Pro- and anticonvulsant actions of morphine and the endogenous opioids: Involvement and interactions of multiple opiate and non-opiate systems

The proconvulsant actions of high doses of systemic morphine are probably mediated by 3 different systems. One of them produces non-convulsant electrographic seizures and can be activated separately from the others both by intracerebroventricular injections as well as microinjections into discrete subcortical areas. The enkephalins and β-endorphin, when administered to the same loci, produce similar effects. Pharmacological evidence suggests that specific opiate receptors of the β-subtype mediate the epileptiform effects produced by this system. The second system mediating proconvulsant effects of systemic morphine is not mediated by stereo-specific opiate receptors. It produces behavioral convulsions, and the GABA-ergic system has been implicated in its action. A third proconvulsant action of systemic morphine can be activated separately from the other two systems by administering this compound with other convulsive agents or manipulations. Specific μ-type opiate receptors are implicated in this effect. In addition to potent proconvulsant effects, systemic morphine also has anticonvulsant properties which are mediated by specific opiate μ-receptors. The conditions under which morphine acts as a proconvulsant rather than an anticonvulsant agent are, as yet, not understood.