Effect of denervation on 'synaptic' ribbon populations in the rat pineal gland

The formation of pineal 'synaptic' ribbons (SR) may be directly related to the adrenergic innervation of the gland. In order to clarify this relationship, SR populations at various times from 12 h to 14 days after pineal denervation were morphometrically analysed by electron microscopy. Pineal denervation was accomplished by bilateral superior cervical ganglionectomy. A decrease in nocturnal pineal SR numbers, indicating a reduction in SR formation, was demonstrated 12 to 24 h after pineal denervation. Seventy-two hours after ganglionectomy SR numbers were comparable with those in nocturnal intact and sham-operated controls. Thereafter, 7 and 14 days after ganglionectomy, SR numbers exceeded nocturnal intact and sham-operated controls. Administration of isoproterenol, a beta-adrenergic receptor agonist, 24 h after denervation significantly increased SR numbers over those in untreated rats denervated 24 h earlier. Thus SR formation remained responsive to adrenergic receptor stimulation in the absence of an intact adrenergic innervation. Further, the increase in SR numbers following subacute (7 to 14 days) denervation indicated that SR formation was not dependent on an intact innervation or the presence of endogenous (pineal) norepinephrine. On the basis of these results, we suggest that SR formation may be related structurally as well as functionally to adrenergic receptors on the rat pinealocyte.