Electrophysiology and pharmacology of striated muscle fibres cultured from dissociated neonatal rat pineal glands

1. Striated muscle fibres were found in each of twenty consecutive pineal glands cultured from individual neonatal rats. 2. In subsequent experiments performed with dissociated cultures of pineal organs pooled from several litters, myotubes were first visible after about 1 week in culture. 3. During the next several weeks the myotubes increased in size, developed crossstriations, and began to twitch spontaneously. 4. The resting membrane potential increased with age in culture. All myotubes studied showed delayed rectification. Action potentials either occurred spontaneously or could be evoked if the membrane were sufficiently polarized. No spontaneous end plate potentials were seen. 5. Acetylcholine (ACh) produced a brief, monophasic depolarizing response. Noradrenaline, serotonin, melatonin, dopamine, and γ-aminobutyric acid (GABA) had no effect on the resting membrane potential when applied iontophoretically. 6. The ACh response was reversibly blocked by 10 -6 M -tubocurarine and irreversibly blocked by 10 -6 M -α-bungarotoxin. Atropine (10 -4 M ) reduced the amplitude and shortened the time course of the ACh response, and 10 -3 M -atropine produced complete but reversible inhibition. 7. We conclude that pineal muscle fibres are electrophysiologically and pharmacologically similar to skeletal muscle fibres in vitro . Although the pineal gland has undetectable levels of ACh, pineal muscle develops ACh receptors but not noradrenaline, serotonin, melatonin, dopamine, or GABA receptors mediating electrophysiological responses, although these latter substances (except dopamine) are found in the pineal.