The long-term goal of the proposed research is to understand mechanisms by which chronic in utero morphine and methadone exposure affect regulation and function of mu opioid receptors (MOR) in respiratory control areas of newborn brainstem. Respiratory depression is induced by endogenous opioid peptides and exogenous opioids that activate MOR. A critical brainstem site for these effects is the nucleus tractus solitarius (NTS), which integrates sensory signals and drives respiratory muscles. Profound disturbance of neonatal breathing is a well-documented consequence of maternal opioid abuse. These neonates exhibit withdrawal hyperventilation and an increased incidence of sudden infant death syndrome (SIDS). The proposed studies are essential for understanding normal respiratory development, drug-induced changes, and effective treatment of pregnant heroin addicts and methadone maintained patients. The exact role of NTS MOR in neonatal congenital narcotic dependence and these respiratory disturbances is unknown. For anatomical, physiological, and pharmacological reasons, the guinea pig is a superb model for study of maternal opioid abuse. Guinea pig kappa opioid receptor has been cloned, but only partial sequences of MOR and delta opioid receptors have been available. However, our laboratory has recently determined the complete guinea pig MOR cDNA sequence. Availability of this sequence will enable us to define for the first time guinea pig MOR pharmacology, and systematically compare it to human and mouse MOR. Research is guided by four hypotheses: 1) guinea pig MOR is functionally similar to human MOR with respect to mu agonist efficacy, binding kinetics, and activation of G-protein, but different from murine MOR; 2) methadone induces respiratory depression and is equipotent to, but of longer duration than morphine in the neonatal guinea pig; 3) chronic in utero morphine and methadone exposure results in increased functional MOR on the cell surface, but decreases the coupling efficiency of MOR with G-proteins in the NTS; 4) chronic in utero morphine and methadone exposure up-regulates MOR mRNA in the NTS. Hypotheses are explored through four specific aims: 1) to compare mu agonist selectivity and potency, and development of cellular tolerance for guinea pig, human and murine MOR each expressed in stably transfected CHO cells; 2) to prove that the respiratory effects of methadone are similar to morphine in the neonatal guinea pig; 3) to study the effects of morphine and methadone on guinea pig MOR in NTS of brainstem sections from guinea pig neonates exposed in utero; and 4) to quantitate MOR mRNA in NTS from guinea pig neonates exposed to morphine and methadone in utero. These studies will provide developmental information on guinea pig NTS MOR following chronic in utero opioid exposure.
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