Injection of apomorphine--a test to predict individual different dopaminergic sensitivity?

Male rats, treated with apomorphine (APO: 2mg/kg s.c.) in an Animex-Motility-Meter, showed individually different motility patterns, each expressed by oral stereotyped behaviour and enhanced locomotor activation reproducible in a second test 4 days later. One group of the rats showed stereotyped sniffing with an increased locomotor activation, S(L,G)-rats, being predominantly "mesolimbic active" rats. The other groups could be classified as mainly licking or gnawing rats, L(S,G)-rats or G(L,S)-rats, with less increase of locomotor activation, resembling predominantly "nigrostriatal active" rats. The G(L,S)-rats seemed to be mostly "nigrostriatal active". In this study the different types of rats were treated with neuroleptic drugs in presence of APO. Haloperidol (HAL; 0.2 and 0.4 mg/kg i.p.) had a clear dose dependent antagonizing effect on APO-induced stereotypies and locomotor activity: a pronounced effect on the L(S,G)- and G(L,S)-rats and less on S(L,G)-rats. In contrast, clozapine (CLO; 10 and 15 mg/kg i.p.) did not antagonize the stereotyped behaviour in the rats tested but showed a characteristic shift in the S(L,G)- and G(L,S)-rats: the predominant stereotypy of these rats, quantified by scoring, changed to licking. In the L(S,G)-rats the predominant licking stereotypy was not changed and the locomotor activity, which was completely antagonized in the S(L,G)- and G(L,S)-rats at both CLO-doses, was affected by 15 mg/kg, only. Furthermore, after the combined treatment with APO and HAL or APO and CLO these rat-types also differed in their amount of ACTH and corticosterone release. DA-1/DA2 and/or DA-1/DA3 receptor mechanisms may be involved in these individually different motility patterns and endocrine reactions. In summary, pretesting of rats with APO and measuring the motility and endocrine parameters may give us information on a preexisting different sensitivity of individuals to react to DAergic stimulation.