Presynaptic adrenergic facilitation of parasympathetic neurotransmission in sympathectomized rat smooth muscle

Abstract

1. Parasympathetic innervation of rat eyelid tarsal smooth muscle normally inhibits sympathetic neurotransmission prejunctionally without significant direct postjunctional effects. Following surgical sympathectomy, parasympathetic stimulation elicits smooth muscle contraction. This study examined the relative contributions of cholinergic and adrenergic mechanisms mediating these contractions. 2. Electrical stimulation of the superior salivatory nucleus, which activates tarsal muscle parasympathetic nerves, elicited large contractions at 2 days postsympathectomy, which were abolished by atropine and were decreased by 65% by α1-adrenoceptor blockade or spinal cord transection. 3. Contractions in response to direct cholinergic stimulation by bethanechol at 2 days postsympathectomy were increased following spinal cord transection (C2) and suppressed by the α1-adrenoceptor agonist phenylephrine, indicating that adrenoceptors on smooth muscle attenuate cholinergic contractions. However, phenylephrine infusion enhanced contractile responses to parasympathetic stimulation. 4. Reverse transcription-polymerase chain reaction revealed α(1D)-adrenoceptor mRNA within pterygopalatine ganglia. 5. At 5 weeks and 14 months postsympathectomy, adrenergic facilitation was significantly less than at 2 days, whereas prazosin-insensitive muscarinic contraction was increased. 6. We conclude that degeneration of sympathetic innervation is followed rapidly by adrenoceptor-mediated prejunctional enhancement of parasympathetic nerve-smooth muscle neurotransmission, which occurs prior to neuroeffector junction formation as determined previously by electron microscopy. Subsequently noradrenergic enhancement is diminished as cholinergic neurotransmission becomes established.