Luteinizing hormone-releasing hormone (LH-RH) cells and their projections in the forebrain of the bat Myotis lucifugus lucifugus

Luteinizing hormone-releasing hormone (LH-RH) neurons and their projections were studied by immunocytochemistry in the brains of little brown bats ( Myotis lucifugus lucifugus: Chiroptera: Vespertilionidae) as a first step in the study of relationships between these neurons and the seasonal reproductive events characteristic of this species. The majority of immunoreactive neurons in adult male, adult female, and fetal bats were ovoid bipolar cells with one thin and one thicker process, both of which gave rise to fine varicose fibers. LH-RH-immunoreactive perikarya were concentrated in the region of the arcuate nuclei in all bats examined. Perikarya were also consistently found dispersed in the mammillary region, anterior hypothalamus, preoptic areas, septum, diagonal band of Broca, and olfactory tracts; they were occasionally observed in the dorsal hypothalamus, organum vasculosum of the lamina terminalis (OVLT), habenula, amygdala, and cingulate gyrus. LH-RH-immunoreactive fibers projected heavily to the median eminence, infundibular stalk, and posterior pituitary. In extrahypothalamic areas, these fibers were especially abundant in the stria medullaris/habenula and stria terminalis/amygdala, but also contributed to the diagonal band of Broca and the olfactory tracts. Immunoreactive fibers that may be components of many different pathways clustered in the rostral septum and permeated the medial hypothalamus. LH-RH-containing fibers frequently entered the subfornical organ, but were observed less often in the OVLT and only occasionally in the pineal. The organization of the LH-RH system in the little brown bat resembles that of primates, but differs considerably from that in the rat. Anatomical characteristics of the LH-RH system in bats thus suggest that this animal may be a particularly suitable species for further study of neuroendocrine control of reproductive function as it may relate to primates, including humans.