Distribution of substance P and neurokinin-1 receptor immunoreactivity in the suprachiasmatic nuclei and intergeniculate leaflet of hamster, mouse, and rat

The circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) re ceives photic information directly via the retinohypothalamic tract (RHT) a nd indirectly from retinally innervated cells in the thalamic intergenicula te leaflet (IGL) that project to the SON. Using standard immunohistochemica l methods, we examined the presence and distribution of substance P (SP) an d the neurokinin-1 receptor (NK-1) in the SON and IGL of rat and determined whether the patterns of immunostaining generalized to the SCN and IGL of S yrian hamster, Siberian hamster, and mouse. Terminals immunoreactive for SP were sparse within the SON of Siberian and Syrian hamsters and mouse but w ere intense in the ventral, retinally innervated portion of the rat SON. Im munostaining for the NK-1 receptor was mainly absent from the SCN of hamste r and mouse. In contrast, a plexus of NK-1-ir cells and processes that was in close proximity to SP-it terminals was found in the ventral SON of the r at. Substance P-it terminals were observed in the IGL of all four species, as were NK-1-ir cells and fibres. Double-labelled IGL sections of hamster o r rat revealed SP-it terminals in close apposition to NK-1-immunostained ce lls and/or fibres. These data indicate that SP could be a neurotransmitter of the RHT in rat, but not in hamster or in mouse, and they highlight poten tial species differences in the role of SP within the SCN circadian pacemak er. Such species differences do not appear to exist at the level of the IGL , where SP-it and NK-1-ir were similar in all species studied.