Heterogeneous sensitivity of cultured dorsal root ganglion neurones to opioid peptides selective for µ - and δ -opiate receptors

Abstract

Opiate-mediated analgesia at the spinal level is thought to involve opiates binding to opiate receptors on primary afferent terminals^1–3 resulting in a selective depression of neurotransmitter release^4–9. Multiple opiate receptor types have been distinguished^11–22 and µ - and δ -opiate receptors, originally described by Kosterlitz et al.^17,18,22, have been demonstrated on primary afferent terminals¹ but the correspondence of these opiate receptors to opiate-mediated depression of transmitter release is unclear. However, opiates binding to receptors present on individual somata of the dorsal root ganglion (DRG) neurones in dissociated cell culture have been reported to reduce the duration and amplitude of calcium-dependent action potentials^8,10. Therefore these opiate receptors might have a function similar to those on primary afferent terminals where a decrease in calcium entry would be correlated with a decrease in transmitter release^23,24. We have now studied the response of DRG neurones to opiate agonists with different affinity for μ- or δ-receptors and our results suggest that both receptor types can mediate decrease in somatic calcium-dependent action potentials but that there is a variable proportion of µ- and δ-receptors on DRG neurones.