A possible role for cyclic guanosine monophosphate in the rat pineal gland.

Adrenergic stimulation of pinealocytes induces an increase of both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). However, for cGMP no biological effects have been demonstrated so far. Therefore we tested the effects of the analog 8-bromo-cGMP on synaptic ribbon numbers and on melatonin synthesis as reflected by N-acetyltransferase (NAT) activity in the rat pineal gland in vitro. Incubation for 6 h with 8-bromo-cGMP did not change the activity of serotonin NAT but it increased the number of synaptic ribbons. These results indicate that cGMP is involved as a second messenger in the regulation of synaptic ribbon numbers in the rat pineal gland.