Maturation of opioid sensitivity of fetal mouse dorsal root ganglion neuron perikarya in organotypic cultures: regulation by spinal cord.

Opioid agonists selectively decrease the duration of the Ca2+ component of the action potential recorded from embryonic dorsal root ganglion neurons in dissociated cell cultures. In contrast, no significant alterations in the action potentials generated by adult dorsal root ganglion neurons in vivo were detected during opioid exposure. In the present study, the perikaryal opioid sensitivity of fetal mouse dorsal root ganglion neurons was analyzed during maturation in organotypic explant cultures. To determine whether spinal cord might influence this sensitivity, neuron perikarya were tested in ganglia grown: (a) in isolation; (b) attached to spinal cord explants; and (c) attached to spinal cord, but decentralized by a dorsal root transection in mature explants 1-2 weeks before the tests. After 2-8 weeks in culture, the duration of the Ba2+-enhanced Ca2+ component of intracellularly recorded action potentials was measured prior to and during bath exposure to the opioid, [D-Ala2, D-Leu5]enkephalin. Sensitive neurons were characterized by a marked, reversible reduction (averaging about 50%) in the duration of the Ca2+ component (which was antagonized by naloxone). The fraction of opioid-sensitive neuron perikarya in dorsal root ganglia grown attached to cord explants was significantly lower (48%) than in ganglia grown isolated (78%) or decentralized in vitro (79%). The mean duration of the Ca2+ component was significantly shorter in ganglion cells which had been grown attached to cord, or subsequently decentralized, compared to cells grown in isolated ganglia (by 24 and 38%, respectively). This difference was even larger in the opioid-insensitive groups. Although opioid-sensitive perikarya in ganglia grown attached to cord had a significantly longer Ba2+-enhanced Ca2+ component than that of insensitive neurons, some of the insensitive perikarya in all 3 types of explant paradigms displayed Ca2+ components which were as prolonged as those of sensitive cells. The results obtained in this study support the hypothesis that the observed decrease in the fraction of opioid-sensitive perikarya during development of fetal mouse dorsal root ganglia is due to regulation by interactions with their central target tissue, the spinal cord. The developmental decrease in the duration of the Ca2+ component of the action potential of these ganglion cells is also enhanced by the presence of the spinal cord. However, regulation of functional opiate receptors and Ca2+ component duration of the ganglion cell perikarya appear to be independent processes.