Differential responses in central dopaminergic activity induced by apomorphine in IPL nude rat

The IPL nude rat, derived by spontaneous mutation from the Sprague-Dawley strain, presents alterations in the prolactin synthesis and secretion due to an increased dopaminergic inhibition. However, there are no reports concerned to central dopamine activity. The corpus striatum is a brain area involved in the development of stereotyped behavior after the activation of mesolimbic and/or nigro-striatal dopamine pathways. In order to identify possible mesolimbic and/or nigro-striatal dysfunctions in the IPL nude rat, we study the spontaneous oral behaviors and the effects of apomorphine-induced dopaminergic activation on stereotyped behavior and neurochemical changes. Males from both strains were injected with saline or apomorphine (2 and 5 mg/kgs.c.) and evaluated during 30 min in a stereotypes oral tests. The corpus striatum and nucleus accumbens were used to measure dopamine (DA), 3,4-dihydroxyphenylalanine (DOPA) and 3,4-dihydroxyphenylacetic acid (DOPAC) by HPLC. The concentrations were expressed as synthesis rate (DA/DOPA) and turnover rate (DOPAC/DA). We observed that the spontaneous gnaw movements were significantly different between the untreated IPL nude and Sprague-Dawley (SD) rats. Apomophine injection decreased the amount of stereotyped gnawing in IPL nude rats at the two doses used, but it induced an increase in SD rats. Apomorphine also caused an enhancement in the number of biting and sniffing without modifying the licking behavior. In addition, modifications of the dopaminergic activity were also observed. Synthesis rate in the striatum of IPL nude rats was higher than in SD rats after the injection of saline. Apomorphine caused a reduction of the synthesis rate in both strains. Turnover rate was significantly lower in the striatum of IPL nude rats than in the SD rats injected with saline. Apomorphine caused an increase in the turnover rate in both strains. Contrary to observed in the striatum, the 2 mg/kg dose of apomorphine caused a significant increase in the DA synthesis rate in nucleus accumbens, while 5 mg/kg decrease it in both strains. The DA turnover rate in the same area was lower in IPL nude than in SD rats after saline injection. Apomorphine enhances the DA turnover rate in both strains. We conclude that the modifications of the oral spontaneous and induced stereotypical patterns observed in the IPL nude rats could be related to the differential responses in dopaminergic activity in the two brain areas examined.