Distinct parafacial regions in control of breathing in adult rats

Recently, based on functional differences, we subdivided neurons juxtaposed to the facial nucleus into two distinct populations, the parafacial ventral and lateral regions, i.e., pF(V) and pF(L). Little is known about the composition of these regions, i.e., are they homogenous or heterogeneous populations? Here, we manipulated their excitability in spontaneously breathing vagotomized urethane anesthetized adult rats to further characterize their role in breathing. In the pF(L), disinhibition or excitation decreased breathing frequency (f) with a concomitant increase of tidal volume (V(T)), and induced active expiration; in contrast, reducing excitation had no effect. This result is congruent with pF(L) neurons constituting a conditional expiratory oscillator comprised of a functionally homogeneous set of excitatory neurons that are tonically suppressed at rest. In the pF(V), disinhibition increased f with a presumptive reflexive decrease in V(T); excitation increased f, V(T) and sigh rate; reducing excitation decreased V(T) with a presumptive reflexive increase in f. Therefore, the pF(V), has multiple functional roles that require further parcellation. Interestingly, while hyperpolarization of the pF(V) reduces ongoing expiratory activity, no perturbation of pF(V) excitability induced active expiration. Thus, while the pF(V) can affect ongoing expiratory activity, presumably generated by the pF(L), it does not appear capable of directly inducing active expiration. We conclude that the pF(L) contains neurons that can initiate, modulate, and sustain active expiration, whereas the pF(V) contains subpopulations of neurons that differentially affect various aspects of breathing pattern, including but not limited to modulation of ongoing expiratory activity.