Postganglionic cholinergic collaterals drive pathological sympathetic hyperactivity

Traditionally, sympathetic ganglia have been viewed as straightforward relay stations, where a single synapse connects cholinergic preganglionic neurons with noradrenergic postganglionic neurons, controlling peripheral organs, blood vessels and the heart. However, Clyburn et al. (2023) demonstrated that our canonical understanding may be oversimplified, uncovering a network of cholinergic collaterals between noradrenergic postganglionic neurons in the stellate ganglia (SG) of mice. They concluded that these intraganglionic collateral connections amplify inputs from sympathetic preganglionic neurons (SPNs), increasing sympathetic outflow to the heart independently from CNS input. Building on these findings, Li et al. (2024) hypothesized that this upregulation in sympathetic outflow from postganglionic neurons may underlie pathological sympathetic dysregulation. In their recent article in The Journal of Physiology, Li's group set out to experimentally determine if remodelling of sympathetic postganglionic neurons increases cholinergic collateral activity, driving cardiac sympathetic hyperactivity seen in cardiovascular diseases such as hypertension. Li et al. implanted miniature osmotic pumps in wild-type mice to continually release either saline (control) or angiotensin II (hypertension) for 1 week. After the infusion period, cardiac-projecting SG neurons were identified, and whole-cell patch clamp recordings were performed. SG neurons from hypertensive mice showed elevated spontaneous EPSC activity compared to control mice, with greater firing frequency but comparable spike amplitudes. Application of TTX, a potent sodium channel blocker, to the ganglia reduced EPSC bursting frequency in both cohorts. This indicated that action potentials within the SG stimulated EPSCs in the control mice and were responsible for the augmented EPSCs in the hypertensive mice. As acetylcholine release was the same between the two cohorts, the question arose whether EPSC hyperactivity resulted from increased intraganglionic collateral connections or cellular hyperactivity itself. To explore this, the authors used a transgenic mouse line (THChATKO) that disabled cholinergic collaterals between noradrenergic neurons in the SG by halting their ability to synthesize acetylcholine (Clyburn et al., 2023). They repeated their original set of experiments, implanting THChATKO mice with osmotic pumps delivering either saline or angiotensin II for 7 days. Whole-cell patch clamp recordings of cardiac-projecting SG neurons revealed that both control and hypertensive THChATKO mice had similar spontaneous EPSC bursting frequency and amplitude. Unlike the wild-type mice, application of TTX to the ganglia from both control and hypertensive THChATKO mice did not further decrease EPSC bursting frequency, indicating that cholinergic collaterals were responsible for both basal EPSCs and the increased EPSC frequency observed in the hypertensive mice. Building on this, the authors assessed if repeated exposure to hypertension altered synaptic activity of SG neurons and if cholinergic collaterals contributed to this alteration. Current-clamp recordings of single cardiac-projecting SG neurons showed an increased number of action potentials in neurons from hypertensive mice compared to control mice at each current injection step. In SG from THChATKO mice, both control and hypertensive conditions showed near-identical numbers of action potentials at each current injection step. This pivotal finding demonstrated that only mice with a fully functional network of cholinergic collaterals show altered synaptic activity within the SG in response to induced hypertension. To confirm that these differences were due to intraganglionic cholinergic collateral activity and not altered intrinsic excitability of SG neurons, the authors measured outward potassium currents (IKA) in SG neurons subject to depolarizing pulses. There were no observed differences in IKA activity between control and hypertensive neurons for both wild-type and THChATKO mice, indicating that intrinsic excitability of SG neurons was not altered and did not contribute to the upregulated synaptic activity seen following induced hypertension. Having validated these findings in cardiac-projecting SG neurons, the authors conducted experiments to determine if hypertension altered the activity of SG neurons projecting to other end organs, specifically brown adipose tissue (BAT). Repeating the protocols used for cardiac-projecting SG neurons, they found comparable results; hypertensive mice had increased EPSC frequency in BAT-projecting SG neurons compared to control animals. BAT-projecting SG neurons from hypertensive THChATKO mice had similar EPSC frequency to control THChATKO mice. Current-clamp experiments in BAT-projecting SG neurons also showed increased action potentials in hypertensive mice compared to control mice, but no difference between hypertensive and control THChATKO mice. From this body of work, the authors concluded that increased interneural transmission within the SG following hypertension was due to the network of cholinergic collaterals, not increased CNS activation or intrinsic excitability of SG neurons. They suggested that these collateral connections might contribute to the sustained sympathetic hyperactivity characteristic of cardiovascular disease, predisposing individuals to a greater risk of dangerous arrhythmias and heart failure. These findings open the door for research into other neurological conditions presenting with pathological sympathetic hyperactivity and raise questions about cholinergic collaterals and the role they may play in disease states as therapeutic gateways. Our group is interested in understanding plasticity of the sympathetic connectome leading to haemodynamic instability after spinal cord injury (SCI). Disruption of descending sympathetic drive from the brainstem after SCI leads to neuroplastic changes within the sympathetic nervous system, causing the formation of aberrant hyperexcitable synapses between SPNs and sensory afferents and hypersensitization of adrenergic receptors on the peripheral vasculature (Phillips & Krassioukov, 2015). These changes prime the sympathetic nervous system to trigger life-threatening blood pressure increases in response to often-innocuous sensory stimuli in a condition known as autonomic dysreflexia (AD) (Eldahan & Rabchevsky, 2018). Patients with AD can experience dramatic increases in blood pressure up to 40 times per day, pathologically stressing their cardiovascular system. Clinically, prevention of AD episodes focuses on blunting sympathetic postganglionic neurotransmission to blood vessels by blocking α1-adrenergic receptors, but these approaches can exacerbate the already pathologically low blood pressure experienced by individuals living with SCI. Thus, there is a need to find therapeutic targets within the nervous system itself to prevent these life-threatening hypertensive insults. Interpolating the findings of Li and colleagues to the field of SCI would suggest that episodic hypertension seen with AD may also trigger neuroplastic changes to the cholinergic collateral network within the SG, increasing sympathetic postganglionic drive to the heart. The cardiovascular consequences of these neuroplastic changes in the SG would be repeated over-excitation of β-adrenergic receptors on the heart, leading to their eventual desensitization. This desensitization would only be further compounded by the repeated transient elevations in circulating catecholamine levels seen during bouts of AD (West et al., 2016). By decreasing the sensitivity of these receptors, cardiac reserve would be diminished, which could increase the risk of further cardiovascular complications, including heart failure (West et al., 2016). As our group has previously stated, this phenomenon of hyperexcitability plasticity in SG cholinergic collaterals after SCI has not been interrogated in detail, and requires further investigation (Miller et al., 2023). In closing, Li and colleagues have elegantly demonstrated that the network of cholinergic collaterals between noradrenergic SG neurons is responsive to chronic hypertension, forming new excitatory connections between noradrenergic postganglionic neurons. The mechanism behind these neuroplastic changes remains unknown, and future work may benefit from exploring if the frequent hypertensive episodes seen with AD after SCI play a role in their development. Doing so may provide a new therapeutic target for treating pathological desensitization of cardiac adrenergic receptors after SCI, potentially reducing heart failure risk for people living with this condition. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. No competing interests declared. D.S.: Conception or design of the work; Drafting the work or revising it critically for important intellectual content; Final approval of the version to be published; Agreement to be accountable for all aspects of the work. No funding.