Effects of amphetamine and apomorphine on locomotor activity after 6-OHDA and electrolytic lesions of the nucleus accumbens septi

Locomotor activity and stereotypy in rats following repeated apomorphine treatments at 1-, 3-, or 7-day intervals

In two experiments, the effects of repeated intermittent administration of a relatively high dose of apomorphine (5 mg/kg) on locomotor activity and/or stereotypic behavior in rats was determined. In Experiment 1, male rats were given ten subcutaneous (SC) injections of apomorphine or vehicle and tested for locomotor activity and stereotypy. The first nine injection test sessions were given at 3-day intervals and the tenth injection test session was given 18 days following the ninth session. In Experiment 2, male rats were tested for locomotor activity following ten SC injections of apomorphine or vehicle with either a one- or seven-day interval between injections. Major findings were as follows: a) apomorphine produced progressively greater increases in locomotor activity with each succeeding injection (i.e., sensitization); b) sensitization to the locomotor activity stimulating effects of apomorphine developed with interinjection intervals of one, three, and seven days; c) the sensitization effect was maintained over the 18-day drug-free break; and d) the effect of apomorphine on stereotypic behavior did not significantly change with repeated injections. These findings indicate that even a single dose of apomorphine induces relatively long-lasting neurobiological changes. Moreover, these findings are consistent with the view that separate neural pathways mediate locomotor activity and stereotypy in rats.

Disruption of latent inhibition by acute administration of low doses of amphetamine

In the latent inhibition (LI) paradigm, nonreinforced preexposure to a stimulus retards subsequent conditioning to that stimulus. Three experiments investigated the effects of acute amphetamine administration on LI in rats. Experiments 1 and 3 used a conditioned emotional response (CER) procedure and Experiment 2 used two-way active avoidance procedure. Experiments 1 and 2 showed that, in both the CER and avoidance procedures, 1.5 mg/kg dl-amphetamine administered either in the preexposure or the conditioning stage alone did not disrupt LI. In contrast, amphetamine administered in both of the stages abolished LI. Experiment 3 showed that the abolition of LI was obtained when the preexposure and conditioning were given 24 hr apart but not when the two stages were given in one session.

Kainic acid lesions of the nucleus accumbens selectively attenuate morphine self-administration

The influence of kainic acid lesions of intrinsic and efferent neurons of the central medial nucleus accumbens on responding simultaneously maintained by food, water and morphine self-administration was assessed. Rats were trained on a multioperant baseline to respond on three different levers that resulted in either a food pellet, the presentation of a water dipper or an infusion of morphine. While responding on the morphine lever was related to dose (0.83-13.2 mg/infusion), increasing concentrations of the drug had little or no effect on responding maintained by food and water before the lesion. Bilateral infusions of the neurotoxin into the nucleus accumbens decreased morphine self-administration but did not appreciably alter food or water intake. Food extinction probes before the lesion produced significant increases in drug intake and decreases in responding on the water lever, but the neurotoxin lesion attenuated the food extinction induced decrease in water intake. These data suggested that kainic acid lesions of the nucleus accumbens decrease the reinforcing efficacy of morphine but do not alter the reinforcing properties of food and water. The neuronal systems potentially involved in mediating the reinforcing effects of environmental events are discussed.

Anatomical analysis of the involvement of mesolimbocortical dopamine in the locomotor stimulant actions of d-amphetamine and apomorphine

Lesion studies employing 6-hydroxydopamine (6-OHDA) suggest that locomotor hyperactivity induced by certain stimulant drugs is dependent on dopaminergic neurotransmission in the nucleus accumbens (NACC). However, studies to date have not adequately controlled for the reported effects of 6-OHDA on baseline (non-drug) activity and on DA levels in other terminal regions. Slow bilateral infusions of 6-OHDA into the NACC, but not into olfactory tubercle (OT) or medial prefrontal cortex (mPFCx), reduced d-amphetamine (0.5 mg/kg SC) hyperactivity and resulted in a "supersensitive" (hyperactive) response to a low dose of apomorphine (0.1 mg/kg SC) in photocell cages. Direct observation revealed no behavioral changes in OT lesioned rats challenged with apomorphine which might correspond to a "denervation supersensitivity" syndrome. Assays of DA and 5-hydroxytryptamine (5-HT) in mPFCx, OT, NACC, and caudate-putamen revealed that 6-OHDA infusion into NACC caused substantial DA loss in NACC, OT and mPFCx, whereas infusion at mPFCx or OT sites depleted DA locally (greater than 85% loss) with little or no remote change. Concentrations of 5-HT were little altered by 6-OHDA, except for a local depletion in mPFCx. The present results confirm the importance of nucleus accumbens DA in the expression of locomotor stimulation induced by apomorphine and d-amphetamine, and suggest that the mPFCx and OT do not make an important contribution.

Role of mesencephalic glycine in locomotor activity

Microinjection of glycine (20 micrograms) into the ventral mesencephalon of rats caused a stimulation of locomotor activity. Microinjection of haloperidol (5 micrograms) into the nucleus accumbens immediately before the glycine injection did not reduce this locomotor stimulation. In rats with 6-hydroxydopamine-induced destruction of mesolimbic dopamine (DA) terminals the ability of apomorphine (0.1 mg/kg, s.c.) to stimulate locomotor activity was reduced by microinjection of the glycine antagonist strychnine into the ventral mesencephalon. Increased glycinergic activity in the ventral mesencephalon therefore appears to stimulate locomotor activity by a mechanism other than the activation of mesolimbic DA neurons.

Adenylate cyclase activity and motor behavior following cerebral ischemia in the unanesthetized gerbil

Five minutes of bilateral carotid occlusion in unanesthetized gerbils produced substantial changes in spontaneous locomotor activity. Behavior was decreased after 1 hr of reperfusion and was increased at 24 hrs post-ischemia. Adenylate cyclase activity was measured in homogenates of frontal cortex and hippocampus at 90 min and 24 hrs following 5 min of cerebral ischemia. Enzyme activity was determined in the absence and presence of the activators guanosine-5'-triphosphate (GTP), guanylyl-5'-imidodiphosphate (GppNHp), isoproterenol (Iso) plus GTP, and forskolin (Fors) plus GTP. Homogenates responded with expected increases over basal adenylate cyclase activity with addition of all activators. An additional small increase in isoproterenol-stimulated activity was observed in frontal cortex homogenates at 90 min post-ischemia. No other significant changes in adenylate cyclase activity were observed after either 90 min or 24 hrs of reperfusion. The substantial increases in locomotor activity evident at 24 hrs after transient ischemia are not associated with measurable changes in adenylate cyclase activity in homogenates of frontal cortex or hippocampus.

Asymmetrical effect of unilateral cortical lesions and amphetamine on DRL-20: a time-loss analysis

Male Sprague Dawley rats received unilateral 1.5 mm diameter focal suction lesions of either the left or right orbito-frontal cortex and were tested for response to 6 doses of amphetamine sulfate ranging from 0.5 to 1.5 mg/kg on a DRL-20 schedule of reinforcement. Right hemispheric lesion animals obtained a greater number of control reinforcements and were more sensitive to amphetamine's disruptive effects, showing a greater dose-related decrease in water rewards obtained than left lesion animals. An analysis method is introduced which combines the interactive effects of premature responses and their IRT value in a way that relates directly to reinforcement attainment. Calculated total session time made unavailable for reinforcement due to premature responding, correlated negatively (r = -.942) with the number of reinforcements obtained.

Nigral dopaminergic neurons: differential sensitivity to apomorphine following long-term treatment with low and high doses of amphetamine

Twice daily injections of 1.0 or 5.0 mg/kg D-amphetamine for 6 consecutive days differentially affected the response of dopaminergic neurons in the substantia nigra pars compacta to challenge injections of apomorphine on the following day. Thus, whereas treatment with the high amphetamine dose produced a dramatic shift to the right of the apomorphine dose-response curve, rats treated with 1.0 mg/kg D-amphetamine responded to apomorphine in the same way as saline-treated controls. These results support the view that long-term treatment with relatively high amphetamine doses is required to produce autoreceptor subsensitivity in the substantia nigra.

Inhibition and facilitation of motor responding of the mouse by actions of dopamine agonists in the forebrain

The actions of dopamine and agonists of dopamine to influence forebrain structures and modify spontaneous locomotion of the mouse were studied, firstly, by injecting agents from different chemical series into the nucleus accumbens (phenylethylamine, aporphine, benzo[g]quinoline, tetralin, dopamine, N-n-propyl-N-phenethyldopamine, N-n-propyl-N-butyldopamine, apomorphine, trans-N-n-propyl-6,7- and -7,8-dihydroxyoctahydrobenzo[g]quinoline, 2-di-n-propylamino-5,6- and -6,7-dihydroxytetralin, 2-di-ethylamino-5,6-dihydroxytetralin) and, secondly, by selecting a potent agent (2-di-n-propylamino-5,6-dihydroxytetralin) for injection into 43 other forebrain areas. Agents from all chemical series were shown to reduce locomotor activity on injection into the nucleus accumbens. The most effective agents were the dialkylated tetralin derivatives and the N-propyl benzo[g]quinoline compound (0.025-0.5 micrograms); inhibition of motor activity generally decreased as dose was increased. The inhibitory effects on motor activity of the propyl substituted tetralin and [g]quinoline were specifically antagonised by sulpiride and/or spiperone (prazosin, yohimbine and methysergide were ineffective). Injections of tetralin (0.1 micrograms) not only into the nucleus accumbens but also into the tuberculum olfactorium, septal nucleus, anterior olfactory nucleus, anteromedial fibre system, claustrum and caudate-putamen could effect inhibition of motor activity. Generally, injections away from these structures were ineffective. It is suggested that small doses of dopamine and agonists of dopamine can influence dopamine receptors which are sensitive to neuroleptic drugs (presynaptic in limbic and striatal regions, but with a possibility of postsynaptic involvement in cortical regions) to effect inhibition of motor activity from a number of discrete areas of the forebrain of the mouse.

Dopamine-rich transplants in rats with 6-OHDA lesions of the ventral tegmental area. I. Effects on spontaneous and drug-induced locomotor activity

In order to investigate the relative contribution of dopaminergic projections to the nucleus accumbens and prefrontal cortex in the regulation of spontaneous and drug-induced locomotor activity, separate groups of rats were prepared with 6-OHDA lesions of the ventral tegmental area alone, or additional grafts of dopamine-rich tissue reinnervating either the nucleus accumbens or medial prefrontal cortex. A fourth unoperated group served as normal controls. The lesions induced no change in spontaneous, daytime activity, but increased overnight activity. The lesioned rats were also hyperactive to apomorphine, while the activational effects of amphetamine were blocked. Grafts of dopamine-rich tissue, whether into the prefrontal cortex or nucleus accumbens, resulted in a significant normalization of both drug responses towards control levels. Neither graft influenced overnight hyperactivity, whereas spontaneous daytime activity was increased above both control and lesion levels by the accumbens grafts alone. The results are interpreted as suggesting that dopaminergic projections to prefrontal cortex and nucleus accumbens are similarly rather than antagonistically involved in the regulation of drug-induced locomotor activation.

Dopamine denervation of frontal cortex or nucleus accumbens does not affect ACTH-induced grooming behaviour

Studies with dopamine (DA) receptor antagonists have implicated brain DA systems in the increased grooming behaviour elicited by intracerebroventricular (i.c.v.) administration of ACTH or exposure to a novel environment. To evaluate the potential contributions of DA terminals innervating frontal cortex and nucleus accumbens, stereotaxically guided injections of 6-hydroxydopamine (6-OHDA) were made into these regions of rat brain following peripheral administration of desmethylimipramine and pargyline. Despite an 88% mean reduction of DA, and 68% mean reduction of noradrenaline in frontal cortex, the amount of grooming behaviour observed either in a novel cage, or after i.c.v. injection of ACTH1-24 was not detectably altered. The lesions also did not affect locomotor activity measured during the scoring of grooming, in an open field, in photocell cages, or in a running wheel, even in the animals most severely depleted of DA. Following nucleus accumbens 6-OHDA infusions, depletions of DA as great as 99% were obtained. Animals with accumbens depletions greater than 90% showed the expected attenuation of amphetamine-stimulated locomotor activity. Nevertheless, they exhibited normal grooming scores in a novel cage, and in response to i.c.v. ACTH1-24. It is concluded that dopaminergic terminals in nucleus accumbens and probably those in frontal cortex are not necessary for the expression of novelty- or ACTH-induced grooming.

Role of nigral dopamine in amphetamine-induced locomotor activity

Dopamine (100 micrograms) injected into the substantia nigra pars reticulata of rats pretreated with the monoamine oxidase inhibitor, pargyline, resulted in a stimulation of locomotor activity. Bilateral injection of the dopamine antagonist haloperidol (5 micrograms) into the substantia nigra pars reticulata resulted in a reduction of the locomotor activity evoked by a low dose of amphetamine (1.25 mg/kg s.c.). These results suggest that the release of dopamine from nigral dendrites is involved in amphetamine-induced locomotor activity.