COMPARISON OF ULTRASTRUCTURAL CHARACTERISTICS OF GONADOTROPIN-RELEASING-HORMONE NEURONS IN PREPUBERTAL AND ADULT MALE-RATS

Gonadotropin-releasing hormone neurons from prepubertal (29-day-old) a nd adult (three-month-old) male rats were demonstrated immunocytochemi cally using the LR1 antibody, and prepared for electron microscopic ex amination. Gonadotropin-releasing hormone neurons were equally immunor eactive in the two age groups, but there were heavy deposits of reacti on product in the outer nuclear envelope of these neurons in prepubert al animals. Point count stereology on electron micrographic montages o f gonadotropin-releasing hormone neurons at x 25,000 was used to compa re the relative proportion of cytoplasm containing various subcellular organelles. More of the cytoplasm was occupied by Golgi apparatus and secretory Vesicles in the prepubertal animals. The representation of mitochondria was equal in the two age groups, while there were more ly sosomes in the gonadotropin-releasing hormone neurons from adult anima ls. The density of synaptic input to the neurons was estimated using q uantitative morphometrics on electron micrographs of three levels of s ection through the neuron, magnified x 25,000. The percentage of the p erikaryal membrane with synaptic contacts was greater in the gonadotro pin-releasing hormone neurons from adults. Most strikingly, there were gonadotropin-releasing hormone terminals on gonadotropin-releasing ho rmone soma of these neurons in prepubertal animals, but not in the adu lts.The highly immunoreactive outer nuclear envelope and relative larg er representation of Golgi and secretory vesicles in gonadotropin-rele asing hormone neurons in prepubertal animals suggest that these cells are actively synthesizing peptides, including gonadotropin-releasing h ormone. The large representation of Golgi apparatus may also reflect t he active biosynthesis of membrane in association with the elaboration of neuronal processes. Gonadotrogin-releasing hormone neurons from pr epubertal animals were less well innervated than those from the adults , suggesting that their integration into the circuitry is not complete at this stage. A striking difference between the two age groups was t he occurrence of gonadotropin-releasing hormone terminals directly on the cell soma, seen exclusively in the prepubertal animals. This might be interpreted as a substrate for the coordination of activity among these neurons, possibly subserving as a mechanism for negative feedbac k which may be in effect during the period prior to puberty.