SELF-ADMINISTRATION OF NEUROKININ SUBSTANCE-P INTO THE VENTROMEDIAL CAUDATE-PUTAMEN IN RATS

There is evidence that the neurokinin substance P plays a role in lear ning and reinforcement processes. Reinforcing effects of substance P w ere found upon injection into several parts of the brain. The aim of t he present study was to gauge possible reinforcing effects of microinj ections of substance P into the ventromedial caudate-putamen in rats. Two different behavioral paradigms were employed. In the first experim ent a two-compartment choice procedure was used and the rats could tri gger substance P injections (500 pg per 5 nl injection volume) into th e ventromedial caudate-putamen by entering one distinctive compartment . During the injection period, substance P-injected animals spent sign ificantly more time in the drug-paired compartment than vehicle-inject ed controls. In the second experiment, nose-poking through a hole in o ne wall of the cage was used as the operant. Rats that could self-admi nister substance P (100 pg per 5 nl injection volume) into the ventrom edial caudate-putamen emitted a significantly higher rate of operant r esponding on the first day of testing and a significantly lower rate o n the third day compared to vehicle-injected animals. The experiments provide evidence that the administration of substance P into the ventr omedial part of the caudate-putamen can have positive reinforcing effe cts, but that repeated injections can have aversive properties. These effects are discussed, firstly, with regard to the possible mechanisms of intrastriatal substance P on striatonigral and striatopallidal out put systems and, secondly, with respect to their possible relevance in the study of the basal forebrain reinforcement system.