Repercussions of anterior hypothalamic lesions on nycthemeral variations of plasma thyroxine and corticosterone in the pigeon

Plasma thyroxine and corticosterone levels were determined by competitive protein binding assay, at 3 hr intervals, throughout the photoperiod. Pigeons were kept in controlled environment (21 +/- 1 degree C; 14L6-20: 10D). Intact controls exhibited low thyroxine (T4) and corticosterone (B) levels for the light phase of the photoperiod. Values were rising during the night, up to a peak at 03 hr. Electrolytic lesions were placed bilaterally in either the nucleus anterior medialis hypothalami or the n. preopticus, or the n. supraopticus. Circadian rhythms of both T4 and B were markedly altered in all lesioned pigeons, with a shift of very high T4 values to the morning times and a complete disorganization of B patterns, with very heterogeneous values. The possibility is raised that anterior hypothalamic formations participate in the endogenous oscillator circuitry in birds.