NOVEL ASSOCIATIONS AMONG GONADOTROPIN-RELEASING-HORMONE NEURONS

GnRH is secreted in bursts into the hypophyseal portal vasculature by a small dispersed population of neurons. The means by which the activi ty of these intrinsically pulsatile cells is coordinated are unknown. This study was initiated as a continuation of our examination of the s ynaptic input to these cells and their anatomical relationships. Brain tissue from female rhesus monkeys and male and female rats was prepar ed for the immunocytochemical demonstration of GnRH. At the light micr oscopic level, GnRH neurons were occasionally found to be in close app osition. Such pairs (or small groups) were randomly distributed throug hout the population of GnRH neurons from the diagonal band of Broca th rough the anterior hypothalamic area in rats and monkeys and in the me dial basal hypothalamus in monkeys. The percentage of neurons found in such associations was small (2-7% in rats and 3-15% in monkeys) and w as independent of the hormonal condition of the animal. GnRH neurons, either singly or in pairs, were serially sectioned for electron micros copic examination. The sparsity of synaptic input to the cell body tha t we had reported earlier on the basis of random sampling was confirme d. No soma had more than a dozen synapses, but none totally lacked inn ervation. The most significant result of serial reconstruction was the discovery of intercellular bridges or passageways between contiguous pairs of GnRH neurons. These were formed by the fusion of processes ex tending from the two cells or by fusion and opening of passageways in the membranes along regions of contiguity between the two cells. They were found in four of seven pairs of neurons examined in the rat and i n four of eight pairs in the monkey. This syncytial arrangement along with GnRH-GnRH synaptic interactions could contribute to the coordinat ion of dispersed influences on these neurons and the propagation of co ordinated pulsatile release of GnRH.