Modulation of Ca²⁺ channels by opioid receptor-like 1 receptors natively expressed in rat stellate ganglion neurons innervating cardiac muscle

Postganglionic sympathetic nerve terminals innervate cardiac muscle and express opioid receptor-like 1 (ORL1) receptors, the most recently described member of the opioid receptor subclass. ORL1 receptors are stimulated by the endogenous heptadecapeptide nociceptin (Noc). To better understand how the signaling events by Noc regulate sympathetic neuron excitability, the goal of the present study was to determine whether sympathetic stellate ganglion (SG) neurons, innervating the heart, natively express ORL1 opioid receptors and couple to Ca²⁺ channels. SG neurons in adult male rats were retrograde-labeled with a fluorescent tracer via injection of the ventricular muscle employing ultrasound imaging. Thereafter, N-type Ca²⁺ channel modulation was investigated using the whole-cell variant of the patch-clamp technique. Exposure of labeled SG neurons to Noc resulted in a concentration-dependent inhibition of Ca²⁺ currents (with an estimated EC₅₀ of 193 ± 14 nM). Pre-exposure of SG neurons to the ORL1 receptor blocker, [Nphe¹,Arg¹⁴,Lys ¹⁵]N/OFQ-NH₂ (UFP-101), significantly decreased the Noc-mediated Ca²⁺ current inhibition. The Ca²⁺ current inhibition was also blocked by pertussis toxin pretreatment, indicating that signaling occurs via Gα_i/o G proteins. Finally, the full-length ORL1 receptor cDNA in SG neurons was cloned and sequenced. Of the two known alternatively spliced variants in rats, sequencing analysis showed that the ORL1 receptor expressed in SG neurons is the short form. Overall, these results suggest that stimulation of postsynaptic ORL1 receptors by Noc in SG neurons regulate cardiac sympathetic activity.