Selective 6OHDA-induced destruction of mesolimbic dopamine neurons: abolition of psychostimulant-induced locomotor activity in rats

Catecholamines and the sleep-wake cycle. I. EEG and behavioral arousal

The exact role of catecholamines (CA) on the sleep-wake cycle is still controversial. Critical analysis of lesion studies tends to suggest a neuromodulatory role for both dopamine (DA) and norepinephrine (NE) on EEG and behavioral arousal. Support for this view is provided by pharmacological studies in which catecholaminergic systems are activated or inhibited. Taken together they show that disturbances in the dynamic balance between neurochemical systems may alter the conditions for wake-triggering mechanisms to express at optimal levels. Large electrolytic or neurotoxic lesions which affect noradrenergic and dopaminergic structures are associated with marked and prolonged EEG changes and decreased behavioral arousal, respectively. In contrast, specific and circumscribed damage restricted to these systems is followed by a transient decrease in waking activity. Thus, results observed after large central lesions are most probably related to destruction of non-catecholaminergic neurons. Inhibition of brain CA synthesis causes behavioral sedation and a decrease in waking activity. Selective pharmacological stimulation of presynaptic alpha-adrenergic (alpha 2) receptors tends to decrease waking, while opposite effects result from alpha 2-receptor blockade. Drugs with agonistic activity at postsynaptic alpha-adrenergic (alpha 1) sites increase EEG desynchronization, but specific blockade of alpha 1-receptor does not result in marked decreases of waking EEG. In contrast, treatments which simultaneously block NE and DA receptors significantly affect waking. Beta-adrenergic receptor blockers show no conclusive effects on waking or sleep. Selective DA-receptor agonists induce biphasic effects, with low doses decreasing and large doses increasing cortical desynchronization and motility. Opposite effects are observed in laboratory animals after injection of specific DA-receptor blockers.

Effect of 6-hydroxydopamine-induced lesions of A10 dopaminergic neurons on aggressive behavior in rats

The present study evaluated the effects of microinjections of 6-hydroxydopamine into the ventral mesencephalic tegmental area (nucleus A10) on aggressive behavior in rats. This treatment resulted in a reduction in foot-shock-induced fighting but failed to influence muricide (mouse-killing) behavior in chronically isolated rats. The general activity of animals tested in the open field was significantly increased two weeks after lesions. These behavioral changes were accompanied by a significant depletion of forebrain dopamine, with no difference between lesioned and sham-lesioned rats in norepinephrine and 5-hydroxyindole acetic acid levels.

Stereotyped behavior elicited by amphetamine in the rat: influences of the testes

Castrating male rats in adulthood increased the duration of stereotyped behavior in responses of 5 mg/kg injections of d-amphetamine sulfate; exogenous treatment with testosterone propionate (TP) reversed this effect. Ovariectomy in adulthood had no effect on stereotypy, but TP injections reduced stereotyped responding by ovariectomized females. Thus testosterone exerts comparable effects on stereotypy elicited by amphetamine in both sexes. Males castrated at 1, 6 or 10 days of age but not males castrated in adulthood displayed levels of stereotyped behavior comparable to those of ovariectomized females when all animals were given TP in adulthood. Control experiments indicated that age of castration rather than time since castration was the critical factor, implying that secretions of the testes early in life exert effects on systems that regulate the responses of adults to amphetamine.

Further studies on the effects of intravenously administered 6-hydroxydopamine on the median eminence of the rat

6-Hydroxydopamine (6-OHDA) given i.v. in a dose of 150 mg/kg to adult male Sprague-Dawley rats produces at 24 h a complete depletion of median eminence (ME) and neurointermediate lobe (NIL) catecholamines (CA), as judged by Falck-Hillarp fluorescence histochemistry, leaving the remainder of the hypothalamus substantially unaltered. Restoration of a normal fluorescence histochemical appearance took place over 35 days, apparently due to regeneration of CA-containing terminals. Administration of desipramine prior to 6-OHDA injection modified its effect; depletion was largely confined to the midline region of the external layer of the ME and to the NIL, suggesting that these regions contain dopaminergic terminals. Microspectrofluorometric methods were used in an attempt to verify this conclusion. It is suggested that this technique provides a model for establishing the role of ME and NIL CA-containing structures in control of pituitary gland function.

Regulatory impairments following selective 6-OHDA lesions of the neostriatum

6-Hydroxydopamine lesions of the ventrolateral (VLC) but not anteromedial (AMC) caudate-putamen in rats resulted in a greater post-operative reduction in body weight and water intake than seen in animals with sham lesions. Once animals had fully resumed spontaneous food and water intake, a series of regulatory challenges were administered, and the AMC rats showed a reduced enhancement of drinking following injection of hypertonic saline. The results are interpreted in terms of a heterogeneous striatal convergence of nigrostriatal and cortical regulatory mechanisms.

The effect of chronic bromocriptine and L-dopa on spiperone binding and apomorphine-induced stereotypy

Chronic treatment with dopamine (DA) agonists has been reported in various paradigms to cause supersensitivity of DA receptors or, contradictorily, subsensitivity of DA receptors. The present study administered 15 mg/kg bromocriptine for 7 days and measured both striatal spiperone binding and apomorphine (AP)-induced stereotypy. A significant decrease in AP-induced stereotypies was observed after chronic bromocriptine treatment, but without a significant parallel decrease in striatal spiperone binding. These results probably do not represent a true agonist-induced subsensitivity, but possibly show that residual bromocriptine in vivo may antagonize AP-induced stereotypy. Since some reports have suggested that L-Dopa may specifically reverse the increases in DA receptor number induced by chronic haloperidol, we also studied the effect of 7 days of L-Dopa treatment after 6-week chronic haloperidol treatment of mice. While chronic haloperidol significantly increased striatal spiperone binding, subsequent L-Dopa treatment did not reverse this biochemical supersensitivity. It is concluded that agonist induction of subsensitivity in the DA system is difficult to reproduce and may depend on highly specific dosage conditions and treatment schedules.

The amphetamine-like releasing effect of the alkaloid (-)cathinone on rat nucleus accumbens and rabbit caudate nucleus

1. (-)Cathinone, a new alkaloid from khat leaves, has recently been reported to have amphetamine-like behavioral effects. In order to investigate the effects of this compound on the cellular level, slices of rat nucleus accumbens, prelabelled with 3H-dopamine, were superfused with solutions of (-)cathinone. 2. It was found that (-)cathinone, as (+)amphetamine, enhanced the release of radioactivity from rat nucleus accumbens tissue. 3. In similar experiments on rabbit caudate nucleus it was observed that the catecholamine reuptake inhibitors benztropine, nomifensine and mazindol were able to block the (-)cathinone-induced release, indicating that the alkaloid has to penetrate to intraneuronal sites in order to evoke release. 4. It is concluded that the pharmacological similarity between (-)cathinone and (+)amphetamine extends to the cellular level and that the behavioral effects of (-)cathinone are due to stimulation of release from central catecholamine storage sites.

Alterations in the stages of continuous amphetamine intoxication produced by lesions of locus coeruleus and substantia nigra

1. Slow-release silicone pellets containing d-amphetamine base were implanted subcutaneously in rats 20 days following radio-frequency lesions of the locus coeruleus (LC), substantia nigra (SN), or control operations. 2. LC-lesioned rats exhibited enhanced motor stereotypies soon after implantation, while SN-lesioned animals showed increased locomotion but decreased stereotypy and anorexia. 3. The later behavioral stages of continuous amphetamine intoxication were attenuated in both lesioned groups as controls then entered the most intense stereotypy, showed the greatest withdrawal to the burrows, and maximally exhibited the increase in socially disruptive behaviors which appear following four days of constant amphetamine intoxication. 4. The integrity of both dopaminergic and noradrenergic systems was necessary for the full expression of the late stages of this animal model of amphetamine psychosis.

Amphetamine psychosis and schizophrenia: a dual model

Several of the behavioral consequences of acute and chronic amphetamine treatment were evaluated and related to the underlying neurochemical correlates of drug treatment. It was suggested that decreased noradrenergic activity after long-term amphetamine treatment influences stimulus sampling, whereas enhanced dopaminergic activity was responsible for the progressive augmentation of stereotypy and self-stimulation behavior observed after long-term exposure to amphetamine. It was hypothesized that amphetamine-induced psychosis and the symptomatology associated with schizophrenia are related to alterations in both norepinephrine and dopamine activity.

Hyperactivity and hypoactivity produced by lesions to the mesolimbic dopamine system

Spontaneous locomotor activity and the locomotor response to amphetamine and apomorphine were studied in rats subjected to either radiofrequency (RF), 6-hydroxydopamine (6-OHDA) or both RF and 6-OHDA lesions of the mesolimbic dopamine (DA) system. Large 6-OHDA lesions of the ventral tegmental area (VTA) or of the nucleus accumbens (N.Acc.) produced hypo-activity in the open field, a complete blockade of the locomotor stimulating effects of D-amphetamine and a profound supersensitive response to apomorphine as measured by a significant increase in locomotor activity as compared to sham-operated animals. In contrast, smaller 6-OHDA lesions of the VTA produced significant increases in spontaneous daytime and nocturnal activity with the biggest effect occurring at the lowest dose. RF lesions to the VTA produced even greater hyperactivity which was blocked by the addition of a 6-OHDA lesion to the N.Acc. The rats with RF lesions to VTA alone that were spontaneously hyperactive remained hyperactive after injection of amphetamine, whereas apomorphine produced a significant decrease in this hyperactivity. In contrast, the rats with the combined RF lesion and N.Acc. 6-OHDA lesion showed a blockade of the locomotor stimulating effects of D-amphetamine and a potentiated response to apomorphine identical to that observed with a N.Acc. lesion alone. All lesion groups revealed massive depletion of DA in the N.Acc. and anterior striatum with significantly greater depletions in those groups showing hypoactivity and hypo-responsiveness to amphetamine. All groups except the N.Acc. 6-OHDA alone group showed significant depletions of DA in the posterior striatum. Thus, limited destruction of the mesolimbic DA system can produce hyperactivity, but more extensive destruction of this system in the region of the N.Acc. and anterior striatum can reverse this hyperactivity and produce a hypo-responsiveness to the locomotor stimulating effects of amphetamine. These results suggest an essential role for dopamine in the expression of spontaneous and stimulant-induced activity. Furthermore, the much larger increase in spontaneous activity in the RF-VTA lesion group as compared to the VTA-6-OHDA groups suggests the presence of an, as yet unidentified, powerful inhibitory influence to the mesolimbic DA system within the midbrain tegmentum.

Reward-induced stereotypy: modulation by the hippocampus

In animals with hippocampal damage, the signaled administration of reward is sufficient to induce the sort of behavioral sterotypy and locomotion that heretofore has been observed only after drug administration. Haloperidol returns these behaviors to normal. The interaction of the hippocampus with reward helps to explain many well-known characteristics of animals with lesions in the hippocampus and may have relevance for catecholamine-based clinical disorders.

Effect of manipulating central catecholamines on puberty and the surge of luteinizing hormone and gonadotropin releasing hormone induced by pregnant mare serum gonadotropin in female rats

We have investigated the effect of manipulating central catecholamines on the timing of puberty (as assessed by vaginal opening) in female rats and the surge of luteinizing hormone (LH) and gonadotropin releasing hormone (GnRH) induced by pregnant mare serum gonadotropin (PMSG) in immature female rats. Manipulation of the catecholamines was carried out with either 6-hydroxydopamine (6-OHDA) administered with or without either desipramine (DMI) or pargyline, or alpha-methyl-p-tyrosine (alpha-MPT). The neonatal administration of 6-OHDA delayed puberty, an effect which was potentiated by pretreatment with DMI and was associated with a reduction in the rate of body growth. Catecholamine fluorescence in animals aged 60--65 days that had been treated with DMI followed by 6-OHDA was diminished only in the caudatus--putamen; treatment with 6-OHDA alone resulted in diminished fluorescence in the hypothalamus and in the intermediate but not the external layer of the median eminence. The neonatal administration of alpha-MPT had no significant effect on either the growth rate or the timing of puberty. Regular oestrous cycle occurred after puberty in animals treated with either 6-OHDA or alpha-MPT. The PMSG-induced LH surge was significantly enhanced by 6-OHDA (administered i.v.) plus DMI, and reduced by 6-OHDA injected in to the lateral ventricle (v). The inhibitory effect of 6-OHDA (v) was reduced by DMI, but in animals given 6-OHDA (i.v.) after pargyline there was a marked reduction in the height of the LH surge. There was a good correlation between the changes in the concentrations of LH in peripheral plasma and the concentrations of GnRH in pituitary stalk plasma in that the PMSG-induced surge of GnRH was significantly increased by 6-OHDA (i.v.) plus DMI and reduced by 6-OHDA (v). In animals treated with 6-OHDA (i.v. plus DMI catecholamine fluorescence was reduced only in the external layer of the median eminence, while after 6-OHDA (v) plus DMI degeneration was seen in the medial forebrain bundle. These results demonstrate a marked difference between the long-term and acute effects of 6-OHDA on the gonadotropin control system. Neonatal treatment with 6-OHDA plus DMI significantly delays puberty and the rate of body growth, but does not affect cyclical gonadotropin release and has no persistent effect on the hypothalamic catecholaminergic systems. The acute administration of 6-OHDA, depending upon the route of administration and whether it is given after DMI, can either potentiate or inhibit the PMSG-induced surge of GnRH and consequently LH by mechanisms which involve destruction, respectively, of either dopaminergic terminals in the median eminence or catecholaminergic fibres in the dorsal hypothalamus.

Apparent serotonergic modulation of the dose-dependent biphasic response of neostriatal neurons produced by D-amphetamine

Previous reports indicate that amphetamine produces a dose-dependent shift to the firing rate of neurons in the anterior neostriatum that parallels the shift in behavior from locomotion to focused stereotypy. To determine if para-chlorophenylalanine (PCPA), which alters the pattern of the behavioral response to amphetamine, also changes the pattern of the neuronal response to the drug, a dose-response analysis was performed on amphetamine-induced changes in unit activity in the anterior neostriatum of rats pretreated 48 h previously with 300 mg/kg PCPA or vehicle. An intraperitoneal injection of 1.0 mg/kg D-amphetamine, which inhibited neostriatal activity in vehicle controls, produced in PCPA-pretreated animals a prolonged excitation. In contrast, the pronounced increase in firing rate produced by 7.5 mg/kg D-amphetamine in control rats was significantly reduced following PCPA pretreatment. Liquid chromatography with electrochemical detection revealed that compared to controls, PCPA produced a significant reduction of serotonin, but not dopamine or norepinephrine, in the telencephalon. The differential effects of PCPA on the action of low and high doses of D-amphetamine in the anterior neostriatum may explain the differential influence of serotonergic systems on amphetamine-induced locomotion add stereotypy.

Gonadectomy and sex differences in the behavioral responses to amphetamine and apomorphine of rats

Following treatment with 5 mg/kg d-amphetamine sulfate or 2 mg/kg apomorphine hydrochloride female rats displayed more intense and longer lasting stereotyped behavior than males. Gonadectomy did not affect the display of stereotyped behavior induced by either drug in either sex. A lower dose of amphetamine (1 md/kg) caused greater stimulation of locomotor activity in females than in males. Castration of males had no effect, but ovariectomy blocked the stimulating effect of amphetamine on activity. By contrast, low doses of apomorphine depressed activity in a dose-dependent manner that was somewhat greater in ovariectomized females than in the other groups. These data add to the growing body of literature demonstrating that gonadal hormones modulate the activity of brain dopamine systems.

Gestational caffeine modifies offspring behaviour in mice

Dams from two strains of mice, BALB/c C57BR were tested during gestation with caffeine, at doses of about 60, 80 and 100 mg/kg/day, in their drinking water. The resulting offspring were behaviourally tested over a 6-month period commencing at age 9 months. When compared with controls, mice from dams that had received caffeine demonstrated longer latencies in a passive avoidance test, and differences were also noted for female C57BR offspring in activity and habituation measures. Having controlled as far as possible for post-natal maternal and environmental effects, the most likely conclusion is that caffeine has a direct pharmacological action on the foetus, and should therefore be classed as a behavioural teratogen in mice.

The effects of radio-frequency lesions of the nucleus accumbens on d-amphetamine-induced locomotor and rearing behavior in rats

A large body of evidence supports the conclusion that mesolimbic dopaminergic neurons, particularly those that innervate the nucleus accumbens (n. ACC), are important for the expression of amphetamine-stimulated locomotor behavior (ASLB). However, a contradictory report (Wirtshafter et al. 1978), stating that bilateral lesions of the n. ACC fail to block ASLB, was based on a general measure of activity that did not distinguish between locomotion and rearing. In the present study, observer ratings of videotaped responses were used to determine the separate effects of 2.0 mg/kg d-amphetamine (d-AMP) on locomotion and rearing in rats with either sham or radio-frequency lesions of the n. ACC. The n.ACC lesions blocked the locomotor stimulation, but not the increased rearing that follows d-AMP administration. These results support the general conclusion that dopaminergic terminals in the n. ACC are important for the expression of ASLB, and further suggest that d-AMP-stimulated locomotion and rearing are mediated through different neural substrates.

Amphetamine-, scopolamine- and caffeine-induced locomotor activity following 6-hydroxydopamine lesions of the mesolimbic dopamine system

As previously reported, 6-hydroxydopamine (6-OHDA) lesions to the region of the nucleus accumbens blocked the locomotor activation induced by low doses of d-amphetamine, and produced a supersensitive locomotor response to the dopamine (DA) agonist, apomorphine. This same lesion, however, failed to block the locomotor activation induced by scopolamine or caffeine. These results suggest that scopolamine and caffeine activate locomotion in the rat by acting independently of presynaptic terminals in the mesolimbic DA system.

Dopaminergic agonists differentially affect open-field activity of rats with A10 lesions

Dopaminergic systems appear to exert considerable control over locomotor activity. Although dopamine neurons are located in relatively close proximity within the mesencephalon, their axons project to more diffuse areas, perhaps reflecting some underlying heterogeneity in their function. The purpose of this study was to determine whether dopamine agonists differentially affect activity by acting upon distinct dopamine systems. Bilateral radio-frequency lesions of area A10 in rats failed to affect spontaneous open-field behavior over a 1-month postoperative period. When injected with 1 mg/kg of apomorphine, however, experimental rats more than doubled their activity as compared to the response of sham-operated controls. In contrast, no difference between the two groups of animals was observed in terms of increased activity following 3 mg/kg of either d-amphetamine or methylphenidate. These results are consistent with previous work indicating the involvement of ventromedial mesencephalic dopamine somata in the control of locomotor activity. The data suggest, however, that systems in addition to the dopaminergic mesolimbic projection are responsible, in part, for the hyperactivity elicited by d-amphetamine or methylphenidate.

Relationships between selective denervation of dopamine terminal fields in the anterior forebrain and behavioral responses to amphetamine and apomorphine

To investigate the relationship between denervation of dopamine (DA) terminal fields in the anterior forebrain and the behavioral responses to amphetamine (1.5 mg/kg) and apomorphine (1 mg/kg), we injected 6-hydroxydopamine (6-OHDA) bilaterally into the anterolateral hypothalamus (ALH) or into specific mesolimbic and anterior striatal terminal fields after pretreatment with desmethylimipramine to protect noradrenergic axons and terminals from 6-OHDA toxicity. After drug testing was completed, the extent of denervation was determined by fluorescent histochemical analysis. When nearly all of the mesolimbicocortical and anteroventral striatal DA terminal fields were denervated by bilateral ALH 6-OHDA, the locomotor response to amphetamine was abolished, and the locomotor and stereotyped sniffing responses to apomorphine were increased. When fewer DA terminal fields were denervated, different results were obtained: the locomotor response to amphetamine decreased or did not change; stereotyped sniffing elicited by apomorphine did not increase or sniffing was replaced by stereotyped licking and biting. The results suggest a mass action relationship between DA terminal fields in the anterior forebrain and the locomotor response to amphetamine. The relationship between the same DA lesions and responses to apomorphine appears to be more complex.

Intracellular analysis of synaptic potentials in rat neostriatum following stimulation of the cerebral cortex, thalamus, and substantia nigra

Intracellular recordings were obtained from neostriatal neurons of unparalyzed male hooded rats anesthetized with urethane. Electrical stimulation of the cerebral cortex (Cx), centromedian-parafascicular area of the thalamus (CMP), and the substantia nigra (SN) elicited monosynaptic excitatory postsynaptic potentials (EPSPs) in neostriatal neurons. Response latencies were, on the average, 3.7 msec, 3.3 msec, and 3.8 msec, for Cx, CMP and SN stimulation, respectively. Over 85% of recorded neurons showed convergence of inputs from all three stimulation sites. The SN induced EPSP sometimes had two components, with the second component beginning 10-15 msec after the first. EPSPs from all three stimulation sites were often followed by inhibitory postsynaptic potentials (IPSPs) lasting from 50-250 msec. Double shock experiments indicated that SN induced EPSPs could be reduced in amplitude by 20-80% when preceded by conditioning stimulation to Cx, CMP or SN. In contrast, the EPSP elicited by Cx stimulation were unaffected by conditioning stimulation. Some recorded neurons were morphologically identified by means of intracellular injection of horseradish peroxidase. All were "medium spiny" neurons. The results of the present study agree well with those of previous studies of cat caudate neurons, and extend them to rat neostriatal neurons.

The effects of dopaminergic agents on the locomotor activity of rats after high doses of methylamphetamine

Treatment of rats with 100 mg/kg/day of methylamphetamine for four days produced long lasting (16 week) depletions of central nervous system levels of dopamine but not of norepinephrine. This methylamphetamine treatment also attenuated the ability of methylamphetamine and apomorphine to produce increases in locomotor activity without shifting the dose-response curve to the right or left. These rats underwent significant decreases in activity after lower doses of haloperidol while the response to L-Dopa was virtually unchanged.

Locomotor activation induced by infusion of endorphins into the ventral tegmental area: evidence for opiate-dopamine interactions

beta-Endorphin in nanomole quantities produced a stimulation of locomotor activity when infused into the region of the dopamine cell bodies of the ventral tegmental area (VTA) in rats. alpha-, gamma-, and des-Tyr-gamma-endorphin produced similar effects, but the D-alanine analogues of alpha and gamma-endorphin produced a larger and longer-lasting activation, presumably reflecting their resistance to degradation. This locomotor activation was reversible by pretreatment with naloxone and by destruction of the terminal projections of the mesocorticolimbic dopamine system originating in the VTA. These results demonstrate that locally infused endorphin can interact with the opioid receptors in the VTA, and they suggest a means by which endorphins activate limbic excitability.

Unilateral mesolimbicocortical dopamine denervation decreases locomotion in the open field and after amphetamine

Unilateral denervation of the mesolimbicocortical dopaminergic (DA) system was produced by microinjections of 6-hydroxydopamine (6-OHDA) into the right or left anterolateral hypothalamus after desmethylimipramine pretreatment. Unilateral DA denervation was confirmed by the marked loss of fluorescent fibers in mesolimbic DA terminal fields in brains processed for catecholamine histofluorescence with the glyoxylic acid-paraformaldehyde method. Such unilateral DA denervation significantly decreased the locomotor exploration of an open field and the locomotor response to d-amphetamine (1.5 mg/kg). These results demonstrate that unilateral mesolimbococortical DA denervation is sufficient, and that bilateral denervation of a single amphetamine or a decrease of locomotion in the open field. The results are consistent with the hypothesis that the mesolimbicocortical DA system energizes behavior, but does not direct it.

Norepinephrine uptake inhibitors as biochemically and behaviorally selective antagonists of the locomotor stimulation induced by indirectly acting sympathomimetic aminetic amines in mice

Pretreatment with the selective noradrenergic uptake inhibitors nisoxetine and desipramine antagonized the locomotor stimulant effect of d-amphetamine without reducing the drug's stereotypy-inducing action. A similar antagonism was observed with imipramine but not with fluoxetine, a selective serotonin uptake inhibitor and structural analog of nisoxetine. The order of potency of antagonism was desipramine greater than nisoxetine greater than imipramine. Nisoxetine also selectively reduced the locomotor activity induced by maximally effective doses of cocaine, d-N-ethyl-amphetamine, and methylphenidate, but not that induced by morphine. Biochemically, nisoxetine blocked the selective reduction in cerebral cortical endogenous and 3H-norepinephrine produced by amphetamine with itself significantly altering either measure. These data support the involvement of norepinephrine in the locomotor stimulant action of indirectly acting sympathomimetic amines.

"Classical" and "atypical" antipsychotic drugs: differential antagonism of amphetamine- and apomorphine-induced alterations of spontaneous neuronal activity in the neostriatum and nucleus accumbens

The ability of clozapine and haloperidol to antagonize the depression of firing rate produced by d-amphetamine and apomorphine in the neostriatum and nucleus accumbens was tested in immobilized, locally anesthetized rats. In the neostriatum, an intraperitoneal injection of 2.5 mg/kg d-amphetamine or 1.0 mg/kg apomorphine produced a prolonged inhibition of neuronal activity that was reversed by a subjsequent injection of either 20 mg/kg clozapine or 2.0 mg/kg haloperidol. An analysis of the onset and magnitude of the blockade revealed that clozapine was more effective than haloperidol in reversing the amphetamine response but that both antipsychotic drugs produced a comparable blockade of the apomorphine-induced depression. Similar results were obtained in the nucleus accumbens. The data indicate that although clozapine acts equieffectively in the neostriatum and nucleus accumbens, this atypical antipsychotic drug, aside from blocking postsynaptic dopamine receptors, may exert at least some of its effects by preventing dopamine release.

Abnormal pattern of amphetamine locomotion after 6-OHDA lesion of anteromedial caudate

6-Hydroxydopamine (6-OHDA) injections into the anteromedial caudate nucleus (AMCN) produced severe loss of dopamine (DA) fibers in this region of the caudate. After a low dose of d'amphetamine (1.5 mg/kg), AMCN 6-OHDA rats made fewer traverses of the length of the activity cage than control rats. In contrast, AMCN 6-OHDA rats interrupted a photocell bema that passed across the middle of the long axis of the activity cage as often as control rats. 6-OHDA injections into the nucleus accumbens (NAc) produced severe loss of DA fibers in NAc without significantly damaging the adjacent anteromedial caudate or olfactory tubercle. After d-amphetamine (1.5 mg/kg), NAc 6-OHDA rats interrupted the photocell beam and traversed the length of the activity cage as frequently as control rats. We conclude that the DA innervation to the anteromedial caudate, but not to the nucleus accumbens, is necessary for that part of the normal locomotor response to a low dose of d'amphetamine that is required for the performance of long traverses of an activity cage.

Thyrotropin-releasing hormone: hyperactivity and mesolimbic dopamine system in rats

The mechanism of stimulatory action of thyrotropin-releasing hormone (TRH) on spontaneous motor activity was investigated in rats. TRH produced a significant hyperactivity with intraperitoneal administration of 20 mg/kg or bilateral injection of 10 micrograms into the nucleus accumbens septi (NAS). Following bilateral injection of 6-hydroxydopamine into the mesolimbic dopamine (DA) pathway, the hyperactivity induced by TRH was not altered, whereas the response to apomorphine given intraperitoneally or DA injected into the NAS was clearly enhanced. The TRH-induced hyperactivity was remarkably suppressed by alpha-methyltyrosine and in contrast, augmented by pargyline. Systemic injection of aminooxyacetic acid in a dose producing behavioral depression reduced markedly the TRH-induced hyperactivity. Bilateral injection of ethanolamine O-sulphate (100 micrograms) into the NAS produced no behavioral depression per se, but remarkably attenuated the hyperactivity response to TRH or DA (20 micrograms) given intraperitoneally or into the NAS. Both TRH (10(-5) and 10(-4) M) and methamphetamine (10(-6)--10(-4) M increased the spontaneous release of 14C-DA from rat NAS slices. These findings suggest that TRH induces hyperactivity by enhancing DA release from nerve terminals in the NAS without a direct stimulation of the post-synaptic DA recptors. TRH and GABA, independently or via interaction between them, may play a reciprocal regulatory role in the activity of the mesolimbic DA system.

Effects of amphetamine and haloperidol on avoidance behavior and exploratory activity

The effect of graded doses of D-amphetamine and haloperidol were tested on retention of a one trial learning passive avoidance response, on extinction of pole-jumping active avoidance behavior and on open-field activity. Low doses of amphetamine (10 microgram/animal) increased passive avoidance latency when given s.c. 1 h prior to the retention test. Higher doses (20 and 1000 microgram/animal) caused a bimodal distribution of avoidance latencies. Haloperidol (0.03 or 1.0 microgram/animal) significantly attenuated passive avoidance behavior. Amphetamine caused a delay of extinction of pole-jumping avoidance behavior in a dose-dependent manner (10, 30 or 90 microgram per rat). Conversely, haloperidol induced a dose-dependent facilitation of extinction (0.03 or 0.1 microgram per rat). Open-field activity was not significantly affected by 30 microgram amphetamine or 0.03 microgram haloperidol; 90 microgram amphetamine significantly increased rearing activity and 0.1 microgram haloperidol decreased ambulation. The data show that passive and active avoidance behavior are sensitive measures to test the activity of psychomotor stimulant and neuroleptic drugs. Exploratory behavior allows more specific behavioral effects to be dissociated from locomotor influences.

Apomorphine-induced locomotor stimulation in developing rats treated with 6-hydroxydopa

Apomorphine-induced locomotor stimulation was investigated in the developing rat following injection with 6-hydroxydopa at birth. Treatment with 6-hydroxydopa potentiated locomotor responsiveness to apomorphine in the 20-day-old rat. The 6-hydroxydopa-treated animal at 30 days, however, was less sensitive to the drug than was the control. Apomorphine again elicited more locomotor stimulation in 6-hydroxydopa-treated animals than in the controls on day 50. These results suggest that the altered sensitivity of dopamine receptors induced with 6-hydroxydopa, is influenced by the onset of activity of other "inhibitory" neurons on day 30.

Dose-dependent biphasic alterations in the spontaneous activity of neurons in the rat neostriatum produced by d-amphetamine and methylphenidate

A dose-response analysis was performed on D-amphetamine- and methylphenidate-induced changes in neuronal activity in the neostriatum of immobilized, phenidate-induced changes in neuronal activity in the neostriatum of immobilized, locally anesthetized rats. Whereas a marked depression of firing rate characterized the response to intraperitoneal injections of 2.5 mg/kg D-amphetamine or 10 mg/kg methylphenidate, increasing the dose (5.0--7.5 mg/kg D-amphetamine and 20--25 mg/kg methylphenidate) shifted this response pattern to a prolonged increase in activity. Both stimulant-induced increases and decreases in neostriatal activity are reversed by 2.0 mg/kg haloperidol. In contrast to the response of neostriatal neurons, the firing rate in the substantia nigra pars compacta was inhibited by both stimulants, even at doses that increased the activity of neurons in the neostriatum. The possible mechanisms underlying these drug-induced changes in firing rate are discussed along with the behavioral implications of a stimulant-induced dose-dependent shift in neostriatal unit activity.

The rotating rodent: a two component system?

The hypothesis that the rotating rat requires both nigro-striatal and mesolimbic dopaminergic components for activity has been tested. 6-Hydroxydopamine lesions were induced unilaterally in either or both the ascending nigrostriatal and mesolimbic dopamine pathway. In one group of rats the nucleus accumbens was destroyed bilaterally with electrolesions. Circling behaviour was only recorded when there was both an imbalance in striatal dopamine concentrations concomitant with stimulation of the limbic dopamine regions. Lesions of the mesolimbic dopamine pathway resulted in changes in drug-induced motor activity but postural asymmetry or circling behaviour was not observed. The results support the hypothesis that the rotating rodent requires two functional dopamine components: striatal dopamine imbalance causing a postural asymmetry and stimulation of mesolimbic dopamine systems providing a locomotor component. The relevance of these components in this animal model is discussed with reference to dopaminergic agonist and antagonist drugs.

Mesolimbic dopamine neurons: effects of 6-hydroxydopamine-induced destruction and receptor blockade on drug-induced rotation of rats

Bilateral injections of 6-hydroxydopamine (6-OHDA) into the nucleus accumbens greatly reduced the dopamine content of this nucleus and the olfactory tubercle and blocked the ipsilateral rotation induced by amphetamine and methamphetamine in rats with unilateral 6-OHDA lesions of the caudate nucleus. In contrast, apomorphine-induced contralateral rotation was enhanced. Similar results were obtained when the destruction of forebrain noradrenergic neurons, normally produced by the nucleus accumbens 6-OHDA lesion, was prevented by desipramine (DMI) pretreatment. Microinjections of the dopamine receptor antagonist heloperidol into the nucleus accumbens did not spread to the olfactory tubercle, as assessed by the distribution of 3H-haloperidol, and blocked circling induced by amphetamine and apomorphine. Amphetamine-induced circling was less effectively blocked by haloperidol injected into the olfactory tubercle. These results suggest that activity at nucleus accumbens dopamine receptors can greatly affect circling behavior, perhaps by amplifying asymmetries of nigrostriatal activity.