POSTSYNAPTIC AND PRESYNAPTIC EFFECTS OF NOREPINEPHRINE IN GUINEA-PIG COLONIC SUBMUCOUS PLEXUS

Intracellular recording techniques were used to investigate the effect s of norepinephrine on submucous neurones in the guinea-pig distal col on. In 81% of the neurones, pressure microejection of norepinephrine p roduced a membrane hyperpolarization associated with a decrease in exc itability and input resistance. Microejection of clonidine (1 mu M) mi micked the norepinephrine-induced hyperpolarization, whereas both phen tolamine (1 mu M) and yohimbine (1 mu M) reversibly suppressed it. Sup erfusion of norepinephrine (1 nM - 10 mu M) hyperpolarized the cells i n a concentration-dependent manner. Norepinephrine and clonidine (1 nM - 10 mu M) caused a concentration-dependent presynaptic inhibition of stimulus-evoked cholinergic fast excitatory postsynaptic potential. S low inhibitory postsynaptic potentials (sISPSs) were induced by focal electrical stimulation of the interganglionic fibre tracts in 43% of t he neurones tested. Superfusion of both phentolamine (1 mu M) and yohi mbine (1 mu M) reduced the sIPSPs while prazosin (1 mu M) had no signi ficant effect. We concluded that norepinephrine acted post- and presyn aptically via alpha(2)-adrenoreceptors to have an inhibitory effect on the guinea-pig colonic submucous. In addition, our study strongly sup ported the role of norepinephrine as a mediator of the sIPSPs. As a re sult, norepinephrine would primarily suppress information transfer wit hin the neuronal circuits in guinea-pig colonic submucosal plexus.