Absence of vasopressin expression by galanin neurons in the golden hamster: implications for species differences in extrahypothalamic vasopressin pathways

In golden hamsters, there is a complete absence of the small diameter vasop ressin (VP) neurons in the bed nucleus of the stria terminalis (BST) and me dial amygdala (Me) which have been shown to exhibit steroid dependency and sexual dimorphism in many other rodent species. In rats, VP in the BST/Me i s always colocalized with the neuropeptide galanin (GAL) and the sex differ ence in VP cell number appears to result from a sex difference in the numbe r of GAL neurons which coexpress VP. Likewise, we reasoned that the species difference in extrahypothalamic VP pathways present in the golden hamster could result from a reduced coexpression of VP by GAL neurons in these regi ons. Here, we used in situ hybridization histochemistry to determine whethe r GAL mRNA expressing neurons are present in the BST and Me of golden hamst ers despite the absence of VP expression in these regions. In addition, we have used slice binding and receptor autoradiography to identify specific G AT,binding sites in the lateral septum, a probable target region of BST/Me neurons, and in situ hybridization to confirm that some of these binding si tes correspond to the GALR1 GAL receptor subtype. Our findings indicate tha t the absence of VP expression in the BST/Me of golden hamsters results fro m a failure of extrahypothalamic GAL neurons to express the VP phenotype, B ecause GAL is expressed in the extended amygdaloid complex and GAL receptor s are present in the septum of golden hamsters, GAL may play a role in modu lating functions previously attributed to BST/Me pathways. (C) 1999 Elsevie r Science B.V. All rights reserved.