Electrophysiologic responses and adenylate cyclase activities of mouse spinal cord-dorsal root ganglion explants rendered tolerant by chronic exposure to morphine or pertussis toxin.

Abstract

Explant cultures of fetal mouse spinal cord with attached dorsal-root ganglia (DRGs) (Fig. 1: Crain 1976) provide a valuable in vitro system for study of neurotransmitter modulation in the CNS. We have previously developed and utilized this system extensively for analysis of the actions of opioids on the electrophysiologic responses of these neurons and for study of the development of tolerance to opioid depressant effects (see review by Crain, 1984). Exposure of fetal mouse spinal cord-DRG explants to opioid alkaloid or peptide agonists resulted in stereospecific, naloxone-reversible, dose-dependent depression of sensory-evoked dorsal-horn synaptic-network responses within a few minutes (e.g. Fig. 2A; Crain et al., 1977, 1978). After chronic exposure to opioids, e.g. 2–3 days in 1 μM morphine (at 35°C), sensory-evoked dorsal-horn responses recovered, and they could then be elicited by DRG stimuli in the presence of opioids at concentrations 10- to 100-fold higher than required to depress a naive expiant (Fig. 2B; (Crain et al., 1979). In addition, these opioid-tolerant explants developed significant cross-tolerance to serotonin (5HT) (Crain et al., 1982). The tolerant state did not develop if the explants were exposed to morphine at lower temperatures (e.g. 20°C for as long as 7 days; Fig. 2C). The data suggest that the sustained decrease in opioid sensitivity observed during chronic opioid exposure at 35°C is mediated by a temperature-dependent metabolic change in these neurons (Crain et al., 1979; Crain, 1984).