Effect of quipazine, a serotonin-like drug, on striatal cholinergic interneurones

Multiple controls exerted by 5-HT2C receptors upon basal ganglia function: from physiology to pathophysiology

Serotonin2C (5-HT2C) receptors are expressed in the basal ganglia, a group of subcortical structures involved in the control of motor behaviour, mood and cognition. These receptors are mediating the effects of 5-HT throughout different brain areas via projections originating from midbrain raphe nuclei. A growing interest has been focusing on the function of 5-HT2C receptors in the basal ganglia because they may be involved in various diseases of basal ganglia function notably those associated with chronic impairment of dopaminergic transmission. 5-HT2C receptors act on numerous types of neurons in the basal ganglia, including dopaminergic, GABAergic, glutamatergic or cholinergic cells. Perhaps inherent to their peculiar molecular properties, the modality of controls exerted by 5-HT2C receptors over these cell populations can be phasic, tonic (dependent on the 5-HT tone) or constitutive (a spontaneous activity without the presence of the ligand). These controls are functionally organized in the basal ganglia: they are mainly localized in the input structures and preferentially distributed in the limbic/associative territories of the basal ganglia. The nature of these controls is modified in neuropsychiatric conditions such as Parkinson's disease, tardive dyskinesia or addiction. Most of the available data indicate that the function of 5-HT2C receptor is enhanced in cases of chronic alterations of dopamine neurotransmission. The review illustrates that 5-HT2C receptors play a role in maintaining continuous controls over the basal ganglia via multiple diverse actions. We will discuss their interest for treatments aimed at ameliorating current pharmacotherapies in schizophrenia, Parkinson's disease or drugs abuse.

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.).

Effect of tianeptine on the hypothalamic somatotropic axis in the conscious sheep

The action of serotonin on growth hormone (GH) secretion is controversial because of interspecies differences and lack of specificity of serotoninergic drugs. Serotonin (5-HT) appears to inhibit GH release in the sheep and in man. We have investigated the site of action of tianeptine, a 5-HT uptake enhancer, in sheep since it is possible to collect hypophysial portal blood for the simultaneous determination of growth hormone-releasing hormone (GHRH) and somatostatin in this species under conscious, unstressed conditions. Tianeptine injection (10 mg/kg i.v.) resulted in a significant, immediate and short-lasting (30 min) increase in peripheral GH (+750%; P < 0.01) and hypophysial portal GHRH (+180%; P < 0.01). No change in the secretion of somatostatin was recorded during the same time. These data suggest that serotoninergic inputs are inhibitory to GH secretion. Tianeptine acts centrally to stimulate GH secretion in the sheep and its effect is mediated through changes in GHRH but not somatostatin release into hypophysial portal blood.

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.

Effects of oxotremorine on inhibitory avoidance behaviour in two inbred strains of mice: interaction with 5-methoxy-NN-dimethyltriptamine

The effects of the cholinergic muscarinic agonist, oxotremorine (0.005, 0.01, 0.02 and 0.04 mg/kg), the serotonergic agonist, 5-methoxy-NN-dimethyltriptamine (5-MeODMT) (0.5, 1 and 2 mg/kg), and their combination, were investigated in C57BL/6 and DBA/2 mice using a one-trial inhibitory avoidance task, drug treatment being given immediately after the acquisition trial. Post-trial administration of oxotremorine facilitated, while post-trial administration of 5-MeODMT inhibited memory retention of both strains in a dose-dependent fashion. The DBA/2 strain was more affected by oxotremorine than the C57BL/6 mice; no strain-dependent sensitivity to serotonergic agonist administration was observed. In both strains, the combination of oxotremorine plus 5-MeODMT inhibited the performance improvement shown by the administration of the cholinergic agonist alone. The facilitatory role of cholinergic stimulation on retention performance was confirmed and an inhibitory action of the serotonergic system on memory processes was suggested. Moreover, the present results support a functional interaction between cholinergic and serotonergic systems on memory consolidation.

Mechanism of the galanin induced increase in acetylcholine release in vivo from striata of freely moving rats

Galanin (GAL) administered intracerebroventricularly (i.c.v.) induced a strong and long-lasting increase in the basal acetylcholine (ACh) release from striata of freely moving rats only when the excitatory corticostriatal input was removed, while its effect was transient in striata of sham-operated rats. This effect was dose-dependent (0.78, 1.56 and 3.12 nmol) and was completely prevented by the GAL receptor antagonist, galantide. GAL injected locally (3.12 nmol) in deafferented striata also induced a persistent increase in ACh release although to a lower extent. The impairment of monoaminergic neurotransmission caused by alpha-methylparatyrosine or p-chlorophenylalanine, respectively inhibitors of catecholamine and serotonin synthesis, completely prevented the rise in ACh output from deafferented striata while the muscarinic antagonist, scopolamine (0.5 mg/kg, s.c.), failed to do it. The data suggest that GAL in the deafferented striatum facilitates basal ACh release through an indirect mechanism. The effect seems to be at least partly mediated by an action of GAL on specific receptors in the striata.

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.

Serotonin regulation of neostriatal tachykinins following neonatal 6-hydroxydopamine lesions

In order to determine whether dopamine mediates the effects of serotonin on tachykinin biosynthesis in the neostriatum, serotonin neurotransmission was altered following depletion of dopamine. Neonatal rats received intracisternal injections of saline or the dopamine neurotoxin 6-hydroxydopamine (6HD). This lesion caused significant reductions in the neostriatum of substance P-like immunoreactivity as well as levels of mRNA coding for preprotachykinin (PPT; the prohormone precursor to tachykinins substance P, neurokinin A and related peptides). Two months later, rats were treated for 5-6 days with saline or the serotonin-uptake inhibitor, zimelidine. Zimelidine treatment of unlesioned animals significantly increased PPT mRNA levels in the neostriatum. However, zimelidine treatment failed to increase PPT mRNA content in 6HD-treated animals. By contrast, neostriatal substance P-like immunoreactivity was restored by zimelidine treatment of 6HD-lesioned animals. These results suggest that an intact nigrostriatal pathway may be required for serotonin neurotransmission to alter PPT mRNA levels in the neostriatum. However, neostriatal tachykinins may be regulated by direct serotonin innervation.

Serotonin, memory, and the aging brain

Serotonin is widely distributed throughout the central nervous system and is implicated in a variety of neural functions such as pain, feeding, sleep, sexual behavior, cardiac regulation and cognition. This paper is concerned with the last of these. Abnormalities of the serotonergic nervous system are well documented in pathologic studies of Alzheimer's disease and there is evidence suggesting that changes in this system occur in association with non-disease aging. Data on the role of serotonin in learning and memory and on the effects of aging on brain serotonin function are reviewed and discussed in relation to pharmacologic treatment strategies for the memory impairments associated with advancing age.

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.

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.

Colchicine administered into the area of the nucleus basalis decreases cortical nicotinic cholinergic receptors labelled by [3H]-acetylcholine

Lesions in the nucleus basalis in the rat are known to decrease presynaptic markers for acetylcholine, including levels of cholineacetyltransferase (CHAT), high affinity uptake of choline and levels of acetylcholinesterase. Effects of lesions of the nucleus basalis on populations of nicotinic and muscarinic receptors are less well understood. After bilateral injection of the neurotoxic agent, colchicine into the nucleus basalis in the rat, levels of CHAT in the cerebral cortex were reduced 44%. Muscarinic cholinergic [( 3H]QNB) and dopaminergic [( 3H]spiroperidol) binding was not changed in the cortex, hippocampus or striatum. However, significant decreases in nicotinic binding sites, labelled by [( 3H]acetylcholine), were observed in the frontal cortex of nucleus basalis treated animals; scatchard plot analysis indicated a significant decrease in the number, but not affinity, of nicotinic binding sites. Colchicine injected into the nucleus basalis had no effect on the binding of [3H]acetylcholine in the hippocampus, but decreased binding of [3H]acetylcholine in the striatum. Subsequent experiments, in which colchicine was administered into the striatum at a site above the nucleus basalis had no significant effect on nicotinic binding in the striatum or frontal cortex. These results support the hypothesis that degeneration of the nucleus-basalis-cortical cholinergic pathway results in a loss of presynaptic nicotinic binding sites in the cortex as well as in the striatum (through transsynaptic degeneration of the cortico-striatal pathway).

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.

Selective breeding for increased cholinergic function: increased serotonergic sensitivity

The effects of the serotonergic antagonist cyproheptadine and the agonist 1(m-chlorophenyl) piperazine (mCPP) on core body temperature, locomotor activity and operant responding for a water reward were determined in two lines of Sprague-Dawley rats selectively bred for differences in sensitivity to the anticholinesterase, diisopropyl fluorophosphate (DFP). Both cyproheptadine and mCPP induced a dose-dependent hypothermia that was significantly greater in the line of rat more sensitive to DFP (the Flinders Sensitive Line--FSL). On the other hand, the mild stimulant effects of cyproheptadine on operant responding and locomotor activity were similar in the two lines, whereas the marked inhibitory effects of mCPP on these two measures were significantly greater in the FSL rats. This study also confirmed that the FSL rats were significantly more sensitive to the hypothermic effects of oxotremorine, a muscarinic agonist, and showed that pretreatment with cyproheptadine reduced the hypothermic effects of oxotremorine to a similar extent in the two lines. These findings indicate that rats selectively bred for increased cholinergic function (FSL) also differ in their sensitivity to serotonergic agonists and antagonists, thereby extending the evidence for cholinergic-serotonergic interactions in the rat.

Serotonin in involuntary movement disorders

Several recent studies have emphasized that serotonergic pathways in the CNS are intimately involved in the modulation of motor behavior, and in the pathophysiology of human involuntary movement disorders. These observations are supported by recent reports demonstrating large serotonergic innervation of the striatum and substantia nigra, and a close interaction between the activity of serotonergic neurons with the dopamine system in the striatum and nigra. In the following communication we summarize evidence demonstrating defective serotonergic functions in a number of human movement disorders and discuss their management with serotonergic drugs.

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.

Evidence for a possible functional interaction between serotonergic and cholinergic mechanisms in memory retrieval

A total of three experiments were conducted. In Experiment 1, the dose-dependent effects of the pretest administration of the serotonergic agonist alaproclate and the selective muscarinic cholinergic agonist oxotremorine, alone and in combination, were assessed in a one-trial inhibitory avoidance task. A clear dose-dependent enhancement of performance was demonstrated as a result of all three treatment conditions, which could not be explained in terms of nonspecific effects of the drugs on behavior in general. In addition, the facilitation of retrieval performance produced by the combined treatment of alaproclate and oxotremorine was observed at dose levels well below those observed following administration of either compound alone. In Experiment 2 attempts were made to block the enhancements of retention resulting from the different treatment conditions (alaproclate, oxotremorine, or the combination of alaproclate and oxotremorine) by pretreating the mice with either scopolamine (a muscarinic cholinergic antagonist) or quipazine (a serotonergic agonist). The results of these experiments indicate that (a) quipazine completely blocked the enhancement of retrieval resulting from alaproclate but not that following oxotremorine or oxotremorine in combination with alaproclate, while (b) scopolamine blocked the enhancement of retrieval resulting from oxotremorine alone as well as that resulting from alaproclate plus oxotremorine but failed to block the memory enhancement resulting from alaproclate. The present results lend further support to the view that both serotonin and acetylcholine play important roles in memory retrieval. More importantly, the results of the present series of experiments provide additional support for a functional interaction between the serotonergic and cholinergic nervous systems in the mediation of behavior.

Modification by physostigmine of the cardiovascular responses to intracerebroventricular administration of 5-hydroxytryptamine in rats

In rats the effect of inhibition of the brain cholinesterase activity on the pressor and heart rate responses to 5-hydroxytryptamine (5-HT), administered into the lateral cerebral ventricle (l.c.v.) was examined. After administration of physostigmine (twice in a small dose of 2.5 micrograms l.c.v., 20 and 15 min before the second injection of 5-HT), the pressor effect of 5-HT (5 micrograms) was strongly reduced or almost abolished, its pure tachycardia was reduced or reversed into a bradycardia and its pure bradycardia was diminished or reversed into a tachycardia. The type of the cardiovascular response to ACh (5 micrograms l.c.v., 20 min after the second administration of 5-HT) indicates that the modification of the cardiovascular response to 5-HT was accompanied by inhibition of the brain cholinesterase activity. Thus, it seems that a functionally competent cholinesterase in the brain is necessary for the generation of the 5-HT-induced pressor response. The present experiments provide further evidence that there is a cholinergic link in the pathway by which serotonergic mechanisms in the preoptic-anterior hypothalamic area rise blood pressure and support the idea that the same link exists in the pathway(s) mediating the heart rate responses to intracerebroventricular administration of 5-HT.

Evidence for a possible interaction between noradrenergic and serotonergic neurotransmission in the retrieval of a previously learned aversive habit in mice

The present series of experiments examined the effects of pretreating pirenperone-injected mice with a variety of non-serotonergic receptor antagonists on retrieval of a one-trial inhibitory (passive) avoidance task. Water-deprived mice were trained to avoid drinking from a water spout located in the avoidance chamber by pairing foot-shock with licks from the water spout. Retention was measured as the suppression of drinking (latency to drink) 48 h later. Pre-test administration of pirenperone (1.0 mg/kg) significantly enhanced retrieval (increased latencies). The suppression of drinking could not be attributed to the non-specific effects of pirenperone on behavior in general, as the performance of non-contigently shocked mice injected with the same dose of pirenperone did not exhibit a similar elevation in latencies. Of the seven pretreatment drugs examined, only phenoxybenzamine (1.0 mg/kg) completely blocked the pirenperone-induced response. Bicuculline (1.0 mg/kg) partially attenuated the enhanced performance resulting from pre-test pirenperone administration. The results suggest that the pirenperone-induced response may be partly due to activation of noradrenergic (alpha) neurotransmission.

The influence of 5-hydroxytryptamine on the release of acetylcholine from guinea-pig brain ex vivo and in vitro

The effect of 5-hydroxytryptamine (5-HT) on the release of acetylcholine (ACh) from the brain of the guinea-pig was investigated in order to determine whether this amine plays a modulatory role on the cortical cholinergic projections. 5-Hydroxytryptamine (0.2-1 mumol), injected intracerebroventricularly (i.c.v.), caused mild excitation, stereotyped movements and ataxia. Simultaneously, it increased the output of ACh from the cortex in a dose-dependent manner. Methysergide (4.2 mumol Kg-1 i.p.) also increased the output of ACh by about 60-80%, but prevented the effect of 5-HT (1 mumol i.c.v.). Metitepine (1-4.2 mumol kg-1 i.p.) increased the output of ACh like methysergide but it changed the facilitation of the release of ACh by 5-HT into inhibition. At the same time the animals became hypothermic, sedated and their electroencephalogram (EEG) was synchronized. Pretreatment with 5,7-HT blocked the increase in release of ACh produced by 5-HT (1 mumol). D-Norfenfluramine (10.4 mumol kg-1) was ineffective alone but reduced the release of ACh in metitepine-pretreated animals. 5-Hydroxytryptamine (10-30 microM) did not affect the efflux of [3H]choline from electrically-stimulated slices of cerebral cortex. The increase in the release of ACh caused by 5-HT, abolished by pretreatment with methysergide and 5,7-HT, may be explained by activation of 5-HT autoreceptors, while the increase of transmitter outflow induced by methysergide may be due to a blockade of 5-HT receptors present on the cholinergic neurones. Metitepine appeared to unmask the tryptaminergic inhibition caused by injection of 5-HT intraventricularly or by the 5-HT-releasing drug, D-norfenfluramine, possibly by acting on the autoreceptors and preventing auto-inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Local cerebral glucose utilisation following indoleamine- and piperazine-containing 5-hydroxytryptamine agonists

Substances with varying structural components have been shown to have 5-hydroxytryptamine (5-HT)-like properties in the CNS. In this study, putative 5-HT agonists with indoleamine moeities--lysergic acid diethylamide (LSD) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)--and with piperazine moieties--quipazine (Quip) and 6-chloro-2-(1-piperazinyl)pyrazine (6-CPP) were administered to rats. Local cerebral glucose utilisation was measured using the [14C]2-deoxyglucose autoradiographic technique. It was found that in most cerebral structures, these substances produced dose-dependent reductions in glucose utilisation. However, Quip and 6-CPP increased glucose utilisation in specific areas of the diencephalon (e.g., nucleus reuniens) and produced a biphasic effect in some but not all extrapyramidal structures (e.g., ventromedial caudate nucleus). No such increases in local cerebral glucose utilisation were measured following LSD or 5-MeODMT administration. These results indicate that although similarities exist between the effects of indoleamine- and piperazine-containing 5-HT agonists on local cerebral glucose utilisation there are also significant differences in the overall patterns of response produced.

Serotonin inhibits acetylcholine release from rat striatum slices: evidence for a presynaptic receptor-mediated effect

Rat brain striatum slices were incubated with [3H]choline, perfused with a physiological buffer, and stimulated by perfusion with a K+-enriched buffer for 2 min. The tritium overflow evoked by K+ was decreased by 5-hydroxytryptamine (serotonin, 5-HT) (maximal inhibition 10(-6) M). This effect of 5-HT was mimicked by several agonists (5-methoxytryptamine, N,N-dimethyl-tryptamine, bufotenin) and blocked by serotonergic antagonists (methiothepin, methysergide, cinanserin) but not by haloperidol; methiothepin and methysergide alone slightly increased the K+-evoked overflow of tritium (3H). Inhibition of the tritium release by 5-HT was not suppressed in the presence of tetrodotoxin (TTX) (10(-6) M). These results suggest that 5-HT tonically inhibits acetylcholine (ACh) release from striatal cholinergic neurons by acting on a presynaptic receptor localized on cholinergic terminals.

Modification of rat brain 5-hydroxytryptamine metabolism by sublethal doses of organophosphate agents

The early and late effects of sublethal doses of two organophosphate agents (paraoxon and soman) on 5-hydroxytryptamine (5-HT) metabolism were investigated in several rat brain areas. Parallel determinations of acetylcholinesterase (AcChE) inhibition were performed. An increase in 5-HT level was observed during the first phase of soman intoxication and a rise in 5-hydroxyindol acetic acid (5-HIAA) appeared in the early and late effects of both anticholinesterase agents with a predominant action in the striatum. These data suggested that paraoxon and soman induce a long-lasting increase in 5-HT turnover. This action cannot be related neither to the AcChE inhibition nor to the acetylcholine level.

Studies on the indirect feedback inhibition of cholinergic neurons triggered by oxotremorine in striatum

Oxotremorine produced 30-75% increases in rat striatal acetylcholine content and 10-15% decreases in choline content at the subtremorogenic doses of 0.34-1.34 mumol/kg, without affecting choline acetyltransferase and acetylcholinesterase activities and the sodium-dependent high affinity uptake of choline. The increase in acetylcholine was blocked by atropine and by reserpine indicating that oxotremorine indirectly influences the intrinsic striatal cholinergic neurons through a monoamine-mediated negative feedback loop. Experiments designed to interfere with neurotransmitter function indicated that noradrenaline and not dopamine or serotonin, mediated the response to oxotremorine.

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

The release of acetylcholine was studied in isolated striatal slices of the rat. The spontaneous and ouabain-stimulated release of acetylcholine was higher in those slices where serotonergic input was somehow impaired: raphe nuclei lesion or p-chlorophenylalanine pretreatment or 5, 7-dihydroxytryptamine pretreatment resulted in a higher release. L-(m-chlorophenyl)-piperazine, a pure serotonin receptor stimulant and D-fenfluramine, a serotonin releaser significantly reduced the release of acetylcholine evoked by ouabain. Serotonin antagonists (cyproheptadine, mianserine and methysergide) prevented the effect of serotonin agonists. When the serotonergic neurons were destroyed either by p-chlorophenylalanine or by 5, 7-dihydroxytryptamine pretreatment D-fenfluramine had no inhibitory action; however, the effect of L-(m-chlorophenyl)-piperazine was not affected. It is suggested that there is a link between serotonergic and cholinergic neurons in the striatum: serotonin released from raphe-striatal neurons is able to inhibit the release of acetylcholine from striatal interneurons.

Effect of morphine on circling behaviour in rats with 6-hydroxydopamine striatal lesions and electrolesions of the raphe nucleus

Morphine antagonized d-amphetamine circling in rats which had received unilateral 6-OHDA lesions of the striatum but failed to reduce the circling in rats with both a unilateral 6-OHDA striatal lesion and a raphe (5-HT) lesion. Naloxone precipitated withdrawal of morphine tolerant rats greatly enhanced d-amphetamine circling when the rats had a 6-OHDA lesion but not when both 6-OHDA and raphe lesions were present. It is concluded that 5-HT is necessary for the morphine-induced inhibition of the circling. The effect of morphine tolerance and naloxone precipitated withdrawal on brain 5-HT function was investigated using a putative 5-HT rotation model in which both a dopamine and a 5-HT agonist were administered to rats with an asymmetrical medial raphe lesion. The findings suggest that chronic treatment with morphine increases striatal 5-HT function.

Comparison of the effects of the stereoisomers of fenfluramine on the acetylcholine content of rat striatum, hippocampus and nucleus accumbens

The (+)- and (-)- isomeric forms of fenfluramine were compared for their effects on rat brain area acetylcholine (ACh) content. The drugs showed similar patterns in increasing ACh content in the accumbens and hippocampus and in being ineffective in the brainstem. The actions differed in the striatum where the (+)-form markedly increased ACh content while the (-)-form produced no change. Both isomer-induced increases in ACh in the accumbens were prevented when 5-HT synthesis was blocked by p-chlorophenylalanine, thus denoting 5-hydroxytryptaminergic mediation of these effects. In striatum, the increase in ACh induced by (+)-fenfluramine was summated with the increase in ACh induced by dopamine receptor stimulation with apomorphine and was not prevented by dopamine receptor blockade with pimozide. On the other hand, apomorphine's effect was blocked by (-)-fenfluramine while pimozide pretreatment unmasked an increase in ACh induced by (-)-fenfluramine. The results favour the notion that there is a population of cholinergic neurons intrinsic to the striatum which is under inhibitory 5-HT regulation and independent of inhibitory dopamine regulation.

Biochemical and behavioral effects of serotonin neurotoxins on the nigrostriatal dopamine system: comparison of injection sites

Unilateral injections of the serotonin neurotoxin, 5,7-dihydroxytryptamine (DHT), at various points along the 5-HT pathway to the forebrain produce a turning syndrome associated with alterations of dopamine synthesis in the ipsilateral striatum. Unilateral injections of DHT into the SN produced an ipsilateral increase in striatal dopamine (DA) turnover and contralateral rotation in response to amphetamine or apomorphine. Injection of DHT into the MFB produced an ipsilateral decrease in striatal DA turnover and tyrosine hydroxylase (TOH) activity, and ipsilateral rotation in response to amphetamine or apomorphine. After the injection of DHT into the SN or MFB, there was a significant correlation between the rates of drug-induced rotation, the decrease in cortical 5-HT turnover, and the change in striatal DA turnover, suggesting that the unilateral change in DA turnover (and, presumably, the increased stimulation of DA receptors) is causally linked to turning. Injection of DHT into the zones of the striatum and GP richest in 5-HT terminals produced the same responses as the MFB-lesioned rats: ipsilateral rotation and a decrease in striatal TOH activity. Injection of DHT into the area of the striatum richest in DA terminals failed to produce rotation or a significant change in TOH activity. We suggest that 5-HT neurons from the raphe nuclei exert a tonic inhibition on the nigrostriatal pathway at the level of the SN through direct synapses on DA neurons, whereas their neostriatal terminals have an indirect effect on DA terminals, perhaps via interaction with cholinergic and GABA-ergic neurons.