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  • 1
    The effects of nociceptin/orphanin FQ (N/OFQ) and opioid receptor agonists on voltage-activated calcium channel currents (ICa) were examined in acutely isolated mouse trigeminal ganglion neurons using whole-cell patch-clamp recordings. These effects were correlated with responses of the neurons to capsaicin and binding of Bandeiraea simplicifolia isolectin B4 (IB4).
  • 2
    Trigeminal neurons were divided into two populations based on the presence (type 2) or absence (type 1) of a prominent T-type ICa. N/OFQ potently (EC50 of 19 nm) inhibited high-voltage-activated (HVA) ICa in most (82 %) small (capacitance < 12 pF) type 1 neurons, but few (9 %) larger (> 12 pF) type 1 neurons. N/OFQ inhibited ICa in few (23 %) type 2 cells, and did not affect the T-type ICa in any cell.
  • 3
    The μ-opioid agonists DAMGO and morphine inhibited ICa in most type 1 neurons, more often (95 %versus 77 %) in the small cells. The inhibition of ICa by DAMGO and morphine was more efficacious in small versus large type 1 neurons. μ-Opioids did not inhibit ICa in type 2 neurons.
  • 4
    Most small type 1 neurons were sensitive to capsaicin (93 %) and bound IB4 (86 %). Fewer larger type 1 neurons responded to capsaicin (30 %) or bound IB4 (58 %). Type 2 neurons did not respond to capsaicin, although some bound IB4 (35 %).
  • 5
    Thus, N/OFQ preferentially inhibits HVA ICa in a subpopulation of small nociceptive trigeminal ganglion neurons that is also highly sensitive to μ-opioid agonists.