A multiaction synapse evoking both EPSPs and enhancement of endogenous bursting

Selective stimulation of two identified input neurons called the 'IV neurons' has a dual influence on the endogenous bursting activity of certain 'PD' motorneurons in the stomatogastric ganglion of the spiny lobster. The effects include: (i) large, conventional and apparently monosynaptic EPSPs; and (ii) enhancement of the endogenous bursting of the pyloric dilator (PD) cells, seen as an increased amplitude of PD oscillations and a higher spiking rate during bursts. The burst enhancement decayed relatively slowly after stimulation ceased, over seconds or tens-of-seconds, depending on stimulus parameters. Modification of the voltage-dependent membrane properties of the PD cells appeared to underlie this effect. The dual-action nature of the IV-to-PD connection was confirmed by selectively blocking the brief EPSP component with 5 x 10(-4) M curare, under which conditions the burst enhancement still persisted. Data from low-Ca2+ experiments were consistent with a conventional mode of synaptic transmitter release underlying the burst enhancement. Enhancement was found to differ significantly from actions of injected current. The IV inputs appear to act on at least two types of synaptic receptors on PD neurons: a curare-sensitive receptor for the brief conventional EPSP, and a curare-resistant receptor for burst enhancement. Analogies may be drawn to the nicotinic and muscarinic cholinergic receptors of vertebrates. These findings may be considered within the contexts of multiaction synapses, modification of cellular properties, and mechanisms for the CNS activation of motor pattern generators.