Sensory stimuli differentially affect in vivo nigral and striatal [H]serotonin release in the cat

The effect of auditory, visual and peripheral nerve stimulation on in vivo [³H]serotonin release in both caudate nuclei and substantiae nigrae was studied in either awake ‘encéphale isolé’ or halothane-anesthetized cats. Release of endogenously synthesized [³H]serotonin was estimated in each brain structure, using push-pull cannulae, continuously superfused with [³H]tryptophan. Bilateral and simultaneous application of click noises and light flashes to awake ‘encéphale isolé’ cats enhanced [³H]serotonin release in both substantiae nigrae but was without effect on striatal [³H]serotonin release. Unilateral, low intensity electrical stimulation of the forepaw of halothane-anesthetized cats diminished [³H]serotonin release in both caudate nuclei and the ipsilateral substantia nigra. No effect was observed in the contralateral substantia nigra. In contrast, high-intensity electrical stimulation of one forepaw increased [³H]serotonin release in the ipsilateral substantia nigra but was without significant effect on release in the other brain areas examined. The results are discussed in light of previously known anatomical data concerning serotoninergic pathways and electrophysiological evidence of the effect of sensory stimuli on serotoninergic neurons. Our findings suggest that serotonin neurons may serve an important function in the integration of sensory information.     

In vivo release of serotonin in two raphe nuclei (raphe dorsalis and magnus) of the cat

The release of ³H-serotonin (³H-5-HT) endogenously synthesized from ³H-tryptophan was estimated in both dorsalis and magnus (MRN) raphe nuclei of anaesthetized “encéphale isolé” cats, by using push-pull cannulae. Resting steady state in the release of ³H-5-HT was observed 30 min after the beginning of superfusion with L-³H-tryptophan. The amounts of ³H-5-HT released in the DRN and the MRN are much greater than those measured simultaneously in the caudate nucleus. A marked increase either in the 5-HT release was seen in the presence of fluoxetine, a potent reuptake blocker of 5-HT, or during local depolarization with potassium chloride. The spontaneous release was diminished by removing Ca⁺⁺ and by adding cobalt to the medium. Tetrodotoxin (TTX) decreased the 5-HT release in the DRN and, based on previously established, blocked the stimulating effect of batrachotoxin. According to the pharmacological characteristics of the monoamine dendritic release determined for dopamine in the substantia nigra [17], our results suggest that 5-HT release processes in the DRN correspond to a release from nerve endings, not from dendrites. The purpose of this study was to determine if the 5-HT released in the DRN is released from either axon terminals or dendrites. Morphological studies performed on the DRN do not consistently demonstrate the high density of serotoninergic varicosities in the DRN. In addition, two types of 5-HT axonal varicosities, characterized by their synaptic or non-synaptic junctions, are present in the brain. The concept that the quantities of 5-HT released could vary from one type compared to the other is discussed.     

Role of thalamic motor nuclei in bilateral regulation of serotoninergic transmission in cat basal ganglia

The effects of unilateral application of GABA (10(-5) M) into thalamic motor nuclei (ventralis medialis-ventralis lateralis, VM-VL) on 5-HT transmission in basal ganglia were investigated in halothane-anaesthetized cats implanted with several push-pull cannulae. The release of [3H]5-HT continuously synthesized from [3H]tryptophan was estimated in both caudate nuclei (CN), both substantia nigrae (SN) and in the dorsal raphe nucleus (DRN). [3H]5-HT release was decreased in the two CN and in the two SN but was enhanced in the DRN when GABA was applied into the VM-VL. These results indicate that thalamic motor nuclei are involved in a bilateral regulation of serotoninergic transmission in the basal ganglia.     

Stimulation of the rat dorsal raphe in vivo releases labeled serotonin from the parietal cortex

In vivo release of labeled serotonin ([³H]5-HT) from the parietal cortex was investigated by cortical cup technique and electrical stimulation of midbrain raphe in rats anesthetized with pentobarbital sodium. The spontaneous efflux of tritium from the parietal cortex preloaded with [³H]5-HT followed a multiphasic exponential course. After 120 min, the rate of efflux appeared to fit the single exponential function (slow phase). Imipramine (10⁻⁶−10⁻³ M) produced a dose-dependent increase in the spontaneous release. When pargyline in concentrations ranging from 10⁻⁴ to 10⁻³ M were added to the medium in the cup, the unchanged [³H]5-HT significantly increased in a dose-dependent manner and the slow declining coefficient of tritium efflux significantly decreased in the presence of 10⁻⁴ pargyline. Stimulation of the rostral two-thirds of the dorsal raphe and the lateral 5-HT bundle originating from the dorsal raphe significantly increased the release of [³H]5-HT and its metabolites while stimulation of the caudal one-third of the dorsal raphe did not produce a significant increase in the release of [³H]5-HT and its metabolites. Stimulation of the median raphe produced no or only a slight increase in the release of [³H]5-HT and its metabolites. These findings are a direct demonstration of the in vivo release of [³H]5-HT from the parietal cortex with stimulation of the dorsal raphe, particularly the rostral two-thirds of the nucleus and provide the neurochemical evidence for the dorsal raphe-cortical 5-HT pathway via the lateral 5-HT bundle.     

Distribution of enkephalin-immunoreactive cell bodies in relation to serotonin-containing neurons in the raphe nuclei of the cat: Immunohistochemical evidence for the coexistence of enkephalins and serotonin in certain cells

In this study we have examined the distribution of enkephalin-like immunoreactive (ELI) cell bodies in the rat raphe nuclei pallidus (NRP), obscurus (NRO), magnus (NRM), pontis and dorsalis (NRD) after intratissular or intraventricular administration of colchicine. All the raphe nuclei examined were observed to contain ELI cell bodies along their whole caudorostral extent. By comparing consecutive sections treated separately with anti-5-HT and enkephalin-antiserum it was observed that certain 5-HT cells in each raphe nucleus contain ELI material. A quantitative estimation was attempted. In NRP and NRo approximately half of the total immunoreactive neuronal population appeared to be immunoreactive for both 5-HT and the enkephalins. In NRM the proportion would be one-third, whereas it seemed almost negligible in NRD. Among the 5-HT cells, approximately two-thirds might be ELI in NRP and NRO, and one-half in NRM.     

Deficits in the mid-brain raphe nuclei and striatum of the AS/AGU rat, a protein kinase C-gamma mutant

The AS/AGU rat carries a recessive mutation (agu) in the gene coding for the gamma isoform of protein kinase C. The rat is characterized by disordered locomotion and progressive dysfunction of the nigrostriatal dopaminergic (DA) system. This dysfunction begins with a failure to release DA within the striatum and culminates in cell loss within the substantia nigra pars compacta. The present study examines another midbrain aminergic system with input to the basal ganglia, the serotonergic (5-HT) raphe-striatal system originating in the dorsal raphe nucleus. By 3 months after birth, there is a very substantial reduction in the extracellular levels of 5-HT in the dorsal caudate-putamen of the mutants compared with controls (c. 70%). This is accompanied by a proportional increase in the levels of the 5-HT metabolite 5-hydroxyindole acetic acid (5-HIAA). At a later age, there are reductions in whole tissue 5-HT (and increases in 5-HIAA) in both the striatum and the region containing the dorsal raphe nucleus, as well as numbers of 5-HT-immunoreactive cells in the dorsal raphe nucleus. The median raphe appears to be unaffected. The results are seen in terms of an initial dysfunction in transmitter release leading to cell death, perhaps through the formation of free radicals or neurotoxins.     

Acetylcholinesterase associated with dopaminergic innervation of the neostriatum: Histochemical observations of a heterogeneous distribution

Unilateral injections of 6-hydroxydopamine that damaged the mesotelencephalic dopaminergic projection resulted in decreased acetylcholinesterase (AChE) staining in the ipsilateral neostriatum. This reduction, which was most apparent in the posterior and medial anterior neostriatum, occurred in rats killed 3 days or more after the 6-hydroxydopamine injection. No hemispheric asymmetries of AChE staining were found in the medial nucleus accumbens septi, olfactory tubercle or amygdala. Thus, some of the AChE in the neostriatum appears to be localized to its dopaminergic innervation; however, the enzyme may be heterogeneously distributed throughout this structure's dopaminergic afferent fibers. AChE seems not to be associated with dopaminergic axons innervating the limbic forebrain.     

Regulation of striatal neuropeptide mRNAs: effects of the 5-HT(2) antagonist SR46349B in adult rats with a neonatal 6-hydroxydopamine lesion.

The intrastriatal injection of 6-hydroxydopamine (6-OHDA) in newborn rats produces a marked striatal dopamine (DA) depletion, accompanied by a serotonin (5-HT) hyperinnervation and an up-regulation of 5-HT receptors. The aim of the present study was to investigate whether the increase in 5-HT(2) receptors could compensate for some of the DA lesion-induced effects, such as the increase in striatal preproenkephalin (PPE) and the decrease in preprotachykinin A (PPT-A) mRNA levels. Three months after the DA lesion, the effect of the selective 5-HT(2) antagonist SR46349B was investigated by a subacute treatment (10 mg/kg, IP, twice per day for 3.5 days). In sham-operated rats, the blockade of 5-HT(2) receptors decreased PPE mRNA levels in the striatum and, by contrast, had no effect on PPT-A mRNA levels. In rats with a unilateral neonatal DA lesion, SR46349B had no more effect on PPE mRNA levels in the intact striatum and was unable to modify the lesion induced-increase in PPE mRNA. The decrease in PPT-A mRNA levels induced by the neonatal DA lesion was not changed after SR46349B treatment in the posterior part of the lesioned striatum. Our results suggest that SR46349B indirectly decreases PPE mRNA levels in striatopallidal neurons in intact animals through a desinhibition of DA neuron activity. This is further evidenced by the lack of PPE mRNA changes in the DA lesioned striatum despite the up-regulation of 5-HT(2) receptor transmission induced in this model. Finally, the absence of any effect of 5-HT(2) antagonist on the expression of PPT-A mRNA in intact animals is discussed. The precise role of 5-HT(2) receptor on PPT-A mRNA biosynthesis after a neonatal lesion should be clarified by further experiments using 5-HT(2) agonists.     

Tectonigral projections in the primate: a pathway for pre-attentive sensory input to midbrain dopaminergic neurons

Much of the evidence linking the short-latency phasic signaling of midbrain dopaminergic neurons with reward-prediction errors used in learning and habit formation comes from recording the visual responses of monkey dopaminergic neurons. However, the information encoded by dopaminergic neuron activity is constrained by the qualities of the afferent visual signals made available to these cells. Recent evidence from rats and cats indicates the primary source of this visual input originates subcortically, via a direct tectonigral projection. The present anatomical study sought to establish whether a direct tectonigral projection is a significant feature of the primate brain. Injections of anterograde tracers into the superior colliculus of macaque monkeys labelled terminal arbors throughout the substantia nigra, with the densest terminations in the dorsal tier. Labelled boutons were found in close association (possibly indicative of synaptic contact) with ventral midbrain neurons staining positively for the dopaminergic marker tyrosine hydroxylase. Injections of retrograde tracer confined to the macaque substantia nigra retrogradely labelled small- to medium-sized neurons in the intermediate and deep layers of the superior colliculus. Together, these data indicate that a direct tectonigral projection is also a feature of the monkey brain, and therefore likely to have been conserved throughout mammalian evolution. Insofar as the superior colliculus is configured to detect unpredicted, biologically salient, sensory events, it may be safer to regard the phasic responses of midbrain dopaminergic neurons as 'sensory prediction errors' rather than 'reward prediction errors', in which case dopamine-based theories of reinforcement learning will require revision.     

Application ofl-glutamic acid and substance P to the substantia nigra modulates in vivo [H]serotonin release in the basal ganglia of the cat

L-Glutamic acid or substance P were applied to the caudate nucleus (CN) or substantia nigra (SN) and their effects on local, spontaneous, in vivo [3H]serotonin ([3H]5-HT) release as well as [3H]5-HT release in the contralateral CN and SN were studied using cats implanted with push-pull cannulae. L-Glutamic acid (5 x 10(-5) M), when applied to the CN or SN inhibited the local release of [3H]5-HT but did not affect release in the contralateral CN and SN. In the SN, the L-glutamic acid effect was blocked by L-glutamic acid diethylester. Substance P (10(-7) M) applied to the SN induced an increase in [3H]5-HT release that was delayed in onset. Furthermore, [3H]5-HT release was elevated in the contralateral CN immediately upon the application of substance P to the SN. These results suggest that L-glutamic acid and substance P may control 5-HT transmission in the basal ganglia.     

Muscarinic receptors on dopamine terminals in the cat caudate nucleus: Neuromodulation of [H]dopamine release in vitro by endogenous acetylcholine

The directly acting muscarinic receptor agonist oxotremorine (1.8–10 μM) produced an increase in electrically evoked [³H]dopamine release from slices of the cat caudate. The maximal increase caused by oxotremorine was 40%, and was antagonized by the muscarinic receptor blocking agent atropine (0.1 μM). Exposure to the acetylcholinesterase (AChE) inhibitor physostigmine (1 μM) resulted in a 50% increase in electrically evoked [³H]dopamine release. The increase caused by physostigmine was also antagonized by atropine (0.1 μM).Atropine did not, however, alter the modulations in [³H]dopamine release mediated by the dopamine autoreceptor: the increase in electrically evoked [³H]dopamine release caused by the dopamine receptor antagonist S-sulpiride (0.1 μM) and the decrease caused by the dopamine receptor agonist pergolide (30 nM) were unaffected by atropine (0.1 μM). These results indicate that the muscarinic receptor-mediated and dopamine autoreceptor-mediated presynaptic effects on [³H]dopamine release are independent.The present results suggest that in the electrically depolarized caudate slice in vitro, released endogenous acetylcholine may interact with muscarinic receptors faciliting depolarization-evoked [³H]dopamine release,ifAChE is inhibited. These muscarinic receptors may be located on dopamine nerve terminals. In the context of present neuroanatomical knowledge, the action of released endogenous acetylcholine on dopamine terminals may be a non-synaptic neuromodulation.     

In vivo modulations by GABA-related drugs of met-enkephalin release in basal ganglia of the cat brain

The influence of the intrapallidal application of GABA-related compounds on the release of Met-enkephalin in the globus pallidus and the caudate nucleus in the two hemispheres was investigated in vivo in the cat. For this purpose, the 4 structures were continously superfused with an artificial CSF through implanted push-pull cannulae and Met-enkephalin released in superfusates was determined using a specific radioimmunoassay.GABA (10–500 μM) reduced the local release of Met-enkephalin during its application but once the amino acid was removed from the superfusing fluid, an increase in the peptide release was observed. Diazepam (10 μM) induced only an inhibitory effect whereas muscimol (1 μM) stimulated Met-enkephalin release. Opposite changes in Met-enkephalin release were also seen with the GABA antagonists, bicuculline methiodide (1 μM) and picrotoxin (10 μM), suggesting that the local regulation of Met-enkephalin released by GABA related compounds ay be mediated by at least two types of GABA receptors.In several cases, the unilateral pallidal application of GABA agonists and antagonists induced significant changes in Met-enkephalin release at distant structures. The most striking effect was observed with diazepam which markedly reduced the peptide release in both caudate nuclei and pallida. These data suggest that GABAergic systems can contribute to some bilateral regulation of striato-pallidal enkephalinergic neurones.     

Heterogeneous distribution of the serotonin 5‐HT1A receptor mRNA in chemically identified neurons of the mouse rostral brainstem: Implications for the role of serotonin in the regulation of wakefulness and REM sleep

The 5‐HT_1A receptor (5‐HT_1AR) plays a key role in the inhibitory influence of serotonin (5‐HT) on rapid eye movement (REM) sleep in rodents. However, the neuronal networks mediating such influence are mostly unknown, notably in the mouse. This led us to map 5‐HT_1AR mRNA, by in situ hybridization histochemistry (ISHH), and to characterize the neuronal phenotype of 5‐HT_1AR mRNA‐positive neurons by dual ISHH and ISHH combined with immunohistochemistry, throughout the mouse rostral brainstem, a pivotal region for the generation of REM sleep and cortical activation. 5‐HT_1AR mRNA was found in most 5‐HT neurons in the dorsal raphe (DR), the median raphe (MnR), the B9, and the interpeduncular (IP) nuclei. 5‐HT_1AR mRNA‐positive neurons were also identified in individualized clusters of γ‐aminobutyric acid (GABA)ergic neurons in the DR and in neurons of an undetermined phenotype in the MnR. In addition, 1) GABAergic neurons of the ventral portion of Gudden's dorsal tegmental nucleus (DTg), the IP, and the caudal portion of the deep mesencephalic nucleus (DpMe), and 2) glutamatergic neurons scattered in the caudal pontine reticular nucleus (PnC) and densely packed in the internal lateral parabrachial subnucleus (PBil) also expressed 5‐HT_1AR mRNA. In contrast, no specific 5‐HT_1AR‐related ISHH signal was generally detected in brainstem cholinergic and catecholaminergic neurons. These results emphasize the role of 5‐HT_1AR as an autoreceptor and the phenotypical heterogeneity of 5‐HT_1AR‐expressing neurons within the DR and the MnR in the mouse brain. They also provide a neuroanatomical basis for understanding the influence of 5‐HT_1AR on REM sleep and wakefulness. J. Comp. Neurol. 518:2744–2770, 2010. © 2010 Wiley‐Liss, Inc.     

Induction of burst firing in ventral tegmental area dopaminergic neurons by activation of serotonin (5-HT)1A receptors: WAY 100,635-reversible actions of the highly selective ligands,flesinoxan and S 15535

This study examined the influence of the highly selective 5-HT_1A receptor ligands, flesinoxan, S 15535, and WAY 100,635, upon the electrical activity of dopaminergic neurons in the ventral tegmental area (VTA), as compared to serotonergic neurons in the dorsal raphe nucleus (DRN) of anesthetized rats. Flesinoxan, a high-efficacy agonist at both pre- and postsynaptic 5-HT_1A receptors, dose-dependently (inhibitory dose (ID)₅₀ = 19.5 μg/kg, i.v.) inhibited the firing of DRN serotonergic neurons. This action was abolished by WAY 100,635 (31 μg/kg i.v.) which is an antagonist at pre- and postsynaptic 5-HT_1A receptors. S 15535, which behaves as an agonist and partial agonist at pre- and postsynaptic 5-HT_1A receptors, respectively, similarly abolished DRN firing in a WAY 100,635-reversible fashion with an ID₅₀ of 6.1 μg/kg, i.v. In contrast to these actions, both flesinoxan (≥500 μg/kg, i.v.) and S 15535 (≥125 μg/kg, i.v.) dose-dependently and monophasically increased the firing rate of dopaminergic neurons in the VTA, with maximal effects of 70.1 ± 17.2% and 33.7 ± 5.3%, respectively. Further, VTA dopaminergic neurons displaying a regular firing pattern were transformed into a bursting mode. This influence of flesinoxan and S 15535 on VTA cells was abolished by WAY 100,635. Administered alone, WAY 100,635 did not significantly modify the activity of either serotonergic or dopaminergic neurons. In conclusion, the present findings show that selective activation of 5-HT_1A receptors not only inhibits serotonergic neurones but also elicits a (possibly related) increase in VTA dopaminergic output. A facilitatory influence of flesinoxan, S 15535, and other selective 5-HT_1A receptor ligands upon mesocortical dopaminergic pathways may contribute to their putative antidepressant properties. Synapse 30:172–180, 1998. © 1998 Wiley-Liss, Inc.     

Differential effects of subchronic treatments with atypical antipsychotic drugs on dopamine D2 and serotonin 5-HT2A receptors in the rat brain

Summary. The effects of 3-week treatment with a typical antipsychotic drug chlorpromazine and three atypical antipsychotic drugs (risperidone, olanzapine and perospirone) on the binding to dopamine D₂ and serotonin 5-HT_2A receptors were examined in the rat stratum and frontal cortex, respectively. Subchronic treatment with chlorpromazine (10 mg/kg) and perospirone (1 mg/kg) significantly increased D₂ receptors, while no increase was observed with lower dose of chlorpromazine (5 mg/kg), perospirone (0.1 mg/kg), risperidone (0.25, 0.5 mg/kg) or olanzapine (1, 2 mg/kg). On the other hand, 3-week administration of chlorpromazine (5, 10 mg/kg) and olanzapine (1, 2 mg/kg) significantly decreased 5-HT_2A receptors, but risperidone (0.25, 0.5 mg/kg) or perospirone (0.1, 1 mg/kg) had no effect. The measurement of in vivo drug occupation for D₂ and 5-HT_2A receptors using N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) suggested that high occupation of 5-HT_2A receptors with lower D₂ receptor occupancy might be involved in the absence of up-regulation of D₂ receptors after subchronic treatment with some atypical antipsychotic drugs. Received September 24, 1999; accepted December 1, 1999     

(125)I]3beta-(4-ethyl-3-iodophenyl)nortropane-2beta-carboxylic acid methyl ester ([(125)I]EINT): a potent and selective radioligand for the brain serotonin transporter.

The binding characteristics of [(125)I]3beta-(4-ethyl-3-iodophenyl)nortropane-2beta-carboxylic acid methyl ester ([(125)I]EINT), a high-affinity selective ligand for the serotonin transporter (5-HTT), and its binding characteristics to rat brain membranes were determined. [(125)I]EINT binding to rat cerebral cortex membranes was saturable and reversible, and its specific binding represented approximately 90% of the total binding. [(125)I]EINT labeled a single site with K(d) = 0.22 +/- 0.03 nM and B(max) = 583 +/- 38 fmol/mg protein. Kinetic analysis revealed a t(1/2) for association and dissociation of 20 and 24 min, respectively. Pharmacological characterization of [(125)I]EINT confirmed its high specificity for the 5-HTT. The pattern of brain region distribution in vivo of intravenously administered [(125)I]EINT indicated greater accumulation of the radioligand in 5-HTT-rich brain regions. However, the signal-to-background ratio was low. Thus, [(125)I]EINT appears to be a useful radioligand for studying the 5-HTT in vitro, but it may not be a good in vivo ligand.     

Autoregulatory properties of dorsal raphe 5-HT neurons: Possible role of electrotonic coupling and 5-HT1D receptors in the rat brain

In the present study, the hypothesis that somatodendritic availability of 5-hydroxytryptamine (5-HT) could be regulated independently of the firing activity of dorsal raphe 5-HT neurons was tested. The 5-HT pathway was electrically stimulated at the level of the ventromedial tegmentum and the ensuing action potentials, recorded in the dorsal raphe, met all criteria for antidromic invasion of 5-HT neurons. The latency of antidromic spikes was current-dependent and the changes in latency were of quantal nature. This observation suggests an electrotonic coupling between 5-HT neurons. Stimulation of the ventromedial tegmentum also induced a decrease in the probability of firing of 5-HT neurons. This reduction in 5-HT neuron firing activity is a 5-HT-mediated response, due to an increased bioavailability of the neurotransmitter in the biophase of somatodendritic 5-HT_1A autoreceptors. The intravenous administration of the 5-HT₁ agonists TFMPP and RU 24969 reduced the duration of suppression of firing induced by the 5-HT-pathway stimulation, without altering the spontaneous firing rate of 5-HT neurons. The effect of TFMPP and RU 24969 on duration of suppression was blocked by (±)mianserin, a drug with high affinity for the rat 5-HT_1D, but not 5-HT_1B, receptors. On the other hand, (−)propranolol, a mixed 5-HT antagonist also blocked the effect of TFMPP. However, the selective 5-HT_1A antagonist (+)WAY 100135 did not alter the effect of TFMPP. These results, in keeping with previous anatomical studies, suggest the existence of electrotonic coupling of 5-HT neurons and indicate that 5-HT release in the rat dorsal raphe nucleus may be controlled independently of firing-regulating 5-HT_1A autoreceptors. They also suggest that 5-HT_1D receptors may play a role in this regulatory function of 5-HT neurons. © 1996 Wiley-Liss, Inc.     

Clozapine, haloperidol, and the D4 antagonist PNU-101387G: in vivo effects on mesocortical, mesolimbic, and nigrostriatal dopamine and serotonin release

Summary. With in vivo microvoltammetry, the dopamine (DA) receptor antagonists, clozapine (D₄/D₂), haloperidol (D₂) and the selective D₄ antagonist, PNU-101387G, were evaluated for their effects on DA and serotonin (5-HT) release within A₁₀ neuronal terminal fields [mesocortical, prefrontal cortex (PFC), mesolimbic, nucleus accumbens, (NAcc)] and within A₉ neuronal terminal fields [nigrostriatal, caudate putamen (CPU)], in chloral hydrate anesthetized rats. Clozapine, which also has 5-HT₂ receptor antagonist properties, significantly (p < 0.001) increased DA release within A₁₀ terminal fields, PFC and NAcc; DA release was not increased by clozapine within A₉ terminals, CPU. Serotonin release was significantly (p < 0.001) increased by clozapine within A₁₀ and A₉ terminal fields. Haloperidol significantly (p < 0.001) increased DA release within PFC, dramatically and significantly (p < 0.001) increased DA release within CPU, but not within NAcc; haloperidol had a small but statistically significant (p < 0.05) increase on 5-HT release within PFC [only at the highest dose studied (2.5 mg/kg)] and within CPU [only at the lowest dose studied 1.0 mg/kg) (p < 0.05)]. The selective D₄ antagonist, PNU-101387G dramatically and significantly (p < 0.001) increased DA release within PFC, modestly, but significantly (p < 0.001) increased DA release within CPU, did not alter DA release within NAcc at the lowest dose studied (1.0 mg/kg) and significantly (p < 0.05) decreased DA release within NAcc at the highest dose studied (1.0 mg/kg). The selective D₄ antagonist did not affect 5-HT release within either A₁₀ or A₉ terminal fields. The present data are discussed in terms of the neurochemistry, antipsychotic activity, and side effect profiles of clozapine and haloperidol, in order to provide comparative profiles for a selective D₄ antagonist, PNU-101387G. Received January 13, 1998; accepted April 27, 1998     

Evidence for a Regulatory Role of Melatonin on Serotonin Release and Uptake in the Pineal Gland

Melatonin has been proposed to exert some regulatory actions within the pineal gland itself. The present study examined the effect of melatonin on the release of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) from rat pineal glands by using an in vitro perifusion system. Melatonin induced a concentration-dependent stimulatory effect on 5-HT secretion from 10^?6 M to 10^?3 M. Maximal effects were obtained with melatonin 10^?3M and concentrations lower than 10^?6 M were without effect. The secretion of 5-HIAA was inhibited by melatonin 10^?3 and 10^?4M, but it was increased when pineals were incubated with 10^?5 and 10^?6 M of melatonin. The indoleamine secretion was also studied on peripherally denervated rat pineal glands. Basal output of 5-HT from these glands was increased when compared with those from control rats. In contrast, the secretion of 5-HIAA was strongly reduced after removal of the sympathetic input to the pineal gland. Melatonin 10^?3 M failed to stimulate 5-HT release from denervated pineal glands, although it inhibited 5-HIAA secretion. In contrast, melatonin 10^?5 M enhanced 5-HT release without altering 5-HIAA output. Fluoxetine, a 5-HT uptake inhibitor, produced similar effects than mM concentrations of melatonin on the indoleamine secretion from control pineal glands, but it had no effect on glands taken from peripherally denervated rats. These data suggest that mM concentrations of the pineal hormone are able to stimulate 5-HT release from the pinealocyte, while mM concentrations of melatonin increase extracellular 5-HT by inhibiting its reuptake in the adrenergic nerve endings. These findings are discussed in relation to the possible role of melatonin regulating the intra- and extracellular availability of 5-HT in the pineal gland and its significance as an autocrine factor.