An electrophysiological and morphological investigation of the projectionsof growth hormone-releasing peptide-6-responsive neurons in the rat arcuate nucleus to the median eminence and to the paraventricular nucleus

Growth hormone-releasing peptide-6 injection induces c-fos messenger RNA ex pression in many arcuate nucleus neurons, and sub-populations of neurons in this region project to the hypothalamic paraventricular nucleus. We examin ed electrophysiologically whether arcuate nucleus neurons that project to t he paraventricular nucleus also project to the median eminence, and whether these neurons are activated by systemic injection of growth hormone-releas ing peptide-6. Of 116 arcuate nucleus neurons tested, 43 were antidromicall y-identified as projecting to the paraventricular nucleus and a further 30 as projecting to the median eminence; these populations displayed distinct electrophysiological characteristics, and contrasting patterns of orthodrom ic response to stimulation of the median eminence and paraventricular nucle us, indicating that these two populations are functionally distinct with li mited communication between them. Only one cell was antidromically-identifi ed as projecting to both these regions. Three of 10 arcuate nucleus neurons that projected to the paraventricular nucleus were activated by injection of growth hormone-releasing peptide-6. In parallel experiments, we examined whether Fos protein expression is induced in arcuate nucleus neurons that project to the paraventricular nucleus, as identified by retrograde-labelli ng with FluoroGold. Immunocytochemical studies revealed that 20% of arcuate nucleus neurons that were retrogradely-labelled from the paraventricular n ucleus were Fos-positive following growth hormone-releasing peptide-6 injec tion, although cells that were both Fos-positive and retrogradely-labelled accounted for less than 5% of the total number of Fos-positive arcuate nucl eus neurons. These results confirm that there is a direct projection from the arcuate nu cleus to the paraventricular nucleus and indicate that growth hormone-relea sing peptide-6 activates some of these neurons. (C) 1999 IBRO. Published by Elsevier Science Ltd.