Synchronization and Frequency Control of Bursting Neurons With Excitatory Synaptic Coupling

RJ Butera, J. C. Smith, and J. Rinzel

Breathing movements in mammals arise from networks of neurons in the brain stem that produce a complex rhythmic pattern. The primary kernel for rhythm generation is located in the pre-Botzinger Complex (Prebot), a subregion of the ventrolateral medulla. A brain slice that contains this region and continues to produce breathing motor output can be isolated in neonatal rats. This rhythmic motor pattern persists after Cl- and K+-dependent synaptic transmission is blocked and is consistent with the hybrid pacemaker-network hypothesis: the pacemaking kernel lies within a subpopulation of neurons with intrinsic bursting properties, and this population interacts with a pattern-formation network produce the complex temporal phase relationships responsible for the motor output driving respiratory activity. Prebot neurons with bursting pacemaker-like properties have been identified in vitro. Experimental evidence suggests that this subpopulation of neurons is coupled by excitatory amino acid (EAA)-mediated synapses and receives EAA-mediated tonic drive. Quantitative data regarding the specific ionic currents of cells in the Prebot are only recently becoming available. We are developing models to complement experimental work in investigating possible mechanisms of rhythm generation. In this present study we investigate the effects of excitatory drive and excitatory synaptic coupling on the synchrony and burst frequency of a population of bursting cells hypothesized to form the Prebot pacemaker kernel. The respiratory oscillator provides an interesting model system to investigate general principles underlying population behavior of coupled bursting neurons.

A minimal model of these neurons is developed. Bursting occurs via a persistent Na+ current with slow inactivation opposed by a K+-dominated leakage current. Frequency control of a pair of cells via excitatory tonic drive (gtonic) and excitatory coupling (gsyn) is examined. For low gsyn, the frequency increases with gtonic. For high gsyn, the frequency may decrease as gtonic is increased. Increasing gsyn reduces burst frequency by prolonging the length of the burst. The origin of synchronous bursting within the population is investigated.

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