Tyrosine hydroxylase in magnocellular neurosecretory neurons. Response to physiological manipulations.

Tyrosine hydroxylase (TH) immunoreactivity is present in a few magnocellular neurons in the supraoptic and paraventricular nuclei of normal Sprague-Dawley and heterozygous Brattleboro rats. Manipulations commonly used to enhance immunostaining of cell bodies, such as colchicine treatment or lesioning of efferent fibers, induced only a slight increase in TH of magnocellular neurons in Sprague-Dawley rats. Seven days after bilateral midbrain transections, which destroyed ascending afferents to paraventricular and supraoptic nuclei from the lower brainstem, there was a substantial increase in TH immunoreactivity in magnocellular neurons. The lesions resulted in a marked increase in the intensity of the staining and in the number of stained cells. Similar increases were induced by giving animals 2% NaCl to drink for 5 days. Neurons in the supraoptic and paraventricular nuclei of homozygous Brattleboro rats show markedly more TH staining than those of normal rats. Vasopressin treatment lowered TH immunoreactivity in magnocellular neurons of Brattleboro rats to normal levels. These observations suggest that extracellular factors, probably afferent inputs, regulate the level of TH immunoreactivity in magnocellular neurons.