Development of cross-tolerance to 5-hydroxytryptamine in organotypic cultures of mouse spinal cord-ganglia during chronic exposure to morphine

Exposure of organotypic explants of mouse spinal cord with attached dorsal root ganglia (DRGs) to low concentrations (～10nM) of 5-hydroxytryptamine (5-HT) markedly depressed sensory-evoked dorsal-horn network responses, resembling the acute effects of opioids in these cultures. Attenuation of cord responses by 5-HT was not prevented by exposure to the 5-HT antagonists, methysergide and cyproheptadiene, nor to the opiate antagonist, naloxone. Explants that had become tolerant to morphine after chronic exposure (1 μM) for > 2 days often developed cross-tolerance to 5-HT. Acute exposure of morphine-tolerant explants to naloxone (1 μM) further attenuated the effects of 5-HT so that the minimum depressant levels of 5-HT were often increased up to 30-fold. Increasing the extra-cellular Ca⁺⁺ concentration (to 5 mM) and/or introduction of 4-aminopyridine markedly antagonized the depressant effects of 5-HT on DRG-evoked cord responses, so that 5-HT levels comparable to those used on morphine-tolerant explants were required to depress naive explants. These depressant effects of 5-HT on cord-DRG explants are consonant with antinociceptive actions of 5-HT administered to dorsal cord $\text{in situ}$. Our data suggest that 5-HT may block neuronal components of dorsal horn networks at similar regions to those that are depressed by opiates, e.g. presynaptic DRG nerve terminals where abundant opiate receptors are located. The marked attenuation of the depressant effects of both 5-HT and opiates on cord-DRG explants by high Ca⁺⁺ raises the possibility that cross-tolerance to 5-HT in morphine-tolerant explants may result from the same neuronal alterations that render dorsal-horn networks tolerant to opiates. Furthermore, the increased degree of cross-tolerance to 5-HT after acute introduction of naloxone in morphine-tolerant cultures may be an expression of opiate dependence.