Neurosecretory material in the supraoptico‐hypophyseal tract of the bat throughout the hibernating and summer periods

Abstract Neurosecretory material in the supraoptico‐hypophyseal tract of 151 hibernating and summer bats (Myotis lucifugus lucifugus) was studied in chromealum‐hematoxylin‐phloxine‐stained serial sagittal sections. Seasonal variations of neurosecretory content throughout the hibernating period (mid‐September through mid‐May) and “active” summer period (June to mid‐September) were observed and the staining density of the neurosecretory material in the supraoptic nucleus, infundibular stem and pars nervosa was “quantitated” visually. Cell counts and measurements also were made to determine if cell number and size changes accompanied seasonal changes in neurosecretory content. A significantly greater staining density of neurosecretory material in the supraoptico‐hypophyseal tract of the hibernating bats, than in the “active” animals was observed. These differences were always greater in the infundibular stem than in the supraoptic nucleus and pars nervosa. Although there was no correlation between seasonal activity and the total number of neurosecretory cells in each brain, there was always a smaller number of dark‐staining cells in the “active” bats than in those that were hibernating. Nucleocytoplasmic ratios of hibernating bats were below those of the active animals. These resulted from smaller nuclear and larger cytoplasmic sizes during hibernation than in the summer. Nucleolar sizes, however, were slightly greater in the hibernating bats than in the active animals. The significance of these results in terms of seasonal activity is discussed.