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1 Department of Anatomy and Neurobiology, University of Vermont, Burlington, VT, USA
2 Department of Biology, Ithaca College, Ithaca, NY, USA
* To whom correspondence should be addressed. E-mail: rodney.parsons{at}uvm.edu.
Mechanisms modulating the pituitary adenylate cyclase activating polypeptide (PACAP)-induced increase in excitability have been investigated using dissociated guinea pig intrinsic cardiac neurons and intact ganglion preparations. Measurements of intracellular calcium (Ca2+) with the fluorescent Ca2+ indicator dye fluo-3 indicated that neither PACAP nor vasoactive intestinal polypeptide (VIP) at either 100 nM or 1 µM produced a discernable elevation of intracellular Ca2+ in dissociated intracardiac neurons. For neurons in ganglion whole-mount preparations kept in control bath solution, local application of PACAP significantly increased excitability, as indicated by the number of action potentials generated by long depolarizing current pulses. However, in a Ca2+-deficient solution in which external Ca2+ was replaced by Mg2+ or when cells were bathed in control solution containing 200 µM Cd2+, PACAP did not enhance action potential firing. In contrast, in a Ca2+-deficient solution with Ca2+ replaced by strontium (Sr2+), PACAP increased excitability. PACAP increased excitability in cells treated with a combination of 20 µM ryanodine and 10 mM caffeine to interrupt release of Ca2+ from internal stores. Experiments using fluo-3 demonstrated that ryanodine/caffeine pretreatment eliminated subsequent caffeine-induced Ca2+ release from intracellular stores whereas exposure to the Ca2+-deficient solution did not. In dissociated intracardiac neurons voltage-clamped with the perforated patch recording technique, 100 nM PACAP decreased the voltage-dependent barium current (IBa). These results demonstrate that in the guinea pig intracardiac neurons, the PACAP-induced increase in excitability apparently requires Ca2+ influx through Cd2+-sensitive calcium permeable channels other than voltage-dependent Ca2+ channels, but not Ca2+ release from internal stores.
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  D. B. Hoover, J. D. Tompkins, and R. L. Parsons Differential Activation of Guinea Pig Intrinsic Cardiac Neurons by the PAC1 Agonists Maxadilan and Pituitary Adenylate Cyclase-Activating Polypeptide 27 (PACAP27) J. Pharmacol. Exp. Ther., October 1, 2009; 331(1): 197 - 203. [Abstract] [Full Text] [PDF]
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