Sequence-specific effects of neurokinin substance P on memory, reinforcement, and brain dopamine activity

Abstract

There is ample evidence that the neurokinin substance P (SP) can have neurotrophic as well as memory-promoting effects. This paper outlines a recent series of experiments dealing with the effects of SP and its N- and C-terminal fragments on memory, reinforcement, and brain monoamine metabolism. It was shown that SP, when applied peripherally (IP), promotes memory (inhibitory avoidance learning) and is reinforcing (place preference task) at the same dose of 37 nmol/kg. Most important, however, is the finding that these effects seemed to be encoded by different SP sequences, since the N-terminal SP1-7 (185 nmol/kg) enhanced memory, whereas C-terminal hepta- and hexapeptide sequences of SP proved to be reinforcing in a dose equimolar to SP. These differential behavioral effects were paralleled by selective and site-specific changes in dopamine (DA) activity, as both SP and its C-, but not N-terminus, increased extracellular DA in the nucleus accumbens (NAc), but not in the neostriatum. The neurochemical changes lasted at least 2 h after injection. These results show that the reinforcing action of peripheral administered SP may be mediated by its C-terminal sequence, and that this effect could be related to DA activity in the NAc. Direct application of SP (0.74 pmol) into the region of the nucleus basalis magnocellularis (NBM) was also memory-promoting and reinforcing, and again, these effects were differentially produced by the N-terminus and C-terminus, supporting the proposed structure-activity relationship for SP's effects on memory and reinfrocement. These results may provide a hypothetical link between the memory-modulating and reinforcing effects of SP and the impairment in associative functioning accompanying certain neurodegenerative processes.