Neurally released pituitary adenylate cyclase‐activating polypeptide enhances guinea pig intrinsic cardiac neurone excitability

Abstract

Intracellular recordings were made in vitro from guinea-pig cardiac ganglia to determine whether endogenous neuropeptides such as pituitary adenylate cyclase-activating polypeptide (PACAP) or substance P released during tetanic neural stimulation modulate cardiac neurone excitability and/or contribute to slow excitatory postsynaptic potentials (sEPSPs). When nicotinic and muscarinic receptors were blocked by hexamethonium and atropine, 20 Hz stimulation for 10 s initiated a sEPSP in all innervated neurones. In 40% of the cells, excitability was enhanced after termination of the sEPSP. This suggested that non-cholinergic receptor-mediated mechanisms contributed to the sEPSP and modulated neuronal excitability. Exogenous PACAP and substance P initiated a slow depolarization in the neurones whereas neuronal excitability was only increased by PACAP. When ganglia were treated with the PAC 1 antagonist PACAP6-38 (500 nM), the sEPSP evoked by 20 Hz stimulation was reduced by ∼50% and an enhanced excitability occurred in only 10% of the cells. These observations suggested that PACAP released from preganglionic nerve terminals during tetanic stimulation enhanced neuronal excitability and evoked sEPSPs. After addition of 1 nM PACAP to the bath, 7 of 9 neurones exhibited a tonic firing pattern whereas in untreated preparations, the neurons had a phasic firing pattern. PACAP6-38 (500 nM) diminished the increase in excitability caused by 1 nM PACAP so that only 4 of 13 neurones exhibited a tonic firing pattern and the other 9 cells retained a phasic firing pattern. These findings indicate that PACAP can be released by tetanic neural stimulation in vitro and increase the excitability of intrinsic cardiac neurones. We hypothesize that in vivo PACAP released during preganglionic firing may modulate neurotransmission within the intrinsic cardiac ganglia.

Differential Activation of Guinea Pig Intrinsic Cardiac Neurons by the PAC1 Agonists Maxadilan and Pituitary Adenylate Cyclase-Activating Polypeptide 27 (PACAP27)

“…The negative chronotropic responses to PACAP were blocked by atropine in both species, suggesting that they occurred through stimulation of intrinsic cholinergic neurons. This conclusion is supported by electrophysiological evidence that PACAP causes slow depolarization of most cholinergic neurons in whole mount preparations of guinea pig atrial ganglia and often triggers the generation of action potentials by these cells (Braas et al, 1998;Tompkins et al, 2007). PACAP-evoked bradycardia in guinea pigs is probably mediated by the PAC1 receptor, because specific PAC1 immunoreactivity has been localized to the plasma membrane of dissociated intrinsic cardiac neurons (Braas et al, 1998).…”

“…For intracellular recordings, the whole mount preparations were pinned in a 2.5-ml Sylgard-lined chamber and superfused continuously (2-3 ml/min) with a modified Krebs' solution containing 10 mM HEPES buffer at 30 -32°C (Hardwick et al, 1995;Braas et al, 1998;Tompkins et al, 2006Tompkins et al, , 2007. Cardiac ganglia were visualized with an inverted microscope equipped with Hoffman optics (Modulation Optics Inc., Greenvale, NY), and individual cardiac neurons were impaled using high-impedance borosilicate microelectrodes (2 M KCl-filled; 60 -100 M⍀).…”

“…Experiments were performed on atrial whole mount preparations containing the intrinsic cardiac ganglia as described previously (Braas et al, 1998;Tompkins et al, 2006Tompkins et al, , 2007. Atrial whole mount preparations were dissected from the heart in ice-cold, standard Krebs' solution (121 mM NaCl, 5.9 mM KCl, 2.5 mM CaCl 2 , 1.2 mM MgCl 2 , 25 mM NaHCO 3 , 1.2 mM NaH 2 PO 4 , and 8 mM glucose), with the pH maintained at 7.4 by aeration with 95% O 2 , 5% CO 2 .…”

Pituitary Adenylate Cyclase-Activating Polypeptide and Its Receptors: 20 Years after the Discovery

“…In contrast, the increased excitability of rat neonatal cardiac neurons induced by PACAP requires coactivation of PAC1-R and VPAC-R and release of Ca 2ϩ from intracellular stores (DeHaven and Cuevas, 2004). PACAP is localized to preganglionic parasympathetic nerves in rat and guinea pig hearts Richardson et al, 2003), and PACAP released at this site acts at postsynaptic PAC1-R to increase excitability of cardiac cholinergic neurons (Tompkins et al, 2007).…”

Autonomic cardiac nerves: literature review

“…This was demonstrated by Tompkins et al [63] in in vitro studies regarding intrinsic cardiac neurons. The exogenous administration of this neuropeptide increased the excitability of cardiac neurons.…”