Morphine effects on spontaneous, nociceptive, antinociceptive and sensory evoked responses of parafasciculus thalami units in morphine naive and morphine dependent rats.

The present experiments used freely behaving animals previously implanted with permanent recording electrodes within the parafasciculus thalami (PF) and stimulation electrodes in nociceptive and antinociceptive areas. The spontaneous and the evoked activity in PF neurons following nociceptive, antinociceptive and sensory stimulation, as well as the effects of morphine and its antagonist naloxone on these inputs in morphine naive and morphine dependent animals, were investigated. The observations demonstrated that the spontaneous activity of PF neurons exhibits variable spontaneous firing rates which are affected by acute and chronic morphine treatment. The PF neuronal population exhibits neurophysiological activity characteristic of morphine dependence, tolerance and withdrawal from morphine. The PF receives mono-, oligo- and polysynaptic inputs from multiple sources, including regions associated with pain pathways which converge on PF cells, as well as from sites involved in pain suppression mechanisms, which supports the hypothesis of a role of the PF as a modulator of pain input. The nociceptive, antinociceptive and sensory inputs are not modified by chronic morphine treatment, but single doses of morphine have remarkable effects on those inputs in morphine naive and morphine dependent animals; naloxone reversed the morphine effects on the evoked activity.