Facilitation of learning by post-trial injection of substance P into the medial septal nucleus

The undeca-neuropeptide substance P (SP) was injected post-trial into the medial nucleus of the septum to test its influence on passive avoidance learning in rats. A facilitation of learning was attained in the alcove avoidance task with doses of 50 ng (37 pmol), 500 ng (370 pmol) and 6.7 microgram (5 nmol) SP and in the step-up avoidance task with 500 ng and 6.7 microgram SP. In the step-down avoidance task no influence of SP was discernible, probably as a consequence of the high retention shown by control animals, masking a possible SP-induced facilitation of learning (ceiling effect). Interestingly, in its effect on learning, SP applied in the septum imitates the effect of electrical septal stimulation, thus providing another example for the parallelism between post-trial SP and electrical brain stimulation which we have demonstrated previously in the substantia nigra, amygdala and lateral hypothalamus.