DIFFERENTIAL REGULATION OF PREPROTACHYKININ-A MESSENGER-RNA EXPRESSION IN STRIATUM BY EXCITATION OF HIPPOCAMPAL-NEURONS

In this report we have studied the influence of hippocampal neurons on neuropeptide mRNA expression in both dorsal and ventral striatum in t he rat. Intrahippocampal unilateral kainic acid injections were perfor med in control animals and in animals with a unilateral 6-hydroxydopam ine-induced dopamine deafferentation of the striatum. In situ hybridiz ation combined with quantitative image analysis was used to study the expression of preprotachykinin A mRNA encoding the neuropeptides subst ance P and neurokinin A. The 6-hydroxydopamine-induced lesion caused a decrease of preprotachykinin A mRNA levels in the ipsilateral dorsal striatum and in both sides of the ventral striatum. In normal rats, th e intrahippocampal kainic acid injection caused a twofold increase in preprotachykinin A mRNA in the limbic parts of the striatum, which are innervated by the hippocampus. No effect of the kainic acid injection was seen in the lateral parts of the dorsal striatum, a region which does not appear to be innvervated by the hippocampus. Animals with a 6 -hydroxydopamine lesion showed a similar kainic acid-mediated increase in preprotachykinin A mRNA in parts of the ventral striatum. In the d opamine-lesioned dorsal striatum and ventral striatum the decreased pr eprotachykinin A mRNA levels were normalized by the intrahippocampal k ainic acid injection. These results show that kainic acid-mediated exc itation of hippocampal neurons causes a dopamine-independent induction of preprotachykinin A mRNA expression in parts of the ventral striatu m, and reverses the dopamine deafferentation-induced decrease of prepr otachykinin A mRNA in both dorsal and ventral striatum. Combined, our results suggest that hippocampal neurons can regulate preprotachykinin A mRNA expression in both the ventral and the dorsal striatum.