Immunohistochemical demonstration of marked changes in the LHRH system of photosensitive and photorefractory European starlings (Sturnus vulgaris).

Immunocytochemistry was used to determine the effects of photoperiod on the LHRH neurosecretory system in the brain of male European starlings. In this species, as in other birds, reproduction is triggered by long daylengths but continued exposure leads to photorefractoriness and to a complete shut-down of the reproductive system. These effects are thought to be mediated through changes in the secretion of LHRH. In starlings exposed to a photoperiod of 11 h light:13 h darkness (11L:13D) and with fully developed testes there was strong immunostaining of both LHRH perikarya (n = 522 +/- 43 S.E.M.) and fibres. Photosensitive short-day (8L:16D) starlings with undeveloped testes had an almost identical distribution of strongly immunoreactive perikarya (n = 523 +/- 62) but there were fewer fibres. In the median eminence, fibre number was reduced significantly (P less than 0.01) by some 30%. In long-day (18L:6D) photorefractory starlings with fully regressed testes there was an even more obvious change in the LHRH system. Perikarya were only weakly immunoreactive and there was a significant (P less than 0.01) reduction in mean diameter from 10 to 6.5 micron. In addition, there was a significant (P less than 0.05) reduction in cell number (312 +/- 62), although this may well result from the fact that some weakly stained cells fell below the limits of resolution and could not be counted. LHRH fibres disappeared almost entirely from the median eminence, and were not visible elsewhere in the brain. The higher neural pathways regulating photorefractoriness induced by long days are unknown but clearly both production of LHRH in the perikarya and release/storage of LHRH in the terminals is being profoundly modified.