ULTRASTRUCTURAL EVIDENCE FOR CHANGES IN SYNAPTIC INPUT TO THE HYPOTHALAMIC LUTEINIZING-HORMONE-RELEASING HORMONE NEURONS IN PHOTOSENSITIVE AND PHOTOREFRACTORY STARLINGS

Neural input to the hypothalamic luteinizing hormone-releasing hormone (LHRH) neurons was investigated in male starlings (Sturnus vulgaris) using electron microscopy combined with immunocytochemistry. Birds (4 to 6 in each group) were sampled at four stages of a photoperiodically induced reproductive cycle: (a) sexually immature but photosensitive, under short days; (b) during sexual maturation after 7 to 25 long day s; (c) during gonadal regression after 35 to 50 long days; and (d) whe n fully photorefractory after 11 months exposure to long days. The len gth of the perikaryal membrane, the number and length of axo-somatic t erminals in contact with it and the number and length of synaptic modi fications within the terminals were measured for a minimum of six LHRH neurons in each brain. The number of axo-somatic terminals per neuron and the number per unit length of perikaryal membrane did not differ in birds of groups (a), (b) and (c), but was significantly greater (P< 0.05) in the fully refractory birds (group d). Similarly, the number o f synaptic modifications was significantly greater (P<0.05) in group ( d) than in the other groups. These results are consistent with increas ed neural input to the LHRH perikarya in photorefractory birds after p rolonged exposure to long days, although there was no indication of a change in input at the time of gonadal regression.