MUSCARINIC MODULATION OF A PATTERN-GENERATING NETWORK - CONTROL OF NEURONAL PROPERTIES

The aim of this article is to investigate the cellular mechanisms unde rlying cholinergic modulation of the pyloric network in the stomatogas tric ganglion (STG) of the Cape lobster Jasus lalandii. Bath applicati on of the muscarinic agonists muscarine, oxotremorine, and pilocarpine on the STG activates a rhythmic pattern from a quiescent pyloric netw ork. The mechanisms of this modulation were investigated on individual pyloric neurons isolated both from synaptic interactions within the n etwork (by photoinactivation of most of the presynaptic neurons and ph armacological blockade of the remaining synapses) and from central inp uts (by a sucrose block of the input nerve). All three muscarinic agon ists activated bursting and plateau properties of all the neurons comp rising the pyloric network. The activation was dose dependent, and was blocked by the muscarinic antagonists activated bursting and plateau properties of all the neurons comprising the pyloric network. The acti vation was dose dependent, and was blocked by the muscarinic antagonis ts atropine, pirenzepine, and scopolamine. The oscillatory behavior tr iggered by the muscarinic stimulation was specific to each type of pyl oric neuron. The isolated neuron AB had the shortest oscillation perio d and depolarizing phase. The constrictor neurons (LP, PY, IC) were th e slowest oscillators, and only oscillated upon hyperpolarizing curren t injection. Under muscarinic modulation, the individual bursting acti vities of the isolated pyloric neurons were of the same type as their activities when isolated from the network but modulated by central inp uts (Bal et al., 1988). The VD neuron is an exception since it was a r apid oscillator in the latter situation and became a slow oscillator w hen modulated by a single muscarinic agonist. To determine the relativ e importance of the muscarinic-dependent bursting properties of the in dividual pyloric neurons in the operation of the intact network, a pro gressive reconstruction of the synaptic circuitry was attempted. We fo und that under certain conditions of muscarinic modulation a new compo site pacemaker could be created, composed of the electrically coupled VD, AB, and PD neurons. This can result in the generation of new pylor ic patterns that were very sensitive to the membrane potential of indi vidual network neurons. The data also confirmed that, in a rhythmic '' pattern-generating network,'' the pacemaker role may not be definitely attributed to a given neuron but instead could be assigned to other n eurons by modulation of their respective oscillatory capabilities.