Differential patterns of basal and naloxone‐evoked dopamine efflux in the rat dorsal and ventral striatum following prolonged‐intermittent exposure to morphine

Abstract

AbstractHypodopaminergia in the ventral striatum is a putative neurobiological correlate of withdrawal in opioid‐dependent individuals. This perspective stands in contrast to brain imaging studies with chronic opioid users showing that naloxone‐enhanced dopamine (DA) release in the dorsal striatum is positively correlated with withdrawal aversion. Here, we examined regional differences in striatal DA function associated with opioid withdrawal in rats exposed to intermittent morphine injections for 31 days. Basal concentrations of DA were reduced (i.e., indicating a hypodopaminergic state) in the ventral striatum on Day 10 of morphine exposure, whereas a more prolonged period of morphine treatment was required to reveal hypodopaminergia in the dorsal striatum on Day 31. The ventral striatum consistently exhibited naloxone‐induced transient reductions in DA below the hypodopaminergic basal levels, whereas morphine enhanced DA efflux. In the dorsal striatum, DA responsivity to naloxone shifted from a significant decrease on Day 10 to a notable increase above hypodopaminergic basal levels on Day 31, corroborating the findings in the human dorsal striatum. Unexpectedly, the magnitude of morphine‐evoked increases in DA efflux on Day 31 was significantly blunted relative to values on Day 10. These findings indicate that prolonged‐intermittent access to morphine results in a sustained hypodopaminergic state as reflected in basal levels in the striatum, which is accompanied by regional differences in DA responsivity to naloxone and morphine. Overall, our findings suggest that prolonging the duration of morphine exposure to 31 days is sufficient to reveal neuroadaptations that may underlie the transition from initial drug exposure to opioid dependence.