or cocaine in utero. Additionally, we assessed the impact of monoaminergic receptor stimulation on evoked ACh release by superfusion with selective 5-HT2, 5-HT3 and D2 receptor antagonists in the presence of FLU-induced and exogenous 5-HT and measuring the capacity of these drugs to reverse inhibitory effects of 5-HT. Given our previous findings of accentuated inhibition of ACh release by 5-HT3 agonism in striata of pCOC-exposed adult rats, we hypothesized that superfusion of endogenous and exogenous 5-HT would lead to greater suppression of evoked ACh release in this group of animals. Our results indicated that ACh release from slices of all prenatal saline (pSAL) rats was inhibited comparably by FLU (10 microM)-elicited increases in endogenous 5-HT or by increases elicited with application of exogenous 5-HT (5 microM). Robust FLU-mediated inhibition of ACh release was evident in slices from pCOC male and pCOC diestrus female rats vs. their respective PSAL control groups. Superfusion of striatal slices with 5-HT (5 microM) produced a pattern of ACh inhibition similar to that produced by FLU; however, the magnitude of ACh inhibition was consistently greater than that observed with FLU. Inhibition of ACh overflow by FLU was blocked by co-superfusion with ketanserin, a 5-HT2 receptor antagonist. ICS-205,930, a 5-HT3 receptor antagonist or sulpiride, a D2 receptor antagonist. Conversely, serotonergic inhibition of ACh overflow was only blocked by a high concentration of ICS-205,930 (5 microM) and was completely reversed by sulpiride (1 microM). Collectively, these findings demonstrate serotonergic modulation of cholinergic neurons varying as a function of prenatal treatment, sex and, for females, phase of estrous. Inhibition of ACh release by 5-HT appears to be mediated by a complex relationship between 5-HT2, 5-HT3 and D2 receptor regulation, as the blockade of any of these receptors reversed the inhibitory effects of FLU on ACh release. Conversely, in the case of exogenous 5-HT-induced inhibition, only blockade of D2 receptors and high concentrations of the 5-HT3 receptor antagonists were capable of reversing monoaminergic inhibition. These data support the hypothesis that the enhanced serotonergic modulation of ACh neurons in pCOC-exposed animals is largely mediated by dopamine (DA) and reflect a major biochemical persistence of neurodevelopmental adaptations elicited by early cocaine exposure.

Sleep-wake effects of meta-chlorophenyl piperazine and mianserin in the behaviorally depressed rat

The present study examined the effects of meta-chlorophenyl piperazine (mCPP) and mianserin on the sleep-wake cycle of the clomipramine-induced behaviorally screened depressed rats. Six-hour polygraphic recordings were made between 06:00 and 12:00 h, after a single injection of either saline or mianserin or mCPP into the lateral cerebral ventricle (i.c.v.) of both the depressed (n=12) and control rats (n=12). The injection of mCPP in the depressed rats caused a significant reduction in the total duration and number of rapid eye movement (REM) sleep episodes while it increased the REM sleep onset latency compared to the control saline injections. The injection of mianserin in the depressed rats also caused a significant reduction in the total duration and number of REM sleep episodes without changing the REM sleep latency. These results demonstrate for the first time that the central administration of mCPP and mianserin could act as an antidepressant in the clomipramine-induced rat model of depression.

The cat neostriatum: relative distribution of cholinergic neurons versus serotonergic fibers

The distribution of choline acetyltransferase (ChAT)-containing neurons and serotonin (5-HT)-containing nerve fibers in the cat neostriatum was investigated by use of immunohistochemical techniques. Both ChAT- and 5-HT-staining techniques were applied to alternate brain sections, thereby allowing a precise comparison of the distribution pattern of ChAT-immunopositive cells (ChAT cells) and 5-HT-immunopositive fibers (5-HT fibers). In the neostriatum, ChAT cells were strongly stained throughout their cell bodies and proximal (first-order) dendrites. The majority of them were multipolar cells with a soma diameter of 20-50 microm (long axis)x10-30 microm (short axis). In the caudate nucleus, ChAT cells were evenly and diffusely distributed except for the dorsolateral region of its rostral half, in which latter region they were distributed in loosely formed clusters. In the rostral portion of the putamen, the density of ChAT-cell distribution was like that in the medial region of the caudate nucleus. In contrast, this distribution was more dense in the caudomedial region of the putamen, adjacent to the globus pallidus. 5-HT fibers in the neostriatum were dark-stained, of quite fine diameter (<0.6 microm), and they contained small, round varicosities (diameter, usually 0.5-1.0 microm, but some >1.0 microm). Such 5-HT fibers were distributed abundantly throughout the caudate nucleus and putamen. In the rostrocaudal portion of the caudate nucleus, their density was high in its dorsal and ventral components, and low in the middle component. Throughout the putamen, 5-HT fibers were distributed homogeneously in the mediolateral and dorsoventral directions. In the caudal portion of the putamen adjacent to the globus pallidus, the 5-HT fibers had a higher density while maintaining their homogenous distribution pattern. In the two main divisions of the striatum, the so-called 'patch' (acetylcholinesterase (AChE)-poor) and 'matrix' (AChE-rich) compartments, there was a near-even distribution of 5-HT fibers and terminals. The above results suggest that the 5-HT-dominated, raphe-striatal pathway is optimally arranged for modulating the activity of both the intrinsic and the projection neurons of the neostriatum.

Prenatal cocaine exposure increases serotonergic inhibition of electrically evoked acetylcholine release from rat striatal slices at adulthood

This study tests the hypothesis that prenatal cocaine (pCOC) exposure (20 mg/kg, bidaily from embryonic days 15-21) modifies 5-HT(3) receptor regulation of electrically-evoked [(3)H]acetylcholine (ACh) overflow from adult male and female (proestrus, diestrus) rat striatal slices. Also, the influence of endogenous dopamine (DA) on serotonin (5-HT) regulation of ACh overflow was determined by assessing the effects alpha-methyl-para-tyrosine (AMPT) pretreatment or sulpiride. Phenylbiguanide (PBG, 5-HT(3) agonist) superfusion dose-dependently inhibited ACh overflow in all groups except the diestrus pCOC group in which there was an enhanced sensitivity to PBG. PBG (10, 30, and 60 microM) produced greater effects in the pCOC male than in the prenatal saline (pSAL) group. The pCOC male group also exhibited greater sensitivity to PBG (30 and 60 microM) than the pCOC proestrus group. PBG inhibition of ACh overflow was comparable in the pSAL male and female (proestrus) groups. PBG inhibition of ACh overflow was greater in the pCOC diestrus group than in the pCOC proestrus (10, 30, and 60 microM), the pSAL diestrus (10 and 30 microM), and the pCOC male (10 microM) conditions. In slices from untreated rats superfused with 30 microM PBG, AMPT pretreatment (68% DA loss) reduced inhibition of ACh overflow, and 1 microM sulpiride increased ACh overflow. ICS205-930 (5-HT(3) antagonist) reduced effectiveness of PBG indicating 5-HT(3) receptor specificity for PBG. In summary, pCOC exposure enhances modulatory effects of 5-HT (via 5-HT(3) receptors) on striatal ACh release in male and females rats and the inhibitory actions of 5-HT(3) receptors are mediated by DA.

Doubly dissociable effects of median- and dorsal-raphé lesions on the performance of the five-choice serial reaction time test of attention in rats

Six experiments examined the effects of selective median (MRN)- and dorsal (DRN)-raphé nucleus lesions on the performance of the five-choice serial reaction time task. In this test rats are required to localize brief visual stimuli presented randomly in one of five locations in approximately 30 min sessions of 100 trials. Both accuracy and latency to respond are measured, as well as the incidence of premature and perseverative responding. Selective 5-HT lesions were induced by intra-raphé infusions of 5,7-dihydroxytryptamine following pretreatment with both a noradrenergic and a dopaminergic re-uptake inhibitor. Analysis of tissue monoamine content demonstrated that the MRN lesion profoundly depleted hippocampal 5-HT (by about 90%) without affecting noradrenaline and dopamine, whereas the DRN lesion primarily depleted (by about 80%) nucleus accumbens and caudate-putamen 5-HT. Rats with 5-HT lesions of the MRN performed the task with a similar degree of accuracy to that exhibited by sham-operated controls. Although the MRN lesion did not affect the latency to respond correctly to the visual targets the lesioned animals collected the food reward significantly faster than the controls. A transient increase in the number of premature responses also resulted from this lesion. In contrast the DRN lesion produced a transient but significant increase in the accuracy of performance, and increased both the speed and the probability of responding. The similarity of the effects following global forebrain 5-HT depletion and the selective DRN lesion suggests that the 5-HT projections of the DRN rather than the MRN may play an important role in impulsive behaviour following 5-HT depletion.

N-methyl-D-aspartate-evoked release of [3H]acetylcholine in striatal compartments of the rat: regulatory roles of dopamine and GABA

The N-methyl-D-aspartate-evoked release of [3H]acetylcholine previously formed from [3H]choline was estimated in striosome- (identified by [3H]naloxone binding) or matrix-enriched areas of the rat striatum using an in vitro microsuperfusion procedure. Experiments were performed in either the absence or the presence of dopaminergic and/or GABAergic receptor antagonists. Although the cell bodies of the cholinergic interneurons were mainly found in the matrix, in the absence of magnesium, N-methyl-D-aspartate (50 microM) stimulated the release of [3H]acetylcholine in both striatal compartments. These responses were blocked by either magnesium, dizocilpine maleate, 7-chlorokynurenate or tetrodotoxin. N-Methyl-D-aspartate responses were concentration-dependent, but the 1 mM N-methyl-D-aspartate response was higher in striosomes than in the matrix. The co-application of D-serine (10 microM) enhanced the 10 microM N-methyl-D-aspartate response in both compartments, but reduced those induced by 1 mM N-methyl-D-aspartate, this reduction being higher in striosomes. The blockade of dopaminergic transmission with the D2 and D1 dopaminergic receptor antagonists, (-)-sulpiride (1 microM) and SCH23390 (1 microM), was without effect on the 50 microM N-methyl-D-aspartate-evoked release of [3H]acetylcholine, but markedly enhanced the 1 mM N-methyl-D-aspartate+D-serine-evoked response in striosomes and to a lesser extent in the matrix. Disinhibitory responses of similar amplitude were observed not only in striosomes but also in the matrix when (-)-sulpiride was used alone, while SCH23390 alone enhanced the 1 mM N-methyl-D-aspartate+D-serine response only in striosomes and to a lower extent than (-)-sulpiride. These results indicate that D2 receptors are mainly involved in the inhibitory effect of dopamine on the 1 mM N-methyl-D-aspartate+D-serine-evoked release of [3H]acetylcholine. They also show that the stimulation of D1 receptors can either reduce (striosomes) or enhance (matrix) this response, since in the latter case the effect induced by the combined application of the D1 and D2 receptor antagonists was smaller than that observed with the D2 receptor antagonist alone. Indicating that released GABA facilitates N-methyl-D-aspartate responses, the blockade of GABAA receptors with bicuculline (5 microM) reduced the 50 microM N-methyl-D-aspartate-evoked release of [3H]acetylcholine in both striatal compartments and the 1 mM N-methyl-D-aspartate+D-serine response in the matrix. These effects result from an inhibition by GABA of the evoked release of dopamine, since the reducing effects of bicuculline on N-methyl-D-aspartate responses were not observed under the complete blockade of dopaminergic transmission by the D1 and D2 receptor antagonists. Further demonstrating a facilitatory role of GABA in the control of N-methyl-D-aspartate-evoked release of [3H]acetylcholine, in the presence of bicuculline, (-)-sulpiride and SCH23390 alone or in combination enhanced, in both compartments, the responses induced not only by 1 mM N-methyl-D-aspartate+D-serine, but also by 50 microM N-methyl-D-aspartate.

5-HT2 receptor regulation of acetylcholine release induced by dopaminergic stimulation in rat striatal slices

The role of 5-hydroxytryptamine (5-HT) receptor subtypes in acetylcholine (ACh) release induced by dopamine or neurokinin receptor stimulation was studied in rat striatal slices. The dopamine D1 receptor agonist SKF 38393 potentiated in a tetrodotoxin-sensitive manner the K(+)-evoked [3H]ACh release while SCH 23390, a dopamine D1 receptor antagonist, had no effect. [3H]ACh release was decreased by the dopamine D2 receptor agonist LY 171555 (quinpirole) and slightly potentiated by the dopamine D2 receptor antagonist haloperidol. The selective neurokinin NK1 receptor agonist [Sar9, met(O2)11]SP also potentiated K(+)-evoked release of [3H]ACh. GR 82334, a NK1 receptor antagonist, blocked not only the effect of [Sar9, met(O2)11]SP but also the release of ACh induced by the D1 receptor agonist SKF 38393. Among the 5-HT agents studied, only the 5-HT2A receptor antagonists ketanserin and ritanserin were able to reduce the ACh release induced by dopamine D1 receptor stimulation. Mesulergine, a more selective 5-HT2C antagonist, showed an intrinsic releasing effect but did not affect K(+)-evoked ACh release induced by SKF 38393. Methysergide and methiothepin, mixed 5-HT1/2 antagonists, as well as ondansetron, a 5-HT3 receptor antagonist, showed an intrinsic effect on ACh release, their effects being additive to that of SKF 38393. 5-HT2 receptor agonists were ineffective. However, the 5-HT2 agonist DOI was able to prevent the antagonism by ketanserin of the increased [3H]ACh efflux elicited by SKF 38393, suggesting a permissive role of 5-HT2A receptors. None of the above indicated 5-HT agents was able to reduce the ACh release induced by the selective NK1 agonist. The results suggest that 5-HT2 receptors, probably of the 5-HT2A subtype, modulate the release of ACh observed in slices from the rat striatum after stimulation of dopamine D1 receptors. It seems that this serotonergic control is exerted on the interposed collaterals of substance P-containing neurons which promote ACh efflux through activation of NK1 receptors located on cholinergic interneurons.

Effect of dotarizine on K(+)-stimulated [3H]-serotonin and [3H]-acetylcholine release from rat hippocampus

1. The effect of the diphenylmethyl-piperazine derivative dotarizine on K(+)-stimulated release of [3H]serotonin ([3H]5-HT) and [3H]acethylcholine ([3H]Ach) in rat hippocampal slices was studied. 2. Dotarizine at a concentration of 10(-6) M significantly decreased the basal [3H]5-HT release and, at a concentration of 10(-5) M, it significantly decreased the K(+)-stimulated [3H]5-HT release compared to vehicle controls. 3. Dotarizine, at a concentration of 5 x 10(-7) M, significantly increased both basal and K(+)-stimulated [3H]Ach release. At higher concentrations (10(-6) and 2 x 10(-6) M), dotarizine did not change the basal release but significantly increased the K(+)-stimulated [3H]Ach release. The effect of dotarizine on K(+)-stimulated [3H]Ach release decreased with increasing dotarizine concentrations. 4. It is speculated that, in addition to its Ca2+ antagonistic activity, dotarizine exerts an antagonistic effect on the presynaptic 5-HT autoreceptors, which could account for the facilitation of [3H]Ach release.

Comparison of the effects of the 5-HT3 receptor antagonists WAY-100579 and ondansetron on spatial learning in the water maze in rats with excitotoxic lesions of the forebrain cholinergic projection system

The effects of the 5-HT3 receptor antagonists. WAY-100,579 and ondansetron (both at doses of 0.001, 0.01 and 0.1 mg/kg s.c.) and the muscarinic receptor agonist arecoline (1.0 mg/kg s.c.), on spatial learning and memory in the water maze were examined in rats after combined S-AMPA lesions to the nucleus basalis and medial septal brain regions. Lesioned rats showed substantially increased latency to find the submerged platform, and spent less time searching in the correct quadrant, and more time circling the periphery of the pool, relative to controls. Lesioned rats treated with WAY-100,579, ondansetron and arecoline exhibited marked improvement in these parameters of learning relative to lesioned animals, with arecoline-treated animals showing the most substantial recovery. Linear dose-related trends of improvement were seen with both of the 5-HT3 antagonists. In probe trials, testing retention of the platform position 24 and 72 h after the end of training, control rats exhibited substantial superiority relative to lesioned rats in accuracy of search in the training quadrant and former platform area, matched by rats treated with arecoline on the first, and by rats treated with the two higher doses of WAY-100,579 and ondansetron on the second probe trial. These results are consistent with our previous studies which demonstrated that another selective 5-HT3 receptor antagonist. WAY-100,289, significantly reversed the cognitive deficits in water maze performance induced by ibotenic acid lesions of forebrain cholinergic projection system. Therefore, selective 5-HT3 receptor antagonists may provide a novel effective therapy for treating cognitive deficits associated with degeneration of central cholinergic neurones, such as Alzheimer's disease or age-associated memory impairment.

Involvement of GABA systems in acetylcholine release induced by 5-HT3 receptor blockade in slices from rat entorhinal cortex

The aim of the present study was to examine the role of 5-HT3 receptors in spontaneous and K(+)-evoked acetylcholine (ACh) release from rat entorhinal cortex and striatal slices. The 5-HT3 receptor antagonists ondansetron and granisetron (0.01-10 microM) produced a concentration-dependent increase in both spontaneous and K(+)-evoked [3H]ACh release in the two brain regions studied. The release of ACh was Ca(2+)-dependent and tetrodotoxin-sensitive. 5-HT3 receptor agonists, such as 2-methyl-5-HT and 1-phenylbiguanide, at concentrations up to 1 microM, did not show any intrinsic effect on [3H]ACh release in both rat brain regions. However, 2-methyl-5-HT, 1 microM, fully blocked the ondansetron-induced enhancement in both basal and K(+)-evoked ACh release, suggesting that 5-HT, through 5-HT3 receptor activation, tonically inhibits ACh release. The possible implication of interposed inhibitory systems in ACh release after 5-HT3 receptor blockade was subsequently analyzed. While the effect of ondansetron was not modified by haloperidol or naloxone, the GABAA receptor antagonist bicuculline produced a marked potentiation of ACh release in the entorhinal cortex but not in the striatum. The results suggest that in this cortical area 5-HT activates 5-HT3 receptors located on GABAergic neurons which in turn inhibit cholinergic function.

Serotonergic modulation of cholinergic function in the central nervous system: cognitive implications

Accumulating evidence suggests that serotonin may modulate cholinergic function in several regions of the mammalian brain and that these serotonergic/cholinergic interactions influence cognition. The first part of this review is an overview of histological, electrophysiological and pharmacological (in vitro, in vivo) data indicating that, in several brain regions (e.g., hippocampus, cortex and striatum), there are neuroanatomical substrates for a serotonergic/cholinergic interaction, and that alterations in serotonergic activity may induce functional changes in cholinergic neurons. In the second part, the review focuses on experimental approaches showing or suggesting that central cholinergic and serotonergic mechanisms are cooperating in the regulation of cognitive functions. These arguments are based on lesion, intracerebral grafting and pharmacological techniques. It is concluded that not all mnesic perturbations induced by concurrent manipulations of the serotonergic and cholinergic systems can be attributed to a serotonergic modification of the cholinergic system. The cognitive faculties of an organism arise from interactions among several neurotransmitter systems within brain structures such as, for instance, the hippocampus or the cortex, but also from influences on memory of other general functions that may involve cerebral substrates different from those classically related to mnesic functions (e.g., attention, arousal, sensory accuracy, etc.).

Serotonergic modulation of anticholinergic effects on cognition and behavior in elderly humans

Cholinergic neurotransmission is thought to be modulated by serotonin as documented in animal and human studies. We examined the effects of the muscarinic antagonist scopolamine (0.4 mg IV) given alone or together with the serotonin mixed agonist/antagonist m-chlorophenylpiperazine (m-CPP, 0.08 mg/kg IV), and the selective 5-HT3 receptor antagonist ondansetron (0.15 mg/kg IV). Ten normal elderly volunteers each received five separate pharmacologic challenges (placebo, ondansetron, scopolamine, scopolamine+ondansetron, and scopolamine+m-CPP). Cognitive, behavioral, and physiologic variables were analyzed using repeated measures analysis of variance. The acute effects of scopolamine in certain cognitive, behavioral, and physiological measures were significantly exaggerated by the addition of m-CPP. Scopolamine's cognitive effects were unaffected by ondansetron at the dose tested, nor did ondansetron given alone affect basal cognitive performance. This pilot study suggests that the serotonin mixed agonist/antagonist m-CPP may influence cholinergic neurotransmission. The changes associated with the combination of scopolamine and m-CPP do not appear to be secondary to simple pharmacokinetic alterations and suggest a complex interaction between the cholinergic and serotonergic systems centrally.

The selective 5-HT3 receptor antagonist, WAY100289, enhances spatial memory in rats with ibotenate lesions of the forebrain cholinergic projection system

The effects of three doses (0.003, 0.03 and 1.0 mg/kg sc) of the 5-HT3 receptor antagonist, WAY 100289, on spatial learning and memory in the water maze were examined in rats before and after ibotenate lesions to the nucleus basalis and medial septal brain regions at the source of cholinergic projections to cortex and hippocampus. The representative cholinergic nicotinic and muscarinic receptor agonists nicotine (0.1 mg/kg) and arecoline (1.0 mg/kg) were also tested for comparison. Both arecoline and nicotine improved initial acquisition in rats before lesioning, in terms of latency to find a hidden platform and accuracy of search strategy. WAY100289 did not affect the performance of normal rats significantly, apart from some non-significant trends towards improvement with the highest dose. However, in animals showing transient navigational deficits in retention and relearning after lesioning, WAY100289 improved performance at all three doses, though ameliorative effects of nicotine and arecoline were more marked also in lesioned rats. These results show that WAY100289 improved spatial learning in animals impaired after lesions to cholinergic projection nuclei, which may reflect an interaction with cholinergic transmission to enhance cognitive function. However, in the present study, WAY100289 appeared to be less effective than direct cholinergic agonists.

In vivo modulation of acetylcholine in the nucleus accumbens of freely moving rats: I. Inhibition by serotonin

Microdialysis was used to characterize the effect of serotonergic input on cholinergic interneurons in the nucleus accumbens (NAC) of freely moving rats. Local infusion of 5-hydroxytryptamine (5-HT) or the serotonin reuptake blocker fluoxetine significantly decreased extracellular acetylcholine (ACh) in the NAC. This decrease in ACh was blocked by the 5-HT1 (and beta-adrenergic) antagonist propranolol. To test suggests that 5-HT inhibits ACh interneurons via one of the 5-HT1 receptor types. The 5HT1A agonist 8-OH-DPAT given systemically again decreased extracellular levels of ACh, and the effect was dose-dependent. The 5-HT1A effect was probably exerted in the NAC, because local infusion of 8-OH-DPAT mimicked systemic injections. These microdialysis results are similar to in vitro studies which suggest an inhibitory impact of 5-HT on ACh release in basal ganglia slices and homogenates. The decrease in extracellular ACh as measured in vivo is apparently mediated, at least in part, through a 5-HT1A receptor in the accumbens. Given the role of the NAC in behavior reinforcement, this 5-HT-ACh interaction may be involved in serotonergic treatment of depression.

Independent effects of cholinergic and serotonergic lesions on acetylcholine and serotonin release in the neocortex of the rat

Rats received a unilateral lesion of the nucleus basalis magnocellularis (NBM) by infusion of ibotenic acid. In addition, the dorsal raphe nucleus was lesioned by infusion of 5,7-dihydroxytryptamine (5,7-DHT). The release of acetylcholine (ACh), choline, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was measured in the frontal neocortex by means of microdialysis. Lesions of the NBM, but not the raphe nucleus, reduced the release of ACh significantly (-47%). The release of 5-HT and 5-HIAA was reduced by raphe lesions (-44% and -79%+), but not by NBM lesions. In no case did the combined lesion affect neurotransmitter release more than a single lesion. These results suggest that serotonergic projections from the dorsal raphe nucleus are not involved in tonic inhibition of ACh release in the neocortex.

Similar ameliorating effects of benzomorphans and 5-HT2 antagonists on drug-induced impairment of passive avoidance response in mice: comparison with acetylcholinesterase inhibitors

Mice were trained to avoid electric shocks by means of step-down type passive avoidance learning tasks, and memory retention was measured 24 h after the training session. Memory impairment (amnesia) was produced by administering either p-chloroamphetamine (PCA), a serotonin (5-HT) releaser or scopolamine (SCOP), a muscarinic cholinoceptor antagonist, 30 min prior to the training session. Benzomorphans, 5-HT2 antagonists and acetylcholinesterase (AChE) inhibitors were administered immediately after the training session. PCA- but not SCOP-induced amnesia was attenuated by the post-training administration of two benzomorphans, (+)N-allylnormetazocine ((+)SKF-10,047) and (+/- )pentazocine ((+/- )PTZ). Similarly, PCA-induced amnesia was reversed by the post-training administration of 5-HT2 antagonists, ritanserin (RIT) and mianserin (MIA), but SCOP-induced amnesia was not. However, the AChE inhibitors, tetrahydroaminoacridine (THA) and physostigmine (PHY) attenuated both PCA- and SCOP-induced amnesia when administered immediately after the training session. These results indicated that benzomorphans and 5-HT2 antagonists have antiamnestic effects in mice, as do AChE inhibitors. In addition, it is interesting that the patterns of ameliorating effect of benzomorphans were similar to those of 5-HT2 antagonists, which differ from those of AChE inhibitors.

Altered serotonergic activity in women with dysphoric premenstrual syndromes

OBJECTIVE

Dysphoric Premenstrual Syndromes (PMS) are quite prevalent and in some women they are severe enough to warrant treatment. Their pathophysiology is still unknown, despite increased interest and research. Here we review the possible role of serotonin in the multidimensional interactive pathophysiology of PMS.

METHOD

Over 170 articles are reviewed. An extensive library search has been conducted and articles are included because of their relevance to: 1) the phenomenology of PMS; 2) the putative association of serotonergic (5-HT) activity with syndromes that occur premenstrually; 3) changes in 5-HT activity along the menstrual cycle, especially the late luteal phase; 4) influence of gonadal hormones on serotonergic functions; 5) endocrine strategies for assessment of 5-HT abnormalities; and 6) treatment studies of PMS with serotonergic agonists.

RESULTS AND CONCLUSIONS

The data presented here suggest that post-synaptic serotonergic responsivity might be altered during the late-luteal-premenstrual phase of the menstrual cycle. Some serotonergic functions of women with PMS might be altered during the entire cycle and be associated with a vulnerability trait. It is hypothesized that gonadal hormones might cause changes in levels of activity of 5-HT systems as part of a multidimensional interactive system. Strategies to evaluate 5-HT activities in the context of the menstrual cycle are discussed--leading to the conclusion that the most promising approach is active stimulation with specific post-synaptic serotonin agonists. Treatment outcome studies of some imperfect compounds that are currently applied as a symptomatic treatment of PMS support the notion that 5-HT is involved in the pathophysiology of these syndromes.

The Psychopharmacology of 5-HT3Receptors

5-HT3 receptors have an exclusive neuronal location and evidence is presented of their involvement in behaviour. 5-HT3 receptor antagonists such as ondansetron, tropisetron and zacopride have provided the critical pharmacological tools to reveal a potent and efficacious ability to regulate disturbed behaviour. Thus the 5-HT3 receptor antagonists will restore to normal rodent and primate behaviour disturbed by increasing limbic dopamine function, aversive situations, cognitive impairments and drug abuse. The remarkable feature of their action is a failure to modify normal behaviour. This unique pharmacological signature has ensured a wide interest in the potential role of the 5-HT3 receptor antagonists in the treatment of schizophrenia, anxiety, age related memory impairment and the problems of withdrawal from drugs of abuse. The preclinical data and preliminary clinical observations are presented.

Effects of combined acetylcholinesterase inhibition and serotonergic receptor blockade on age-associated memory impairments in rats

We recently reported that post-training administration of serotonergic receptor antagonists attenuated the inhibitory-avoidance memory deficits normally exhibited by aged rats. In the present study, we determined whether a subeffective dose of the serotonergic type-2 receptor antagonist, ketanserin, would augment the facilitative effects produced by the acetylcholinesterase inhibitor, physostigmine, on memory in aged rats using the same task. The drugs were injected intraperitoneally alone, or in combination, immediately following training. Retention testing occurred 24 hours following training. A dose-dependent enhancement of memory was demonstrated as a result of the two treatment conditions (physostigmine 0.01-10.0 micrograms/kg, ketanserin 1.0 mg/kg + physostigmine 0.001-0.01 micrograms/kg). The facilitation of memory produced by the combined treatment was observed at doses well below those required to produce a similar effect when each drug was administered alone. The results provide additional evidence for an interaction between the cholinergic and serotonergic neurotransmitter systems in learning and memory, and may have important implications in the treatment of age-related memory impairments.

Effect of selective opiate antagonists on striatal acetylcholine and dopamine release

We investigated the effect of selective opiate antagonists on striatal acetylcholine (ACh) and dopamine (DA) release. The mu-receptor antagonist beta-funaltrexamine (beta-FNA), the delta-antagonist naltrindole (NTI), and the kappa-antagonist norbinaltorphimine (nor-BNI) were used to selectively block different subtypes of opiate receptors. The experiments were carried out on isolated superfused striatal slices of rats, loaded with [3H]choline or [3H]dopamine. beta-FNA and NTI significantly enhanced the electrical field stimulation-evoked release of ACh but only if the dopaminergic input had been impaired either by chemical denervation or D2 dopamine receptor blockade. By contrast, neither the selective nor nonselective antagonists had any modulatory effect on the release of dopamine. It is concluded, therefore, that the release of ACh is tonically controlled by endogenous opioid peptide(s) through the stimulation of mu- and delta-opiate receptors located on cholinergic axon terminals, in addition to the tonic control by DA.

5-Hydroxytryptamine3 receptors sited on cholinergic axon terminals of human cerebral cortex mediate inhibition of acetylcholine release

Synaptosomes prepared from freshly obtained human cerebral cortex and labeled with [3H]choline have been used to investigate the modulation of [3H]acetylcholine ([3H]ACh) release by 5-hydroxytryptamine (5-HT). The Ca(2+)-dependent release of [3H]-ACh occurring when synaptosomes were exposed in superfusion to 15 mM KCl was inhibited by 5-HT (0.01-1 microM) in a concentration-dependent manner. The effect of 5-HT was mimicked by 1-phenylbiguanide, a 5-HT3 receptor agonist, but not by 8-hydroxy-2-(di-n-propylamino)tetralin, a 5-HT1A receptor agonist. The 5-HT3 receptor antagonists tropisetron and ondansetron blocked the effect of 5-HT, whereas spiperone and ketanserin were ineffective. It is suggested that cholinergic axon terminals in the human cerebral cortex possess 5-HT receptors that mediate inhibition of ACh release and appear to belong to the 5-HT3 type.

Serotonergic influence on cholinergic-induced analgesia: differences in two inbred strains of mice

C57BL/6 (C57) and DBA/2 (DBA) inbred mice showed different analgesic responses to cholinergic stimulation. The simultaneous administration of muscarinic and serotonergic agonists, oxotremorine and 5-methoxy-NN-dimethyltryptamine (5-MeODMT), lowered the antinociceptive effect of the cholinergic drug in DBA mice, while no effects were detectable in the C57 strain. These results suggest a strain-dependent behavioural effect of the interaction of cholinergic and serotonergic neuronal systems.

In Vivo Neurochemical Evaluation of Striatal Serotonergic Hyperinnervation in Rats Depleted of Dopamine at Infancy

Destruction of nigrostriatal dopamine (DA) neurons with 6-hydroxydopamine (6-OHDA) early in development results in hyperinnervation of striatum by the serotonergic afferents deriving from the dorsal raphe nucleus. We have used in vivo microdialysis to investigate the degree to which serotonergic neurotransmission in striatum is altered by this increase in the density of serotonin (5-HT) terminals. The effects of several manipulations known to influence 5-HT function on extracellular 5-HT and 5-hydroxyindoleacetic acid in striatum were compared in adult rats treated neonatally with 6-OHDA and in intact adult rats. Basal levels of 5-HT in extracellular fluid (ECF) of striatum were similar in neonatally DA-depleted rats and in intact rats. Perfusion with the 5-HT reuptake blocker, fluoxetine (100 microM), increased 5-HT in striatal ECF of neonatally DA-depleted rats to levels that were threefold greater than those achieved in intact rats. Likewise, K(+)-depolarization of the 5-HT terminals (100 mM in perfusate) or systemic administration of the 5-HT releaser, (+/-)-fenfluramine (10 mg/kg i.p.), increased the concentration of 5-HT in striatal ECF of neonatally DA-depleted rats to levels approximately threefold greater than those observed in striatum of intact rats. These findings indicate that the 5-HT hyperinnervation of striatum that takes place in rats depleted of DA at infancy is associated with an increased capacity for neurotransmitter release in this system. Concomitant increased in high-affinity 5-HT uptake may prevent the occurrence of any measurable changes in the resting concentration of 5-HT in striatal ECF.

m-Chlorophenylpiperazine as a probe of serotonin function

m-Chlorophenylpiperazine (mCPP) is the most extensively used probe of serotonin function in psychiatry. This article reviews its in vitro and in vivo properties in animals, normal human subjects, and psychiatric patients. mCPP is a safe, reliable, direct 5-hydroxytryptamine (5HT) agonist, which may be used to evaluate 5HT receptor sensitivity. It causes a consistent, dose-dependent elevation of ACTH, cortisol, and prolactin levels in both animals and humans, as well as increased body temperature in man. It also causes a variety of behavioral effects, depending on the population studied. These effects are probably 5HT receptor-related, although specific 5HT receptor subtype mechanisms have not yet been established. mCPP may be considered an important addition to armamentarium of 5HT receptor probes, which is especially useful until more selective 5HT receptor agonists have been tested.

Naloxone enhances the release of acetylcholine from cholinergic interneurons of the striatum if the dopaminergic input is impaired

Naloxone significantly enhanced the release of radioactive acetylcholine ([3H]ACh) from rat striatal slices loaded with [3H]choline either when the nigrostriatal pathway had been destroyed by 6-hydroxydopamine or when the D2 dopamine receptors had been inhibited by sulpiride. This in vitro study supplies the first neurochemical evidence, that, in addition to D2-receptor-mediated dopaminergic tonic control, there is opiate-receptor mediated presynaptic modulation of striatal ACh release, possibly by endogenous enkephalin released from local neurons. Such modulation occurs under conditions in which the dopaminergic input is impaired.

Antagonism by antidepressant drugs of the inhibitory effect of trifluoromethylphenylpiperazine (TFMPP) on [3H]acetylcholine release in rat or guinea-pig hippocampal synaptosomes

The effects of antidepressant drugs on the m-trifluoromethylphenylpiperazine (TFMPP)-induced inhibition of K(+)-evoked [3H]acetylcholine (3H-ACh) release were studied in rat or guinea-pig hippocampal synaptosomes. The serotonergic agonist TFMPP dose-dependently inhibited the K(+)-evoked release of 3H-ACh in rat hippocampus (IC50 = 53 microM). Chlorimipramine (5-500 nM), a typical tricyclic antidepressant, and minaprine (1-100 nM), an atypical antidepressant drug, partially antagonized the effect of TFMPP on 3H-ACh release in a dose-dependent manner. Other antidepressants (imipramine, citalopram, indalpine, fluoxetine, fluvoxamine, oxaprotiline, mianserine, nomifensine), at concentrations ranging from 10 to 500 nM, produced similar effects. Drugs with no antidepressant effect, such as chlorpromazine, clobazam, and cocaine (50, 100 and 500 nM), were without significant influence on the TFMPP effect. In guinea-pig hippocampal synaptosomes, minaprine (50 nM) also reduced the TFMPP-induced inhibition of 3H-ACh release, whilst clobazam (50 nM) was inactive. These results suggest that antidepressant drugs interact in vitro with heterologous serotonergic presynaptic receptors on cholinergic nerve terminals in rat and guinea-pig hippocampus.

Effects of serotonergic denervation on the density and plasticity of brain muscarinic receptors in the rat

This study investigated whether serotonergic lesion may affect density, sensitivity, and plasticity of muscarinic receptors in hippocampus and cerebral cortex. Intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) in rats produced a 90% reduction in cortical and hippocampal 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents. In these brain areas, the 5,7-DHT lesion did not affect the overall density of muscarinic receptors or those of M1 and non-M1 muscarinic receptor subtypes as assayed using [3H]N-methylscopolamine ([3H]NMS), [3H]pirenzepine, and [3H]NMS in the presence of pirenzepine, respectively. In addition, the binding of the muscarinic agonist [3H]oxotremorine-M (OXO-M), taken as an indirect index of coupling efficiency of non-M1 receptors with G-proteins, did not change significantly in cortex and hippocampus of 5,7-DHT-lesioned rats. Similarly, carbachol-induced accumulation of [3H]inositol phosphates (InPs) in hippocampal miniprisms showed no significant differences between tissues from 5,7-DHT-lesioned and sham-operated rats. In sham-operated rats, an intraperitoneal (i.p.) injection of scopolamine (10 mg/kg once daily) during 21 days caused an increased density of [3H]NMS binding sites in cortex (+20%) and hippocampus (+26%). This up-regulation was restricted to non-M1 receptors subtypes. In 5,7-DHT-lesioned rats, chronic scopolamine failed to modify significantly the density of cortical or hippocampal M1 or non-M1 receptors. These results suggest 1) that 5-HT denervation did not affect the density and sensitivity of muscarinic receptors and 2) that the ability of cortical and hippocampal non-M1 receptors to up-regulate following repeated injection of scopolamine requires the integrity of 5-HT neurons terminating in these brain structures.

5-HT1A agonists increase and 5-HT3 agonists decrease acetylcholine efflux from the cerebral cortex of freely-moving guinea-pigs

1. The influence of 5-hydroxytryptamine1A (5-HT1A), 5-HT2 and 5-HT3 agonists and antagonists on acetylcholine (ACh) release from the cerebral cortex was studied in freely moving guinea-pigs. 2. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.01-1 mg kg-1, s.c.) caused the 5-HT syndrome and dose-dependently increased ACh release. Ru 24969 (1-10 mg kg-1, s.c.) shared the same effects, but it was less potent. (-)-Propranolol (5 mg kg-1) and metitepine (2 mg kg-1) prevented these behavioural and neurochemical responses. 3. (+/-)-1(4-Iodo-2,5-dimethoxyphenyl)2-aminopropane (DOI) up to 2 mg kg-1 did not modify ACh release and ketanserin (0.5 mg kg-1) was ineffective on 5-HT-induced changes of ACh outflow. 4. 2-Methyl-5-HT (500 micrograms, i.c.v.) and 5-HT (500 micrograms, i.c.v.) plus metitepine (2 mg kg-1, s.c.) inhibited the gross behaviour and ACh release. ICS 205-930 (0.5 mg kg-1) prevented these responses. 5. 2-Methyl-5-HT, up to 10 mumols 1(-1), and 8-OH-DPAT, up to 0.1 mumols 1(-1), (like 5-HT) did not change [3H]-choline efflux from cerebral cortex slices. 6. These results suggest that exogenous 5-HT and related selective agonists modulate guinea-pig cortical cholinergic structures through 5-HT1A and 5-HT3 receptors. The stimulation of 5-HT1A autoreceptors may lead to disinhibition of the cholinergic cells, tonically inhibited by the tryptaminergic control. Conversely, the stimulation of 5-HT3 receptors inhibits ACh release, possibly through an interneurone. No direct 5-HT modulation of the cholinergic nerve endings was found.

Presynaptic serotonin receptors in the central nervous system

Presynaptic 5-HT autoreceptors have been identified in any region of the mammalian CNS containing 5-HT nerve terminals that has been investigated for this purpose. They belong to the 5-HT1B receptor subclass in the rat and to the 5-HT1D subclass in the pig, guinea pig, and probably man. The presence and operation of presynaptic 5-HT autoreceptors have been proven by the ability of 5-HT receptor agonists to inhibit 5-HT release and of 5-HT receptor antagonists not only to competitively antagonize this effect but also to disclose the autoinhibitory effect of endogenous 5-HT by blocking the autoreceptor, thus interrupting the negative feedback loop. There is evidence that presynaptic 5-HT autoreceptors are operative in vivo. Presynaptic inhibitory 5-HT heteroreceptors have also been identified in various brain regions of the rat. DA nerve terminals in the striatum and nucleus accumbens as well as GLU nerve terminals in the cerebellum are endowed with such receptors, which were either not yet classified (DA neurone) or represent a not yet specified 5-HT1 subtype (GLU neurone). Release-inhibiting 5-HT receptors on the acetylcholine nerve terminals in the hippocampus are of the 5-HT1B subtype, and those in the striatum were not yet classified in detail. A 5-HT heteroreceptor mediating stimulation of release occurs on rat striatal DA nerve terminals; it belongs to the 5-HT3 class. Thus, presynaptic inhibitory 5-HT auto- and heteroreceptors as well as presynaptic excitatory 5-HT heteroreceptors are involved in the regulation of transmitter release in the brain.

The effects of ondansetron, a 5-HT3 receptor antagonist, on cognition in rodents and primates

The selective 5-HT3 receptor antagonist, onansetron, has been assessed in three tests of cognition in the mouse, rat and marmoset. In a habituation test in the mouse, ondansetron facilitated performance in young adult and aged animals, and inhibited an impairment in habituation induced by scopolamine, electrolesions or ibotenic acid lesions of the nucleus basalis magnocellularis. Arecoline failed to improve basal performance in young adult mice but inhibited the impairment caused by scopolamine and lesions of the nucleus basalis magnocellularis. In the T-maze reinforced alternation task in rats, ondansetron and arecoline antagonised a scopolamine-induced impairment. In an object discrimination and reversal learning task in the marmoset, assessed using a Wisconsin General Test Apparatus, ondansetron improved performance in a reversal learning task. We conclude that ondansetron potently improves basal performance in rodent and primate tests of cognition and inhibits the impairments in performance caused by cholinergic deficits.

Minaprine antagonises the serotonergic inhibitory effect of trifluoromethylphenylpiperazine (TFMPP) on acetylcholine release

The serotonin agonist, m-trifluoromethylphenylpiperazine (TFMPP), inhibited the K+-evoked release of [3H]acetylcholine ([3H]ACh) from rat hippocampal synaptosomes. The inhibitory effect of TFMPP was blocked by the non-selective 5-HT1 antagonist, methiothepin, but was not affected by ketanserin, mesulergine or spiperone. The 5-HT3 antagonist, MDL 72222, slightly reversed the inhibitory effect. The antidepressant, minaprine, did not modify the basal release of [3H]ACh but it antagonised the inhibitory effect of TFMPP on the K+-evoked release. The maximal reversal was found at 0.3 microM minaprine. These results suggest that minaprine interacts with heterologous presynaptic 5-HT1B receptors. A new approach is thus opened to the study of the mechanism of action of antidepressant drugs.

Acetylcholine release from rat hippocampal slices is modulated by 5-hydroxytryptamine

Experiments were performed with slices of rat hippocampus in order to investigate whether the release of acetylcholine in this area is modulated through 5-hydroxytryptamine (5-HT) receptors. The slices were prelabeled with [3H]choline then stimulated electrically twice for 4 min each at a frequency of 3 Hz. The overflow of tritium evoked was inhibited by exogenous 5-HT in a concentration-dependent manner. The 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HC1 ((+/-)-DOI), did not mimic 5-HT. The effect of 5-HT was antagonized by methiothepin but not by the 5-HT2 antagonist, ketanserin. The 5-HT1 agonist, 5-methoxy-3-[1,2,3,6-tetrahydropyridin-4-yl]-1H-indole (RU 24969), inhibited the electrically evoked overflow of tritium, whereas the 5-HT1A-selective agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), was ineffective. Methiothepin itself, but not ketanserin, increased the evoked overflow of tritium. In contrast, the overflow was inhibited by the 5-HT uptake blocker, 6-nitroquipazine. The evoked overflow was also reduced by d-fenfluramine, a serotonin releaser. The concentration-inhibition curve for d-fenfluramine was shifted to the right by methiothepin. It is concluded that the release of ACh in rat hippocampus may be tonically inhibited by 5-HT through the activation of receptors, possibly belonging to the 5-HT1B subtype.

Effect of 5-hydroxytryptamine on [3H]-acetylcholine release from guinea-pig striatal slices

1. The effect of 5-hydroxytryptamine (5-HT) on spontaneous and electrically-evoked tritium efflux was studied in guinea-pig caudate nucleus slices preloaded with [3H]-choline. 2. 5-HT, 10-300 mumol l-1, temporarily increased the spontaneous tritium efflux (as well as the endogenous acetylcholine (ACh) release) and, after 15 min perfusion, inhibited it. The facilitatory effect of 5-HT on spontaneous efflux was increased while the inhibitory effect did not occur in slices taken from dopamine-depleted guinea-pigs. 3. The increase in spontaneous tritium efflux by 5-HT was blocked by methiothepin, methysergide (pA2 8.7) and by the selective 5-HT2 antagonist, ritanserin (pA2 6.7). 4. The inhibition of spontaneous tritium efflux by 5-HT was prevented by methysergide and methiothepin but not by ritanserin and (-)-propranolol. 5. 5-HT, 100 mumol l-1, inhibited the electrically-evoked tritium efflux and this effect was unchanged in dopamine-depleted slices. 6. The inhibition of electrically-evoked tritium efflux by 5-HT was blocked by methiothepin and methysergide but not by (-)-propranolol or ritanserin. 7. These results suggest that 5-HT may exert a rapid and transient (excitatory) and a more prolonged (inhibitory) control over striatal cholinergic neurones.

5-HT3 receptors mediate inhibition of acetylcholine release in cortical tissue

The release of cerebral acetylcholine from terminals in the cerebral cortex has been shown to be regulated by 5-hydroxytryptamine (5-HT) but it is not known which subtype of the 5-HT receptor is involved. 5-HT receptor agonists increase acetylcholine levels in vivo, indicating a reduced turnover, and reduce release of acetylcholine from striatal slices in vitro. Depleting 5-HT by inhibiting synthesis or by destroying the neurons containing 5-HT potentiates acetylcholine release, and increases acetylcholine turnover in the cerebral cortex and hippocampus. Selective antagonists for the 5-HT3 receptor subtypes which seem to have effects on mood and activity may exert their effect through the regulation of acetylcholine release in the cortex and limbic system. Radioligand binding studies show a high density of 5-HT3 receptors in the cholinergic-rich entorhinal cortex and we provide evidence that a reduction in cortical cholinergic function can be effected in vitro by 5-HT3 receptors.

Induction of purposeless chewing behaviour in rats by 5-HT agonist drugs

The 5-HT agonist m-chlorophenylpiperazine (m-CPP; 1-16 mg/kg i.p. or s.c.), trifluoromethylphenylpiperazine (TFMPP; 2-16 mg/kg i.p.) and quipazine (2.5-20 mg/kg i.p.) increased purposeless chewing behaviour in rats. However, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.025-4 mg/kg s.c.) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT; 0.25-8 mg/kg s.c.) were without effect on chewing behaviour. Chewing behaviour induced by m-CPP (6 mg/kg s.c.) was antagonised by pretreatment with the 5-HT antagonists methiothepin and mianserin, but not by ketanserin or spiperone, or ICS 205-930. m-CPP (6 mg/kg s.c.)-induced chewing behaviour was also antagonised by pretreatment with (-)-propranolol (20 mg/kg). Pretreatment with the anticholinergic drugs benzhexol (2.5 mg/kg), and scopolamine (1 mg/kg) antagonised m-CPP (6 mg/kg s.c.)-induced chewing behaviour, but methylscopolamine (1 mg/kg) had no effect. These data support the role of 5-HT receptors in the mediation of purposeless chewing behaviour and suggest an interaction between brain 5-HT and acetylcholine systems.

Inhibition of striatal acetylcholine release by endogenous serotonin

We have examined the hypothesis that endogenous serotonin (5-HT) exerts an inhibitory influence on the release of acetylcholine (ACh) in striatum. Striatal slices were prepared from adult rats, preincubated with [3H]choline, superfused, and exposed to electrical field stimulation. The stimulation-induced overflow of tritium into the superfusate was used as a measure of ACh release. We observed that fluoxetine, an inhibitor of 5-HT uptake, reduced ACh overflow in slices prepared from caudal striatum, an area of high 5-HT concentration, but not in slices from rostral striatum, an area of low 5-HT concentration. Moreover, methysergide, a 5-HT antagonist, increased ACh efflux in caudal but not rostral striatum. Finally, direct activation of 5-HT receptors with the 5-HT agonist, quipazine, inhibited stimulation-induced ACh overflow in both rostral and caudal striatum. These results suggest that endogenous 5-HT normally is capable of inhibiting striatal ACh release, and that the extent of the modulation is related to the degree of serotonergic innervation. In addition, 5-HT receptors capable of modulating ACh release are present in 5-HT-poor rostral striatum, as well as in 5-HT-rich caudal striatum.

Inhibition of striatal acetylcholine release by serotonin and dopamine after the intracerebral administration of 6-hydroxydopamine to neonatal rats

The intraventricular administration of 6-hydroxydopamine (6-OHDA) depletes the striatum of dopamine (DA). When given to rat pups at an early age, the toxin also increases striatal serotonin (5-HT) content. In the accompanying report we observed that endogenous 5-HT, like DA, exerts an inhibitory influence on the release of acetylcholine (ACh) from striatal slices prepared from control animals and that the extent of this inhibition is related to the degree of serotonergic innervation of the region being examined. To determine whether this hyperinnervation was accompanied by an increase in serotonergic influence on ACh release, striatal slices were prepared from adult rats, preincubated with [3H]choline, superfused, and exposed to electrical field stimulation. The efflux of tritium into the superfusate was used as a measure of ACh release. In confirmation of previous reports, we observed that direct and indirect agonists of DA and 5-HT both reduced ACh overflow from control slices, whereas overflow was increased by antagonists of these amines. Slices prepared from rats given 6-OHDA-induced lesions as adults were responsive to each of these pharmacological manipulations, as well. In contrast, ACh overflow from slices prepared from animals lesioned with 6-OHDA as neonates was not modified by either dopaminergic or serotonergic drugs. These results suggest that the serotonergic hyperinnervation of striatum produced by neonatal 6-OHDA is accompanied by a loss of the inhibitory influence of endogenous 5-HT and DA on striatal ACh release and, thus, provide no evidence for a role for either transmitter in the behavioral sparing associated with such lesions.