Cheryl J Schindler

Prenatal morphine exposure differentially alters learning and memory in male and female rats.

The present study tested the hypothesis that exposure to morphine on prenatal days 11-18 impairs performance on tasks requiring learning and memory in adult male and female rats. In Experiment 1, a symmetrical maze was used to measure learning. In Experiment 2, an eight-arm radial maze was used to assess working spatial memory. The results of Experiment 1 demonstrated that prenatal morphine exposure reduces the time needed to complete the trials, but does not affect the accuracy of performance in male rats. In contrast, prenatal drug treatment had no effects on either the time or the accuracy of performance in female rats. In Experiment 2, both male and female morphine-exposed rats needed more time to complete regular trials (no delay) than controls; however, morphine exposure in male rats did not affect performance on tasks requiring memory, measured with delay trials, but hindered it in ovariohysterectomized (OVX) female rats. In OVX females, replacement injections of both estrogen and progesterone restored the impairment of performance on delay trials produced by prenatal morphine exposure. Thus, the present study demonstrates that prenatal morphine exposure differentially alters performance of adult male and female rats on tasks requiring learning and spatial memory.

Impact of maternal morphine and saline injections on behavioral responses to a cold water stressor in adult male and female progeny.

The purpose of the present study was to test the effects of maternal morphine and saline injections on chronic cold water stress responses in three groups of adult male and female rats: prenatally morphine-exposed adult progeny, prenatally saline-exposed adult progeny, and control groups. All male rats were gonadally intact, and female rats were ovariectomized (OVX) in adulthood, and half of them were injected with estradiol benzoate (EB). All animals were exposed to a cold water stressor daily for 2 weeks and tested before (baseline) and after (stress effects) the chronic cold water stressor in a swim test and an open field test. In the swim test, both adult males and OVX, EB-treated adult females born to mothers injected with morphine or saline displayed more floating behavior during the swim test than their controls, both before and after the cold water stressor. Male rats exposed to morphine or saline prenatally also spent more time struggling during the swim tests than controls, and this was further increased after the cold water stressor. In the open field test, males and OVX, EB-treated females born to morphine- or saline-injected mothers were less active and displayed fewer rearings than controls. No differences were observed in OVX females as a result of prenatal injections. Thus, the present study demonstrates that maternal injections, regardless of injection content, induce long-lasting effects on stress responsiveness in adult progeny.

Prenatal morphine exposure alters susceptibility to bicuculline seizures in a sex- and age-specific manner

Bicuculline was used to investigate seizure susceptibility in pre- and peripubertal male and female rats exposed prenatally to morphine. Morphine-exposed males showed increased seizure susceptibility at prepubertal and decreased susceptibility at peripubertal ages. There was no difference in seizure susceptibility in morphine-exposed females at either age. Therefore, the present data suggest that males are more vulnerable than females to morphine-induced insults during prenatal brain development.

Field-specific changes in hippocampal opioid mRNA, peptides, and receptors due to prenatal morphine exposure in adult male rats.

Alterations in the opioid system in the hippocampal formation and some of the possible functional consequences were investigated in adult male rats that were prenatally exposed to either saline or morphine (10 mg/kg twice daily on gestational days 11-18). In situ hybridization and Northern blots were used to measure proenkephalin and prodynorphin mRNA, and radioimmunoassays quantified proenkephalin- and prodynorphin-derived peptide levels in the dentate gyrus, CA3, and CA1 subfields of the hippocampal formation. Prenatal morphine exposure in male rats decreases proenkephalin and increases prodynorphin mRNA selectively in the granule cell layer of the dentate gyrus. Similarly, met-enkephalin peptide levels are decreased and dynorphin B peptide levels are increased in the dentate gyrus but not CA3 or CA1 of prenatally morphine-exposed males. In addition, there are decreases in dynorphin-derived peptides in the CA3 subfield. Receptor autoradiography revealed increases in the density of micro but not delta receptor labeling in discrete strata of specific hippocampal subfields in morphine-exposed males. Because alterations in the hippocampal opioid system suggest possible alterations in the excitability of the hippocampal formation, changes in opioid regulation of seizures were examined. Morphine exposure, however, does not alter the latency to onset or number of episodes of wet dog shakes or clonic seizures induced by infusion of 10 nmol [D-Ala2, MePhe4, Gly-ol5]enkephalin into the ventral hippocampal formation. Interestingly, a naloxone (5 mg/kg) injection 30 min before bicuculline administration reverses the increased latency to onset of clonic and tonic-clonic seizures in morphine-exposed males. Thus, the present study suggests that exposure of rats to morphine during early development alters the hippocampal opioid system, suggesting possible consequences for hippocampal-mediated functions.

Mu-opioid receptors in seizure-controlling brain structures are altered by prenatal morphine exposure and by male and female gonadal steroids in adult rats

The present study used autoradiography to examine the effect of prenatal morphine exposure on mu-opioid receptor density in epileptic seizure-controlling brain structures including the substantia nigra pars compacta (SNC), substantia nigra pars reticulata (SNR), superior colliculus (SC), and subthalamic nucleus (STN) of adult male and female rats. The results demonstrate that prenatal morphine exposure increases the mu-opioid receptor density in the SNC and STN, but not in the SNR or in the SC of gonadally intact adult male rats. The density of mu-opioid receptors in the SNC and STN is, however, decreased following gonadectomy in morphine-exposed males, and testosterone treatment fails to restore this decrease to the level of gonadally intact males. Further, in the SC, the density of mu receptors was lower in both saline-exposed, gonadectomized (GNX) and GNX, TP-treated males and in morphine-exposed, GNX, TP-treated males relative to gonadally intact saline- and morphine-exposed males, respectively. In ovariectomized (OVX) female rats, the same prenatal morphine exposure increases the mu-opioid receptor density in the SNC and SNR, but decreases it in the STN. The density of mu-opioid receptors is also decreased in the SNC and SC of OVX estrogen-treated females and in the SNR and SC of OVX, progesterone-treated females. Thus, the present study demonstrates that mu-opioid receptors in seizure-controlling brain structures are sex-specifically altered by prenatal morphine exposure in adult progeny. Further, prenatal morphine exposure alters gonadal hormone effects on the density of mu receptors in adult, OVX females.

Prenatal morphine exposure decreases susceptibility of adult male rat offspring to bicuculline seizures.

The purpose of this study was to investigate the effect of prenatal exposure to morphine (5-10 mg/kg twice daily on days 11-18 of gestation) on bicuculline seizure susceptibility and to examine the interaction of prenatal morphine exposure and hormonal background in adult male rats. The data demonstrate that prenatal morphine exposure does not affect clonic but decreases susceptibility to tonic-clonic bicuculline seizures in intact male rats. Thus, the present data support our previous work demonstrating alterations in seizure susceptibility of adult morphine-exposed animals.

Cholera toxin B decreases bicuculline seizures in prenatally morphine- and saline-exposed male rats

Prenatal morphine exposure on gestation days 11-18 alters bicuculline-induced seizures in rats during development and in adulthood. Adult, morphine-exposed male progeny exhibit an increased latency to bicuculline seizures, which can be reversed by administration of the opioid receptor antagonist naloxone. In chronically morphine-treated adult mice, cholera toxin B (CTX-B) can reverse the effects of chronic morphine administration. Therefore, the present study investigated whether prenatally morphine-exposed rats show a similar response to CTX-B as chronically morphine-treated adult rodents. Prenatally morphine-, saline- and unexposed male progeny were tested for seizure susceptibility with a 7.5-mg/kg intraperitoneal injection of bicuculline in adulthood. CTX-B or saline was injected subcutaneously at 24, 12, and 0.5 h before bicuculline injection. CTX-B decreased the occurrence of bicuculline-induced seizures in both prenatally saline- and morphine-exposed but not unexposed rats. Furthermore, three, but not one, saline injections administered at 12-h intervals prior to bicuculline administration reversed the increase in seizure latency in prenatally morphine-exposed adult males, suggesting an altered responsiveness of the stress system. The present study demonstrates that CTX-B can decrease the occurrence of bicuculline seizures in prenatally stressed rats and that increased seizure latencies in prenatally morphine-exposed male rats may be related to stress responses.

Bicuculline seizure susceptibility and nigral GABAA α1 receptor mRNA is altered in adult prenatally morphine-exposed females1

Prenatal morphine exposure (5-10 mg/kg twice daily on gestation days 11-18) can adversely affect neurological development, including seizure susceptibility. The present study examines the effects of prenatal morphine exposure on seizure susceptibility to the GABA antagonist and convulsant bicuculline and GABA(A) alpha(1) receptor mRNA in the substantia nigra (SN) of female rats. The results demonstrate that prenatally morphine-exposed ovariectomized (OVX) females and OVX females with estradiol benzoate (EB) replacement have an increased latency to seizure onset compared to controls. In addition, prenatal morphine exposure decreases the area covered by grains of GABA(A) alpha(1) receptor mRNA in the anterior SN in both OVX and EB+progesterone (P)-treated groups, and decreases the number of GABA(A) alpha(1) receptor mRNA-labeled cells/field in EB females. Furthermore, prenatally morphine- and saline-exposed EB and EB+P females had decreased GABA(A) alpha(1) receptor mRNA-labeled cells/field in the anterior SN compared to OVX animals of the same prenatal exposure. These results demonstrate that the long term effects of prenatal morphine exposure in female rats is dependent on their hormonal status, and suggest that seizure susceptibility may be altered via neuropharmacological changes in the GABA system in the SN.

Cortical and striatal μ-opioid receptors are altered by gonadal hormone treatment but not by prenatal morphine exposure in adult male and female rats

The cerebral cortex (CX), cingulate CX (cgCX), and striatum (STR) play an important role in locomotion, cognition, emotion, and reward-motivated behaviors, and are altered by prenatal morphine exposure. We have demonstrated that delta-opioid receptors in the CX and STR of adult male and female rats are altered by prenatal morphine exposure and gonadal hormonal treatment. Because morphine binds with greater affinity to mu- than delta-opioid receptors, the present study examined the effect of prenatal morphine exposure on mu-opioid receptor density in the CX, cgCX, and STR of adult male and female rats using receptor autoradiography. In Experiment 1, three groups of adult male rats were analyzed: intact, gonadally intact; GNX, gonadectomized; and TP, GNX and testosterone propionate (TP)-treated. In Experiment 2, four groups of adult females were analyzed: OVX, ovariectomized; EB, OVX and estradiol benzoate (EB)-treated; P, OVX and progesterone (P)-treated; and EB+P, OVX and EB- and P-treated. In male rats, GNX and TP males had lower mu-opioid receptor densities in all three brain regions than gonadally intact males regardless of prenatal drug exposure. In female rats, OVX, EB+P-treated females had lower mu-opioid receptor density in the STR than OVX only females regardless of prenatal drug exposure. There were no drug or gonadal hormone effects in the CX or in the cgCX of female rats. Thus, the present study demonstrates that gonadal hormones, and not prenatal morphine exposure, alter the density of mu-opioid receptors in the CX, cgCX, and STR of adult male and female rats.

Alterations of prenatal morphine exposure in μ-opioid receptor density in hypothalamic nuclei associated with sexual behavior

Our previous work demonstrated that prenatal morphine exposure twice daily during gestational days 11-18 differentially alters male and female sexual behavior. One possible explanation may be that prenatal morphine exposure alters the sexual behavior via alterations of mu-opioid receptors in brain regions involved in reproductive function and behavior, including the ventromedial nucleus of the hypothalamus (VMH), arcuate nucleus (ARC), and medial preoptic area (mPOA). In experiment 1, mu-opioid receptor density was analyzed in three groups of adult male rats: gonadally intact, gonadectomized (GNX), and GNX and testosterone 17beta-propionate-treated (TP). In experiment 2, mu-opioid receptor density was analyzed in four groups of adult female rats: ovariectomized (OVX), OVX and estradiol benzoate-treated (EB), OVX and progesterone-treated (P), and OVX and EB- and P-treated (EB+P). Experiment 1 demonstrated that prenatal morphine exposure lowered the mu-opioid receptor density in the mPOA of adult, gonadally intact and in TP males, while this difference was not apparent in GNX male rats. Experiment 2 demonstrated that prenatal morphine exposure increased mu-opioid receptor density in OVX females, while decreasing it in EB females in the VMH. When compared to our previous sexual behavior data, the present results demonstrate that at least some changes in sexual behavior of adult male and female rats prenatally exposed to morphine may be related to alterations in mu-opioid receptors in brain regions controlling sexual behavior.