Neuroscience Letters, 51 (1984) 171-176 171 Elsevier Scientific Publishers Ireland Ltd. NSL 02977 SUPRAMAMMILLARY AFFERENTS TO GUINEA PIG HIPPOCAMPUS CONTAIN SUBSTANCE P-LIKE IMMUNOREACTIVITY CHRISTINE GALL and LYNN SELAWSKI Department of Anatomy, University of California - Irvine, Irvine, CA 92717 (U.S.A.) (Received June 1st, 1984; Revised version received July 24th, 1984; Accepted July 26th, 1984) Key words: hippocampus - supramammillary region - substance P - hypothalamus - neuropeptides The origin of substance P immunoreactive (SPI) axons in guinea pig hippocampus was analyzed using immunocytochemical techniques combined with transections and retrograde transport of fluorescent dye. A unilateral depletion of hippocampal axonal SPI was observed following ipsilateral transection of rostral hippocampus and fibria suggesting that the vast majority of SPI axons in hippocampus are extrin- sic afferents which enter the structure from the septal pole. The combined use of immunocytochemistry and fluorescent dye transport demonstrated the supramammillary region of the hypothalamus to be the only area where dye-labeled hippocampal afferent neurons also exhibited SPI. These data indicate that the supramammillary region is the principal source of SPI axons in guinea pig hippocampus and, most probably, in the hippocampus of other animals (squirrel, cat, monkey) sharing a similar pattern of ax- onal SPI. Recent work has demonstrated dramatic interspecies differences in the distribu- tion of hippocampal substance P-like immunoreactivity (SPI). While virtually no immunoreactive elements are observed in the hippocampus of the rat or mouse [1, 6] a very complex pattern of perikaryal and axonal immunoreactivity is observed in the hippocampus of guinea pig, squirrel, cat and monkey [4, 5, 10] (Fig. 1). In all of these latter animals the most striking feature of the immunoreactive labeling pat- tern is a plexus of immunoreactive axons which courses within the supragranular and proximal molecular layer of the dentate gyrus. It is not clear from the distribu- tion of these immunoreactive axons alone whether they arise from an extrinsic source such as the medial septal nucleus [2, 7] or the supramammillary region of the hypothalamus [11, 12], or whether they arise from intrinsic hippocampal neurons, such as the SPI neurons of the hilar region [4]. The present study was con- ducted to determine the origin of these substance P-immunoreactive afferents to the dentate gyrus granule cells in the guinea pig. Adult (300-500 g) male guinea pigs (Hartley strain) were used. In 3 animals a unilateral transection was made through the rostral hippocampus and fimbria 7-14 days before sacrifice. The remaining animals received multiple intrahippocampal in- jections of either Fast Blue or Propidium Iodide fluorescent dye. Each injected hip- pocampus received three separate injections of either 0.15/~1 of 7°7o Fast Blue (FB) 0304-3940/84/$ 03.00 © 1984 Elsevier Scientific Publishers Ireland Ltd.