Selectivity of antagonists for the Cys-loop native receptors for ACh, 5-HT and GABA in guinea-pig myenteric neurons

1. The three most common Cys-loop receptors expressed by myenteric neurons are nACh, 5-HT₃ and GABA_A. To investigate the function of these proteins researchers have used channel inhibitors such as hexamethonium (antagonist of nACh receptors), ondansetron (antagonist of 5-HT₃ receptors), picrotoxin and bicuculline (both antagonists of GABA_A receptors). The aim of this study was to investigate the specificity of these inhibitors on Cys-loop receptors of primary cultured neurons obtained from the guinea-pig small intestine. The whole-cell configuration of the patch clamp techniques was used to record membrane currents induced by ACh (I_ACh), 5-HT (I_5-HT) and GABA (I_GABA) in the absence and the presence of various concentrations of hexamethonium, ondansetron, picrotoxin or bicuculline.
2. The three Cys-loop receptors present in enteric neurons are expressed independently and they do not cross-desensitized. Hexamethonium inhibited I_ACh without affecting I_5-HT and I_GABA. Ondansetron inhibited I_5-HT and also I_ACh but did not affect I_GABA. Picrotoxin and bicuculline inhibited I_5-HT, I_ACh and I_GABA with different potency, being the lowest potency on 5-HT₃ receptors. All these inhibitory effects were concentration dependent and reversible.
3. Our observations showed that except for hexamethonium, all other inhibitors used here show different degrees of selectivity, which has to be considered when these antagonists are used in experimental studies aimed to investigate the functions of these receptors. In particular, in tissues expressing nACh receptors because these are the targets of all other inhibitors used here. The low potency of picrotoxin and bicuculline to inhibit 5-HT₃ receptors suggests that these receptors are heteromeric proteins.