Novel two-rhythm generator theory of breathing in mammals

For many years, the prevailing theory has been that the mammalian respiratory network is built in layers like an onion with, at its centre, a single rhythmogenic kernel that controls the timing of inspiration, expiration and airway muscle activity. The smallest anatomically contiguous network of neurones that remains able to generate a respiratory-like rhythmically active circuit was assumed by most investigators to represent this rhythmogenic kernel. A group of neurones with the requisite properties was identified in 1991 within a small region of the ventrolateral medulla in neonate rats and named the pre-Botzinger complex (preBotC) (Smith et al. 1991). The preBotC consists of at least two classes of synaptically interconnected glutamatergic neurones endowed with intrinsic bursting properties (Pena et al. 2004). However, in less reduced preparations (e.g. the in vitro brainstem–spinal cord preparation) the preBotC rhythmogenic kernel appears to be activated and/or paced by a distinct set of neurones (preinspiratory, pre-I, neurones) located more rostrally in the ventral respiratory column below the facial motor nucleus (parafacial respiratory group, pfRG) (Onimaru & Homma, 2003). The concept developed by Onimaru and colleagues since the late 1980s is that mammalian respiratory rhythm generation relies on two nests of interconnected pacemaker neurones with the pre-I neurones serving as master oscillator. However, this concept of respiratory rhythmogenesis has remained until recently within the general theoretical framework of a single rhythm generator that drives all respiratory outflows. In this issue of The Journal of Physiology, Janczewski & Feldman (2006) present neurophysiological data that support a different view of mammalian respiratory rhythm generation and of the role played by the pfRG in this process. Using decerebrate neonate rats treated with a pharmacological cocktail of ketamine (an NMDA receptor antagonist) and fentanyl (a μ-opiate agonist), they observed that expiratory motor activity remained regular whereas inspiratory activity occurred at integer multiples of the expiratory period (quantal pattern). Arguing that the quantal pattern of inspiratory activity is unlikely to result from selective gating of the inspiratory motor outflow at the motor or premotor neurone level, the authors conclude that their experimental conditions have uncovered the existence of separate rhythm generators for inspiration and expiration. Based on the differential effect of brain transections on inspiratory and expiratory activity in their preparation, the authors confirm that the preBotC region is the dominant site for inspiratory rhythm generation and they propose that the pfRG contains the core of the expiratory rhythm generating network. The notion that breathing in vertebrates depends on coupled oscillators that have originated in two separate rhombomeres is congruent with prior evidence on amphibian breathing (Vasilakos et al. 2005). Eerie similarities exist between the postulated pre-I oscillator of mammals and the buccal/gill oscillator of frogs on one hand and between the preBotC I oscillator of mammals and the putative lung oscillator of frogs on the other (Vasilakos et al. 2005). These similarities include the selective sensitivity of the preBotC and frog lung oscillator to opiates and the quantal nature of these outflows in the presence of submaximal doses of such drugs (Janczewski et al. 2002; Vasilakos et al. 2005). Yet, the mammalian pre-I oscillator and the buccal/gill oscillator of frogs probably did not originate in the same rhombomeres and are therefore unlikely to be homologous (Vasilakos et al. 2005). Janczewski & Feldman's results suggest therefore that a more rostral and distinct rhythm generator (the pre-I oscillator) may have evolved in mammals to control expiratory activity. Janczewski and Feldman's interpretations are internally logical and extremely appealing from a comparative physiological standpoint but several issues still need clarification. For example, the opiate agonist fentanyl does not actually uncouple the two hypothetical rhythm generators since inspiration is not free-running relative to expiratory activity. Inspiration just misses beats as it were (quantal breathing). Thus, although two rhythm generators (expiratory and inspiratory) may indeed exist, they are still conjoined twins at this stage. Secondly, the pfRG is anatomically coextensive with the retrotrapezoid nucleus (RTN), a region that contains chemosensitive neurones that are further activated by peripheral chemoreceptor stimulation and may encode a significant portion of the chemical drive to breathe (Guyenet et al. 2005). It is therefore possible than the transections performed by Janczewski & Feldman caudal to the RTN could have eliminated expiratory activity by suppressing the chemical drive to this particular outflow rather than by eliminating an expiratory rhythm generator per se. The authors discard this interpretation because the transection did not eliminate rhythmic inspiratory activity, but their experimental evidence is not definitive. The autonomic literature contains many examples of sequential coronal transections causing inhibition then reappearance of an outflow probably due to the removal of tonic inhibitory drives located somewhere between the transections (Morrison, 2004). In conclusion, the study by Janczewski & Feldman (2006) provides seminal evidence for a two-rhythm generator theory of mammalian breathing and their results provide yet another view of the role of the RTN/pfRG in respiration. Undoubtedly, each perception of the RTN/pfRG, namely central chemoreception, and inspiratory and expiratory rhythm generation, is correct in its own experimental context. What is needed is a more detailed understanding of this region of the brain. A clarification of the synaptic mechanisms responsible for quantal breathing in vertebrates would also help to better understand the cellular basis of the two-rhythm generator theory of breathing.