Non-additive interaction between nicotinic cholinergic and P2X purine receptors in guinea-pig enteric neurons in culture

1. Acetylcholine (ACh)-activated currents and their interaction with ATP-ac tivated currents were studied in primary cultures of myenteric neurons from guinea-pig small intestine using patch clamp techniques. Peak currents cau sed by co-application of ACh (1 mM) and ATP (300 mu M) were 78 +/- 2% of th e sum of currents activated by each agonist alone (P < 0.05, n = 29). Rever sal potentials measured during co-application of ACh and ATP did not differ from those measured during application of ACh or ATP alone. Addition of BA PTA (10 mM) to the pipette solution or replacement of extracellular Ca2+ wi th Na+ did not prevent occlusion. 2. Responses caused by co-application of 5-HT (300 mu M), acting at 5-HT3 r eceptors, and ACh (3 mM) or ATP (1 mill) were additive (94 +/- 3 or 96 +/- 4%, respectively, of the sum of currents activated by 5-HT and ACh or ATP a lone; P > 0.05). Currents caused bSr GABA (1 mM), acting at GABA(A) recepto rs, and ACh (3 mM) or ATP (1 mM) were also additive (105 +/- 4 or 100 +/- 3 %, respectively, of the sum of currents activated by GABA and ACh or GABA a nd ATP applied separately; P > 0.05). 3. Single channel currents caused by ACh and ATP in the same outside-out pa tches were less than additive (85 +/- 10 % of the predicted sum, P < 0.05). 4. P2X receptors and nicotinic cholinergic receptors (nAChRs) are linked in a mutually inhibitory manner in guinea-pig myenteric neurons. The function al interaction does not involve ligand binding sites, Ca2+-dependent mechan isms, a change in the driving force for Na+ or cytoplasmic signalling mecha nisms.