IDENTIFICATION AND FUNCTIONAL-SIGNIFICANCE OF NICOTINIC CHOLINERGIC RECEPTORS IN THE RAT PINEAL-GLAND

The existence and subunit identification of the nicotinic cholinergic receptors in the rat pineal gland were examined by autoradiography, us ing [H-3]cytisine and [I-125]alpha-bungarotoxin as labelled ligands. T he experiments performed with radioactive cytisine did not reveal spec ific binding, while iodinated alpha-bungarotoxin disclosed moderate sp ecific binding density, suggesting that the nicotinic cholinergic rece ptor in the rat pineal is structurally organized with the alpha7 or al pha8 subunits present, the only ones that bind alpha-bungarotoxin with high affinity. In vitro functional experiments using pineal explants demonstrated that the binding site may represent a readily accessible nicotinic cholinergic receptor. Nicotine, though having no effect per se on the synthesis and release of melatonin, significantly diminished , in a dose-dependent manner, the norepinephrine-stimulated melatonin accumulation. This effect could be blocked by coincubation with the ch olinergic antagonist d-tubocurarine, suggesting that the nicotinic cho linergic receptor in the rat pineal could be involved in the functiona l regulation of the gland.