Modulation of sympathetic preganglionic neuron activity via adrenergic receptors

The sympathetic preganglionic neurons (SPNs) play a key role in the sympathetic nervous system. Previous reports have suggested that norepinephrine (NE) directly affects SPNs via both inhibitory hyperpolarization interactions mediated by α2 receptors and excitatory depolarization interactions mediated by α1 receptors. It remains poorly understood, however, whether the excitability of SPNs can be inhibited indirectly (presynaptically) as well as directly (postsynaptically). We intracellularly recorded 41 SPNs using the whole-cell patch-clamp technique in spinal cord slice preparations of neonatal rats. We examined the effects of NE or dexmedetomidine hydrochloride (Dxm) (α2-adrenergic receptor agonist) on SPNs by analyzing the excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). EPSPs were dominant in 15 SPNs (EPSP-SPNs) and IPSPs were dominant in 7 SPNs (IPSP-SPNs) at baseline. We were unable to analyze the postsynaptic potentials in the other 19 SPNs, due to high frequency of action potential firings (firing-SPNs). At baseline, the membrane potentials and resistances of each type of SPN were similar. NE (1  μM) gradually depolarized the EPSP-SPNs and IPSP-SPNs (P   < 0.001) and NE significantly increased the EPSP frequency of the EPSP-SPNs (P   < 0.05). Dxm (10  nM) after application of NE decreased the EPSP frequency of the EPSP-SPNs (P  <  0.001) and the EPSP voltage and IPSP voltage of the IPSP-SPNs (P  <  0.05). In 5 of the 19 firing-SPNs, NE induced membrane hyperpolarization (P   < 0.05) and completely inhibited firings. Dxm had no effect in these neurons. The SPNs received inhibitory modulation through α2-adrenergic receptors. Some SPNs can be directly inhibited via effects independent of the α2 receptors.