Pineal muscarinic phosphoinositide response: pertussis toxin resistant signaling with very low receptor number.

Autoradiography revealed very low density of muscarinic receptors in the rat pineal gland. Yet, the magnitude of phosphoinositide hydrolysis elicited with 0.1 mM carbachol was similar to that seen with 1 mM norepinephrine. The cholinergic and adrenergic phosphoinositide responses were fully additive. The cholinergic signal was insensitive to pertussis toxin both in vivo and in vitro and persisted in pineals cultured for 5 days. The data expand our previous finding on functional muscarinic acetylcholine receptors in the rat pineal gland.