Mutual inhibition between Bötzinger-complex bulbospinal expiratory neurons detected with cross-correlation in the decerebrate rat

Abstract

We examined the synaptic connections between pairs of Bötzinger-complex, bulbospinal expiratory neurons in decerebrate rats. All were antidromically activated from the spinal cord at the C2-C3 border. Cross-correlation histograms of 18 ipsilateral pairs showed troughs on both sides of time zero (8) and to one side of time zero (4); most (12) were accompanied by peaks at time zero. Similarly, cross-correlation histograms of the contralateral pairs (12) showed troughs on both sides of time zero (3) and to one side of time zero (3); few (2) were accompanied by peaks at time zero. We considered the troughs in these cross-correlation histograms to be evidence of inhibition between the neurons and sought confirmation of the inhibitory connection. First, using the antidromic activation stimulus, we computed post-stimulus histograms of the extracellularly recorded discharge for six neurons and found that three showed troughs. Then, we continued this approach, computing post-stimulus averages of the membrane potentials recorded intracellularly from these neurons after iontophoresis of chloride to reverse inhibitory synaptic potentials. Depolarising potentials were observed in 15 of 16 of these averages. We interpreted these as reversed inhibitory post-synaptic potentials and concluded that Bötzinger-complex, bulbospinal expiratory neurons inhibit one another in rats as they do in cats.