Serotonin modulates muscimol- and baclofen-elicited inhibition of cerebellar Purkinje cells

The Cerebellar Dopaminergic System

In the central nervous system (CNS), dopamine (DA) is involved in motor and cognitive functions. Although the cerebellum is not been considered an elective dopaminergic region, studies attributed to it a critical role in dopamine deficit-related neurological and psychiatric disorders [e.g., Parkinson's disease (PD) and schizophrenia (SCZ)]. Data on the cerebellar dopaminergic neuronal system are still lacking. Nevertheless, biochemical studies detected in the mammalians cerebellum high dopamine levels, while chemical neuroanatomy studies revealed the presence of midbrain dopaminergic afferents to the cerebellum as well as wide distribution of the dopaminergic receptor subtypes (DRD1-DRD5). The present review summarizes the data on the cerebellar dopaminergic system including its involvement in associative and projective circuits. Furthermore, this study also briefly discusses the role of the cerebellar dopaminergic system in some neurologic and psychiatric disorders and suggests its potential involvement as a target in pharmacologic and non-pharmacologic treatments.

Serotonergic control of GABAergic inhibition in the lateral amygdala

Much evidence implicates the serotonergic regulation of the amygdala in anxiety. Thus the present study was undertaken to characterize the influence of serotonin (5-HT) on principal neurons (PNs) of the rat lateral amygdala (LA), using whole cell recordings in vitro. Because inhibition is a major determinant of PN activity, we focused on the control of GABAergic transmission by 5-HT. IPSCs were elicited by local electrical stimulation of LA in the presence of glutamate receptor antagonists. We found that 5-HT reduces GABA_A inhibitory postsynaptic currents (IPSCs) via presynaptic 5-HT_1B receptors. While the presynaptic inhibition of GABA release also attenuated GABA_B currents, this effect was less pronounced than for GABA_A currents because 5-HT also induced a competing postsynaptic enhancement of GABA_B currents. That is, GABA_B currents elicited by pressure application of GABA or baclofen were enhanced by 5-HT. In addition, we obtained evidence suggesting that 5-HT differentially regulates distinct subsets of GABAergic synapses. Indeed, GABA_A IPSCs were comprised of two components: a relatively 5-HT-insensitive IPSC that had a fast time course and a 5-HT-sensitive component that had a slower time course. Because the relative contribution of these two components varied depending on whether neurons were recorded at proximity versus at a distance from the stimulating electrodes, we speculate that distinct subtypes of local-circuit cells contribute the two contingents of GABAergic synapses. Overall, our results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.NEW & NOTEWORTHY We report that 5-HT, acting via presynaptic 5-HT_1B receptors, attenuates GABA_A IPSCs by reducing GABA release in the lateral amygdala (LA). In parallel, 5-HT enhances GABA_B currents postsynaptically, such that GABA_B inhibitory postsynaptic currents (IPSCs) are relatively preserved from the presynaptic inhibition of GABA release. We also found that the time course of 5-HT-sensitive and -insensitive GABA_A IPSCs differ. Together, these results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.

Altered cerebellar organization and function in monoamine oxidase A hypomorphic mice

Monoamine oxidase A (MAO-A) is the key enzyme for the degradation of brain serotonin (5-hydroxytryptamine, 5-HT), norepinephrine (NE) and dopamine (DA). We recently generated and characterized a novel line of MAO-A hypormorphic mice (MAO-A(Neo)), featuring elevated monoamine levels, social deficits and perseverative behaviors as well as morphological changes in the basolateral amygdala and orbitofrontal cortex. Here we showed that MAO-A(Neo) mice displayed deficits in motor control, manifested as subtle disturbances in gait, motor coordination, and balance. Furthermore, magnetic resonance imaging of the cerebellum revealed morphological changes and a moderate reduction in the cerebellar size of MAO-A(Neo) mice compared to wild type (WT) mice. Histological and immunohistochemical analyses using calbindin-D-28k (CB) expression of Purkinje cells revealed abnormal cerebellar foliation with vermal hypoplasia and decreased in Purkinje cell count and their dendritic density in MAO-A(Neo) mice compared to WT. Our current findings suggest that congenitally low MAO-A activity leads to abnormal development of the cerebellum.

Treatment of Ataxia in Cortical Cerebellar Atrophy with the GABAergic Drug Gabapentin

The aim of this work was to investigate the efficacy of the GABAergic drug gabapentin in the treatment of the cerebellar signs caused by cortical cerebellar atrophy (CCA). Ten patients with CCA received gabapentin in single doses of 400 mg in an open-label study; thereafter, daily administration of 900-1,600 mg of gabapentin was continued during at least 4 weeks. An ataxia scale based on clinical findings was used to evaluate the cerebellar signs at baseline and after administration of the drug. A statistically significant improvement of the ataxia scores was found after single doses of 400 mg of gabapentin and after the administration of 900-1,600 mg of this drug during 4 weeks, as compared to the results obtained at baseline. An important clinical amelioration was also evident. Gabapentin has been demonstrated to be capable of improving the cerebellar signs in cases of CCA, after single doses and after continued administration of the drug during 4 weeks. GABAergic enhancement or supplementation could play an important role in the treatment of diseases of the cerebellar cortex associated with a deficit of GABA.

Serotonergic neuromodulation in the cerebellar cortex: cellular, synaptic, and molecular basis

The cerebellum, like most sensorimotor areas of the brain, receives a serotonergic innervation from neurons of the reticular formation. It is well established that local application of serotonin modulates the firing rate of cerebellar Purkinje cells in vivo and in vitro, but the mechanisms by which serotonin affects the cerebellar function are still poorly understood. Whereas interactions between serotonin, glutamate, and GABA have been reported to increase or decrease the firing frequency of Purkinje cells, there is little evidence for a modulation of excitatory and inhibitory synapses by serotonin in the cerebellar cortex. Changes in the intrinsic electrical properties of Purkinje cells upon application of serotonin have also been reported, but their impact on Purkinje cell firing is unclear. The recent finding that serotonin specifically modulates the activity of Lugaro cells, a class of inhibitory interneurons of the cerebellar cortex, offers new insights on the action of this neuromodulator. The peculiar axonal projection and specific interneuronal targets of the Lugaro cells suggest that the action of serotonin might occur upstream of Purkinje cells through a resetting of the computational properties of the cerebellar cortex. Understanding the mechanisms of the serotonergic modulation of the cerebellar cortex is of clinical relevance, as abnormal serotonin metabolism has been observed in animal models and pathological cases of motor disorders involving the cerebellum, and as chronic intravenous administration of L-5-hydroxytryptophan (5-HTP), a precursor of serotonin, was the first treatment shown to improve significantly cerebellar symptoms.

In vivo modulation of vagal-identified dorsal medullary neurones by activation of different 5-Hydroxytryptamine(2) receptors in rats

1. In in vivo experiments, DOI (a 5-HT(2) receptor agonist), MK-212 (a 5-HT(2C) receptor agonist), and BW-723C86 (a 5-HT(2B) receptor agonist) were applied by ionophoresis to neurones in the rat nucleus tractus solitarius (NTS) receiving vagal afferent input. 2. The majority of the putative 'monosynaptically' vagal activated cells were inhibited by both MK-212 (4/6) and DOI (2/4), but unaffected by BW-723C86 (12/14). In contrast, 'polysynaptically' activated NTS cells were excited by both BW-723C86 (13/19) and DOI (9/10). Inactive 'intermediate' cells were inhibited by BW-723C86 (9/12), MK-212 (5/6) and DOI (3/4), whilst active cells of this group were excited by BW-723C86 (7/13) and DOI (5/5). 3. The selective 5-HT(2B) receptor antagonist LY-202715 significantly reduced the excitatory actions of BW-723C86 on 'intermediate' and 'polysynaptic' cells (13/13), but not the inhibitory effects observed on inactive Group 2 cells (n=5) whereas the selective 5-HT(2C) receptor antagonist RS-102221 reversed the inhibitory effects of MK-212 and DOI on 'monosynaptic and 'intermediate' neurones. 4. Cardio-pulmonary afferent stimulation inhibited two of four putative 'monosynaptically' activated calls and all four inactive intermediate cells. These were also inhibited by DOI and MK-212. In contrast, cardio-pulmonary afferents excited all five active intermediate cells and all six putative 'polysynaptically' activated NTS cells, while all were also previously excited by BW-723C86 and/or DOI. 5. In conclusion, these data demonstrate that neurones in the NTS are affected differently by 5-HT(2) receptor ligands, in regard of their vagal postsynaptic location, the type of cardio-pulmonary afferent they receive and the different 5-HT(2) receptors activated.

Effects of cocaine administration on receptor binding and subunits mRNA of GABA(A)-benzodiazepine receptor complexes

The effects of intermittent intraperitoneal (i.p.) administration of cocaine (20 mg/kg) on GABA(A)-benzodiazepine (BZD) receptors labeled by t-[(35)S]butylbicyclophosphorothionate (TBPS), and on several types of mRNA subunits were investigated in rat brain by in vitro quantitative receptor autoradiography and in situ hybridization. Phosphor screen imaging with high sensitivity and a wide linear range of response was utilized for imaging analysis. There was a significant decrease in the level of alpha 1, alpha 6, beta 2, beta 3, and gamma 2 subunits mRNA, with no alteration of [(35)S]TBPS binding in any regions in the brain of rats at 1 h following a single injection of cocaine. In chronically treated animals, the mean scores of stereotyped behavior were increased with the number of injections. The level of beta 3 subunit mRNA was decreased in the cortices and caudate putamen, at 24 h after a final injection of chronic administrations for 14 days. In the withdrawal from cocaine, the frontal cortex and hippocampal complexes showed a significant increase in [(35)S]TBPS binding and alpha1 and beta 3 subunit mRNA in the rats 1 week after a cessation of chronic administration of cocaine. These findings suggest that the disruption of GABA(A)-BZD receptor formation is closely involved in the development of cocaine-related behavioral disturbances. Further studies on the physiological functions on GABA(A)-BZD receptor complex will be necessary for an explanation of the precise mechanisms underlying the acute effects, development of hypersensitization, and withdrawal state of cocaine.

Serotonin-driven long-range inhibitory connections in the cerebellar cortex

Disturbances of the serotoninergic neuromodulation in the cerebellar cortex have been involved in several types of ataxia, but the physiological action of serotonin in this structure remains poorly understood. We report that in slices of the rat cerebellar vermis, serotonin triggers the firing of an inhibitory interneuron presynaptic to Golgi cells. The Lugaro cell, a neglected interneuronal type, satisfies the expected criteria for this input, whereas basket cells, stellate cells, or Golgi cells do not. Lugaro cells are selectively excited by serotonin, and their firing behavior (sustained steady frequency in the 5-15 Hz range) resembles the pattern of occurrence of serotonin-evoked IPSCs in Golgi cells. Immunohistochemical stainings and single cell reconstructions show that Lugaro cell axons form a parasagittal plexus but also extend long transverse branches that run parallel to the parallel fibers and are partly myelinated. Electrophysiological data suggest that these transverse axons participate in synaptic contacts of the Lugaro cells with Golgi cells, and we calculated that in the intact cerebellum a given Lugaro cell contacts >100 Golgi cells. Serotonin modulation of Lugaro cells may constitute an intracortical switch involved in information patterning at the level of Golgi cells and granule cells populations, and particularly in synchronizations recorded along the transverse axis in vivo.

Cannabinoid receptor modulation of synapses received by cerebellar Purkinje cells

The high density of cannabinoid receptors in the cerebellum and the degradation of motor coordination produced by cannabinoid intoxication suggest that synaptic transmission in the cerebellum may be strongly regulated by cannabinoid receptors. Therefore the effects of exogenous cannabinoids on synapses received by Purkinje cells were investigated in rat cerebellar slices. Parallel fiber-evoked (PF) excitatory postsynaptic currents (EPSCs) were strongly inhibited by bath application of the cannabinoid receptor agonist WIN 55212-2 (5 microM, 12% of baseline EPSC amplitude). This effect was completely blocked by the cannabinoid CB1 receptor antagonist SR 141716. It is unlikely that this was the result of alterations in axonal excitability because fiber volley velocity and kinetics were unchanged and a cannabinoid-induced decrease in fiber volley amplitude was very minor (93% of baseline). WIN 55212-2 had no effect on the amplitude or frequency of spontaneously occurring miniature EPSCs (mEPSCs), suggesting that the effect of CB1 receptor activation on PF EPSCs was presynaptically expressed, but giving no evidence for modulation of release processes after Ca(2+) influx. EPSCs evoked by climbing fiber (CF) stimulation were less powerfully attenuated by WIN 55212-2 (5 microM, 74% of baseline). Large, action potential-dependent, spontaneously occurring inhibitory postsynaptic currents (sIPSCs) were either severely reduced in amplitude (<25% of baseline) or eliminated. Miniature IPSCs (mIPSCs) were reduced in frequency (52% of baseline) but not in amplitude, demonstrating suppression of presynaptic vesicle release processes after Ca(2+) influx and suggesting an absence of postsynaptic modulation. The decrease in mIPSC frequency was not large enough to account for the decrease in sIPSC amplitude, suggesting that presynaptic voltage-gated channel modulation was also involved. Thus, while CB1 receptor activation reduced neurotransmitter release at all major classes of Purkinje cell synapses, this was not accomplished by a single molecular mechanism. At excitatory synapses, cannabinoid suppression of neurotransmitter release was mediated by modulation of voltage-gated channels in the presynaptic axon terminal. At inhibitory synapses, in addition to modulation of presynaptic voltage-gated channels, suppression of the downstream vesicle release machinery also played a large role.

5-HT potentiation of the GABA(A) response in the rat sacral dorsal commissural neurones

1. The modulatory effect of 5-hydroxytryptamine (5-HT) on the gamma-aminobutyric acid(A) (GABA(A)) response was investigated in the neurones freshly dissociated from the rat sacral dorsal commissural nucleus (SDCN) using the nystatin perforated patch recording configuration under the voltage-clamp conditions. 2. 5-HT potentiated GABA-induced Cl- current (IGABA) without affecting the reversal potential of IGABA and the apparent affinity of GABA to its receptor. 3. Alpha-Methyl-5-HT mimicked the potentiation effect of 5-HT on IGABA while ketanserine blocked it. 1-Oleoyl-2-acetyl-glycerol (OAG) potentiated IGABA, and the effect of 5-HT on IGABA was occluded by OAG pretreatment. In the presence of chelerythrine, 5-HT failed to potentiate IGABA, suggesting that protein kinase C (PKC) is involved in the pathway through which the activation of the 5-HT2 receptor potentiates the IGABA. 4. The facilitatory effect of 5-HT on IGABA remained in the presence of BAPTA-AM. LiCl also had no effect on 5-HT-induced potentiation of IGABA. 5. H-89, genistein, okadaic acid and pervanadate all had no effects on 5-HT potentiation of IGABA. Pertussis toxin treatment for 6-8 h did not block the facilitatory effect of 5-HT on IGABA. 6. The present results show that GABA(A) receptor in the rat SDCN could be modulated in situ by 5-HT, one of the major transmitters involved in the supraspinal control of nociception, and that the phosphorylation of GABA(A) receptor by PKC may be sufficient to support such modulation. The results also strongly support the hypothesis that the cotransmission by 5-HT and GABA has an important role in the spinal cord.

Buspirone, a serotonergic 5-HT1A agonist, is active in cerebellar ataxia. A new fact in favor of the serotonergic theory of ataxia

We have previously proposed a serotonergic hypothesis for cerebellar ataxia and mentioned that the levorotatory form of 5-hydroxytryptophan, a serotonin precursor, is partially active in subtypes of cerebellar ataxia, including cerebellar cortical atrophy (CCA). It has been demonstrated that 5-HT1A serotonergic receptors play an important role in the control of Purkinje cells discharges and in the inhibition of the release of glutamate by cerebellar glutamatergic terminals. To test further the serotonergic hypothesis of cerebellar ataxia, we administered buspirone, a 5-HT1A agonist usable in human medicine, in a randomized double blind drug placebo trial for 4 months. Nineteen patients with CCA were included; nine patients were given placebo and 10 Buspirone, at the mean dose of 0.69 mg/kg. The evaluation of ataxia was based on a static and a kinetic ataxia scale, fully quantitative measures and the evaluation of the sway path and area at posturography. At 4 months, a significant effect of buspirone was observed for drug induced gains of the kinetic score, two items of the static score, and the maximum duration of standing upright with feet together. These results indicate that a novel chemical therapeutic approach is possible for cerebellar ataxia; moreover, they support the existence of a link between cerebellar ataxia and disturbances of the serotonergic cerebellar system, especially a serotonergic deficit.

Serotonin modulates ethanol-induced depression in cerebellar Purkinje neurons

In the present study, we found that local application of serotonin (5-HT) potentiated ethanol-induced depressions of the spontaneous activity of Purkinje neurons in urethane-anesthetized rats. 5-HT also potentiated depressions induced by gamma-aminobutyric acid; however, this modulatory response was quantitatively smaller than 5-HT-induced potentiation of ethanol depression. Previous reports suggested that the release of 5-HT can be regulated by presynaptic 5-HT autoreceptors. We found that local application of methiothepin, which may induce 5-HT overflow through the inhibition of presynaptic autoreceptors, facilitated ethanol-mediated responses. This methiothepin effect was greatly diminished in neonatally 5,7-dihydroxytryptamine-lesioned animals, suggesting a presynaptic mechanism was involved. We also found that the 5-HT1A antagonist UH301 did not attenuate 5-HT-facilitated ethanol reactions. On the other hand, local application of 5-HT1B agonist CGS12066B potentiated ethanol-induced depression. Taken together, our data suggest that 5-HT can modulate ethanol-mediated electrophysiological depression, possibly mediated through 5-HT1B receptors in the cerebellum.

Identification of central 5-HT and 5-HT1A receptors in the turtle brain (Chrysemys picta)

Radiologand binding studies were undertaken in the turtle whole brain, cerebellum and raphe using the selective radioligands [3H]5-hydroxytryptamine trifluoroacetate ([3H]5-HT) amd [3H]+/-8-hydroxy-2-(di-N-propylamino) tetralin hydrobromide ([3H]DPAT) to identify serotonin (5-HT) receptors and the specific 5-HT1A receptor subtype. Scatchard analysis identified a nanomolar affinity binding site for [3H]5-HT (12 nM) in turtle whole brain assays. A low affinity 5-HT1A site (102 nM) was also identified in turtle whole brain assays, with a higher affinity site noted in binding studies performed with tissue from the inferior raphe (20 nM). The difference in affinity for 5-HT receptors in reptilian versus mammalian brain may prove characteristic of lower vertebrate brains with implications for the physiologic effects of this neurotransmitter in the central nervous system.

Modulation of GABAA receptor function by G protein-coupled 5-HT2C receptors

Two classical neurotransmitters, 5-hydroxytryptamine (5-HT) and GABA, coexist in neurons of the medulla oblongata, and activation of 5-HT receptors modulates GABAA receptor function in neurons of the ventral tegmental area, substantia nigra and cerebellum. We now report that activation of 5-HT2C receptors produces a long-lasting (20-90 min) inhibition of GABAA receptors in Xenopus oocytes coexpressing both types of receptors 5-HT2C receptors caused a approximately 60% decrease in the GABAA receptor Emax without affecting the EC50 or Hill coefficient. Intracellular microinjection of 500 microM BAPTA blocked, whereas microinjection of inositol 1,4,5-triphosphate mimicked the inhibitory action of 5-HT2C receptors. The inhibition was independent of the GABAA receptors subunit composition; receptors containing alpha 2 beta 1, alpha 1 beta 1 gamma 2L, and alpha 2 beta 1 gamma 2S were inhibited to the same extent by 5-HT2C receptor activation. Moreover, GABAA receptors composed of wild-type alpha 2 plus mutant beta 1(S409A) subunits were inhibited to the same extent as wild-type receptors. The nonspecific protein kinase inhibitor, staurosporine, and the inhibitor of serine/threonine protein phosphatases, calyculin A, did not block the inhibitory effects of 5-HT2C receptors. The results with these inhibitors, taken together with those obtained with GABAA receptors with different subunit compositions, suggest that protein kinases or serine/threonine phosphatases are not involved in this GABAA receptor modulatory process. Thus, we propose that 5-HT2C receptors inhibit GABAA receptors by a Ca(2+)-dependent, but phosphorylation independent, mechanism and that 5-HT and GABA may act as cotransmitters to regulate neuronal activity. Furthermore, disruption of the cross-talk between these receptors may play a role in the anti-anxiety actions of 5-HT2 receptor antagonists.

Methamphetamine facilitates ethanol-induced depressions in cerebellar Purkinje neurons of prazocin- or DSP4-treated rats

Methamphetamine (MA) and ethanol (EtOH) are two commonly abused drugs. Previous behavioral studies indicated that MA may synergistically alter EtOH responses. In the present study, we found that local application of MA did not potentiate ethanol-induced depressions of the spontaneous activity of Purkinje neurons in urethane-anesthetized rats. We and others previously found that, in cerebellar Purkinje neurons, EtOH and gamma-amino-butyric acid (GABA)-mediated depressions can be enhanced by norepinephrine (NE) acting via beta-adrenergic receptors while these responses are decreased by activation of alpha-adrenergic receptors. In the present experiment, after blocking alpha-adrenergic receptors with prazocin, MA significantly enhanced EtOH responses in most of neurons studied. It has been reported that MA may directly and indirectly enhance alpha-adrenergic and beta-adrenergic receptor-mediated responses. The present study may suggest that MA can negatively modulate (antagonize) the depressant effects of ethanol via the alpha-adrenergic receptor, which oppose the positive modulatory mechanism (potentiation of EtOH depression) via actions of the beta-adrenergic receptors. We found that lesioning NE neurons with N-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4), a selective noradrenergic neurotoxin, enhance the MA-facilitated ethanol responses, suggesting that this action of MA may not require NE. Since it has been reported that MA increases serotonin (5-HT) and catecholamine release from their nerve terminals, MA may potentiate EtOH depressions by facilitating the release of NE and 5-HT. Taken together, our data suggested that MA may modulate EtOH responses via catecholaminergic and serotonergic mechanisms in cerebellar Purkinje neurons.

Synaptic glutamate release by postnatal rat serotonergic neurons in microculture

Serotonergic neurons are thought to play a role in depression and obsessive compulsive disorder. However, their functional transmitter repertoire is incompletely known. To investigate this repertoire, intracellular recordings were obtained from 132 cytochemically identified rat mesopontine serotonergic neurons that had re-established synapses in microcultures. Approximately 60% of the neurons evoked excitatory glutamatergic potentials in themselves or in target neurons. Glutamatergic transmission was frequently observed in microcultures containing a solitary serotonergic neuron. Evidence for co-release of serotonin and glutamate from single raphe neurons was also obtained. However, evidence for gamma-aminobutyric acid release by serotonergic neurons was observed in only two cases. These findings indicate that many cultured serotonergic neurons form glutamatergic synapses and may explain several observations in slices and in vivo.

Effects of serotonin and the 5-HT2/1C receptor agonist DOI on neurons of the cerebellar dentate/interpositus nuclei: possible involvement of a GABAergic interneuron

The present study was designed to examine the effects of iontophoretically applied serotonin (5-HT) on neurons of the cerebellar dentate/interpositus nuclei in an in vitro slice preparation and to determine if the 5-HT2/1C receptor subtype could be responsible for mediating any effects noted with 5-HT. 5-HT and the 5-HT2/1C-selective agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) were iontophoretically applied alone and during superfusion of the 5-HT2/1C-selective antagonist, ritanserin. 5-HT and DOI elicited either inhibition or excitation of the spontaneous activity of dentate/interpositus neurons. An inhibitory response was induced by both compounds in the majority of cells responding. Ritanserin significantly attenuated the inhibitory response elicited by both 5-HT and DOI. In addition, the inhibitory response to DOI was significantly attenuated by the gamma-aminobutyric acid (GABA) antagonists, bicuculline and picrotoxin. Our results suggest that the 5-HT2/1C receptor subtype may be partially responsible for mediating 5-HT-induced inhibition of dentate/interpositus neurons, possibly via activation of GABAergic interneurons.

Normal serotonin but elevated 5-hydroxyindoleacetic acid concentration in cerebellar cortex of patients with dominantly-inherited olivopontocerebellar atrophy

Beas-Zarate and coworkers (Eur. J. Pharmacol., 198 (1991) 7-14) recently reported markedly reduced concentration of presynaptic serotonin neurotransmitter markers in cerebellum of rodents which had suffered destruction of the inferior olivary-cerebellar (climbing fibre) projections by the neurotoxin 3-acetylpyridine; these experimental animal data suggested that serotonin might be one of the neurotransmitters released by climbing fibres. We measured the concentration of serotonin and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in autopsied cerebellar cortex of 14 patients with dominantly-inherited olivopontocerebellar atrophy (OPCA) who all had near-total degeneration of the inferior olivary climbing fibres. As compared with the controls, mean concentration of serotonin in cerebellar cortex of the OPCA patients was normal whereas 5-HIAA levels (+79%, P less than 0.02) and 'turnover' ratio 5-HIAA/serotonin (+148%, P less than 0.05), on average, were significantly elevated. These data do not support the notion that serotonin is a predominant neurotransmitter of the human climbing fibre. However, the markedly elevated serotonin turnover ratio suggests the possibility of increased serotonergic neuronal activity, which might alter, and perhaps improve, the functioning of the preserved cerebellar cortical neurones in OPCA.