Presynaptic modulation of dopamine-induced locomotor activity of oxotremorine in nucleus accumbens of the rat

Intra-medial prefrontal cortex injections of scopolamine increase instrumental responses for cocaine: an intravenous self-administration study in rats

The present experiments examined the effects of muscarinic cholinergic receptor blockade in the nucleus accumbens (NAC) and medial prefrontal cortex (MPC) on intravenous cocaine self-administration. Adult male Sprague-Dawley rats were implanted with chronic indwelling jugular catheters and guide cannulae stereotaxically aimed at the NAC or MPC. The rats were then given the opportunity to intravenously self-administer cocaine (0.8 mg/kg/infusion) during daily 2-h sessions. Intra-NAC microinjections of methyl-scopolamine (2, 4, 8, 16, and 32 microg/side) or vehicle did not affect either the number of lever presses made or infusions delivered. On the other hand, intra-MPC injections of scopolamine significantly increased responding, although there was only a trend for an increase in the number of cocaine infusions. The effects of intra-MPC injections of scopolamine (8 and 16 microg/side) on locomotor activity were also evaluated. Intra-MPC injections of scopolamine (16 microg/side) produced significant increases in locomotor activity. However, these same microinjections decreased locomotor activity when the animals also received cocaine (15 mg/kg, i.p.). These results suggest that cholinergic neurotransmission at muscarinic receptors in the MPC is involved in regulating cocaine-maintained responding.

Rats self-administer carbachol directly into the nucleus accumbens

The potential reinforcing effect of the muscarinic cholinergic agonist carbachol within the nucleus accumbens (ACB) was examined in female Wistar rats by using the technique of intracranial self-administration. Rats dose dependently self-administered solutions of 0.0-6.6 mM (in a volume of 100 nL per injection) directly into the ACB. Rats self-administered the 3.3 and 6.6 mM doses significantly more than the group given only vehicle. The caudate putamen did not support reliable self-administration of the 6.6-mM dose. Rats exhibited preference for the lever that produced infusions of 3.3 and 6.6 mM carbachol into the ACB over the lever that had no consequence. The self-infusion of the 6.6-mM dose into the ACB was inhibited by the coadministration of the muscarinic antagonist scopolamine (0.25 mM), but not by the nicotinic antagonist mecamylamine (6.6 mM). The present results suggest that direct activation of muscarinic receptors within the ACB supports self-administration and could result from reinforcement or from elicitation of a novel stimulus.

The effect of cholinergic stimulation in the nucleus accumbens on locomotor behavior

The present study was designed to investigate the effect of cholinergic stimulation of the nucleus accumbens (NA) on locomotor behavior. Microinjection of the cholinergic agonist, carbachol, into the NA resulted in a dose-dependent increase in spontaneous locomotor activity. Pretreating rats with the nicotinic antagonist, mecamylamine, significantly reduced the carbachol-induced locomotor response, whereas pretreatment with the muscarinic antagonist, atropine, was ineffective. Injection of the dopamine antagonist, haloperidol, prior to stimulation of the NA with carbachol, significantly attenuated the motor stimulant effect of carbachol. Evidence has accumulated to suggest that the increased locomotor response elicited from NA stimulation is mediated by GABAergic efferents from the NA to the substantia innominata (SI). Pretreatment with the GABA agonist, muscimol, into the SI significantly antagonized the locomotor hyperactivity induced by carbachol injection into the NA. Taken together, these data indicate that carbachol produces a motor stimulant effect by activating nicotinic receptors which may modulate the release of dopamine in the NA and inhibit the GABAergic efferents to the SI.

Stimulatory effect of N-methyl aspartate on locomotor activity and transmitter release from rat nucleus accumbens

N-Methyl-aspartate (NMA), an agonist at central glutamate receptors, elicited prolonged and intense locomotor activity when injected into the nucleus accumbens septi (NAS) in subconvulsive doses (3-10 micrograms bilaterally). This effect was antagonised by intra-accumbens injection of the specific NMA antagonist, aminophosphonovaleric acid (APV) in a dose (3.0 micrograms bilaterally) that was without intrinsic effect when given on its own. Intra-accumbens injection of APV also suppressed locomotor hyperactivity elicited by intra-accumbens injection of DA (50 micrograms bilaterally) in rats pretreated with nialamide. In vitro release of [3H]-acetylcholine in accumbens tissue slices was significantly increased in the presence of NMA (30 microM) or N-methyl-D-aspartate (NMDA) (15 microM). Both effects were antagonised by APV (30 microM). Similar results were obtained with tissue slices of rat corpus striatum. These results suggest that locomotor stimulation by intra-accumbens NMA is mediated by an action on the mesolimbic dopaminergic neuron, either directly or via a cholinergic interneuron. In addition, activity at the glutamate synapse may be enhanced by the presence of DA affecting glutamate release and/or reuptake.

Drug-induced modulation of locomotor hyperactivity induced by picrotoxin in nucleus accumbens

Locomotor hyperactivity was induced in rats by bilateral injection of picrotoxin (PIC) into the nucleus accumbens (NAC) followed by intraperitoneal (IP) or intra-accumbens (IA) injection of agents affecting dopamine (DA), acetylcholine, serotonin, or GABA receptors. IP injection of haloperidol and diazepam attenuated PIC-induced hypermotility in a dose-dependent manner. Low (sedative) doses of the DA agonists apomorphine (APO) and lisuride, or pretreatment with reserpine abolished PIC-induced hypermotility. Independent of a preceding IA injection of PIC, higher IP doses of APO produced the well-known locomotor effect. LSD, and the atypical neuroleptic, sulpiride, potentiated PIC-induced hypermotility strongly whereas clozapine was ineffective. IA injection of carbachol or haloperidol, in doses which antagonized hypermotility induced by APO IP, did not influence PIC-induced hypermotility. The atypical neuroleptics, clozapine and sulpiride, and the benzodiazepine, diazepam, inhibited PIC-induced hypermotility. The results suggest that there is a complex involvement of GABA, DA and serotonin functions in the effectuation of PIC-induced hypermotility and that PIC-induced hypermotility may be affected by DA-sensitive structures situated outside the NAC.

The cholecystokinin analogue, caerulein, does not modulate dopamine release or dopamine-induced locomotor activity in the nucleus accumbens of rat

We have investigated the effect of the cholecystokinin (CCK) analogue, caerulein, in the nucleus accumbens on dopamine (DA) release and locomotor activity. Caerulein (10(-9)-10(-4) M) did not affect the resting or K+-evoked release of [14C]DA, although concentrations within this range have been shown to modulate transmitter release in cerebral cortex. When injected bilaterally into the nucleus accumbens, caerulein (30 ng-1 microgram) did not affect spontaneous locomotor activity as compared to vehicle-injected controls. Similarly, bilateral injections of caerulein (1 microgram) into the nucleus accumbens did not modulate locomotor hyperactivity induced by bilateral injections of DA (20 microgram) into the nucleus accumbens. It is concluded that CCK in the nucleus accumbens has no direct effect on dopamine release or dopamine-stimulated locomotor activity.

Locomotor hyperactivity caused by dopamine infusion into the nucleus accumbens of rat brain: specificity of action

Rats selected as high-activity and low-activity responders to the hyperactivity-inducing action of peripherally administered (-)N-n-propylnorapomorphine [(-)NPA] were subject to intra-accumbens infusion of dopamine, noradrenaline, serotonin, acetylcholine and GABA (0.48 microliters/h, 25 micrograms/24 h, 13 days). Locomotor activity was measured during infusion and for a minimum of 35 days thereafter. After discontinuation of infusion the animals' responsiveness to (-)NPA was also assessed and, on the 2nd day of withdrawal, sensitivity to the hyperactivity-inducing action of acute intra-accumbens dopamine was determined. Dopamine caused a biphasic pattern of hyperactivity during infusion with peaks of responding between days 2-5 and 8-12: normal values returned after withdrawal of infusion. However, 2-3 weeks after withdrawal of intra-accumbens dopamine infusion animals showed reversed responding to (-)NPA challenge, the initial low-active animals giving a high-active response and high-active animals giving low-activity. Infusions of noradrenaline, serotonin, GABA and acetylcholine produced some increase in locomotor activity towards the termination of infusion, but no treatment could replicate the first hyperactivity peak and no treatment, after withdrawal, could reverse the responsiveness to (-)NPA of high- and low-active animals. Acute injections of dopamine into the nucleus accumbens showed that the infusion of the different neurotransmitter substances caused change within that nucleus. Nevertheless, changes in locomotor behaviour following the infusion of dopamine into the nucleus accumbens are specific for dopamine.