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 days; (c) during gonadal regression after 35 to 50 long days; and (d) when fully photorefractory after 11 months exposure to long days. The length of the perikaryal membrane, the number and length of axo‐somatic terminals in contact with it and the number and length of synaptic modifications 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