Presence of NK3-sensitive neurones in different proportions in the medial habenula of guinea-pig, rat and gerbil.

Electrophysiological recordings were made from neurones of the medial habenula (Mhb) in brain slices obtained from guinea-pig, rat and gerbil brain. The selective NK3 agonist, senktide, was used to determine the relative number of NK3-sensitive-neurones in the Mhb of each species. The proportion of neurones responding to NK1 (Sar9Met(O2)11SP) and NK2 (beta Ala8NKA(4-10) agonists was also assessed. All (65/65) of the guinea-pig Mhb neurones tested were excited by the NK3 agonist, but NK1 and NK2 agonists were without effect. NK3 responses in the guinea-pig were not altered by the presence of a selective NK1 antagonist, CP-99,994. NK1, NK2 and NK3 agonists were without effect on Mhb neurones from gerbil brain slices. In agreement with findings from previous studies, a population of rat Mhb neurones responded to NK1 or NK3 agonists alone or were excited by both. These data show that there is a difference in both the number of NK-sensitive neurones and the type of NK response found in the medial habenula of the three species. The high sensitivity to an NK3 agonist, combined with the apparent lack of NK1 and NK2 responses in the guinea-pig Mhb makes this preparation ideal for studies of central NK3-mediated events.