Glycine activates myenteric neurones in adult guinea-pigs

1. We studied the effects of glycine on myenteric neurones and muscle activity in the colon and stomach of adult guinea-pigs. 2. Intracellular recordings revealed that myenteric neurones responded to local microejection of glycine (1 mM) with a fast, transient membrane potential depolarisation (57 % of 191 colonic neurones and 26 % of 50 gastric neurones). Most glycine-sensitive neurones had ascending projections and were choline acetyltransferase immunoreactive. Glycine preferentially activated neurones with a late afterhyperpolarisation (AH-neurones) and tonic spiking neurones with fast synaptic inputs (tonic S-neurones) but less frequently phasic 8-neurones and inexcitable (non-spiking) neurones. The depolarisation had a reversal potential at -19 +/- 13 mV, which was increased by 18 +/- 10 % upon lowering extracellular chloride concentration and decreased by 38 +/- 14 % in furosemide (frusemide, 2 mM). 3. Strychnine (300 nM) reversibly abolished the glycine-induced depolarisation and the Cl- channel blocker picrotoxin (100 muM) reduced the amplitude of the depolarisation by 55 +/- 6 %. The glycine effect was a postsynaptic response because it was not changed after nerve blockade with tetrodotoxin (1 pi) or blockade of synaptic transmission in reduced extracellular [Ca2+]. The effect was specific since the response was not changed by the nicotinic antagonists hexamethonium (200 mum) and mecamylamine (100 mum), the GABA(A) receptor antagonist bicuculline (10 muM), the NMDA antagonist MK-801 (20 muM) or the 5-HT3 antagonist ICS 205930 (1 muM). 4. Glycine (1 mM) induced a tetrodotoxin- and strychnine-sensitive contractile response in the colon; the contractile response in the stomach was tetrodotoxin insensitive. 5. Glycine, activated myenteric neurones in the adult enteric: nervous system through strychnine-sensitive mechanisms. The glycine-evoked depolarisation was caused by Cl- efflux and the maintenance of relatively high intracellular chloride concentrations involved furosemide-sensitive cation-chloride co-transporters.