Circling behaviour in the rat following unilateral injections of p-chlorophenylalanine and ethanolamine-O-sulphate into the substantia nigra.

Circling behaviour following the unilateral intranigral injection of the gamma-aminobutyric acid (GABA) transaminase inhibitor ethanolamine-O-sulphate, (EOS), or the tryptophan hydroxylase inhibitor p-chlorophenylalanine (pCPA) was compared, both for spontaneous circling and for circling induced by apomorphine administration. EOS-injected rats exhibited spontaneous contralateral rotation for up to 3 h after such injections. pCPA-injected rats showed only a slight ipsilateral postural asymmetry, both groups of rats elicited ipsilateral rotation upon apomorphine challenge. Increased ipsilateral dopamine release within the corpus striatrum accompanied this response. It is proposed that GABA and 5-HT may be closely related in modifying the nigrostriatal impulse flow.