Spontaneous and cortically-evoked unit activity in the caudate nucleus after systemic morphine.

The ability of systemically administered morphine to alter spontaneous and cortically-evoked single unit activity in rat caudate nucleus was assessed in this study. The animals were prepared with permanently implanted microelectrode guide cannulas and cortical stimulating electrodes. For recording sessions, they were pretreated with phenobarbital to allow mounting in the stereotaxic device. Morphine was administered intravenously in incremental doses from 1 to 30 mg/kg. Neither spontaneous activity nor evoked firing was altered in approximately half the units. Spontaneous activity was depressed in 12 of 33 cells and stimulated in 5 of 33 cells. Evoked activity was reduced in 7 of 36 units and enhanced in 9 of 36 cells. A large proportion of the responses to morphine were not reversed by 1 mg/kg naloxone, especially the changes in cortically-evoked firing. These results demonstrate a multiplicity of changes in spontaneous and cortically-evoked activity of caudate nucleus neurons following systemically administered morphine.