Activation of tryptophan hydroxylase by stimulation of central serotonergic neurons.

Electrical stimulation of serotonergic neurons in the raphe nuclei of the midbrain or medulla enhances the formation of serotonin in vivo in the terminal projections of these neurons [I]. The acceleration of serotonin synthesis results from an increase in the conversion of tryptophan to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase [2,3] [EC 1.14.16.4 tryptophan-5-monooxygenase: L-tryptophan, tetrahydropteridine: oxygen oxidoreductase (5-hydroxylating)], the initial and rate-limiting enzyme in the reaction sequence leading to the formation of serotonin. It is likely that this increase in serotonin synthesis is due to an activation of tryptophan hydroxylase itself since this has previously been shown to occur with in vitro depolarization of rat brain slices [4-7]. Earlier attempts to demonstrate an increase in tryptophan hydroxylase activity in vitro after in vivo electrical stimulation of serotonergic nuclei have, however, been unsuccessful [3]. We now present preliminary evidence that in vivo electrical stimulation of the serotonergic dorsal raphe nucleus produces an increase in the activity of cortical tryptophan hydroxylase, measured in vitro, that parallels increases both in the in vivo to 5-HTP (in the presence of an inhibitor of aromatic amino acid accumulation of 5-hydroxyindoleacetic acid (5-HIAA), the conversion of tryptophan decarboxylase) and in the metabolite of serotonin. In these experiments, male Sprague-Dawley rats (450-550 g) were anesthetized with chloral hydrate (400 mg/kg, i.p.) and placed in a small animal stereotaxic apparatus (David Kopf Instruments). Stimulating electrodes (size O0 insect pins from Carolina Biological Supply), insulated to within 0.5 mm of the tip, were positioned in the dorsal raphe nucleus according to coordinates from the atlas of Pellegrino et al. [8] (anterior-posterior, +0.3 mm; lateral, 0 mm; depth, -1.3 mm). The animals were grounded by a probe in the rectum. Stimulation consisted of constant current menopolar square waves (200 uamps, 1.5 msec pulse width, 10 Hz) delivered from a Grass S-48 stimulator coupled to a Grass Constant Current Unit. Experimental animals received 30 min of continuous stimulation while shams were unstimulated. In one group of animals in vivo accumulation of 5-HTP was measured in order