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., pFV and pFL. 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 pFL, disinhibition or excitation decreased breathing frequency (f) with a concomitant increase of tidal volume (VT), and induced active expiration; in contrast, reducing excitation had no effect. This result is congruent with pFL neurons constituting a conditional expiratory oscillator comprised of a functionally homogeneous set of excitatory neurons that are tonically suppressed at rest. In the pFV, disinhibition increased f with a presumptive reflexive decrease in VT; excitation increased f, VT and sigh rate; reducing excitation decreased VT with a presumptive reflexive increase in f. Therefore, the pFV, has multiple functional roles that require further parcellation. Interestingly, while hyperpolarization of the pFV reduces ongoing expiratory activity, no perturbation of pFV excitability induced active expiration. Thus, while the pFV can affect ongoing expiratory activity, presumably generated by the pFL, it does not appear capable of directly inducing active expiration. We conclude that the pFL contains neurons that can initiate, modulate, and sustain active expiration, whereas the pFV contains subpopulations of neurons that differentially affect various aspects of breathing pattern, including but not limited to modulation of ongoing expiratory activity.