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Tyrosine hydroxylase and dopamine‐β ‐hydroxylase: distribution in the individual rat hypothalamic nuclei

THE HYDROXYLATION of tyrosine by tyrosine hydroxylase (L-tyrosine, tetrahydropteridine: Oz oxidoreductase (2 hydroxylating); EC 1.14.3a) (LEVITT et al., 1965) and the 0-hydroxylation of dopamine by dopamine-p hydroxylase (EC 1.14.2.1) (KAUFMAN & FRIEDMAN, 1965) are essential steps in the formation of the neurotransmitter noradrenaline. Tyrosine hydroxylase (MLJSACCHIO et af., 1969) and dopamine-p-hydroxylase (UNDENFRIEND & CREVELING, 1959) have been found to occur in brain, as well as in peripheral adrenergic nerves and in the adrenal medulla. Both enzymes (COYLE, 1972: REIS & MOLINOFF, 1972) are present in nerve terminals and are unevenly distributed in brain. Dopamine-P -hydroxylase is widely distributed in the central nervous system, with a pattern of regional distribution that parallels the levels of noradrenaline (REIS & MOLINOFF, 1972). It is probably produced in adrenergic cell bodies (AXELROD, 1972) and is concentrated in noradrenaline containing axon terminals. The highest dopamine-P-hydroxylase activity is measured in the brain stem and hypothalamus (COYLE & AXELROD, 1972). Tyrosine hydroxylase activity is highest in the striatum and in the hypothalamus (COYLE, 1972), regions with a high dopamine content (POPOV et al., 1967). The simultaneous development of a sensitive enzymatic-isotopic assay for dopamine and noradrenaline (COYLE & HENRY, 1973) and of a technique for removal of individual hypothalamic nuclei (PALKOVITS, 1973) has allowed us to determine the distribution of these catecholamines within the rat hypothalamus (PALKOVITS et a] . , 1974). Both amines were found to be unevenly distributed throughout the hypothalamus, and the individual nuclei were heterogeneous as regards their content of the amines. These observations together with the availability of sensitive methods for the measurement of tyrosine hydroxylase and dopamine-P-hydroxylase (COYLE, 1972; MOLINOFF et a!., 1971) prompted us to the study of the distribution of these enzymes in the rat hypothalamus. Sprague-Dawley male rats (Zivic Miller) weighing 20&250 g, were killed by decapitation and the brains were immediately removed and frozen on dry ice. Hypothalamic nuclei were removed from frozen serial sections of the brain according to the method of PALKOVITS ef af. (1974) with special stainless steel needles.