Jason G. Umans

Evidence from opiate binding studies that heroin acts through its metabolites.

The relative affinity to opiate receptors of heroin, 6-acetylmorphine and morphine was estimated by determining their ability to displace specifically bound 3H-naltrexone from rat brain opiate binding sites. In vitro hydrolysis of heroin to 6-acetylmorphine was monitored in the binding assay filtrate by use of a quantitative HPLC procedure. The rate of heroin hydrolysis was significantly slower at 0 degrees C than at 37 degrees C. The displacement of 1 nM 3H-naltrexone by unlabeled ligand at concentrations ranging from 7 to 500 nM was measured at 0 degrees C for 120 minutes, yielding IC50 values of heroin = 483 nM, 6-acetylmorphine = 73 nM and morphine = 53 nM. When the binding data for heroin were recalculated to include the displacement that could be attributed to the 6-acetylmorphine derived from heroin degradation during the incubation, all of the apparent heroin binding was accounted for by the 6-acetylmorphine. These results are consistent with previous reports of the low binding affinity of morphine congeners (e.g., codeine) that lack a free phenolic 3-hydroxyl group and support the view that heroin is a prodrug which serves to determine the distribution of its intrinsically active metabolites, 6-acetylmorphine and morphine.

Effects of opiates on fetal behavioral activity in utero

The effects of methadone and morphine on fetal behavior were studied following their administration to either the mother (5micrograms/kg/min) or fetus (0.5micrograms/kg maternal wt./min) using the chronically-instrumented pregnant ewe model. Fourteen unanesthetized, unrestrained pregnant ewes between 130-141 days gestation (term 145 days) were studied 9-28 days following the surgical implantation of cannulae and electrodes to monitor fetal heart rate, blood pressure, breathing movements, eye movements, muscular and electrocortical activity. The three behavioral states of quiet sleep, rapid eye movement sleep and arousal could be distinguished using these parameters. Both drugs, by both routes, resulted in fetal arousal, largely at the expense of quiet sleep. While morphine and methadone were equipotent in suppressing quiet sleep following infusion to the fetus, methadone was three times more potent than morphine after maternal infusion. This potency difference is consistent with the results of prior pharmacokinetic studies demonstrating that fetal exposure to maternally administered methadone is approximately three times greater than that to maternally administered morphine.

Heroin: Analgesia, toxicity and disposition in the mouse☆☆☆

The acute toxicity of heroin (3,6-diacetylmorphine, DAM) and its metabolites 6-acetylmorphine (AM) and morphine (M) following intravenous (i.v.) and intracerebroventricular (i.c.v.) administration and their tailflick-test analgesic activity following i.c.v. administration were studied in mice. After i.c.v. administration, M was 2.5-3 times more potent as a naloxone-reversible analgesic than either DAM or AM. DAM and AM provoked a naloxone-sensitive respiratory depressant lethality (i.c.v.) while M (i.c.v.), and all three drugs given i.v., caused convulsions prior to death. The dose-response and naloxone antagonism studies suggest that the receptor mechanisms which may subserve opiate convulsions differ from those mediating either analgesia or depressant lethality. Studies of DAMs disposition in vivo and in vitro suggest that unhydrolyzed DAM may contribute to its own pharmacodynamic profile after i.v., but not subcutaneous (s.c.) administration to mice.

The relationship between maternal and fetal plasma protein binding of methadone in the ewe during the third trimester

The binding of methadone to maternal and fetal plasma proteins was determined throughout the third trimester in the pregnant ewe. Blood was sampled from chronic indwelling catheters placed in the maternal aorta and fetal aorta. Methadone binding was determined by use of equilibrium dialysis with (3H)-methadone. Maternal binidng ranged from 50.4 to 89.5%, with a mean of 76.2 +/- 1.3 (SE)%. Fetal binding was initially significantly lower than maternal binding, but increased rapidly in the last two weeks before parturition. Prior to 130 days gestation, the ratio of fetal binding to maternal binding was 0.40 +/- 0.03. This binding ratio increased to 0.82 +/- 0.08 in the last few days of pregnancy. Preliminary results suggested that maternal binding was higher in the early post-partum period. These results demonstrate that the relationship between maternal and fetal plasma binding of methadone changes rapidly towards the end of pregnancy, and fetal binding approaches maternal binding at parturition.

The graded and quantal nature of opioid analgesia in the rat tailflick assay.

Opioid agonists routinely increase the latency to respond in the rodent tailflick assay. The nature of this effect was investigated in 5 experiments using several parametric variations and routes of administration. Morphine and methadone were found to produce both quantal and graded effects in all experiments. In cases where quantal effects were observed, the majority of animals also responded in a graded manner during subsequent testing. The increase in latency to respond in the tailflick assay produced by opioid agonists is not accurately characterized as predominantly quantal.

Effects of thyrotropin-releasing hormone in the fetal lamb

Thyrotropin-releasing hormone causes neurobehavioral arousal and stimulates breathing in adult, newborn, and preterm experimental animals. Its effects on behavioral state, breathing, blood pressure, and heart rate were studied in the chronically instrumented late term fetal lamb. Fetal intravenous administration of thyrotropin-releasing hormone resulted in behavioral arousal with electrocortical desynchronization, increased body and eye movements, rapid and deep breathing movements, and a transient bradycardia followed by prolonged tachycardia, associated with an increase in both systolic and diastolic blood pressure. The effects were similar following intracisternal administration of thyrotropin-releasing hormone. The effects of thyrotropin-releasing hormone on behavior, but not breathing, was abolished in the presence of muscarinic blockade. Thyrotropin-releasing hormone may play a role in the modulation of central regulation of cardiovascular, respiratory, and behavioral activity in the fetus.