PHYSIOLOGY OF MUSCARINIC SYNAPSES IN SYMPATHETIC GANGLIA

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The long-term goal of this project is to elucidate the integrative function of muscarinic and peptidergic synapses in bullfrog sympathetic ganglia. The bullfrog was chosen for study because its anatomical features are optimal for electrophysiological experiments that can be performed at the cellular level and directly related to the control of vascular tone and glandular secretion. The proposed experiments exploit the fact that paravertebral sympathetic neurons in the bullfrog are organized into 2 major subsystems, B and C, that can be identified by their axonal conduction velocities, and selectively activated due to the anatomical separation of their preganglionic inputs. These cell types are further distinguished by their expression of muscarinic synapses and neuropeptides, and by their functional roles. The experiments will employ electrophysiological methods to study isolated preparations containing either ganglia and their end-organs, ganglia alone, or end-organs alone, and to study dissociated neurons in primary cell culture. The proposal has 3 specific aims: 1) To test the hypothesis that co-transmitters endow the vasomotor C system with variable synaptic gain that is regulated by temporal patterns of preganglionic activity. The hypothetical model postulates that synapses in the ganglia and at end-organs function as 2 variable gain stages in series. In ganglia, acetylcholine (ACH) and luteinizing hormone releasing hormone (LHRH) are co-released to produce a nicotinic epsp, a slow muscarinic ipsp, and a slow peptidergic epsp. Temporal interactions between the 2 slow synaptic potentials are hypothesized to control ganglionic gain. In the periphery, it is postulated that the release of the co-transmitters epinephrine (EPI) and neuropeptide Y (NPY) is differentially regulated by activity. When NPY is released, it enhances end-organ gain by potentiating vascular responses to EPI. 2) To test the hypotheses that the B system innervates cutaneous mucous glands and that calcitonin gene-related peptide (CGRP) is both a ganglionic co-transmitter that mediates a slow epsp and a trophic factor that regulates the expression of nicotinic ACH receptors. 3) To test the hypothesis that LHRH released by preganglionic C fibers acts heterosynaptically to increase the gain of synapses between pre- and postganglionic B neurons. Results of the proposed studies will provide a conceptual framework for understanding the integrative function of synaptic co-transmission in more complex circuits including mammalian autonomic ganglia and the brain.