Neurophysiological evidence for tolerance and dependence on opiates: simultaneous multiunit recordings from septum,...

Multiunit activity was recorded simultaneously from the septum (Spt), medial thalamus (PF-CM complex), and caudate nucleus (CN) in freely behaving rats previously implanted with permanent nichrone semimicroelectrodes (62 mu in diameter). A challenge dose of morphine (10 mg/kg) and its antagonist (naloxone 1.0 mg/kg) was examined in the same animals while they were morphine naive and after they had become morphine physically dependent. Electrophysiological observations indicated that it would be possible to identify three physiological phenomena: 1) activity related to morphine dependency; 2) activity related to tolerance; and 3) activity related to withdrawal. Experimental observations yielded three different electrophysiological patterns of activity related to tolerance. Each of the three central sites (Spt, PF-CM, and CN) exhibited a different response pattern to morphine in morphine physically dependent animals.