COMPARISON OF THE BEHAVIORAL-EFFECTS OF INFUSION OF CARBACHOL AND ACETYLCHOLINESTERASE INTO THE RAT SUBSTANTIA-NIGRA

It has been postulated for many years that acetylcholinesterase (AChE) may play a nonclassical role in the substantia nigra, unrelated to it s ability to hydrolyse acetyicholine. In this study the behavioural ef fects of unilateral infusion of AChE and a cholinergic agonist, carbac hol, were compared. Carbachol induced ipsiversive circling over a very short time scale (minutes), whereas AChE induced contraversive circli ng, but over a longer time course--l0 days. Both agents showed selecti vity of response within the substantia nigra: acetylcholinesterase was only effective when infused into the most rostral region of the subst antia nigra and its effects were limited to the pars compacta. In cont rast, carbachol had effects in both the pars compacta and reticulata, with a graded sensitivity to carbachol in the rostralicaudal plane: in fusions into rostral regions induced high rates of circling compared t o more caudal areas, suggesting that the cfiolinergic input to the sub stantia nigra is not homogenous, but greater in rostral regions. This disparity between the effects of carbachol and AChE would, therefore, suggest that AChE is not exerting its long-term behavioural actions vi a a cholinergic mechanism, both in terms of time course of the respons e and the areas within the substantia nigra sensitive to these agents.