Gonadotropin-releasing hormone immunoreactive neurons with access to fenestrated capillaries in mouse brain

Gonadotropin-releasing hormone (GnRH) producing neurons which have access to fenestrated capillaries were identified through a combination of indirect immunofluorescence for GnRH with a fluorescein-taged second antibody and histochemical demonstration of the localization of blood borne and retrogradely transported horseradish peroxidase. In the mouse, GnRH positive neurons were present in the septum, which includes neurons originating from the nervus terminalis, the nucleus medialis and triangularis septi, and the nucleus of the diagonal band. Also, GnRH immunoreactive neurons could be seen in the lateral anterior hypothalamus, the nucleus preopticus medianus, the rostral nucleus periventricularis hypothalami and, to a lesser extent, the nucleus preopticus medialis. Single GnRH positive neurons were found in the nucleus supraopticus, the bed nucleus of the stria terminalis and the cingulate cortex. GnRH neurons which showed uptake of horseradish peroxidase were located in all of these regions and intermingled with unlabeled GnRH neurons. No preferential topographical concentration of GnRH neurons with access to the fenestrated vasculature was apparent. In animals in which GnRH secretion was stimulated by castration for 2 weeks, 65% of all GnRH neuronal perikarya contained horseradish peroxidase. This was reduced to 35% after a 2-week treatment of ovariectomized animals with 10 micrograms/day estradiol while the total number of immunoreactive GnRH cells remained unchanged. No differences in the number of GnRH-horseradish peroxidase positive cells was seen when the dose of horseradish peroxidase of the survival time were increased. While the presence of certain GnRH neurons with dual actions via collaterals cannot be excluded, the results suggest that there are two populations of GnRH neurons, one with access to fenestrated capillaries which is probably related to neurosecretory endocrine regulation of anterior pituitary gonadotropin secretion, and one without access to fenestrated capillaries which is probably related to intracerebral neurotransmission only.