Brainstem and suprapontine structures controlling cardiorespiratory function are reasonably well defined, but many subtleties of the intricate neural circuitry are yet to be revealed. Contemporary experimental approaches allow for elegant delineation of nodal connectivity underpinning integrative behaviours. The highly co-ordinated contemporaneous activation of respiratory motor and autonomic outputs at rest and in response to stress is crucial to survival. In this issue of Experimental Physiology, Malheiros-Lima et al. (2022) extend our understanding of pontomedullary communication in the control of breathing and blood pressure. Medullary catecholaminergic C1 neurons and ventrolateral pontine adrenergic A5 neurons are two key nodes in the brainstem network controlling cardiorespiratory function. Both are strongly implicated in chemoreflex control of respiratory and sympathetic outflows, but the nature of the interaction between the nodes is not fully appreciated. In a sophisticated experimental series, Malheiros-Lima et al. (2022) set out to establish: (1) whether C1 neurons projecting directly to A5 neurons are glutamatergic (excitatory); (2) whether retrograde ablation of C1 neurons via the A5 region reduces hypoxia-dependent activation of C1 neurons; and (3) whether ionotropic glutamatergic receptor blockade in A5 neurons decreases the cardiorespiratory effects of selective activation of C1 neurons. The experimental findings demonstrate a crucial link between C1 and A5 neurons in the cardiorespiratory control network. In adult male rats, lentiviral transfection of catecholaminergic (tyrosine hydroxylase-positive) neurons in the rostral ventrolateral medulla (C1 neurons) with a fluorescent protein provided confirmation that transduced C1 terminals, which are predominantly glutamatergic, make direct contact with ipsilateral A5 neurons of the pons, amongst other sites, such as the A6 locus coeruleus. Retrograde destruction of C1 neurons was achieved by microinjection of a selective poison unilaterally into the A5 region, which decreased ipsilateral C1 catecholaminergic neurons by 25% compared with the contralateral neuronal count. Exposure to hypoxia for 3 h elicited the expected increases in c-Fos expression in C1 neurons; however, after selective poisoning, the density of c-Fos-positive non-catecholaminergic C1 neurons was unchanged, whereas the total number of c-Fos-positive catecholaminergic neurons was reduced compared with the contralateral population. Bilateral injections of kynurenic acid into the A5 region abrogated, in part, the increase in blood pressure, splanchnic sympathetic nerve activity and phrenic nerve motor activity elicited by selective optogenetic activation of C1 catecholaminergic neurons (transfected with channelrhodopsin2 and activated by laser). The study reveals that medullary C1 neurons activated by hypoxia project directly to pontine A5 neurons. Moreover, glutamatergic receptor blockade in the A5 node suppressed sympathetic and respiratory responses to selective C1 neuronal activation. Although C1 neurons are traditionally considered catecholaminergic, possessing the enzymatic portfolio to produce various catecholamines, Malheiros-Lima et al. (2022) found that > 90% of the varicosities in the A5 region are glutamatergic. The physiological effects of selective pharmacological blockade of glutamate neurotransmission corroborate the neuroanatomical observations suggesting that the C1–A5 connection is mediated via glutamate and accounts for an appreciable amount of C1-dependent cardiorespiratory activation. A5 neurons might also have capacity for a direct independent contribution owing to their intrinsic chemosensitivity (Kanbar et al., 2011), which appears to be a ubiquitous feature of brainstem neural circuits. There is growing evidence for a contribution by C1 neurons to breathing (Malheiros et al., 2018), extending to the respiratory response to hypoxia (Malheiros et al., 2020). The present study extends this work, revealing a contribution by the C1–A5 pathway to respiratory motor drive, shown by retardation in phrenic motor amplitude in response to hypoxia after glutamatergic inhibition in the A5 region. The precise link between A5 neurons and other pontine and medullary sites implicated in the chemoreflex-dependent drive to breathe remains to be determined, but the lack of effect of A5 inhibition on hypoxia-dependent tachypnoea might exclude connectivity with rhythmogenic sites, such as the pre-Bötzinger complex. Confusingly, however, previous work reported that A5 lesions do not affect ventilatory responses to hypoxia in conscious rats nor the phrenic nerve response in the in situ working heart–brainstem preparation (Taxini et al., 2017). This suggests that anaesthesia affects the relevant circuitry behaviour and, as such, is a confounder in the present study. Thus, it appears that the C1–A5 pathway forms an intrinsic component of the chemoreflex circuitry mediating sympathoexcitation and the pressor response to stress, with a potential contribution to the accompanying respiratory excitation. Pontine adrenergic neurons project to most structures of the brain in a highly organized manner. Therefore, it might be that specific elements of the fight or flight response, beyond cardiorespiratory adjustments, are also manifest through the C1–A5 axis. We can anticipate continued elegant work from this research group striving to unravel these complex connections. Let's keep in touch! None declared.
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Sep 1, 2023
Einav Fleck 
Open Access