1. Choline acetyltransferase activity and [3H]quinuclidinyl benzylate-binding sites were detected in the pineal gland of normotensive Wistar—Kyoto rats and of spontaneously hypertensive rats.

2. In vitro, muscarinic activation by pilocarpine increased the pineal metabolic production of hydroxyindole derivatives up to 5-hydroxytryptamine and produced a less marked stimulation of melatonin biosynthesis.

3. Electrical field stimulation of pineal gland slices caused similar metabolic effects.

4. Muscarinic blockade with atropine inhibited the effects on hydroxyindole metabolism.

5. [3H]Quinuclidinyl benzylate-binding sites, indicative of muscarinic receptors, were more numerous, and basal 5-hydroxytryptamine and melatonin levels were higher, in the pineal gland of spontaneously hypertensive rats compared with Wistar—Kyoto rats.

6. The atropine-sensitive metabolic effects of pilocarpine and electrical field stimulation on the pineal gland were increased in spontaneously hypertensive rats compared with Wistar-Kyoto rats.

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