naloxone and iberiotoxin

Fast excitatory postsynaptic potentials (fEPSPs) occur in bursts in the myenteric plexus during evoked motor reflexes in the guinea-pig ileum in vitro. This study used electrophysiological methods to study fEPSPs during stimulus trains to mimic bursts of synaptic activity in vitro. The amplitude of fEPSPs or fast excitatory postsynaptic currents (EPSCs) declined (rundown) during stimulus trains at frequencies of 0.5, 5, 10 and 20 Hz. At 0.5 Hz, fEPSP or fEPSC amplitude declined by 50% after the first stimulus but remained constant for the remainder of the train. At 5, 10 and 20 Hz, synaptic responses ran down completely with time constants of 0.35, 0.21 and 0.11 s, respectively. Recovery from rundown occurred with a time constant of 7 s. Mecamylamine, a nicotinic cholinergic receptor antagonist, or PPADS, a P2X receptor antagonist, reduced fEPSP amplitude, but they had no effect on rundown. Responses caused by trains of ionophoretically applied ATP or ACh (to mimic fEPSPs) did not rundown. Blockade of presynaptic inhibitory muscarinic, adenosine A1, opioid, alpha2-adrenergic and 5-HT1A receptors or pertussis toxin (PTX) treatment did not alter rundown. Antidromic action potentials followed a 10-Hz stimulus train. Iberiotoxin (100 nM), a blocker of large conductance calcium activated K+ (BK) channels, did not alter rundown. These data suggest that synaptic rundown is not due to: (a) action potential failure; (b) nicotinic or P2X receptor desensitization; (c) presynaptic inhibition mediated by pertussis-toxin sensitive G-proteins, or (d) BK channel activation. Synaptic rundown is likely due to depletion of a readily releasable pool (RRP) of neurotransmitter.

Topics: Acetylcholine; Action Potentials; Adenosine A1 Receptor Antagonists; Adenosine Triphosphate; Adrenergic alpha-2 Receptor Antagonists; Animals; Dose-Response Relationship, Radiation; Drug Interactions; Electric Stimulation; Estriol; Excitatory Postsynaptic Potentials; GABA Antagonists; Guinea Pigs; Idazoxan; Ileum; In Vitro Techniques; Iontophoresis; Male; Mecamylamine; Myenteric Plexus; Naloxone; Narcotic Antagonists; Neural Inhibition; Neurons; Nicotinic Antagonists; Peptides; Picrotoxin; Piperazines; Platelet Aggregation Inhibitors; Pyridoxal Phosphate; Serotonin 5-HT1 Receptor Antagonists; Theophylline; Time Factors

It is known that mu-agonists inhibit electrical field stimulation (EFS)-evoked ACh release from longitudinal muscle myenteric plexus (LMMP) preparation of guinea pig ileum when muscarinic autoinhibition does not fully work. In the present study, the possible role of K+ channels in the mechanisms of mu-agonists-induced inhibition and autoinhibition of ACh release was studied. In the presence of atropine, which blocks the autoinhibition, non-selective K+ channel blockers, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), reversed the inhibitory effect of mu-agonists, morphine and [D-Ala2, N-Me-Phe4, Gly5-ol] enkephalin, on EFS-evoked ACh release, but not that of kappa-agonist U-50,488. Apamin, iberiotoxin or glibenclamide did not affect the inhibition of ACh release by morphine. On the other hand, in the absence of atropine (under the autoinhibition working condition), 4-AP increased EFS-evoked ACh release, but atropine did not further increase ACh release in the presence of 4-AP. In contrast, although TEA did not affect EFS-evoked ACh release, atropine increased ACh release in the presence of TEA. These results suggest that the inhibitory effects of mu-agonists and muscarinic autoinhibition on the ACh release are associated with activation of different types of K+ channels in the guinea pig LMMP preparations: the former is associated with 4-AP- and TEA-sensitive K+ channels and the latter is associated with 4-AP- but not TEA-sensitive K+ channels.

Topics: 4-Aminopyridine; Acetylcholine; Analgesics, Opioid; Animals; Apamin; Atropine; Drug Interactions; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Glyburide; Guinea Pigs; Ileum; In Vitro Techniques; Male; Morphine; Muscarinic Antagonists; Myenteric Plexus; Naloxone; Narcotic Antagonists; Neuromuscular Junction; Peptides; Potassium Channel Blockers; Potassium Channels; Receptors, Muscarinic; Receptors, Opioid, mu; Tetraethylammonium