A comparison of the behavioural consequences of chronic stimulation of dopamine receptors in the nucleus accumbens of rat brain effected by a continuous infusion or by single daily injections

Abstract

Mesolimbic dopamine (DA) receptors of the rat were subject to a chronic (13 day) stimulation effected either by single, repeated daily injections or by infusion from osmotic minipumps of 1.56–50 μg DA bilaterally into the nucleus accumbens. Rats were preselected according to their responsiveness to the hyperactivity inducing effect of the DA agonist (−)N-n-propylnorapomorphine [(−)NPA], and the two groups categorised as ‘high’ and ‘low’ activity responders were used in the present studies. Whilst the stimulation of mesolimbic DA receptors by a single acute injection or by a 24 h infusion was ineffective to modify locomotor responding, the repetition of the daily injections or the continuation of the infusion caused dose-related increases in locomotor activity of both groups of rats. The spectrum of enhanced activity responding depended on the mechanism of stimulation, a ‘biphasic’ or ‘monophasic’ pattern resulting from the infusion or daily injections respectively. It was an important observation that a repetitive biting behaviour developed concomitant to the hyperactivity during daily DA injections, a response not observed during infusion, indicating that the nature of the receptor stimulation, continuous or pulsatile, can dictate the development of a motor response. The effects of discontinuing the chronic intra-accumbens stimulation on responsiveness to (−)NPA was the same whether the chronic stimulation was achieved by repeated injection or infusion. However, the consequences of DA withdrawal critically depended on the initial classification of rat activity according to (−)NPA. Thus, initially ‘high’ response animals reversed to ‘high’ responsiveness. Such data may indicate that mesolimbic DA can act to regulate motor performance rather than simply exerting a facilitatory role.