Characterization of unit activity recorded from septum, thalamus, and caudate following incremental opiate treatment

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Abstract
The effects of a wide range of morphine doses and of its antagonist, naloxone, on spontaneous multiunit discharges in freely moving rats were recorded simultaneously from the septum (Spt), medial thalamus (CM‐PF complex), and caudate nucleus (CN). A high percentage of neurons in these three areas are affected by morphine. Neurons in the CM‐PF complex exhibited a greater number of morphine‐induced changes (104/145) than did those in the caudate nucleus (79/160), or in the septum (67/150). The morphine‐induced changes exhibited dose‐related patterns: the three structures examined in the present study exhibited four response patterns to incremental doses of morphine: either a monophasic effect, an increase or decrease in firing rate, or a biphasic effect; ie, lower morphine doses induced a decrease in activity, whereas higher doses induced an increase in firing rate. There was no observed correlation between the response patterns in the three regions. The technique provides a tool with which to identify and classify the specific response patterns induced by morphine in specific brain regions, and the results may indicate that each region plays a different physiological role in the effects induced by morphine.