1. The mechanisms for induction of bistable plateau potential properties by a set of serotonergic/cholinergic peripheral stretch receptor cells [gastropyloric receptor (GPR) cells] were examined in the crab stomatogastric ganglion (STG) with the use of intracellular recording techniques. 2. GPR cell stimulation evoked nicotinic excitatory postsynaptic potentials (EPSPs) and induced plateau potential capability in the dorsal gastric (DG) motor neuron. The plateau potential could be triggered during a GPR train either by the summating nicotinic EPSPs or by brief intracellular current injection. After pharmacological blockade of nicotinic and muscarinic receptors, a slow depolarization in response to GPR stimulation was revealed. Prolonged plateau potentials could still be evoked after this treatment. Local application of serotonin (5-HT; 10 microM to 1 mM) mimicked the noncholinergic plateau inducing effects of GPR stimulation in the DG motor neuron. 3. The synergistic action of acetylcholine (ACh) and 5-HT was examined by stimulating the GPR cells at different frequencies (1-20 Hz). The plateau induction was present down to 2 Hz. The time to onset for triggering a plateau during a GPR train was determined by the co-released ACh. 4. The 5-HT-evoked slow depolarization persisted in tetrodotoxin (TTX; 0.1-1 microM), and the DG motor neuron could still produce a plateau potential on brief depolarization in the absence of the spike-generating mechanism. 5. In normal TTX-containing saline, the 5-HT-evoked depolarization was accompanied by a weak and variable decrease in apparent input conductance. After substituting one-half of the extracellular sodium with either Trisma-HCl or choline, the decrease in apparent input conductance became more pronounced. This decrease was converted to an increase in apparent input conductance when extracellular Ca2+ was replaced with Mg2+. 6. Under voltage-clamp conditions, local application of 5-HT caused a slow inward current of prolonged duration in DG. The current versus voltage relationship had an inverted U-shape with no apparent reversal potential in the entire voltage range investigated (-90 to -5 mV). The 5-HT-induced changes in input conductance showed a complex voltage dependence, with a conductance decrease from moderately depolarized voltages. 7. Extracellular Cs+ (2-4 mM) caused the DG to hyperpolarize 2-4 mV from rest, whereas lowering extracellular Ca2+ caused it to depolarize 7-15 mV. The combined action of low extracellular Ca2+ and 2-4 mM Cs+ caused an almost complete block of the slow 5-HT-evoked depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)