Activation of μ-opioid receptors in the ventral pallidum (VP) is important for the induction of behavioral sensitization to morphine in rats. The present study was designed to ascertain if neurons within the VP demonstrate sensitization at a time when morphine-induced behavioral sensitization occurred (ie 3 or 14 days after five once-daily injections of 10 mg/kg i.p. morphine) in rats. Western blotting was used to evaluate transcription factors altered by opiates, CREB and ΔFosB. CREB levels did not change in the VP, but there was a significant decrease in levels of its active, phosphorylated form (pCREB) at both 3-and 14-days withdrawal. ΔFosB levels were elevated following a 3-day withdrawal, but returned to normal by 14 days. This profile also was obtained from nucleus accumbens tissue. In a separate group of similarly treated rats, in vivo electrophysiological recordings of VP neuronal responses to microiontophoretically applied ligands were carried out after 14-days withdrawal. The firing rate effects of local applications of morphine were diminished in rats withdrawn from i.p. morphine. Repeated i.p. morphine did not alter GABA-mediated suppression of firing, or the rate enhancing effects of the D1 dopamine receptor agonist SKF82958 or glutamate. However, VP neurons from rats withdrawn from repeated i.p. morphine showed a higher propensity to enter a state of depolarization inactivation to locally applied glutamate. Overall, these findings reveal that decreased pCREB in brain regions such as the VP accompanies persistent behavioral sensitization to morphine and that this biochemical alteration may influence the excitability of neurons in this brain region.

Repeated, intermittent exposure to drugs of abuse results in response enhancements to subsequent drug treatments, a phenomenon referred to as sensitization. As persistent neuronal sensitization may contribute to the long-lasting consequences of drug abuse, characterizing the neuroanatomical substrates of sensitization is providing insights into addiction. It is known that the ventral tegmental area (VTA) is necessary for induction, and expression involves the nucleus accumbens (NAc). We reveal here that the ventral pallidum (VP), a brain region reciprocally innervated by the VTA and the NAc, is a critical mediator of opiate-induced behavioral sensitization. Blockade of VP m-opioid receptors (via intra-VP CTOP injections) negated the ability of systemic administration of the opiate, morphine to induce motor sensitization, and for sensitized rats to subsequently express enhanced responding to a morphine challenge. Intra-VP morphine was sufficient to induce motor sensitization, and this sensitization was expressed following 17 days of withdrawal. Rats with a treatment history of intra-VP morphine demonstrated cross-sensitization to a challenge injection of systemically administered morphine. Conversely, repeated systemic treatments of morphine cross-sensitized to an intra-VP morphine challenge. These results indicate that activation of VP m-opioid receptors is sufficient to evoke behavioral sensitization and that these receptors are necessary for sensitized responding to systemic morphine. The study pioneers the concept that both development and expression of drug-induced sensitization are regulated by the VP. Thus, the VP is likely an important contributor to neuronal adaptations that underlie addiction.

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Common neurobiological substrates contribute to the progressively increased behavioral effects (i.e., sensitization) that occur with repeated intermittent treatments of cocaine and morphine. Consequently, repeated exposure to cocaine can augment responding to morphine (termed cross-sensitization). Drug-induced sensitization in rats may model aspects of the dysfunction in motivation that are imposed by addiction. The ventral pallidum (VP) is involved in motivated behaviors and its function is altered by acute administration of cocaine and morphine, but the effects of repeated drug exposure remain unknown. Targeting this paucity, the present study evaluated electrophysiological changes in the VP of rats exposed to five once-daily cocaine treatments (15 mg/kg i.p.). This regimen also induced behavioral-sensitization that was expressed 3 days later when the rats received either an acute injection of cocaine (15 mg/kg i.p.) or morphine (10 mg/kg i.p.). VP neurons recorded in vivo 3 days after the repeated cocaine treatment regimen demonstrated increased excitatory responding to microiontophoretic applications of morphine and glutamate. The maximal effect (E max ) was increased without altering potency, suggesting a change in the functional efficacy of the respective receptor systems. This did not represent a potentiation in transmission in general, for the effects of GABA were diminished. The results provide the first evidence for cellular adaptation in the VP after a sensitizing drug treatment paradigm and reveal that crosssensitization of drug-induced behaviors temporally correlates with changes in VP neuronal responding. These findings advance an emerging theme that alterations in the VP may contribute to the increased motivation for drug seeking that occurs in drug-withdrawn addicts.