Pre-sympathetic neuron dysfunction – time to reconsider increased intrinsic activity as the cause of neurogenic hypertension

In this issue, there is a paradigm-shifting article (Almado et al., 2014) that examines the effect of chronic intermittent hypoxia (CIH) (which stimulates the development of neurogenic hypertension) on the intrinsic electrophysiological properties of rostral ventrolateral medulla (RVLM) pre-sympathetic neurons in juvenile rats. The authors provide the first experimental evidence that both pre-sympathetic and phrenic nucleus-projecting neurons of the RVLM exhibit the characteristics of intrinsic pacemakers in juvenile rats. Significantly, the authors present experimental data demonstrating that the stress of chronic intermittent hypoxia (CIH), which evokes neurogenic hypertension, produces no change in the electrophysical properties of these RVLM neurons. These findings challenge the prevailing hypothesis that increased activity of pre-sympathetic neurons is the source of enhanced sympathetic drive during neurogenic hypertension. This study is the first report of the impact of CIH on the neural activity of RVLM neurons in juvenile animals, in which there is full expression of functional ion channels and synaptic receptors compared to neonatal rats in which prior studies have been conducted. As such, these findings, generated in animals of a significantly older age than was previously reported, are likely more representative of the impact of CIH on RVLM neural activity in the fully developed adult brain than studies conducted in neonatal animals. Owing to the significant global adverse health impact of hypertension, studies such as these, which suggest new directions in which to direct studies designed to elucidate the mechanisms driving neurogenic hypertension have potential high significance for human health.

It has been long held that pre-sympathetic neurons in the RVLM are the source of sympathetic activity and it has been postulated that changes in the intrinsic properties (i.e., increased activity) of these neurons are the main causal mechanism for the development of sympathetic overactivation and neurogenic hypertension. There are considerable challenges in measuring the activity of these pre-sympathetic neurons – as such this hypothesis is pre-dominantly supported by data generated in neonatal rats exhibiting a still developing central nervous system. Recently work by several pioneering groups (Degacheva et al., 2013; Gao & Derbenev, 2013) has facilitated the study of this neuronal population in brainstem slices from juvenile rats – a significant technical achievement. The studies presented by Almado et al., in this issue of Experimental Physiology utilize the same technical advancements to study the underlying activity of pre-sympathetic neurons in the RVLM of juvenile animals. To test the hypothesis that changes in RVLM neurons represent the source of neurogenic hypertension the author’s utilized chronic intermittent hypoxia (CIH), a well-established model of neurogenic hypertension (Zocaal et al., 2008, Zoccal et al., 2009, Moraes et al, 2013). To test this hypothesis the authors employed retrograde labeling of 1) RVLM pre-sympathetic neurons and, 2) RVLM/BöTC phrenic nucleus-projecting neurons in juvenile rats and whole-cell patch clamp techniques in brain-stem slices of animals subjected to CIH or normoxia for 10 days.

The authors report that RVLM pre-sympathetic neurons projecting to the thoracic spinal cord and RVLM neurons projecting to the phrenic motor nucleus exhibit intrinsic pacemaker activity. This finding has important implications for our understanding of sympathetic drive – suggesting these neurons are acting as the central generator of sympathetic efferent activity. It is likely this finding has not been reported previously due to external factors impacting the results obtained in earlier studies e.g., presence of anesthetic, temperature, comparative immaturity of neonatal rat brains. These findings extend the work of previous pioneering studies conducted examining RVLM neurons in juvenile rats that did not examine the firing properties of RVLM pre-sympathetic neurons (Dergacheva et al., 2013) or focused on kidney-related RVLM pre-sympathetic neurons (Gao & Derbenev, 2013). The authors have extensively verified that their experimental paradigm of neurogenic hypertension, chronic intermittent hypoxia, evokes hypertension that correlates with increased sympathetic activity (Zocaal et al., 2008, Zoccal et al., 2009, Moraes et al, 2013). Using this model the authors have previously demonstrated that CIH-induced sympathetic overactivity occurs in the late expiratory period (Zoccal et al., 2008) - suggesting increased activity of neurons termed the augmenting expiratory neurons, findings in accordance with the hypothesis of neurogenic hypertension being driven by increased neural activity. In contradiction to this hypothesis, in the current study, following CIH the authors did not detect any change in the intrinsic electrophysical properties of either RVLM pre-sympathetic neurons or RVLM/BöTC phrenic nucleus-projecting neurons. These novel data demonstrate that the stress of CIH, which evokes neurogenic hypertension, produces no changes in the neuronal excitability of either of these neuronal populations.

The current studies provide three novel pieces of evidence 1) it is now technically possible to conduct whole-cell patch-clamp recordings from multiple RVLM sub-populations in juvenile rats, 2) RVLM pre-sympathetic and phrenic nucleus-projecting neurons present pacemaker activity and, 3) in the CIH model of sympathetic overactivity and hypertension the intrinsic activity of these neurons remains unchanged. These findings will enable further studies to be conducted in juvenile rats, in which the ion channels and synaptic receptors are fully expressed and functional – potentially providing further ground breaking insights into neural activity under control and hypertensive conditions. Collectively, these studies disprove the concept that that in the CIH experimental model of neurogenic hypertension an increase in activity of pre-sympathetic neurons underlies the increase in sympathetic activity. Instead the intriguing hypothesis that modulation in synaptic excitatory inputs to RVLM cells may underlie sympathoexcitation is proposed. These findings present an alternative future direction for investigations into the mechanisms underlying enhanced sympathetic activity - which remain to be elucidated and require investigation and confirmation across multiple animal models of neurogenic hypertension.