Central CO2 chemoreception and integrated neural mechanisms of cardiovascular and respiratory control

In this review, we ex-amine why blood pressure (BP) and sympathetic nerve activity (SNA) increase during a rise in central nervous system. (CNS) Pco 2 (central chemoreceptor stimulation). CNS acidification modifies SNA by two classes of mechanisms. The first one depends on the activation of the central respiratory controller (CRG) and causes the much-emphasized respiratory modulation of the SNA. The CRG prob-ably modulates SNA at several brain stem or spinal locations, but the most important site of interaction seems to be the caudal ventrolateral medulla (CVLM), where unidentified components of the CRG periodically gate the baroreflex. CNS Pco2 also influences sympathetic tone in a CRG-independent manner, and we propose that this process operates differently according to the level of CNS Pco2. In normocapnia and indeed even below the ventilatory recruitment threshold, CNS Pco2 exerts a tonic concentration-dependent excitatory effect on SNA that is plausibly mediated by specialized brain stem chemoreceptors such as the retrotrap-ezoid nucleus. Abnormally high levels of Pco2 cause an aversive interoceptive awareness in awake individuals and trigger arousal from sleep. These alerting responses presumably activate wake-promoting and/or stress-related pathways such as the orexinergic, noradrenergic, and serotonergic neurons. These neuronal groups, which may also be directly activated by brain acidification, have brainwide projections that contribute to the CO 2-induced rise in breathing and SNA by facilitating neuronal activity at innumerable CNS locations. In the case of SNA, these sites include the nucleus of the solitary tract, the ventrolateral medulla, and the preganglionic neurons.8 page(s

Retrotrapezoid nucleus, respiratory chemosensitivity and breathing automaticity

Breathing automaticity and CO₄ regulation are inseparable neural processes. The retrotrapezoid nucleus (RTN), a group of glutamatergic neurons that express the transcription factor Phox2b, may be a crucial nodal point through which breathing automaticity is regulated to maintain CO₄ constant. This review updates the analysis presented in prior publications. Additional evidence that RTN neurons have central respiratory chemoreceptor properties is presented, but this is only one of many factors that determine their activity. The RTN is also regulated by powerful inputs from the carotid bodies and, at least in the adult, by many other synaptic inputs. We also analyze how RTN neurons may control the activity of the downstream central respiratory pattern generator. Specifically, we review the evidence which suggests that RTN neurons (a) innervate the entire ventral respiratory column and (b) control both inspiration and expiration. Finally, we argue that the RTN neurons are the adult form of the parafacial respiratory group in neonate rats.10 page(s

Photostimulation of retrotrapezoid nucleus Phox2b-expressing neurons in vivo produces long-lasting activation of breathing in rats

The retrotrapezoid "nucleus" (RTN), located in the rostral ventrolateral medullary reticular formation, contains a bilateral cluster of ~1000 glutamatergic noncatecholaminergic Phox2b-expressing propriobulbar neurons that are activated by CO₂ in vivo and by acidification in vitro. These cells are thought to function as central respiratory chemoreceptors, but this theory still lacks a crucial piece of evidence, namely that stimulating these particular neurons selectively in vivo increases breathing. The present study performed in anesthetized rats seeks to test whether this expectation is correct. We injected into the left RTN a lentivirus that expresses the light-activated cationic channel ChR2 (channelrhodopsin-2) (H134R mutation; fused to the fluorescent protein mCherry) under the control of the Phox2-responsive promoter PRSx8. Transgene expression was restricted to 423 ± 38 Phox2b-expressing neurons per rat consisting of noncatecholaminergic and C1 adrenergic neurons (3:2 ratio). Photostimulation delivered to the RTN region in vivo via a fiberoptic activated the CO₂-sensitive neurons vigorously, produced a long-lasting (t₁/₂ = 11s) increase in phrenic nerve activity, and caused a small and short-lasting cardiovascular stimulation. Selective lesions of the C1 cells eliminated the cardiovascular response but left the respiratory stimulation intact. In rats with C1 cell lesions, the mCherry-labeled axon terminals originating from the transfected noncatecholaminergic neurons were present exclusively in the lower brainstem regions that contain the respiratory pattern generator. These results provide strong evidence that the Phox2b-expressing noncatecholaminergic neurons of the RTN region function as central respiratory chemoreceptors.14 page(s

The molecular basis for T-type Ca2+ channel inhibition by G protein beta2gamma2 subunits

Gbetagamma, a ubiquitous second messenger, relays external signals from G protein-coupled receptors to networks of intracellular effectors, including voltage-dependent calcium channels. Unlike high-voltage-activated Ca(2+) channels, the inhibition of low-voltage-activated Ca(2+) channels is subtype-dependent and mediated selectively by Gbeta(2)-containing dimers. Yet, the molecular basis for this exquisite selectivity remains unknown. Here, we used pure recombinant Gbetagamma subunits to establish that the Gbeta(2)gamma(2) dimer can selectively reconstitute the inhibition of alpha(1H) channels in isolated membrane patches. This inhibition is the result of a reduction in channel open probability that is not accompanied by a change in channel expression or an alteration in active-channel gating. By exchanging residues between the active Gbeta(2) subunit and the inactive Gbeta(1) subunit, we identified a cluster of amino acids that functionally distinguish Gbeta(2) from other Gbeta subunits. These amino acids on the beta-torus identify a region that is distinct from those regions that contact the Galpha subunit or other effectors.status: publishe

Protein Kinase A Activity Controls the Regulation of T-type CaV3.2 Channels by Gβγ Dimers*S⃞

Low voltage-activated (LVA), T-type, calcium channels mediate diverse biological functions and are inhibited by Gβγ dimers, yet the molecular events required for channel inhibition remain unknown. Here, we identify protein kinase A (PKA) as a molecular switch that allows Gβ2γx dimers to effect voltage-independent inhibition of Cav3.2 channels. Inhibition requires phosphorylation of Ser1107, a critical serine residue on the II-III loop of the channel pore protein. S1107A prevents inhibition of unitary currents by recombinant Gβ2γ2 dimers but does not disrupt dimer binding nor change its specificity. Gβγ dimers released upon receptor activation also require PKA activity for their inhibitory actions. Hence, dopamine inhibition of Cav3.2 whole cell current is precluded by Gβγ-scavenger proteins or a peptide that blocks PKA catalytic activity. Fittingly, when used alone at receptor-selective concentrations, D1 or D2 agonists do not elicit channel inhibition yet together synergize to inhibit Cav3.2 channel currents. We propose that a dual-receptor regulatory mechanism is used by dopamine to control Cav3.2 channel activity. This mechanism, for example, would be important in aldosterone producing adrenal glomerulosa cells where channel dysregulation would lead to overproduction of aldosterone and consequent cardiac, renal, and brain target organ damage