Effects of regional micro-injection of morphine on somatosensory, motor, and analgesic aspects of septal hyperreactivity: A multivariate assessment

Abstract

Rats with septal lesions demonstrate a quantitative increase in reactivity to noxious stimuli as well as a qualitative change in their responsiveness to non-noxious stimuli, responding as if both were noxious. Morphine analgesia is characterized by a diminished reactivity to noxious stimuli without a change in the sensory threshold, the patient responding to a noxious stimulus as if it were non-noxious. Many of the same brain structures involved in aspects of septal hyperreactivity are also involved in morphine analgesia, and the endogenous opiate system may provide a common site of action for these effects. Rats given either septal or sham lesions were injected with either morphine or vehicle, followed by either naloxone or vehicle, in either the corticomedial amygdala, posterior hypothalamus, or ventral hippocampus and were tested on a battery of behavioral tests designed to measure reactivity to both noxious and non-noxious stimuli. Discriminant function analysis revealed that the greatest effect of morphine was to attenuate septal hyperreactivity to non-noxious stimuli, and that the drug effects were site-dependent. Naloxone showed site-dependent antagonism of the morphine effect as well as unspecified independent effects of its own. The data were discussed in terms of distribution and availability of endogenous opiate receptors, and a functional differentiation between naloxone-sensitive and naloxone-insensitive receptors was suggested.