Figure 6. A model illustrating the sites of action of CCK, SP, CGRP and noradrenaline in guinea pig gallbladder. Noradrenaline and CCK act presynaptically on vagal cholinergic neurones. Their actions, respectively, inhibit and enhance synaptic transmission. SP and CGRP are thought to be released from extrinsic sensory neurones and act postsynaptically causing slow EPSPs. RI = nicotinic receptor, R2 = tachykinin NK-3 receptor, R3 = CGRP receptor, R4 = ?2- adrenoreceptor and R5 = CCK-A receptor.