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 S-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 μM) reduced the amplitude of the depolarisation by 55 ± 5%. The glycine effect was a postsynaptic response because it was not changed after nerve blockade with tetrodotoxin (1 μM) 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 μM) and mecamylamine (100 μM), the GABA_A receptor antagonist bicuculline (10 μM), the NMDA antagonist MK-801 (20 μM) or the 5-HT₃ antagonist ICS 205930 (1 μM). 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.