Stimulus-elicited investigation in apomorphine-treated gerbils

Abstract

When the dopamine receptor stimulant, apomorphine hydrochloride (0.03–3.0 mg/kg, sc), was injected into Mongolian gerbils (Meriones unguiculatus), low doses produced changes in behavior that were qualitatively dissimilar to those produced by higher doses. The behaviors observed included one test of the duration and frequency of investigation of a novel object; a second test of the duration and frequency of investigation of five holes in the floor (one of which concealed an odor source); and the rate of crossing lines of a grid on which motion pictures of the behavior were projected to provide a measure of locomotor activity. After the highest dose of apomorphine (3.0 mg/kg) investigatory and activity measures were severely decreased. After 1.0 or 2.0 mg/kg doses of this putative dopamine agonist, investigation of the holes, as well as the duration, but not the frequency, of investigation of a novel object, were decreased. The stability of the latter frequency measure indicates that this drug does not disrupt all attention directed towards novel stimuli. However, a deficit in selective attention was indicated by the lack of inhibition of responding on subsequent trials. A lack of selectivity of responding to novel stimuli was also supported by the observation that gerbils receiving a range of doses of apomorphine did not consistently investigate the odor hole more than the other neutral holes. Gerbils receiving a moderate dose (0.3 mg/kg) of apomorphine were impaired only on some of the measures and not on others. The selective effects of 0.3 mg/kg apomorphine were evaluated further in another experiment and, in addition, compared in gerbils pretreated with the potent and selective dopamine blocker, pimozide. Again, there was no impairment by apomorphine of the duration of investigation of an object, but there was a depressant effect on frequency of object investigation, duration and frequency of investigation of the holes, and of locomotor activity. Pimozide itself also had a slight, nonsignificant depressant effect that was not additive with the effects of apomorphine. When both drugs were given, pimozide blocked the apomorphine effects. Thus, a moderate dose of apomorphine can depress investigatory behavior as well as locomotor activity (as has been reported previously), consistent with the earlier proposal that low dose effects of apomorphine that differ qualitatively from higher dose effects may be due to the stimulation of hypothetical “presynaptic” dopamine receptors.