“Synaptic” ribbon (SR) populations in the rat pineal gland were analyzed morphometrically at various ages from birth to early adulthood. The rats were born and raised in continuous light or continuous darkness. SR numbers were high irrespective of the presence or absence of light during the early neonatal period. However, SR numbers decreased substantially during the second neonatal week of continuous light, remaining low at subsequent ages, including early adulthood. In contrast, SR numbers apparently remained elevated during continuous darkness at various ages from the early neonatal period to early adulthood. Stimulation of pineal adrenergic receptors with L‐isoproterenol during continuous light resulted in an acute, reversible increase in SR numbers; blockade of these receptors with L‐propranolol during continuous darkness resulted in an acute decrease in SR numbers. These results indicate that light‐mediated changes in SR populations were (1) age related and (2) related to some component of the pineal beta‐adrenergic receptor mechanism. A hypothesis that SR formation may be related to beta‐adrenergic receptor‐associated membrane turnover is presented.
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