Effect of ergot drugs on central 5-hydroxytryptamine neurons: Evidence for 5-hydroxytryptamine release or 5-hydroxytryptamine receptor stimulation

In combined biochemical and functional studies it has been possible to show that ergocornine (0.5-5 mg/kg) and the ergolene derivative (5R,8R)-8-(4-p-methoxyphenyl-1-piperazinylmethyl)-6-methylergolene (PTR 17402; MPME) (0.25-5 mg/kg) reduce in a dose-dependent way brain 5-hydroxytryptamine (5-HT) turnover in rat as evaluated with the tryptophan hydroxylase inhibitor, alpha-propyl-dopacetamide (H 22/54), whereas 2-Br-alpha-ergocryptine (CB 154; Br-EC) had no effect on brain 5-HT turnover. Effects on 5-HT receptor activity were evaluated using the extensor hindlimb reflex of acutely spinalized rats. It was found that ergocornine increased the 5-HT receptor activity independent of presynaptic 5-HT stores and that it didnot have any effects on uptake, retention and spontaneous overflow of 3-H-5-HT in vitro but reduced the fiedl stimulation-induced release of 3-H-5-HT in vitro. Therefore, it is suggested that ergocornine is a 5-HT recpetor-stimulating agent, an effect which may lead to reduction of nervous impulse flow in the 5-HT neurons and subsequently of 5-HT release and turnover. MPME, on the other hand, seems to increase 5-HT receptor release of 5-HT stores, mainly from extragranular sites. Thus, the increase in extensor reflex activity found after MPME was reduced by reserpine and H 22/54 and enhanced by nialamide and in vitro MPME markedly increased 3-H-5-HT overflow in cortical slices of nialamide-pretreated rats and inhibited uptake and retention of 3-H-5-HT (EC50 equals 1.6 times 10-minus 6 M) in cortical slices of normal rats. Inhibition of the 5-HT membrane pump does not seem to be of any major importance, since chlorimipramine was only weakly active on the extensor reflex in the pharmacological models used and since MPME did not block but rather enhanced the 5-HT depletion caused by 4-methyl-alpha-ethyl-m-tyramine. It is suggested that MPME is a releaser of extragranular 5-HT stores leading to increased 5-HT receptor activity and reduction of 5-HT turnover in the same way as indicated for ergocornine. This new ergolene derivative may represent a new class of antidepressant drugs acting via release of extragranular 5-HT stores.     

Effect of acute and chronic tri-iodothyronine (T3) administration to rats on central 5-HT and dopamine-mediated behavioural responses and related brain biochemistry

The effects of both acute and chronic T₃ administration (100 μg/kg, s.c.) on behavioural responses in the rat mediated by 5-hydroxytryptamine (5-HT) and dopamine (DA) were examined 24 hr after the last injection of hormone at a time when serum T3 levels had almost returned to control values. Concomitant changes in the biochemistry of the 5-HT and DA systems were also examined.Administration of T₃ produced complex changes in both central DA and 5-HT systems. Both acute and chronic T₃ treatment enhanced the hyperactivity response to tranylcypromine (TCP) plus L-DOPA, with chronic treatment giving a much greater enhancement. Paradoxically, the accumulation of brain DA at 20 and 30 min after L-DOPA in chronically treated rats was less than in the controls. In order to study possible T₃ effects at the DA postsynaptic receptor, behavioural responses to the DA agonist apomorphine were measured. The activity responses were slightly attenuated in chronically treated rats only. Furthermore, the cataleptogenic effect of haloperidol (0.3 mg/kg) was significantly potentiated in chronically T3-treated rats only. Because striatal [³H]-spiroperidol binding was unaltered in chronically treated rats, it is thought that T₃ may produce an altered sensitivity postsynaptic to striatal DA receptors, or a change in activity of a modulatory neuronal system.It was found that 24 hr after either acute or chronic T₃ injection, the hyperactivity response to TCP + L-tryptophan (L-TP) was markedly enhanced [350% (chronic) and 220% (acute) increase above control activity]. A similar result was obtained for the 5-HT agonists (quipazine and 5-MeODMT).There was no change in whole brain 5-HT or TP accumulation measured 75 min following TP injection in the chronically-treated animals. This, together with enhanced 5-HT postsynaptic agonist responses, may be due to primary changes occurring at or beyond DA neurones, which appear to be interposed between 5-HT neurones and the site expressing the behavioural response.     

The action of single and repeated doses of p-bromo-methamphetamine on the monoamine content and turnover rate in rat brain

The effect of p-bromo-methamphetamine (pBrMA) on serotonergic and catecholaminergic neurones in the rat brain was studied. Acute (15 mg/kg, s.c.) and chronic (30 × 15 mg/kg, s.c.) administration of the drug caused a long-lasting decrease in serotonin (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) content of whole brain. In contrast the catecholamine (CA) levels remained unchanged. Detailed studies of the 5-HT and CA content of brain regions showed a time-dependent decrease in 5-HT after acute injection, and a marked and persistent reduction in 5-HT and 5-HIAA after chronic treatment. In both injection regimens the content of noradrenaline (NA) was enhanced in the cortex. The turnover rate of 5-HT, dopamine (DA) and NA was reduced after acute treatment. Repeated injections caused a decrease in 5-HT turnover, while that of NA remained unchanged. However, the turnover rate of DA was enhanced. The present data appears to confirm the previous suggestion that after chronic treatment, when the rats show improvement in their learning performances, the serotonergic tone of the brain is decreased while dopaminergic tone is increased.     

Discriminative stimulus properties of quipazine

Rats were trained to discriminate quipazine (2.5 mg/kg) from saline in a two-lever, drug discrimination task. After high levels of accuracy (i.e. 90% correct choice responding) were attained, dose and time parameters of the quipazine cue were studied during extinction test sessions. Quipazine was highly discriminable at doses of 1.25 and 0.94 mg/kg. With decreasing doses, the percentage of responding on the quipazine lever declined in a dose-related fashion. By varying the normal 30-min time interval between injection and testing, the onset and duration of action of quipazine were determined to be about 5–15 min and 60–90 min respectively.After injections with novel drugs, animals were tested during extinction sessions. Lysergic acid diethylamide, (LSD), at doses of 0.08 mg/kg and 0.10 mg/kg, transferred completely to the quipazine cue (i.e. above 95% quipazine responding); neither d-amphetamine (1.0 and 2.0 mg/kg) nor apomorphine (0.5 and 1.0 mg/kg) showed any transfer to the quipazine cue.By administering various neurotransmitter blockers in conjunction with quipazine and testing during extinction sessions, it was found that the putative central 5-HT antagonists cyproheptadine (1.0 mg/kg), methysergide (1.0 mg/kg), and methiothepin (1.0 mg/kg) produced significant decreases in the discriminability of quipazine. The dopamine antagonists haloperidol (0.05 and 0.10 mg/kg) and fluphenazine (1.0 mg/kg) were without effect. It was concluded that LSD and quipazine have a common mechanism of action which may (or may not) involve 5-HT; a role for DA was not indicated.     

Antagonism of baclofen's effects on rat striatal 5-HT metabolism: A model for 5-HT agonistic properties?

The increase in the rat striatal concentration of 5-hydroxyindoleacetic acid (5-HIAA) elicited by baclofen was antagonized by the 5-HT antagonists pipamperone (10/30 mg/kg i.p.), cyproheptadine (30 mg/kg i.p.), methiothepin (1 mg/kg i.p.), and GP 50 302 (1/3 mg/kg i.p.), but not by cinanserin (1–30 mg/kg i.p.), pizotifen (1–10 mg/kg i.p.), spiroperdol (0.1–1 mg/kg i.p.), or haloperidol (0.1–1 mg/kg i.p.). The 5-HT agonists, m-chlorophenylpiperazine (1 mg/kg i.p.) and MK 212 (3/10 mg/kg i.p.) also showed an antagonistic effect. Methysergide (5–20 mg/kg i.p.) and quipazine (2.5/5 mg/kg i.p.) were previously shown to act similarly, whereas mianserin (5–20 mg/kg i.p.) was inactive and methergoline at lower doses (0.25–0.5 mg/kg i.p.) increased the effect of baclofen, which was reversed at higher doses (1 mg/kg i.p.). The alterations by these compounds of the 5-HT increase elicited by baclofen were more or less similar; however, they were less clear-cut and occurred at higher doses. These interactions were not the result of interferences of the compounds with the absorption, distribution, or metabolism of baclofen nor with its effect on the nigrostriatal dopaminergic system, since the increase in dopamine concentrations it caused was not affected by any of the compounds. A comparison of our results with published data on the antagonism of 5-hydroxytryptophan-induced head twitches, on spiroperidol or 5-HT displacement, on 5-HT-stimulated adenylate cyclase, and with electrophysiological results suggests that the antagonistic effect of compounds interfering with the 5-HIAA elevating action of baclofen is not related to 5-HT receptor blocking properties of these drugs, Instead, it seems to be much more related to 5-HT agonists properties. It is speculated that this model might reveal presynaptic agonistic properties of drugs, but more data are needed to confirm or reject this.     

Hallucinogenic phenylethylamines: interactions with serotonin turnover and receptors

The effects of derivatives of phenylethylamine and α-methyl-phenylethylamine )2,5-dimethoxy-4-methylamphetamine, p-methoxyamphetamine, mescaline, p-methoxyphenylethylamine, 3,4-dimethoxyphenylethylamine) have been studied on 5-hydroxytryptamine (5-HT) turnover and receptors in the rat brain and spinal cord. Both p-methoxyamphetamine (pMA) and 2,5-dimethoxy-4-methylamphetamine (STP, DOM) in doses of 5 mg/kg but not even high doses of mescaline, 50 mg/kg, reduced 5-HT turnover in the CNS. this reduction did not appear to be related to irreversible MAO inhibition but was probably due to direct stimulation of 5-HT receptors. Evidence for receptor stimulation was obtained on the extensor hindlimb reflex of the acutely spinalized rat; p-methoxyphenylethylamine (pMPEA) but not 3,4-dimethoxyphenylethylamine (DIMPEA) directly stimulated 5-HT receptors. The 5-HT receptor stimulation induced by DOM and pMA, via a feedback mechanism probably inhibits the presynaptic 5-HT neurons with subsequent reduction of 5-HT release and 5-HT turnover. We propose the hypothesis that the 5-HT receptor stimulation induced by the psychotomimetic phenylethylamines is mainly responsible for their pharmacological and hallucinogenic properties.     

Effects of baclofen on striatal 5-HT mediated by the nigrostriatal dopamine system.

In rats, baclofen injected unilaterally into the substantia nigra increased dopamine (DA) and serotonin (5-HT) levels in the ipsilateral striata. Homovanillic acid levels first decreased and then increased to a maximum after 4 h. The content of 3,4-dihydroxyphenylacetic acid also increased after a latency of about 1 h. The effects of baclofen on striatal DA metabolism are consistent with the assumption that impulse flow in these DA neurones was diminshed by the drug. DA metabolites were also found in greater quantities in the contralateral striata, probably owing to compensatory activation of these DA neurones. Baclofen injected intranigrally did not increase the striatal 5-HT content in rats with a 6-hydroxydopamine-induced lesion of the nigrostriatal DA pathway. Our results suggest the presence of DA receptors on striatal 5-HT terminals or on interneurones synapsing axo-axonally with them.     

The effect of niaprazine on the turnover of 5-hydroxytryptamine in the rat brain

Niaprazine (60 mg/kg i.p.) increased rat brain 5-hydroxyindole acetic acid (5-HIAA) concentrations 30 min after treatment, and reduced them at 3-8 hr after treatment. Rat brain 5-hydroxytryptamine (5-HT) levels were unchanged. Niaprazine also produced a short-lasting depletion of rat brain noradrenaline (NA) and dopamine (DA). Pretreatment with alpha-phenyl-alpha-propyl-benzeneacetic acid, 2-(diethylamino) ethyl ester hydrochloride (SKF 525A) (75 mg/kg i.p.) potentiated the increase in 5-HIAA and depletion of catecholamines produced 1 hr after niaprazine, but abolished the reduction in 5-HIAA produced 8 hr after the drug. This suggested that a metabolite might be responsible for the delayed reduction in 5-HIAA levels. A potential metabolite, p-fluoro-phenylpiperazine (FPP) (5-40 mg/kg i.p.) reduced rat brain 5-HIAA and 3,4-dihydroxyphenyl acetic acid (DOPAC), and inhibited 5-HT and NA uptake in vitro. Unlike niaprazine, FPP produced no behavioural sedation, but in large doses produced a behavioural syndrome indicative of serotonergic stimulation. Studies of the metabolism of 14C-niaprazine in rats indicated the presence of a urinary metabolite with the same chromatographic characteristics as FPP. These results suggest that niaprazine itself depletes brain catecholamines and increases 5-HT turnover, while a metabolite, FPP, subsequently reduces the turnover of 5-HT and DA.     

The effects of quipazine on 5-HT metabolism in the rat brain

Summary Since quipazine is a potent 5-HT agonist in peripheral organs, its possible stimulatory effects on serotoninergic receptors in the rat brain were investigated. Quipazine administration (10 mg/kg, i.p.) induced a significant decrease in the synthesis and turnover rates of serotonin in the brain stem as well as in the forebrain. It is not likely that these changes were mediated by a negative feed-back mechanism triggered by adirect action of quipazine on central 5-HT postsynaptic receptors. Indeed, in contrast to LSD and 5-methoxy-N,N-dimethyltryptamine, this compound failed to activate the 5-HT sensitive adenylate cyclase in colliculi homogenates of newborn rats. However, quipazine exerted direct effects on serotoninergic terminals. It inhibited competitively the reuptake process in synaptosomes (Ki =1.38×10^–7 M) and stimulated the K⁺ evoked release of newly synthesized³H-5-HT in slices of the brain stem. Injected in vivo in a dose which affected 5-HT uptake and release, quipazine did not modify MAO activity. However, this activity was noncompetitively inhibited by high concentration of the drug in vitro (Ki=3.0×10^–5 M). These actions are very likelyindirectly responsible for the stimulation of central 5-HT receptors.     

Synthetic melanocyte stimulating hormone release-inhibiting factor (MIF). Part III: effect of L-prolyl-N-methyl-D-leucyl-glycinamide and MIF on biogenic amine turnover.

The effect of the factor that inhibits the release of melanocyte stimulating hormone (MSH), i.e., L-prolyl-L-leucyl-glycinamide (MIF), and L-prolyl-N-methyl-D-leucyl-glycinamide, an analog, on brain norepinephrine (NE), dopamine (DA) and serotonin (5-HT) turnover was examined in rats. The analog (40 mg/kg i.p.), in a fashion similar to MIF (40 and 5 mg/kg i.p.), increased brain DA turnover; only MIF (40 mg/kg i.p.) increased endogenous DA levels. The analog (40 and 5 mg/kg i.p.) decreased brain NE turnover; MIF at the same doses was ineffective. Neither MIF nor the analog affected rat brain 5-HT turnover or the 5-HTP-induced behavioural syndrome in the mouse. These results indicate that the analog, like MIF, exerts effects on central catecholamine turnover. The different biochemical profile of the analog compared to MIF may be importance with regard to potential clinical use in the treatment of Parkinson's disease and depression.     

Effects of pyrroxan and chlordiazepoxide on biogenic amine metabolism in the rat brain

Pyrroxan (20 mg/kg, i.p.), a new potential antianxiety agent, increased brain norepinephrine (NE) turnover in rats, reflecting a possible central -adrenergic receptor blocking activity. In contrast, chlordiazepoxide (20 mg/kg, i.p.), a widely used antianxiety agent, did not alter the NE turnover. Pyrroxan did not affect overall DA turnover although it did appear to accelerate DA turnover initially. The initial potentiation of DA turnover may indicate a short-lasting blocking action on DA receptors. In comparison, chlordiazepoxide (20 mg/kg, i.p.) decreased the turnover rate of DA. Effects of both drugs on 5-HT indicate a decrease in turnover with no significant monoamine oxidase activity or blockade of the 5-HT reuptake mechanism. Both drugs antagonized the decline in intraventricularly-injected ¹⁴C-5-HT, indicated by higher rat brain ¹⁴C-5-HT. Neither drug caused consistent changes in endogenous 5-HT, 5-hydroxyindoleacetic acid, or tryptophan levels. Neither drug potentiated the behavioral effects of l-Dopa nor increased the 5-HTP behavioral syndrome in the mouse. Pyrroxan may be expected to exhibit a spectrum of activity between that of minor and major tranquilizers, characterized by antianxiety action together with sedative or tranquilizing activity.     

Effect of dihydroergosine (DHESN) on the serotoninergic system and behaviour: Is DHESN a new antidepressive agent?

Acute (50.0 mg/kg) and repeated (0.1–10.0 mg/kg) administration of dihydroergosine (DHESN) to rats over 5 days lowered the concentration of 5-HIAA in the brain. DHESN given acutely increased the brain 5-HT in p-CPA-treated animals and diminished the probenecid-induced increase in brain 5-HIAA. In pargyline-treated rats DHESN enhanced the 5-HT/5-HIAA ratio. DHESN administered to rats repeatedly over 5 days decreased the level of 5-HT in blood platelets, and in vitro at concentrations of 10^-4 M and 10^-3 M inhibited the uptake of [¹⁴C]-5-HT in platelets. DHESN (10.0–100.0 mg/kg) potentiated the 5-HT syndrome produced in rats by pargyline and 5-HTP. This potentiation was blocked with cyproheptadine but not with haloperidol. DHESN (1.0 and 10.0 mg/kg) lowered the locomotor activity of rats and 10.0 mg/kg DHESN also reduced the duration of immobility in rats forced to swim in a restricted space. The results indicate that DHESN, like antidepressants, decreases the turnover of serotonin in the brain and potentiates the 5-HT-mediated behaviour. This might suggest that the drug should be further investigated for its potential antidepressive properties.     

Neurochemical effects of chronic co-administration of ritanserin and haloperidol: comparison with clozapine effects

The effects of chronic treatment with clozapine (20 mg/kg per day), ritanserin (0.5 mg/kg per day), haloperidol (0.5 mg/kg per day), or the combination of haloperidol and ritanserin, on dopamine (DA) and serotonin (5-HT) metabolism were studied. Chronic haloperidol treatment decreased DA metabolism in nucleus caudatus. Chronic ritanserin treatment failed to alter striatal or mesolimbic DA metabolism but decreased the concentrations of 5-HT and 5-hydroxyindoleacetic acid in the nucleus raphe dorsalis. The effects of chronic haloperidol were not altered by concomitant ritanserin administration. In comparison, chronic clozapine treatment affected neither DA nor 5-HT metabolism. These results show that the biochemical effects of chronic haloperidol treatment on the major ascending DA neurons cannot be modulated by concomitant 5-HT₂ receptor blockade.     

Paradoxical decrease of brain 5-HT turnover by metergoline,a central 5-HT receptor blocker

Summary Since metergoline (1-methyl-8-beta-carbobenzyloxy-aminomethyl-10-alpha-ergoline) is a potent 5-HT antagonist in peripheral organs, its possible blocking effects on 5-HT receptors in the rat brain were investigated. In vitro, metergoline inhibited both the specific high affinity binding of ³H-5-HT onto synaptosomal membranes (IC 50=18 nM) and the stimulating effect of 10 M 5-HT on the adenylate cyclase activity in colliculi homogenates from newborn rats (IC 50=12 M). In vivo, the administration of metergoline (10 mg/kg i.p., 60 min before death) resulted in a significant decrease in the ³H-5-HT binding capacity of synaptosomal membranes from the forebrain of adult rats. Taken together, these data clearly indicated that metergoline is a potent blocker of some serotoninergic receptors in the rat brain. Surprisingly, the changes in 5-HT turnover occurring in the brainstem and in the forebrain 1 h after metergoline (2–10 mg/kg) treatment were similar to those normally induced by a central 5-HT agonist: both the rate of 5-HT utilisation and that of 5-HT synthesis were significantly decreased. These changes were in contrast to the acceleration of 5-HT turnover induced by the administration of another potent central 5-HT antagonist, methiothepin. These results are discussed in relation to the possible existence of several types of serotoninergic receptors in the rat brain. It is possible that the positive feedback regulation of 5-HT turnover is triggered by the blockade of serotoninergic receptors sensitive to methiothepin, but not to metergoline.     

Long-term 5-HT reuptake blockade,but not monoamine oxidase inhibition,decreases the function of terminal 5-HT autoreceptors: an electrophysiological study in the rat brain

Summary 5-HT-containing terminals possess autoreceptors which modulate the release of 5-HT into the synaptic cleft. Tritiated imipramine ([³H]IMI), and more specifically [³H]citalopram and [³H]paroxetine, bind to a site associated with the 5-HT reuptake carrier on the 5-HT terminals. The function of terminal 5-HT autoreceptors is decreased following long-term treatment with the 5-HT reuptake blocker citalopram. The present study was undertaken to determine whether an increased synaptic availability of 5-HT or, the occupation of the [³H]IMI site, were responsible for this modification. Unitary extracellular recordings were obtained from CA₃ dorsal hippocampus pyramidal neurons under chloral hydrate anesthesia in rats treated daily with fluoxetine (10 mg/kg/day × 14 days), a selective 5-HT reuptake blocker, or clorgyline (1 mg/kg/day × 21 days), an inhibitor of type A monoamine oxidase. The function of the terminal 5-HT autoreceptors was assessed by comparing the effectiveness of the electrical stimulation of the ascending 5-HT pathway on the firing activity of hippocampus pyramidal neurons prior to, and following, the administration of methiothepin, an antagonist of the terminal 5-HT autoreceptor, and, by determining the ratio of effectiveness of 0.8 Hz (S1) and 5 Hz (S2) stimulations. Long-term administration of fluoxetine or clorgyline both increased the efficacy of the stimulation of the 5-HT pathway. However, the enhancing effect of methiothepin on the efficacy of the stimulation was attenuated by the fluoxetine, but not by the clorgyline, treatment. The reduction of the function of the terminal 5-HT autoreceptor by the long-term fluoxetine treatment was further indicated by the decreased ratio of effectiveness of the 0.8 and 5 Hz stimulations as compared to that of control rats. To verify that the reuptake blockade per se could not account for the increased synaptic efficacy of 5-HT projections following long-term fluoxetine, the drug was administered acutely to naive rats. It did not increase the efficacy of the stimulation of the 5-HT pathway. Two conclusions are drawn from these results: 1) the increased efficacy of 5-HT synaptic transmission by long-term treatment with antidepressant 5-HT reuptake blockers is not directly due to 5-HT reuptake blockade, but rather, to a reduced function of the terminal 5-HT autoreceptor; 2) the latter phenomenon cannot be ascribed to an increased availability of 5-HT in the synaptic cleft as it was not produced by long-term clorgyline treatment. Hence, these results suggest that long-term occupation of the [³H]IMI site might result in a decreased function of terminal 5-HT autoreceptors.Send offprint requests to Claude de Montigny     

Ca2+ dependency of serotonin and dopamine release from CNS slices of chronically isolated rats

 Much evidence points to a significant involvement of the classical neurotransmitters 5-HT and DA in affective disorders with possible changes in different structures of the CNS and also at different levels of the signal transduction chain, i.e., receptor, synthesis, uptake or release. We have used chronic isolated housing as an animal model of depression. These isolated rats enabled the study of KCl-induced release of 5-HT and DA from nucleus accumbens, prefrontal cortex and hippocampal slices. The following questions were addressed: first, if there is a change in the depolarization dependent release of DA and 5-HT from these CNS structures, and second, if the release is through the classical exocytotic mechanism. A significant increase in KCl stimulated release of 5-HT was observed in chronically isolated animals when compared to controls. 5-HT release was completely abolished from controls or isolated animals, when slices were incubated with Krebs containing zero Ca²⁺/10 mM Mg²⁺, the inorganic Ca²⁺ channel blockers, Cd²⁺ or Ni²⁺ and the calmodulin inhibitor, trifluoperazine. The organic Ca²⁺ channel blockers nifedipine and D-600 were less effective in inhibiting the stimulated 5-HT release. KCl stimulated DA release was only significantly increased from hippocampus slices, of isolated, but not control animals. This release was also highly Ca²⁺-dependent. The basal release of DA and 5-HT was similar in control and isolated animals and was not affected by the Ca²⁺ channel antagonists. The results suggest that extracellular Ca²⁺-dependent release of 5-HT and, to a lesser degree, of DA, is increased in this chronic animal model of depression in several CNS structures. Received: 21 October 1997 / Final version: 7 February 1998     

Effects of carbamazepine on 5-hydroxytryptamine function in rodents

The effects of carbamazepine (CBZ) on brain 5-hydroxytryptamine (5-HT) function were investigated in rodents pretreated with CBZ acutely or for 14 days. In behavioural experiments, mice pretreated with 14 days CBZ showed increased 5-HT₂-mediated head twitch behaviour after injection of carbidopa (25 mg/kg) followed by 5-hydroxytryptophan (5-HTP, 100 mg/kg). However, no change in head twitches after 5-methoxy,N,N,-dimethyltryptamine (5MeODMT 5.0 mg/kg), a direct agonist, was observed. Chronic CBZ administration to rats did not alter either the behavioural syndrome induced by 8-hydroxy-2-dipropy-laminotetralin (8-OH-DPAT, 1.0 mg/kg), an index of post-synaptic 5-HT_1A responses, or hypothermia after 8-OH-DPAT (0.5 mg/kg) which is thought to reflect presynaptic 5-HT_1A activity. Both hyperactivity and the behavioural syndrome seen after tranylcypromine (20 mg/kg) followed byl-tryptophan (100 mg/kg) were decreased by prior treatment with CBZ (14 days). Accumulation of 5-HTP after administration of the amino acid decarboxylase inhibitor NSD1015 (100 mg/kg) was decreased after acute CBZ (50 mg/kg) in hippocampus. However, after 14 days oral treatment no change in this measure of 5-HT synthesis was seen, in either hippocampus or frontal cortex.CBZ (50 µM) added to superfused brain slices did not affect potassium-stimulated [³H]-5-HT release. However, hippocampal slices from rats pretreated with CBZ (14 days) showed increased potassium-stimulated [³H]-5-HT release. CBZ (14 days) did not alter 5-HT₂ binding in rat frontal cortex. These results indicate a depressant effect of acute CBZ upon presynaptic 5-HT activity and an increase in presynaptic 5-HT function when CBZ is given for 14 days. Post-synaptic 5-HT function was, however, not altered by CBZ.     

Possible mechanism of the hypotensive action of 2,6-dichlorobenzylidene aminoguanidine: evidence for central noradrenaline receptor stimulation

The effects of 2,6-dichlorobenzylidene aminoguanidine acetate (DCBAG, Wy 8678, Wyeth Laboratories) have been studied on central monoamine neurons of rats in chemical and physiological experiments. Catecholamine (CA) and 5-hydroxytryptamine (5-HT) turnover was studied by the use of the tyrosine hydroxylase inhibitor, α-methyltyrosine methylester (H 44/68) and the tryptophane hydroxylase inhibitor, α-propyldopacetamide (H 22/54). The degree of CA and 5-HT depletion was studied biochemically and histochemically. Arterial pressure, heart and respiration rate were studied in rats anaesthetized with halothane, and the flexor hindlimb reflex was tested in acutely spinalized rats.Biochemically and histochemically, a dose dependent reduction of H 44/68-induced noradrenaline (NA) depletion was found both in the cerebral cortex and in the hypothalamus suggesting a reduction of NA turnover by DCBAG in doses of 0.1–10 mg/kg. The H 44/68-induced disappearance of dopamine (DA) and the H 22/54-induced disappearance of 5-HT was not changed by DCBAG, 1 mg/kg, but a significant reduction was found after 10 mg/kg. These results suggest a decrease of DA and 5-HT turnover with higher doses. The flexor hindlimb reflex, which is dependent on NA receptor activity, was increased by DCBAG (1–20 mg/kg) in spite of previous reserpine and H 44/68 treatment. It is therefore suggested that the decrease in NA turnover observed could be due to a NA receptor stimulating effect of DCBAG, eliciting a feedback to reduce NA release and turnover.DCBAG (5–500 μg/g) produced a dose-dependent reduction in arterial pressure and heart rate; higher doses also reduced respiratory rate. The CA receptor blocking agent chlorpromazine (5 mg/kg), but not the DA receptor blocking agent spiroperidol (1 mg/kg), blocked the effects of DCBAG on arterial pressure and heart rate. These findings suggest that DCBAG lowers arterial pressure and heart rate by its NA receptor stimulating action. These results give further evidence for the existence of a central NA vasodepressor mechanism.     

Evidence for a Role of Transporter-Mediated Currents in the Depletion of Brain Serotonin Induced by Serotonin Transporter Substrates

Serotonin (5-HT) transporter (SERT) substrates like fenfluramine and 3,4-methylenedioxymethamphetamine cause long-term depletion of brain 5-HT, while certain other substrates do not. The 5-HT deficits produced by SERT substrates are dependent upon transporter proteins, but the exact mechanisms responsible are unclear. Here, we compared the pharmacology of several SERT substrates: fenfluramine, d-fenfluramine, 1-(m-chlorophenyl)piperazine (mCPP) and 1-(m-trifluoromethylphenyl)piperainze (TFMPP), to establish relationships between acute drug mechanisms and the propensity for long-term 5-HT depletions. In vivo microdialysis was carried out in rat nucleus accumbens to examine acute 5-HT release and long-term depletion in the same subjects. In vitro assays were performed to measure efflux of [³H]5-HT in rat brain synaptosomes and transporter-mediated ionic currents in SERT-expressing Xenopus oocytes. When administered repeatedly to rats (6 mg/kg, i.p., four doses), all drugs produce large sustained elevations in extracellular 5-HT (>5-fold) with minimal effects on dopamine. Importantly, 2 weeks after dosing, only rats exposed to fenfluramine and d-fenfluramine display depletion of brain 5-HT. All test drugs evoke fluoxetine-sensitive efflux of [³H]5-HT from synaptosomes, but d-fenfluramine and its bioactive metabolite d-norfenfluramine induce significantly greater SERT-mediated currents than phenylpiperazines. Our data confirm that drug-induced 5-HT release probably does not mediate 5-HT depletion. However, the magnitude of transporter-mediated inward current may be a critical factor in the cascade of events leading to 5-HT deficits. This hypothesis warrants further study, especially given the growing popularity of designer drugs that target SERT.     

Rapid tolerance to the motor effects of p-chloroamphetamine in rats

Studies were designed to test the hypothesis that tolerance to the effect of p-chloroamphetamine (PCA) on motor activity in rats would develop with repeated injections. In related biochemical studies the effects of single or repeated doses of PCA on the in vitro synaptosomal uptake of³H-NE and³H-DA and on the in vivo metabolism of intraventricularly administered³H-NE and³H-DA were investigated. The administration of 10 mg/kg of PCA induced a complex behavioral syndrome, which was quantified by scoring specific symptoms after direct observation. In agreement with previous data, this syndrome appears to be mediated by a release of 5-HT since pretreatment with PCA prevented its development on subsequent injection of the drug. After the administration of lower doses of PCA, total motor activity as measured in activity cages increased, and tolerance to this effect also developed rapidly. For example, pretreatment with 5 mg/kg of PCA greatly attenuated the stimulant effect of a subsequent dose of 3 or 5 mg/kg of the drug. Moreover, the degree of tolerance was the same if the time between the 2 injections was 1 day or 2 weeks, suggesting that 5-HT release is also involved in the tolerance to the motor effects of lower doses of the drug. Moreover, biochemical studies of the response of catecholaminergic neurons to PCA suggest that tolerance does not develop to the effects on DA and NE neurons on repeated injection of PCA.