An investigation of amphetamine-induced release of 5-HT from rat hippocampal slices

Functional Interplay between Dopaminergic and Serotonergic Neuronal Systems during Development and Adulthood

The complex integration of neurotransmitter signals in the nervous system contributes to the shaping of behavioral and emotional constitutions throughout development. Imbalance among these signals may result in pathological behaviors and psychiatric illnesses. Therefore, a better understanding of the interplay between neurotransmitter systems holds potential to facilitate therapeutic development. Of particular clinical interest are the dopaminergic and serotonergic systems, as both modulate a broad array of behaviors and emotions and have been implicated in a wide range of affective disorders. Here we review evidence speaking to an interaction between the dopaminergic and serotonergic neuronal systems across development. We highlight data stemming from developmental, functional, and clinical studies, reflecting the importance of this transmonoaminergic interplay.

An intact dopaminergic system is required for context-conditioned release of 5-HT in the nucleus accumbens of postweaning isolation-reared rats

We investigated the effect of the tyrosine hydroxylase inhibitor, alpha-methyl-para-tyrosine (AMPT) on extracellular dopamine and 5-HT levels in the nucleus accumbens of group- and isolation-reared rats. Microdialysis with high-performance liquid chromatography-electrochemical detection was used to quantify dopamine and 5-HT efflux in the nucleus accumbens following foot shock and in association with a conditioned emotional response (CER). Isolation- and group-reared rats received i.p. injections of either saline (0.9%) or AMPT (200 mg/kg) 15 h and 2 h prior to sampling. There was no significant difference between saline-treated isolation- or group-reared rats for basal efflux of dopamine or 5-HT, however as expected, AMPT-treatment significantly reduced dopamine efflux in both groups to an equivalent level (50-55% saline-treated controls). Exposure to mild foot shock stimulated basal dopamine efflux in saline-treated groups only, although the effect was significantly greater in isolation-reared rats. In AMPT-treated rats, foot shock did not affect basal dopamine efflux in either rearing group. Foot shock evoked a prolonged increase in 5-HT efflux in both isolation- and group-reared saline-treated rats but had no effect on 5-HT efflux in AMPT-treated rats. In response to CER, isolation-rearing was associated with significantly greater efflux of both dopamine and 5-HT in saline-treated rats, compared to saline-treated, group-reared controls. However in AMPT-treated rats, efflux of dopamine or 5-HT did not change in response to CER. These data suggest that unconditioned or conditioned stress-induced changes in 5-HT release of the nucleus accumbens are dependent upon intact catecholaminergic neurotransmission. Furthermore, as the contribution of noradrenaline to catecholamine efflux in the nucleus accumbens is relatively minor compared to dopamine, our findings suggest that dopamine efflux in the nucleus accumbens is important for the local regulation of 5-HT release in this region. Finally, these findings implicate the isolation-enhanced presynaptic dopamine function in the accumbens with the augmented ventral striatal 5-HT neurotransmission characterized by isolation-reared rats.

Anticonvulsant action of hippocampal dopamine and serotonin is independently mediated by D2 and 5-HT1A receptors

The present microdialysis study evaluated the anticonvulsant activity of extracellular hippocampal dopamine (DA) and serotonin (5-HT) with concomitant assessment of the possible mutual interactions between these monoamines. The anticonvulsant effects of intrahippocampally applied DA and 5-HT concentrations were evaluated against pilocarpine-induced seizures in conscious rats. DA or 5-HT perfusions protected the rats from limbic seizures as long as extracellular DA or 5-HT concentrations ranged, respectively, between 70-400% and 80-350% increases compared with the baseline levels. Co-perfusion with the selective D(2) blocker remoxipride or the selective 5-HT(1A) blocker WAY-100635 clearly abolished all anticonvulsant effects. These anticonvulsant effects were mediated independently since no mutual 5-HT and DA interactions were observed as long as extracellular DA and 5-HT levels remained within these protective ranges. Simultaneous D(2) and 5-HT(1A) receptor blockade significantly aggravated pilocarpine-induced seizures. High extracellular DA (> 1000% increases) or 5-HT (> 900% increases) concentrations also worsened seizure outcome. The latter proconvulsive effects were associated with significant increases in extracellular glutamate (Glu) and mutual increases in extracellular monoamines. Our results suggest that, within a certain concentration range, DA and 5-HT contribute independently to the prevention of hippocampal epileptogenesis via, respectively, D(2) and 5-HT(1A) receptor activation.

The effects of antipsychotic drugs on serotonergic activity in the rat hippocampus

The serotonergic activity in hippocampus was investigated following acute and chronic treatment with the antipsychotic drugs haloperidol and risperidone. Acute administration of risperidone, the serotonin(2) (5-HT(2)) receptor antagonist ketanserin, and the dopamine (DA)-D(2) receptor antagonist raclopride increased the 5-hydroxyindoleacetic acid/serotonin (5-HIAA/5-HT) ratio. In contrast, acute administration of haloperidol did not affect this ratio. Chronic administration of risperidone maintained the increased 5-HIAA/5-HT ratio; a challenge dose of risperidone after the chronic treatment and the subsequent washout period also maintained the increased ratio. Chronic administration of haloperidol as well as a challenge dose of haloperidol following chronic treatment did not affect the serotonergic activity in hippocampus. Administration of ketanserin or raclopride after chronic treatment and the washout period induced an additional increase in the 5-HIAA/5-HT ratio in risperidone-treated rats. Moreover, a challenge dose of ketanserin, but not raclopride, increased the 5-HIAA/5-HT ratio in haloperidol-treated rats. The present results indicate that acute and chronic treatment of haloperidol or risperidone modified serotonergic activity in the hippocampus in a different way. Moreover, the augmentation of serotonergic activity induced by risperidone did not seem to be solely related to dopaminergic or serotonergic properties and may be of particular relevance for the amelioration of schizophrenia symptoms.

Dopaminergic input is required for increases in serotonin output produced by behavioral activation: an in vivo microdialysis study in rat forebrain

Previous research has demonstrated that pharmacological stimulation of postsynaptic dopamine D2 receptors produces increases in serotonin output. The present study explored whether this relationship also holds under physiological conditions. Accordingly, we examined the effects of D2 receptor blockade or unilateral dopamine depletion on behaviorally induced increases in extracellular serotonin levels in the corpus striatum and prefrontal cortex of freely moving rats using in vivo microdialysis. Extracellular levels of dopamine and serotonin, as well as behavioral activity, were increased by both mild tail pinch and the light-dark transition. Tail pinch-induced increases in serotonin levels (39+/-3% and 53+/-5% in the corpus striatum and prefrontal cortex, respectively), but not the accompanying behavioral changes, were blocked by local application of the D2 receptor antagonist raclopride (10 microM). D2 receptor blockade also disrupted the positive relationship between striatal serotonin levels and behavioral activity of animals across the light-dark transition (r=0.93 without raclopride, r=0.24 in presence of raclopride). Unilateral 6-hydroxydopamine lesion of the nigrostriatal dopaminergic system also abolished increases in striatal serotonin output induced by both tail pinch and light-dark transition. A negative correlation was observed between the degree of striatal dopamine depletion and tail pinch-induced increases in serotonin efflux (r= - 0.88). Thus, both a local blockade of postsynaptic D2 receptors and striatal dopamine depletion prevented increases in serotonin output that normally accompany behavioral activation. These data indicate that the increases in the forebrain serotonin output produced by two distinct physiological/environmental manipulations appear to be largely dependent upon intact local dopaminergic neurotransmission.

Involvement of dopamine D2 receptors in apomorphine-induced facilitation of forebrain serotonin output

The effect of systemic administration of the nonselective dopamine receptor agonist apomorphine on efflux of serotonin (5-hydroxytryptamine, 5-HT) in striatum and hippocampus of freely moving rats was examined using in vivo microdialysis. 5-HT efflux was increased by a moderate dose of apomorphine sufficient for a postsynaptic dopaminergic effect (0.5 mg/kg, s.c.), but not by a lower dose (0.1 mg/kg, s.c.), that acts preferentially on presynaptic dopamine receptors. This effect was blocked by a dopamine D2 receptor antagonist raclopride, administered either systemically or locally into striatum, but not by a 5-HT1A receptor antagonist N-¿2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl¿-N-(2-pyridinyl) cyclohexanecarboxamide 3HCI (WAY-100635). This indicates that dopamine D2 receptors, and not 5-HT1A receptors, mediate the facilitatory effect of apomorphine, and that this effect occurs at the nerve terminal level. Behavioral effects of apomorphine outlasted the concomitant changes in 5-HT efflux, suggesting that these changes resulted from dopaminergic receptor activation, rather than from the drug-induced behavioral arousal.

Brain structures involved in the behavioral stimulant effect of central serotonin release

Drugs such as p-chloroamphetamine or a combination of tranylcypromine and tryptophan release serotonin in the central nervous system and produce a behavioral serotonin syndrome. However, in the presence of methysergide or following destruction of descending spinal serotonergic projections by 5,7-dihydroxytryptamine, central serotonin release produces hyperlocomotion. This supports the hypothesis that release of serotonin in the brain promotes locomotion but that the expression of this effect can be blocked by concomitant intraspinal effects of serotonin release. Hyperlocomotion induced by serotonin release is attenuated or blocked by: (a) pretreatment with p-chlorophenylalanine; (b) acute surgical lesions of the basal diencephalon; (c) chronic lesions of the ventromedial midbrain tegmentum by local injection of 5,7-dihydroxytryptamine; and (d) acute surgical decortication. Medial decortication tends to be more effective then lateral decortication. Hyperlocomotion produced by methamphetamine is also attenuated or blocked by acute basal diencephalic lesions or decortication. It is suggested that ascending serotonergic and dopaminergic projections collaborate in the generation of spontaneous voluntary motor activity.

Functional regulation by dopamine receptors of serotonin release from the rat hippocampus: in vivo microdialysis study

The functional regulation by dopamine (DA) receptors of serotonin (5-HT) release from the rat hippocampus was investigated by use of in vivo microdialysis. Dialysate 5-HT levels were reduced by co-perfusion of 10 microM tetrodotoxin (TTX) and were elicited by K+ (60 and 120 mM) stimulation in a concentration-dependent manner. Local perfusion (10 microM) and peripheral administration (20 mg/kg, i.p.) of fluoxetine produced increases in 5-HT levels. These results indicate that the spontaneous 5-HT levels in the rat hippocampus can be used as indices of neuronal origin from the serotonergic nerve terminals. The nonselective dopamine (DA) receptor agonist apomorphine (1, 10 and 100 microM), when perfused through the probe over a period of 40 min, increased 5-HT release in a concentration-dependent manner. Apomorphine-induced (100 microM) increases in 5-HT release was abolished by pretreatment with the selective D2 receptor antagonist, S(-)-sulpiride (1 and 10 microM), but not prevented by pretreatment with the selective D1 receptor antagonist, R(+)-SCH-23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2, 3, 4, 5-tetrahydro-1H-3-benzazepine) (1 microM). S(-)-Sulpiride and R(+)-SCH-23390 by themselves did not alter the spontaneous 5-HT levels. The 5-HT release was elevated by perfusion of the selective DA reuptake inhibitor GBR 12909 (1-[2-[bis(4-fluorophenyl) methoxy]ethyl]-4-[3-phenyl-propyl]piperazine) (1, 10 and 100 microM), indicating the possibility of not only exogenous but also endogenous DA-mediated facilitatory effects on 5-HT release in vivo. The 5-HT release was also elevated by perfused (+/-)-PPHT ((+/-)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin) (1, 10 and 100 microM), the selective D2 receptor agonist, in a concentration-dependent manner. On the other hand, (+/-)-PPHT (100 microM) failed to increase 5-HT release in catecholamine (CA)-lesioned rats pretreated with 6-hydroxydopamine (6-OHDA)(200 micrograms/rat, i.c.v.). The (+/-)-PPHT-induced (100 microM) increase in 5-HT release was prevented not only by pretreatment with 10 microM S(-)-sulpiride but also by pretreatment with the alpha 2-adrenoceptor antagonist idazoxan (10 microM). These findings suggest that the functional regulation of 5-HT release via D2 receptors exists in the rat hippocampus. Furthermore our results indicate that the facilitatory effect of 5-HT release via D2 receptors may be mediated indirectly by noradrenergic neurons, but not mediated directly through D2 receptors located on serotonergic nerve terminals.

Functional interaction between serotonin and other neuronal systems: focus on in vivo microdialysis studies

In this review, the functional interactions between serotonin (5-HT) and other neuronal systems are discussed with the focus on microdialysis studies in the mammalian brain (mainly rats). 5-HT release is negatively regulated not only by somatodendritic 5-HT1A and terminal 5-HT1B (5-HT1D) autoreceptors but also by alpha 2-adrenergic and mu-opioid heteroreceptors that are located on serotonergic nerve terminals. 5-HT by itself is involved in the inhibitory effects of noradrenaline release and the facilitatory regulation of dopamine release via multiple 5-HT receptors. Acetylcholine release appears to be regulated by inhibitory 5-HT1B heteroreceptors located on cholinergic nerve terminals. Long-term treatment with 5-HT-uptake inhibitors and noradrenaline-uptake inhibitor produces desensitization of 5-HT1A autoreceptors and alpha 2-heteroreceptors, respectively, which may be related therapeutically to the delayed onset of the effects of antidepressants. Some microdialysis studies have predicted that the combination of a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist might produce much greater availability of 5-HT in the synaptic cleft in terms of much faster induction of subsensitivity of 5-HT1A autoreceptors. Clinical trials based on this hypothesis have revealed that combination therapy with a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist ameliorated the therapeutic efficacy in depressive patients. Taken together, neurochemical approaches using microdialysis can contribute not only to clarification of the physiological role of the serotonergic neuronal systems but also might be a powerful pharmacological approach for the development of therapeutic strategies.

Reverse tolerance to amphetamine evokes reverse tolerance to 5-hydroxytryptophan

Repeated intermittent administration of amphetamine in mice caused reverse tolerance to 5-hydroxy-L-tryptophan (5-HTP)-induced head twitch, as well as to amphetamine-induced stereotypy. The repeated administration of 5-HTP alone also resulted in reverse tolerance in the head-twitch test. Daily pretreatment with haloperidol prior to amphetamine administration blocked the development of both reverse tolerance to amphetamine and to 5-HTP, whereas daily pretreatment with cyproheptadine prior to amphetamine blocked only the reverse tolerance to 5-HTP. On the other hand, 5-HTP-induced reverse tolerance was blocked by daily pretreatment with cyproheptadine, but not with haloperidol. There appears to be no difference in the persistence of the reverse tolerance to 5-HTP, whether induced by amphetamine or by 5-HTP; in both instances, the persistence does not correlate with the persistence of reverse tolerance to amphetamine. The data suggest that the reverse tolerance to amphetamine and the associated reverse tolerance to 5-HTP are independent events, both of which are mediated by dopaminergic mechanisms.

5-Hydroxytryptamine involvement in the locomotor activity suppressant effects of amphetamine in the mouse

d-Amphetamine, in doses lower than required to increase motor activity, reduced mouse spontaneous locomotor activity when this was assessed using cages equipped with photocell units, using treadwheels, or the measurement of spontaneous climbing behaviour. Acute treatments with the serotonergic agonists quipazine and 5-hydroxy-DL-tryptophan also reduced wheel running activity, spontaneous locomotor activity assessed using photocell cages, and spontaneous climbing behaviour; fenfluramine caused a similar effect. Pretreatment with 5-hydroxy-DL-tryptophan enhanced the inhibitory effects of d-amphetamine. A 3-day treatment with fenfluramine, or lesions of the median raphe nucleus (but not the dorsal raphe nucleus) abolished the ability of d-amphetamine to reduce motor activity in the three test systems. It is concluded that low doses of d-amphetamine can reduce locomotor activity and that the effects may be mediated via an enhancement of the release of 5-hydroxytryptamine from the system arising in the median raphe nucleus.