Sympathetic preganglionic neurones (SPNs) convey sympathetic activity flowing from the CNS to the periphery to reach the target organs. Although previous in vivo and in vitro cell recording studies have explored their electrophysiological characteristics, it has not been possible to relate these characteristics to their roles in cardiorespiratory reflex integration. We used the working heart–brainstem preparation to make whole cell patch clamp recordings from T3–4 SPNs (n = 98). These SPNs were classified by their distinct responses to activation of the peripheral chemoreflex, diving response and arterial baroreflex, allowing the discrimination of muscle vasoconstrictor-like (MVClike, 39%) from cutaneous vasoconstrictor-like (CVClike, 28%) SPNs. The MVClike SPNs have higher baseline firing frequencies (2.52 ± 0.33 Hz vs. CVClike 1.34 ± 0.17 Hz, P = 0.007). The CVClike have longer after-hyperpolarisations (314 ± 36 ms vs. MVClike 191 ± 13 ms, P < 0.001) and lower input resistance (346 ± 49 MΩ vs. MVClike 496 ± 41 MΩ, P < 0.05). MVClike firing was respiratory-modulated with peak discharge in the late inspiratory/early expiratory phase and this activity was generated by both a tonic and respiratory-modulated barrage of synaptic events that were blocked by intrathecal kynurenate. In contrast, the activity of CVClike SPNs was underpinned by rhythmical membrane potential oscillations suggestive of gap junctional coupling. Thus, we have related the intrinsic electrophysiological properties of two classes of SPNs in situ to their roles in cardiorespiratory reflex integration and have shown that they deploy different cellular mechanisms that are likely to influence how they integrate and shape the distinctive sympathetic outputs.

Hypertension is associated with pathologically increased sympathetic drive to the vasculature. This has been attributed to increased excitatory drive to sympathetic preganglionic neurons (SPN) from brainstem cardiovascular control centers. However, there is also evidence supporting increased intrinsic excitability of SPN. To test this hypothesis, we made whole cell recordings of muscle vasoconstrictor-like (MVClike) SPN in the working-heart brainstem preparation of spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats. The MVClike SPN have a higher spontaneous firing frequency in the SH rat (3.85 ± 0.4 vs. 2.44 ± 0.4 Hz in WKY; P = 0.011) with greater respiratory modulation of their activity. The action potentials of SH SPN had smaller, shorter afterhyperpolarizations (AHPs) and showed diminished transient rectification indicating suppression of an A-type potassium conductance (IA). We developed mathematical models of the SPN to establish if changes in their intrinsic properties in SH rats could account for their altered firing. Reduction of the maximal conductance density of IA by 15–30% changed the excitability and output of the model from the WKY to a SH profile, with increased firing frequency, amplified respiratory modulation, and smaller AHPs. This change in output is predominantly a consequence of altered synaptic integration. Consistent with these in silico predictions, we found that intrathecal 4-aminopyridine (4-AP) increased sympathetic nerve activity, elevated perfusion pressure, and augmented Traube-Hering waves. Our findings indicate that IA acts as a powerful filter on incoming synaptic drive to SPN and that its diminution in the SH rat is potentially sufficient to account for the increased sympathetic output underlying hypertension.

Young, spontaneously hypertensive (SH) rats show an increased level of sympathetic activity that predates the development of hypertension (Simms et al 2009 J Physiol).We have examined the properties of muscle vasoconstrictor sympathetic preganglionic neurones (MVC‐SPN) in neonatal SH and Wistar rats. Whole‐cell recordings were obtained from SPN (Wistar n=169, SH n=76; aged p9–14) in T3–4 segment of the working heart‐brainstem preparation.SPN of SH rats showed a higher firing frequency (1.7 vs 3.0Hz) and smaller AHP amplitude. When analysed by functional subgroup it was apparent that these difference were secondary to changes in the excitability of MVC‐SPN. These showed significantly higher firing frequencies (2.2 vs 3.5Hz) with increased respiratory modulation. Additionally, the SH MVC‐SPN had shorter, smaller AHPs, lower input resistances and a weaker transient rectification (IA).Voltage clamp analysis of spontaneous excitatory synaptic events in MVC‐SPN showed similar frequencies and amplitudes of EPSCs. There was a modest preponderance of large EPSCs in SH rats (>30pA, <0.3Hz) which alone did not account for the increased spike discharge.In summary, the MVC SPN of neonatal SH rats are intrinsically more excitable. These changes occur prior to, and may play a causal role in, the development of hypertension.

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A population of sympathetic preganglionic neurones (SPN) recorded in vitro show gap junction‐mediated oscillations in membrane potential that underlie their spike discharge (Logan et al, 1996, J Physiol), but the role of such coupling in vivo is uncertain. We made whole‐cell recordings from functionally identified SPN in the rat (aged p5‐16) working heart‐brainstem preparation (WHBP, (Paton, 1996, J Neurosci Meths)) to examine the neural mechanisms generating their firing activity.108 SPNs identified in the T3‐4 segment were functionally categorised by responses to activation of peripheral chemoreflex (NaCN, ia) and diving response (cold to snout). Muscle vasoconstrictor (MVC, 38%) SPN were activated by both tests and converseley cutaneous vasoconstrictor (CVC, 27%) SPN were inhibited by both. MVCs had a higher baseline firing frequency in comparison with CVC (2.6±0.3 vs. 1.4±0.3 Hz; p=0.007) and shorter after‐hyperpolarization (156±10 vs. 296±39 ms, p=0.0001). MVC SPN showed a respiratory‐modulated pattern of firing generated by a torrent of kynurenate‐sensitive, excitatory synaptic inputs. By contrast, the activity of CVC SPN was driven by large, slow, biphasic membrane potential oscillations (n=14 of 15 examined) that resembled filtered action potentials which were relatively insensitive to membrane potential, not blocked by spinal kynurenate and were indistinguishable from those previously reported in vitro. Antidromic activation of CVC SPN evoked short latency depolarization confirming the presence of coupling.These findings indicate that gap junctional coupling plays an important functional role in the patterning and synchronization of the output from CVC but not MVC SPN.Funded by BHF.