Morphine (intracerebroventricular) activates spinal systems to inhibit behavior induced by putative pain neurotransmitters

Abstract

Previous investigations find that morphine administered i.c.v. induces antinociception directly at supraspinal sites and indirectly via activation of descending spinal systems. Independent experimentation suggests substance P and N-methyl-D-aspartate (NMDA) administered intrathecally (i.t.) can act as putative pain neurotransmitters to stimulate afferent pathways mediating nociception. The present studies were designed to determine whether a functional link exists between these observations. Mice were administered morphine i.c.v. 15 min before i.t. injections of substance P or NMDA. Additional investigations utilized coadministration of substance P or NMDA i.t. with one of several antagonists. Morphine administered i.c.v. inhibited both substance P- and NMDA-induced behavior in a dose-dependent manner. Coadministration of noradrenergic or adenosine receptor antagonists with substance P or NMDA i.t. dose-dependently reversed morphine (i.c.v.)-mediated inhibition. Methysergide injected i.t. caused significant, but only partially effective, antagonism of the effects of morphine (i.c.v.). Naloxone coadministered i.t. was effective in reversing morphine (i.c.v.)-mediated inhibition of NMDA-induced behavior, but ineffective in the substance P assay. These data demonstrate a functional link between activation of descending systems mediating antinociception by morphine (i.c.v.) and inhibition of putative pain neurotransmitters by spinally active antinociceptive agents. The potential involvement of serotonergic and opioid spinal systems is not clear, but noradrenergic and adenosine spinal pathways appear to play an important role in the indirect actions of morphine (i.c.v.). Differences in the inhibition of NMDA- and substance P-induced behavior also provide evidence for the presence of substance P and NMDA receptors in separate afferent pathways transmitting nociceptive stimuli.