ROLE OF GANGLIONIC COTRANSMISSION IN SYMPATHETIC CONTROL OF THE ISOLATED BULLFROG AORTA

1. The relation between preganglionic activity and arterial tone was s tudied in preparations of bullfrog lumbar sympathetic ganglia 7-10 and the dorsal aorta. 2. Two or more stimuli evoked contractions when app lied to the preganglionic C, but not the B pathway Contractions were b locked when transmission in ganglia 9 and 10 was disrupted by cutting the sympathetic chain or adding (+)-tubocurarine. Contractions were an tagonized by postganglionic action of guanethidine, but not by phentol amine or suramin. 3. Aortic responses to short trains (10-100 stimuli) were half-maximal at 0.3-0.5 Hz, saturated near 1 Hz and had a minimu m latency of 8.9 s. By contrast, responses to 300 stimuli were half-ma ximal at 1 Hz and became 2.5-fold larger at 10 Hz. 4. Exogenous lutein izing hormone releasing hormone (LHRH) potentiated preganglionically e voked contractions. Endogenous LHRH mediated contractions evoked by 10 Hz stimulation in (+)-tubocurarine. These responses had a longer late ncy than in normal Ringer solution and were blocked by [D-pGlu(1),D-Ph e(2),D-Trp(3,6)]-LHRH. The LHRH antagonist did not alter contractions evoked by continuous stimulation in normal Ringer solution or by burst s of stimuli in hexamethonium. 5. Exogenous neuropeptide Y (NPP) poten tiated neurogenic contractions and responses to adrenaline. Benextrami ne blocked contractions produced by nerve stimulation, adrenaline and NPY, but not ATP. 6. The results show that contractions of the isolate d aorta are tuned to physiological frequencies of activity in sympathe tic C neurones. Peptidergic cotransmission in the ganglia can increase arterial tension, but not during synchronous activation of primary ni cotinic synapses. It is suggested that the physiological role of LHRH arises from interactions with subthreshold nicotinic EPSPs and that po stganglionic release of NPY shifts frequency tuning of the circuit dur ing prolonged activity.