Behavioral and developmental changes associated with prenatal opiate receptor blockade

Adolescent opiate exposure in the female rat induces subtle alterations in maternal care and transgenerational effects on play behavior

The non-medical use of prescription opiates, such as Vicodin(®) and MSContin(®), has increased dramatically over the past decade. Of particular concern is the rising popularity of these drugs in adolescent female populations. Use during this critical developmental period could have significant long-term consequences for both the female user as well as potential effects on her future offspring. To address this issue, we have begun modeling adolescent opiate exposure in female rats and have observed significant transgenerational effects despite the fact that all drugs are withdrawn several weeks prior to pregnancy. The purpose of the current set of studies was to determine whether adolescent morphine exposure modifies postpartum care. In addition, we also examined juvenile play behavior in both male and female offspring. The choice of the social play paradigm was based on previous findings demonstrating effects of both postpartum care and opioid activity on play behavior. The findings revealed subtle modifications in the maternal behavior of adolescent morphine-exposed females, primarily related to the amount of time females' spend nursing and in non-nursing contact with their young. In addition, male offspring of adolescent morphine-exposed mothers (MOR-F1) demonstrate decreased rough and tumble play behaviors, with no significant differences in general social behaviors (i.e., social grooming and social exploration). Moreover, there was a tendency toward increased rough and tumble play in MOR-F1 females, demonstrating the sex-specific nature of these effects. Given the importance of the postpartum environment on neurodevelopment, it is possible that modifications in maternal-offspring interactions, related to a history of adolescent opiate exposure, plays a role in the observed transgenerational effects. Overall, these studies indicate that the long-term consequences of adolescent opiate exposure can impact both the female and her future offspring.

Chronic exposure to the opioid growth factor, [Met5]-enkephalin, during pregnancy: maternal and preweaning effects

The opioid peptide, [Met(5)]-enkephalin (termed opioid growth factor, OGF), is an autocrine growth factor that serves as a constitutively active inhibitory agent. OGF crosses the placenta and depresses DNA synthesis in the fetus. The role of OGF in pregnancy and parturition, and the influence exerted on prenatal and neonatal features of the offspring, were studied in rats. Females received daily injections of 10 mg/kg OGF throughout gestation; all offspring were cross-fostered to lactating noninjected dams at birth. No effects on the length of gestation, course of pregnancy, behavior of the pregnant dam, maternal weight gain, or food and water intake throughout gestation were recorded in OGF-treated mothers. Moreover, nociceptive response in these females was not altered by chronic OGF exposure, and no signs of physical dependence or withdrawal could be observed following a challenge by the opioid antagonist naloxone. Litter size and the number of live births per litter of OGF-treated mothers were reduced by 25% from control subjects and a fourfold increase in stillborns was noted for mothers receiving OGF compared to control levels. Histopathologic analysis confirmed the stillborns to have died in utero. OGF-exposed neonates were normal in body weight and crown-to-rump length, but these pups were observed to be lethargic and cyanotic, and had subnormal weights of many organs. Body weights of 10-, 15-, and 21-day-old OGF-exposed rats were reduced 11-27% from control levels. Wet and dry organ weights of the rats maternally subjected to OGF were decreased from control values in six of the eight organs evaluated at 10 days. At weaning, some organs were subnormal in weight. These data lead us to hypothesize that a native opioid peptide-OGF-is integral to certain aspects of maternal, neonatal, and postnatal well-being, and that disruptions in this opioid peptide have serious repercussions on the course of pregnancy and fetal outcome.

Regulation of mouse kappa opioid receptor gene expression by retinoids

The effect of retinoids on the expression of kappa opioid receptor (KOR) gene was examined in normal and transgenic animals. KOR-lacZ transgene expression was specifically elevated in KOR-positive areas of the developing CNS by depleting vitamin A from animal diets. The endogenous KOR mRNA species, including all three isoforms, were also upregulated by depleting vitamin A in developing animals. Change in the expression of isoforms a and b is similar in prenatal stages but differs during postnatal development. Interestingly, upregulation of isoform c is most significant postnatally. The regulation of KOR gene by vitamin A was substantiated in a mouse embryonal carcinoma P19 culture system in which retinoic acid (RA), the most potent ingredient of vitamin A, was able to suppress the expression of all the three KOR isoforms and KOR protein. The RA-mediated suppression was blocked by an RA receptor antagonist and a histone deacetylase (HDAC) inhibitor. By using a reporter transfection assay in P19 cells, the potential genetic element responsible for RA-mediated suppression of KOR gene expression was located to intron 1 of the mouse KOR gene, which could also be blocked by HDAC inhibitor. Furthermore, suppression of KOR gene expression by RA in P19 cells appeared to be an indirect event and required protein synthesis. A role of RA in KOR gene regulation during developmental stages was discussed.

Naltrexone is not detected in preweaning rats following transplacental exposure: implications for growth modulation

Extracts of brain and heart from rats at birth and postnatal days 2 and 10 were evaluated for naltrexone following maternal injection of 50 mg/kg opioid antagonist throughout gestation. Samples were prepared by ultrafiltration, lyophilized, reconstituted in mobile phase, and separated by reversed-phase high performance liquid chromatography with ultraviolet detection. Qualitative analysis revealed the presence of naltrexone in tissues from neonates, but not in rats of 2 and 10 days, that were transplacentally exposed to drug. These results confirm earlier reports showing that naltrexone, maternally administered, passes through the placenta and enters the fetus. Moreover, the data suggest that the somatic and neurobiological acceleration observed in offspring exposed to naltrexone during gestation is not due to opioid receptor blockade during the postnatal period.

Opioid receptor blockade during prenatal life modifies postnatal behavioral development

The ontogeny of physical characteristics, spontaneous motor, and sensorimotor behaviors of preweaning rats, as well as ambulation and emotionality at weaning (day 21) were studied in rats exposed to 50 mg/kg naltrexone (NTX) or saline (controls) daily throughout gestation by maternal administration; all animals were cross-fostered to untreated mothers at birth. Morphine challenge tests and nociceptive measures revealed that this dosage of opioid antagonist blocked opioid receptors for 24 h. At birth and weaning, animals in the NTX group weighed 12 and 20%, respectively, more than control offspring. The age at which a specific physical characteristic, spontaneous motor behavior, or reflex initially appeared and the age at which 100% of the animals demonstrated a particular characteristic/behavior often were accelerated in animals prenatally exposed to NTX. The frequency of ambulation was subnormal in the NTX group, and the frequency and/or incidence of rearing, grooming, wet-dog shakes, and defecation were reduced from normal levels in these opioid antagonist-exposed rats. These results imply that interactions of endogenous opioid systems during embryogenesis are determinants of somatic, physical, and behavioral development in postnatal life.

Transplacental transfer of naltrexone in rats

Extracts of fetal (20 days gestation) brain, heart, and liver were evaluated for naltrexone in rats 1 hour following maternal injection of 50 mg/kg opioid antagonist; adult plasma from the pregnant rats was analyzed. Samples were prepared by ultrafiltration, lyophilized, reconstituted in mobile phase, and separated by reversed phase high-performance liquid chromatography with ultraviolet detection. This qualitative analysis revealed the presence of naltrexone in all fetal tissues, as well as in adult plasma. These results indicate naltrexone, maternally administered, passes through the placenta and enters the fetus. The data would suggest that reports concerning somatic and neurobiological acceleration in offspring exposed to naltrexone during gestation may be the result of a direct opioid antagonist action in the fetus.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. The specific topics covered this year include stress: tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.