Estrogenic regulation of proenkephalin mRNA expression in the ventromedial hypothalamus of the adult male rat

Cocaine sensitization in male rats requires activation of estrogen receptors

Gonadal steroids play a modulatory role in cocaine use disorders, and are responsible for many sex differences observed in the behavioral response to cocaine. In females, it is well established that estradiol enhances the behavioral response to cocaine. In males, we have recently shown that testosterone enhances sensitization to cocaine but its mechanism of action remains to be elucidated. The current study investigated the contribution of DHT, a non-aromatizable androgen, and of estradiol, in regulating cocaine-induced sensitization in male rats. Gonadectomized (GDX) male rats treated with estradiol sensitized to repeated cocaine administration, while GDX rats treated with DHT did not, implicating estradiol in cocaine sensitization. Furthermore, intact male rats treated with the antiestrogen ICI 182,780 did not show sensitization to repeated cocaine. This study demonstrates the pivotal role of estradiol in cocaine-induced neuroplasticity and neuroadaptations in the rodent brain.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Sex hormone activity in alcohol addiction: integrating organizational and activational effects

There are well-known sex differences in the epidemiology and etiopathology of alcohol dependence. Male gender is a crucial risk factor for the onset of alcohol addiction. A directly modifying role of testosterone in alcohol addiction-related behavior is well established. Sex hormones exert both permanent (organizational) and transient (activational) effects on the human brain. The sensitive period for these effects lasts throughout life. In this article, we present a novel early sex hormone activity model of alcohol addiction. We propose that early exposure to sex hormones triggers structural (organizational) neuroadaptations. These neuroadaptations affect cellular and behavioral responses to adult sex hormones, sensitize the brain's reward system to the reinforcing properties of alcohol and modulate alcohol addictive behavior later in life. This review outlines clinical findings related to the early sex hormone activity model of alcohol addiction (handedness, the second-to-fourth-finger length ratio, and the androgen receptor and aromatase) and includes clinical and preclinical literature regarding the activational effects of sex hormones in alcohol drinking behavior. Furthermore, we discuss the role of the hypothalamic-pituitary-adrenal and -gonadal axes and the opioid system in mediating the relationship between sex hormone activity and alcohol dependence. We conclude that a combination of exposure to sex hormones in utero and during early development contributes to the risk of alcohol addiction later in life. The early sex hormone activity model of alcohol addiction may prove to be a valuable tool in the development of preventive and therapeutic strategies.

Anabolic-androgenic steroid effects on nociception and morphine antinociception in male rats

The purpose of this study was to investigate the effects of acute and chronic administration of anabolic-androgenic steroids (AAS) on nociception and morphine antinociception in acute pain models, as well as on chronic inflammatory nociception. In Experiment 1, adult, gonadally intact male rats were injected s.c. for 28 days with either 5 mg/kg testosterone (T), dihydrotestosterone (DHT), stanozolol (STAN), or safflower oil vehicle (N=12-25/group). On day 28, rats in each group were tested on acute thermal and mechanical nociceptive assays, before and after morphine treatment. In Experiment 2, rats in each group (N=8-10/group) were injected with mineral oil or complete Freund's adjuvant (CFA) into one hindpaw after 28 days of AAS treatment, and then tested for thermal hyperalgesia, mechanical allodynia, inflammation and locomotor suppression intermittently for 28 days. Experiment 3 replicated nociceptive measurements in Experiments 1 and 2, but with a single AAS or vehicle injection occurring 3h prior to testing (N=10-12/group). While chronic AAS administration tended to decrease body weight gain and alter reproductive organ weights in the expected manner, it did not significantly alter acute nociception nor attenuate the development of various chronic pain indices after CFA administration. Morphine antinociceptive potency was significantly decreased by chronic DHT on the hot plate test only. Acute AAS administration also did not significantly alter acute or chronic nociception, or morphine antinociceptive potency. Comparisons between acute and chronic AAS administration suggest that steroid tolerance did not occur in rats treated with AAS chronically. Taken together, these data do not support the hypothesis that AAS exposure alters nociception or morphine antinociception in gonadally intact males.

Opioid modulation of cell proliferation in the ventricular zone of adult zebra finches (Taenopygia guttata)

Besides modulating pain, stress, physiological functions, motivation, and reward, the opioid system has been implicated in developmental and adult mammalian neurogenesis and gliogenesis. In adult male songbirds including zebra finches, neurons generated from the ventricular zone (VZ) of the lateral ventricles are incorporated throughout the telencephalon, including the song control nuclei, HVC, and area X. Although the endogenous opioid met-enkephalin is present in neurons adjacent to the VZ and is upregulated in song control regions during singing, it is not known whether the opioid system can modulate adult neurogenesis/gliogenesis in zebra finches. We used quantitative RT-PCR and in situ hybridization to demonstrate that μ- and δ-opioid receptors are expressed by the VZ of adult male zebra finches. Treating cultured VZ cells from male birds with the opioid antagonist naloxone led to an increase in cell proliferation measured by 5-bromo-2-deoxyuridine incorporation, whereas administering met-enkephalin had the opposite effect, compared with saline-treated cultures. Systemically administering naloxone (2.5 mg/kg body wt) to adult male zebra finches for 4 d also led to a significant increase in cell proliferation in the ventral VZ of these birds, compared with saline-treated controls. Our results show that cell proliferation is augmented by naloxone in the VZ adjacent to the anterior commissure, suggesting that the endogenous opioids modulate adult neurogenesis/gliogenesis by inhibiting cell proliferation in songbirds.

Acute HPA axis response to naltrexone differs in female vs. male smokers

BACKGROUND

Both opioid antagonist administration and cigarette smoking acutely increase hypothalamic-pituitary-adrenal (HPA) axis activity as measured by adrenocorticotropic hormone (ACTH) and cortisol levels. However, male and female smokers may differ in their response to the opioid antagonist naltrexone, which may be partially mediated by sex differences in HPA axis function. Smokers, as a group, have frequently been shown to have HPA axis dysfunction, which may have relevance to the course and maintenance of nicotine dependence. The purpose of this study was to examine possible sex differences in HPA axis function by comparing stress-hormone response to naltrexone within healthy male and female smokers. Additionally, exploratory analyses compared the combined effects of naltrexone and cigarette smoking on hormonal responsivity between the sexes.

METHOD

Thirty-eight healthy smokers (22 men) were tested in two separate morning sessions after 12h of smoking abstinence. For women, self-reports of menstrual cycle information were obtained prior to each session (date of last menstruation, cycle length, reproductive phase, etc.). Each participant received 50mg naltrexone or placebo capsule (in random order) and plasma levels of ACTH and cortisol were assessed at regular intervals for several hours. A subgroup of 12 participants underwent a similar, additional session in which they smoked a single cigarette three hours after naltrexone administration.

RESULTS

Naltrexone significantly increased ACTH and cortisol levels in women, but not men (DrugxSexxTime, p<0.05). A post hoc analysis suggested that women at an estimated 'high estrogen' phase had a greater cortisol response (DrugxEstrogen level, p<0.05) than those at an estimated 'low estrogen' phase. Exploratory analyses showed that smoking a single cigarette potentiated naltrexone-induced increases in ACTH (p<0.05) and cortisol (p<0.01) in all participants.

CONCLUSION

The findings support the hypothesis that women are more sensitive to opioid antagonism at the level of the HPA axis. Although further studies are needed to examine mechanisms underlying these responses, both results may have clinical implications for the use of naltrexone as a treatment for nicotine dependence.

Ovarian steroids modulate leu-enkephalin levels and target leu-enkephalinergic profiles in the female hippocampal mossy fiber pathway

In the hippocampal formation (HF), the enkephalin opioids and estrogen are each known to modulate learning and cognitive performance relevant to drug abuse. Within the HF, leu-enkephalin (LENK) is most prominent in the mossy fiber (MF) pathway formed by the axons of dentate gyrus (DG) granule cells. To examine the influence of ovarian steroids on MF pathway LENK levels, we used quantitative light microscopic immunocytochemistry to evaluate LENK levels in normal cycling rats and in estrogen-treated ovariectomized rats. Rats in estrus had increased levels of LENK-immunoreactivity (ir) in the DG hilus compared to rats in diestrus or proestrus. Rats in estrus and proestrus had higher levels of LENK-ir in CA3a-c compared to rats in diestrus. Ovariectomized (OVX) rats 24 h (but not 6 or 72 h) after estradiol benzoate (EB; 10 microg) administration had increased LENK-ir in the DG hilus and CA3c. Electron microscopy showed a larger proportion of LENK-labeled small terminals and axons in the DG hilus compared to CA3 which may have contributed to region-specific changes in LENK-ir densities. Next we evaluated the subcellular relationships of estrogen receptor (ER) alpha, ERbeta and progestin receptor (PR) with LENK-labeled MF pathway profiles using dual-labeling electron microscopy. ERbeta-ir colocalized in some LENK-labeled MF terminals and smaller terminals while PR-ir was mostly in CA3 axons, some of which also showed colocalization with LENK. ERalpha-ir was in dendritic spines, but no colocalization with LENK-labeled profiles was observed. The present studies indicate that estrogen can modulate LENK in subregions of the MF pathway in a dose-and time-dependent manner. These effects might be triggered by direct activation of ERbeta or PR in LENK-containing terminals.

Actions of sex steroids on behaviours beyond reproductive reflexes

The actions of sex steroids in the brain have been shown, from molecular to systems levels, to control reproductive behaviour in a wide range of vertebrates. It has become increasingly clear that gonadal steroid hormones have regulatory functions which extend far beyond the direct coordination of an animal's physiological state and its display of sexual behaviour. While some of these actions may include changes in mood or other behavioural measures, such as exploration or excitability, sex steroid hormones also influence neural plasticity, neuronal activity and, possibly, learning and memory, as reflected by long-term potentiation or age-related deficits. Here we describe two systems that have been used to explore the non-reproductive roles of gonadal steroid hormones. The first of these is to examine the oestrogen-sensitive opioid peptide gene expression in the hypothalamus. Currently, we are attempting to identify the types of behaviour which may be altered consequent to the oestrogenic induction of the preproenkephalin gene. The second approach involves studying the effects of progesterone at the neuronal cell membrane and characterizing the metabolites of progesterone which have benzodiazepine-like actions in the brain. A number of studies suggest that this may provide an alternative mechanism through which progesterone can influence mood or behaviour.

Pronociceptive and antinociceptive effects of estradiol through endogenous opioid neurotransmission in women

Prominent interindividual and sex-dependent differences have been described in responses to sustained pain and other stressful stimuli. Variations in mu-opioid receptor-mediated endogenous opioid neurotransmission may underlie some of these processes. We examined both baseline mu-opioid receptor levels and the activation of this neurotransmitter system during sustained pain using positron emission tomography in a sample of young healthy men and women. Women were studied twice, during low and high estrogen states. The high-estrogen state was associated with regional increases in baseline mu-opioid receptor availability in vivo and a greater activation of endogenous opioid neurotransmission during the pain stressor. The latter did not differ from that obtained in males. During the low estrogen condition, however, significant reductions in endogenous opioid tone were observed at the level of thalamus, nucleus accumbens, and amygdala, which were associated with hyperalgesic responses. Estrogen-associated variations in the activity of mu-opioid neurotransmission correlated with individual ratings of the sensory and affective perceptions of the pain and the subsequent recall of that experience. These data demonstrate a significant role of estrogen in modulating endogenous opioid neurotransmission and associated psychophysical responses to a pain stressor in humans.

Interface of physical and emotional stress regulation through the endogenous opioid system and mu-opioid receptors

Unraveling the pathways and neurobiological mechanisms that underlie the regulation of physical and emotional stress responses in humans is of critical importance to understand vulnerability and resiliency factors to the development of a number of complex physical and psychopathological states. Dysregulation of central stress response circuits have been implicated in the establishment of conditions as diverse as persistent pain, mood and personality disorders and substance abuse and dependence. The present review examines the contribution of the endogenous opioid system and mu-opioid receptors to the modulation and adaptation of the organism to challenges, such as sustained pain and negative emotional states, which threaten its internal homeostasis. Data accumulated in animal models, and more recently in humans, point to this neurotransmitter system as a critical modulator of the transition from acute (warning signals) to sustained (stressor) environmental adversity. The existence of pathways and regulatory mechanisms common to the regulation of both physical and emotional states transcend classical categorical disease classifications, and point to the need to utilize dimensional, "symptom"-related approximations to their study. Possible future areas of study at the interface of "mind" (cognitive-emotional) and "body" (physical) functions are delineated in this context.

Chronic estrogenic drug treatment increases preproenkephalin mRNA levels in the rat striatum and nucleus accumbens

Estrogens modulate the expression of preproenkephalin (PPE) in the hypothalamus but little is known for other brain regions. The present study investigated the effect of hormonal withdrawal and replacement therapy on PPE expression in the striatum, nucleus accumbens and cortex. Ovariectomized Sprague-Dawley rats were treated for 2 weeks with estradiol, a specific ligand for estrogen receptor alpha (ERalpha), 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) and estrogen receptor beta (ERbeta) 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN), or the selective estrogen receptor modulators (SERMs) tamoxifen and raloxifene. Brain PPE mRNA levels, measured by in situ hybridization, were high in the striatum and nucleus accumbens compared to the low expression in the cortex. Ovariectomy decreased uterine weights compared to intact uterus, which was corrected by estradiol and PPT. Tamoxifen and raloxifene partially stimulated uterine weights while DPN left it unchanged. In the anterior, median and posterior striatum and in the core and shell of the nucleus accumbens, ovariectomy decreased PPE mRNA levels compared to intact rats, this was corrected by estradiol treatment except for the posterior striatum. PPT, DPN, tamoxifen and raloxifene reproduced the estradiol effect. In the prefrontal and cingulate cortices, neither ovariectomy nor treatments changed PPE mRNA levels. These results show for the first time that estradiol increases PPE mRNA in the striatum and nucleus accumbens. This effect is observed also with estrogen receptor agonists for the ERalpha and ERbeta as well as with SERMs.

Influence of hormonal status on enkephalin-degrading aminopeptidase activity in the HPA axis of female mice

Opioids are involved in the regulation of hypothalamus-pituitary-adrenal (HPA) axis activity under physiological conditions. In the present work, we analyzed the influence of ovariectomy and estradiol (E), progesterone (P) or estradiol plus progesterone (E+P) replacement on soluble (S) and membrane-bound (MB) enkephalin-degrading aminopeptidase activity (EDA) in the HPA axis. Female mice (Balb/C) were distributed in 15 groups of 10 animals each: sham-operated controls (C), ovariectomized controls (OV-C), and ovariectomized mice treated with increasing doses of E (10, 20 or 40 mg/kg), P (100, 200 or 400 mg/kg) or E+P (10+100, 20+200 or 40+400 mg/kg). In hypothalamus, ovariectomy increased both S and MB EDA activities, whereas E replacement returned them to control levels, although MB EDA activity increased again after the replacement with 40 mg/kg E. P replacement increased S EDA activity, but returned MB EDA activity to control levels. The replacement of E+P returned both S and MB EDA activities to control levels, although MB EDA activity was lower than control values after the replacement with 10+100 mg/kg E+P. In pituitary, neither ovariectomy nor the replacement of E or E+P changed S EDA, although the highest concentrations of P increased S EDA activity. However, ovariectomy increased MB EDA and E replacement returned the activity to control or below control levels, depending on the concentration used. However, P administration returned the activity to control or below control levels depending on the concentration used, although 200 mg/kg P had no effects on MB EDA. E+P replacement returned pituitary MB EDA activity to control levels. In adrenal glands, ovariectomy did change either S or MB EDA. However, E, P or E+P replacement decreased S EDA activity in different degrees, depending of the dose administrated. No changes were detected in MB EDA after hormone replacement. These results indicate that female steroid hormones influence EDA activity at different levels of HPA axis.

Effect of testosterone and season on proenkephalin messenger RNA expression in the preoptic area of the hypothalamus in the ram

Enkephalin appears to exert an inhibitory action on LH secretion, but whether testosterone regulates enkephalin gene expression is unknown. This study tested the hypothesis that testosterone and/or season modulate preproenkephalin mRNA expression in specific areas of the hypothalamus. Romney Marsh rams were castrated (wethers) either during the breeding season or nonbreeding season and received intramuscular injections of either oil or testosterone propionate (five/group). Blood samples were taken for the assay of plasma LH and testosterone. Preproenkephalin mRNA expression was quantified in hypothalamic sections by in situ hybridization. Mean plasma LH concentrations were reduced and the interpulse interval for LH pulses was greater in testosterone propionate-treated wethers compared with oil-treated wethers, with no change in LH pulse amplitude. Testosterone propionate treatment reduced proenkephalin expression in the diagonal band of Broca, the caudal preoptic area, and the bed nucleus of the stria terminalis. Seasonal differences in proenkephalin expression were observed in the bed nucleus of the stria terminalis, lateral septum, periventricular nucleus, and paraventricular nucleus. No differences were observed between treatments in seven other regions examined. We conclude that testosterone and season regulate proenkephalin mRNA levels in the preoptic area/hypothalamus in the ram in a region-specific manner.

Gender differences in proenkephalin gene expression response to delta9-tetrahydrocannabinol in the hypothalamus of the rat

Chronic exposure to delta9-tetrahydrocannabinol (delta9-THC) produces an activation of preproenkephalin (PENK) gene expression in the rat hypothalamus. The levels of circulating gonadal steroids concurrently modulate this neuropeptide in male and female rats. However, whether gonadal steroids regulate delta9-THC effects on PENK gene expression in the hypothalamus of male and female rats remains unknown. To test this hypothesis, experiments were carried out on intact, 2-week-gonadectomized, 1-week-gonadectomized, 1-week-dihydrotestosterone (DHT) replaced male rats, and 2-week-gonadectomized, 1-week-gonadectomized, 1-week-oestradiol replaced female rats. One week after hormonal replacement, animals were treated with vehicle or delta9-THC (5 mg/kg/day, i.p. 7 days). In males, delta9-THC administration to intact animals induced PENK mRNA in the paraventricular nucleus (PVN) and ventromedial nucleus (VMN) of the hypothalamus. Orchidectomy did not affect basal PENK mRNA levels in the PVN, but reduced PENK mRNA levels in the VMN. However, delta9-THC treatment induced PENK gene expression to the same extent in both hypothalamic nuclei of intact, castrated and DHT-replaced males. In females, ovariectomy decreased PENK gene expression in PVN and VMN. delta9-THC administration increased PENK gene expression in castrated females, but had no effect in the oestradiol-replaced group. Taken together, these results suggest gender differences in the response of chronic exposure to cannabinoids on PENK gene expression in the hypothalamus. Furthermore, it appears that alterations in opioid gene expression induced by cannabinoids in female rats depend upon the presence or absence of circulating oestradiol.

Prepro-thyrotropin releasing hormone 178-199 immunoreactivity is altered in the hypothalamus of the Wistar-Kyoto strain of rat

The rat prepro-thyrotropin releasing hormone (TRH) 178-199 is derived from prepro-TRH by the actions of the endopeptidases, prohormone convertase 1 (PC1) and PC2. PPTRH 178-199 attenuates the synthesis and secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary both in vitro and in vivo, suggesting an inhibitory action on hypothalamic-pituitary-adrenal (HPA) axis function. This peptide also acts centrally to increase activity and decrease anxiety related behaviors. To elucidate the involvement of this peptide in these functions, we have compared the expression of PPTRH 178-199, PPTRH mRNA, and PC1 and PC2 mRNAs in the Wistar-Kyoto (WKY) and Wistar strains of rat. WKY rats have been shown to possess neuroendocrine abnormalities (HPA hyper-activity) and hyper-emotional behavioral characteristics. Immunohistochemical analysis of PPTRH 178-199 demonstrated significant strain differences in the paraventricular nucleus (PVN) of the hypothalamus and the parastrial nucleus (PSN). WKY rats had significantly greater numbers of immunoreactive (IR) cell body profiles (P<0.0005) than Wistar rats in the PVN and a significantly lower fiber density (P<0.002) in the PSN. Levels of PPTRH, PC1, and PC2 mRNA were not different between strains in any brain region examined. These data suggest that altered levels of PPTRH 178-199 in WKY rats could cause, at least in part, the hyper-activity of the HPA axis and the hyper-emotional behavioral characteristics seen in this rat strain. Such data fit with the hypothesis that PPTRH 178-199 is involved in the regulation of the HPA axis and behavior.

Reduction of zif268 messenger RNA expression during prolonged withdrawal following "binge" cocaine self-administration in rats

Cocaine self-administration increases dopamine efflux and neuronal activity in the mesocorticolimbic dopamine system compared with experimenter-administered cocaine. Following a prolonged cocaine self-administration binge, dopamine efflux in the nucleus accumbens is attenuated and behaviors emerge that are indicative of anhedonia and anxiety. The neuronal correlates of these behavioral and neurochemical effects of a cocaine binge were assessed using in situ hybridization histochemistry to detect changes in zif268 messenger RNA expression. Rats were fitted with intravenous catheters; one group was trained to self-administer cocaine (0.5mg/injection), then allowed continuous access to cocaine during a 16h binge, while yoked animals received either saline or cocaine according to the same schedule. Measurement of tactile startle responses and ultrasonic distress calls either immediately after termination of cocaine access or one or 14 days later confirmed peak withdrawal at 24h after the binge. The level of zif268 messenger RNA was lower upon termination of cocaine self-administration than in both yoked treatment groups in the ventral tegmental area and hippocampus. In contrast, zif268 messenger RNA expression increased in the periaqueductal gray matter one day after termination of passive cocaine treatment, coincident with enhanced expression of ultrasonic vocalizations. Zif268 messenger RNA expression decreased over time in the nucleus accumbens core and infralimbic cortex, with reduced expression observed in the nucleus accumbens core, caudatoputamen, hippocampus and amygdala 14 days after termination of cocaine self-administration. The results suggest that withdrawal following a cocaine self-administration binge produces a long-lasting reduction of constitutive zif268 messenger RNA expression in mesocorticolimbic brain regions related to the nucleus accumbens. The relatively greater effect in animals that self-administered cocaine implies a relationship of certain regional responses to behavioral conditioning.

Fenfluramine-induced increase in preproenkephalin mRNA levels in the striatum: interaction between the serotonergic, glutamatergic, and dopaminergic systems

Fenfluramine (FE) is a halogenated amphetamine derivative that has been used in the treatment of obesity. It has been suggested that the effects of FE on the striatum are mediated by serotonergic mechanisms. However, several major afferent systems may be involved, and administration of FE may be useful to study interactions between these systems. In this work, the effects of FE on striatopallidal neurons and the possible involvement of the major striatal afferent systems were studied in rats by determination of FE-induced changes in striatal levels of preproenkephalin (PPE) mRNA using in situ hybridization. Injection of FE induced a significant increase (60%) in striatal levels of PPE mRNA. This increase was blocked by pretreatment with the D(1) dopamine receptor antagonist SCH-23390 or with the NMDA glutamate receptor antagonist MK-801, or by lesion of the serotonergic system with 5,7-dihydroxytryptamine or p-chlorophenylalanine. In 6-hydroxydopamine lesioned rats, the lesion-induced increase in PPE mRNA levels was not affected by injection of FE, but was reduced by simultaneous serotonergic deafferentation. The results suggest that the serotonergic, glutamatergic, and dopaminergic system interact to increase striatal PPE mRNA levels after FE administration.

Sex differences in estrogenic regulation of preproenkephalin mRNA levels in the medial preoptic area of prepubertal rats

Opioids have been implicated in sexual differentiation of the brain and in the regulation of reproductive behavior and endocrinology of mammals. Previous studies have indicated that estrogen administration in adults regulates preproenkephalin MRNA levels in several hypothalamic brain nuclei. We have determined preproenkephalin mRNA levels in estrogen-treated juvenile male and female rats to investigate the developmental pattern of estrogenic regulation of enkephalinergic neurons in the medial preoptic area. Rats were treated with estradiol benzoate (20 microgram/kg/day) or oil from day 21 to 23. Sections of the medial preoptic area (mPOA) were studied by in situ hybridization histochemistry at the single cell level and quantified with the assistance of an image analysis system. Our data indicate that males contain higher levels of preproenkephalin mRNA per neuron than females. In addition, our results indicate that estrogen causes an upward shift in the amount of mRNA expressed per cell, females demonstrating a greater response to estrogen than males. An increase in soma cell area following estrogen treatment was observed only in female mPOA enkephalinergic neurons. Taken together, these results indicate a sex difference in total preproenkephalin levels and in estrogenic regulation of preproenkephalin mRNA in the POA of juvenile rats. These results are discussed in relation to the differential role opioids may play in male and female reproductive physiology.

Chronic administration of cannabinoids regulates proenkephalin mRNA levels in selected regions of the rat brain

This study was designed to examine the interactions between the cannabinoid and enkephalinergic systems in the rat brain. To this aim, we have examined the effects of subchronic (5 days) administration (10 mg.kg-1.day-1; i.p.) of delta 9 -tetrahydrocannabinol (THC) or R-methanandamide (AM356) and chronic (18 days) administration with the synthetic cannabinoid receptor agonist CP-55,940 (1 mg.kg-1.day-1; i.p) on proenkephalin (PENK) mRNA levels in several brain regions of the rat. Twenty micrometer brain sections from striatum, nucleus accumbens, paraventricular nucleus, ventromedial nucleus, periaqueductal grey matter and mammillary nucleus were hybridized with an oligonucleotide probe complementary to PENK using in situ hybridization technique. Subchronic administration of THC or AM356 increased PENK mRNA levels in the ventromedial nucleus of the hypothalamus, (82%) and (39%), in the periaqueductal grey matter, (97%) and (49%), and mammillary nucleus, (43%) and (9%), respectively. In contrast, both drugs were without effect in the striatum and nucleus accumbens. On the other hand, chronic administration of CP-55,940 increased PENK mRNA levels in the striatum (44%), nucleus accumbens (25%), paraventricular (31%) and ventromedial nuclei of the hypothalamus (41%). These results revealed that chronic cannabinoid administration increases opioid gene expression in the rat central nervous system and suggest an interaction between the cannabinoid and enkephalinergic systems that may be part of a molecular integrative response to behavioral and neurochemical alterations that occur in cannabinoid drug abuse.

Acute and repeated ECS treatment increases CRF, POMC and PENK gene expression in selected regions of the rat hypothalamus

The purpose of this study was to investigate the effects of acute and repeated electroconvulsive shock (ECS) on corticotropin releasing factor (CRF), proopiomelanocortin (POMC) and proenkephalin (PENK) gene expression in selected regions of the brain and pituitary of the rat. Acute ECS increased CRF gene expression in the paraventricular nucleus (PVN) by 20%, an effect that was further enhanced to 38% when rats received repeated ECS treatment. Acute and repeated ECS increased POMC gene expression in the arcuate nucleus (ARC) by 49-59% but failed to alter these mRNA levels in the anterior lobe (AL) of the pituitary gland. PENK gene expression was increased by 35% in the nucleus accumbens (NA) and by 180% the ventromedial nucleus (VMN) after acute or repeated ECS treatment but no significant changes were found in the PVN or striatum (ST). Taken together, these results indicate a differential CRF and opioid gene expression regulation after acute or repeated ECS treatment that may be relevant to their therapeutic or side effects in depression.

Estrogen alters proenkephalin RNAs in the paraventricular nucleus of the hypothalamus following stress

Gonadal steroids modulate activity of the hypothalamo-pituitary-adrenal axis (HPA) following stress, but the regulatory pathways of this modulation are unknown. A possible site of action is the synthesis of CRH and/or enkephalin in cells of the paraventricular nucleus of the hypothalamus (PVN). To investigate this possibility, we utilized two stressors, i.p. hypertonic saline injection (HSI) or exposure to novel environment, and examined the response of CRH or c-fos mRNAs and proenkephalin (PPE) mRNA and heteronuclear RNA (hnRNA, primary transcript). Male rats were gonadectomized and treated with estrogen or dihydrotestosterone propionate (DHTP) for 2 weeks. In situ hybridization revealed that novelty or HSI elevated levels of PPE hnRNA and c-fos mRNA in the PVN. Estrogen attenuated the elevation of PPE hnRNA in the PVN following HSI, and enhanced the c-fos mRNA response to novelty. In contrast, DHTP did not affect PPE hnRNA, but inhibited the c-fos mRNA response to novelty. These data indicate that in male rats estrogen receptor but not androgen receptor may modulate the endocrine stress response by altering PPE transcription in the PVN and that this effect depends on the type of stressor.

Alterations in the estrogen sensitivity of hypothalamic proenkephalin mRNA expression with age and prenatal exposure to alcohol

Studies suggest that exposure to alcohol in utero causes reproductive and neuroendocrine deficits in adult female rats. The ventromedial nucleus of the hypothalamus (VMN) is an estrogen-sensitive brain region which is regarded as a primary locus for modulating female reproduction. Proenkephalin (PE) mRNA expression in the VMN is dramatically increased by estrogen and this elevation is thought to be involved in modulating female reproductive behavior and neuroendocrine function. To examine whether prenatal alcohol exposure has long-term effects on the ability of estrogen to influence hypothalamic PE mRNA levels, female rats at 2-3, 6-7 or 15-18 months of age, derived from alcohol- or control-fed dams, were studied. 7 days following ovariectomy, animals received either estrogen or sham treatment for 2 days prior to sacrifice. PE mRNA levels in the VMN and striatum were determined by in situ hybridization histochemistry. Film autoradiogram density, numbers of PE mRNA-expressing cells and exposed silver grains/cell were analyzed. Estrogen treatment increased hybridization density, the number of PE mRNA-expressing cells and PE mRNA (grains) level/cell in the VMN of normal adult female rats. In old rats, estrogen increased the number of PE mRNA-expressing cells without up-regulating PE mRNA grain density/cell. In fetal alcohol-exposed (FAE) female rats, the number of cells that expressed PE mRNA did not increase following estrogen treatment at any age. Elevation of grain density/cell following estrogen was observed in FAE animals but only at 7-8 months of age. Overall, these data indicate that the estrogen responsiveness of PE mRNA expression in the VMN declines with age and, furthermore, prenatal exposure to alcohol blunts estrogen's effects on PE mRNA expression in the adult VMN. These finding may help to explain the mechanisms underlying the loss of reproductive function observed in FAE females.

Effect of morphine on proenkephalin gene expression in the rat brain

Previous studies indicate that an acute injection of morphine does not effect the level of opioid peptides and their mRNA in the brain. However, due to the presence of a large pool of mRNA and possible opposing changes in turnover rate it is often difficult to visualize the transitory and relatively small alterations in gene transcription by examining mRNA level. Therefore, in situ hybridization with probes directed against intronic sequences to measure the primary transcript of proenkephalin (PPE) mRNA (heteronucleic RNA, hnRNA) in the rat brain following morphine administration was used in this study. The distribution of the hybridization signal of probes against both the A and B intron of the PPE gene were identical and coincide with the distribution PPE mRNA. Thus, to increase the sensitivity of this assay both probes were concurrently hybridized. Female and male Sprague-Dawley rats were gonadectomized and injected with morphine (10 mg/kg, SC). We detected no changes in PPE mRNA levels in the striatum, olfactory tubercle (OT) and n. accumbens core (NAC) at any time following morphine administration. However, from 0.5 h until 24 h following morphine injection, the levels of PPE hnRNA in NAC and OT but not in the dorsal striatum were significantly decreased. The level of c-fos mRNA was increased only the dorsal striatum following morphine injections. These data show that morphine administration can acutely change opioid peptide gene transcription. The observed decrease of PPE hnRNA levels for 24 h following a single morphine injection may indicate its importance for the development of acute and chronic dependence. However, the significance of these alterations in PPE gene transcription in term of the acute effect of morphine is not clear, because the steady-state level of mRNA was not changed.

Androgens selectively modulate C-fos messenger RNA induction in the rat hippocampus following novelty

We have previously shown that androgen receptors are found in high concentrations in hippocampal CA1 pyramidal cells. To begin to explore the possible roles for androgen receptors in this area of the brain, we studied the effects of endogenous and exogenous androgen on the behaviourally induced expression of cellular immediate early gene messenger RNAs. Adult male Fischer 344 rats were either gonadectomized, gonadectomized and given two Silastic capsules of dihydrotestosterone propionate at the time of surgery, or left intact. Three weeks later, animals were placed into a novel open field for 20 min. This behavioural paradigm caused region- and gene-specific increases of c-fos, jun-B, c-jun and zif268 messenger RNA in the hippocampus as determined by semi-quantitative in situ hybridization histochemistry. The removal of circulating androgen by gonadectomy potentiated, whereas dihydrotestosterone treatment of castrates attenuated, the behaviourally induced expression of c-fos messenger RNA in the CA1 region of the hippocampus. No changes in c-fos messenger RNA expression were detected in the CA3 or dentate gyrus regions where androgen receptor levels are low. Androgen status did not affect either the basal or stimulated expression of Jun-B, c-Jun or zif268 messenger RNA in any of the three cellular regions of the hippocampus examined. These results implicate androgen receptors in modulating the active response of hippocampal neurons to a behaviourally relevant stimulus. Since the products of cellular immediate genes can function to alter an array of downstream genes, the modulation of these genes in the hippocampus by gonadal hormones may have important ramifications for hippocampal function.

Opposite changes in the expression of enkephalin in the amygdala and hypothalamus after lesions of the bed nucleus of the stria terminalis in the rat

The expression of enkephalin in neurons of the rat forebrain was studied by in situ hybridization and immunohistochemistry after unilateral injections of ibotenic acid into the bed nucleus of the stria terminalis. Initially, we observed that the destruction of nerve cell bodies in this nucleus resulted in a prominent bilateral increase in the number of neuronal perikarya immunoreactive for [Met]enkephalin in the lateral/basolateral amygdaloid complex-especially in the anterior division of the latter nucleus-as compared with NaCl-injected rats. In a separate set of experiments, this effect was associated with a significant (two times) enhancement of the number of nerve cell bodies containing preproenkephalin A messenger RNAs in the same amygdaloid nucleus ipsilateral to the injection, as compared with controls. In the hypothalamus of both experimental and control rats, the nerve cell bodies immunoreactive for [Met]enkephalin were few since the animals were not pretreated with colchicine, and the effects of the lesion were difficult to appreciate. However, using in situ hybridization, numerous nerve cell bodies containing preproenkephalin A messenger RNAs were detected bilaterally in the perifornical area, the paraventricular (parvocellular division) and the ventromedial nuclei of the hypothalamus. In the latter nucleus, the lesion of the bed nucleus of the stria terminalis resulted in a strong decrease (about two times) in the number of labelled cell bodies as compared with the controls, whereas no significant changes were found bilaterally in the paraventricular nucleus. In agreement with some data of the literature, our results indicate that the bed nucleus of the stria terminalis plays an important role in the regulation of neuropeptide genes expression in certain regions of the limbic system. Such a role is often exerted by nerve fibres afferents to the nerve cell bodies considered. However, from numerous neuroanatomical data of the literature, it appears more probable that the induction or inhibition of the expression of enkephalin in presynaptic neurons is due to the disappearance of their postsynaptic target in the bed nucleus of the stria terminalis.

Membrane-bound tyrosine aminopeptidase activities in the rat brain throughout the estrous cycle

Puromycin sensitive and insensitive membrane-bound aminopeptidase activity levels during the estrous cycle in several brain areas have been described in this research. We have found the highest aminopeptidase M activity levels during the proetrous stage in the hypothalamus, the amygdala and the pituitary gland. Since this enzyme has been involved in opioid peptide metabolism, it is suggested that aminopeptidase M could play a part in the decrease in the inhibitory influence of the endogenous opioids peptides that participate in the LH surge.

Hormonal regulation of androgen receptor messenger RNA in the medial preoptic area of the male rat

In the adult male rat, androgen and estrogen synergize in the regulation of male reproductive behaviors. To explore some of the molecular mechanisms underlying this synergism we examined the distribution and hormonal regulation of androgen receptor (AR) and estrogen receptor (ER) mRNAs in the medial preoptic area (MPOA) and bed nucleus of the stria terminalis (BST) of the adult male rat. Using in situ hybridization, AR and ER mRNAs were found to be distributed in overlapping but unique patterns. The highest density of AR mRNA was found in the central part of the medial preoptic n. and the principal n. of the BST. Gonadectomy (GDX) of adult male rats caused an increase in hybridization density in both brain areas after 4 days followed by a decrease after 2 months. In contrast, ER mRNA was increased following GDX and remained high regardless of length of time. Treatment of adult GDX'd males with dihydrotestosterone (DHT) reversed the effects of GDX on AR mRNA at both the short and long-term castrate but had no effect on ER mRNA in both the MPOA and BST. Estrogen treatment increased AR mRNA in the long-term castrate only and decreased ER mRNA in both long- and short-term castrates. Immunocytochemical detection of AR revealed a similar distribution to AR mRNA; however, AR immunoreactivity was reduced in the MPOA and BST after both short- and long-term GDX. In vitro [3H]DHT binding in cytosols of the preoptic area showed appreciable binding but there was no effect of length of time following GDX. These data show that the pattern of regulation of AR mRNA is unique to this receptor type and does not follow the pattern of regulation of the ER mRNA. Furthermore, although the distribution of AR mRNA and AR protein coincide within the MPOA, changes in mRNA levels as a result of castration or hormone treatment do not result in corresponding changes in binding. This mismatch between mRNA and binding suggests a complex regulation of AR beyond simply changes in transcription.

The hypothalamic ventromedial nucleus sends a met-enkephalin projection to the preoptic area's periventricular zone in the female rat

The female but not the male rat possesses a dense network of methionine enkephalin (m-Enk) fibers in the periventricular zone of the preoptic area (pePOA). The potential source of these fibers was determined by injection of the tracer fluorogold, FAu, into the preoptic area of adult female rats. Twenty-four hours before they were killed, the rats were administered colchicine (intraventricularly) to enable immunocytochemical visualization of m-Enk cells. Upon examination of the brains with fluorescence microscopy, double-labeled cells showing fluorogold and immunofluorescence for m-Enk were consistently observed in the preoptic area, the ventrolateral division of the ventromedial nucleus of the hypothalamus (VMHvl) and nearby medial tuberal area (MTA), the arcuate nucleus, periventricular area of the hypothalamus, perifornical area, and dorsomedial nucleus of the hypothalamus. A series of lesion and knife cut experiments using glass, Halasz, and wire knives determined that the pePOA m-Enk fibers arose from the hypothalamus, near or within the VMH. Ibotenic acid lesions further determined that the source of the m-Enk projection was the VMHvl with a possible additional contribution from the MTA.

Proenkephalin gene regulation in the neuroendocrine hypothalamus: a model of gene regulation in the CNS

During the past decade, a great deal of progress has been made in studying the mechanisms by which transcription of neuropeptides is regulated by second messengers and neural activity. Such investigations, which have depended to a great extent on the use of transformed cell lines, are far from complete. Yet a major challenge for the coming decade is to understand the regulation of neuropeptide genes by physiologically and pharmacologically relevant stimuli in appropriate cell types in vivo. The proenkephalin gene, a member of the opioid gene family, has served as a model to study regulated transcription, not only in cell lines, but also in central (e.g., hypothalamic) and peripheral (e.g., adrenal) neuroendocrine tissues. Here we review regulation of proenkephalin gene expression in the hypothalamus. Several approaches, including in situ hybridization, use of transgenic mice, and the adaptation of electrophoretic mobility shift assays to complex tissues, have played critical roles in recent advances. A summary of possible future developments in this field of research is also presented.

N-methyl-D-aspartate R1 messenger RNA and [125I]MK-801 binding decrease in rat spinal cord after unilateral hind paw inflammation

Recent evidence suggests that N-methyl-D-aspartate receptors play an important role in the etiology and maintenance of chronic nociception. Previous studies have demonstrated that tissue injury or stimulation of nociceptive afferent projections results in the expansion of receptive fields, hyperalgesia and C-fiber-induced wind-up, events that can be inhibited by N-methyl-D-aspartate antagonists. This study examines the effect of unilateral hind paw inflammation on N-methyl-D-aspartate R1 messenger RNA and [125I]dizocilpine maleate binding in the L4-L5 segments of the lumbar spinal cord of rats. Spinal cords were examined at 7.5 h, three, seven and 20 days after injection of the left hind paw with 120 microliters of complete Freund's adjuvant. N-methyl-D-aspartate R1 messenger RNA, as measured with in situ hybridization, was observed to decrease bilaterally in laminae I, II and X of the lumbar spinal cord. This decrease was evident in laminae I and II at 7.5 h and three days after hind paw injection. In lamina X, a postinjection decrease in hybridization signal was observed at 7.5 h and seven days. A bilateral decrease in [125I]dizocilpine maleate binding was observed in laminae I and II at three, seven and 20 days after paw injection. This observed decrease in binding at the N-methyl-D-aspartate receptor suggests a compensatory mechanism by which N-methyl-D-aspartate-mediated nociceptive events may be modulated.

NMDA R1 mRNA distribution in motor and thalamic-projecting sensory neurons in the rat spinal cord and brain stem

The N-methyl-D-aspartate (NMDA) receptor is important in both sensory and motor neurotransmission. In this study we examine NMDA R1 mRNA hybridization signal over individual sensory and motor neurons in the spinal cord and brain stem. A significantly greater quantity of NMDA R1 mRNA was present in motor neurons of the lumbar spinal cord and hypoglossal nucleus compared to thalamic projecting sensory neurons in the spinal cord dorsal horn, the spinal trigeminal nucleus pars caudalis and the cuneate and gracile nuclei. No significant difference in the quantity of NMDA R1 mRNA was observed between sensory neurons known to relay predominantly nociceptive information (trigeminothalamic and spinothalamic tract neurons) and that relay predominantly touch and proprioceptive information (dorsal column neurons).

Castration increases [125I]MK801 binding in the hippocampus of male rats

This study examines the effect of castration and androgen replacement on [125I]MK801 binding in the hippocampus. In castrated male rats, [125I]MK801 binding was significantly increased in both the stratum oriens and radiatum and the pyramidal cell layer of CA1. In contrast, no increase in [125I]MK801 binding was observed in the stratum oriens and radiatum of CA1 of castrated rats that were treated with dihydrotestosterone. No change in [125I]MK801 binding was observed in the CA3 region or dentate gyrus after castration. The observed increase in [125I]MK801 binding in pyramidal cell neurons within CA1 suggests that androgens may potentially affect hippocampal function by modulating pyramidal cell NMDA receptors.

Distribution of NMDAR1 receptor subunit mRNA and [125I]MK-801 binding in the hypothalamus of intact, castrate and castrate-DHTP treated male rats

This study examines NMDAR1 receptor subunit mRNA expression and [125I]MK-801 binding in hypothalamic and limbic nuclei of intact, castrate and castrate-dihydrotestosterone propionate (DHTP)-treated male rats. In intact rats, the highest levels of NMDAR1 mRNA were observed in the supraoptic, suprachiasmatic, ventromedial and arcuate nuclei. Low levels of hybridization were observed in the bed nucleus of the stria terminalis, lateral preoptic area, lateral hypothalamic area and lateral septum. In castrated rats both NMDAR1 mRNA and [125I]MK-801 binding are significantly decreased in the lateral septum compared to castrate rats treated with DHTP, a non-aromatizable androgen. NMDAR1 mRNA was also significantly decreased in the supraoptic nucleus of castrate rats when compared to castrate rats treated with DHTP. These data suggest that androgens may modulate NMDA receptor function in some parts of the central nervous system.

Somatostatin-immunoreactive neurones in the hypothalamic ventromedial nucleus possess oestrogen receptors in the male and female rat

Neurones containing oestrogen receptors (ERs) in the ventrolateral division of the hypothalamic ventromedial nucleus (VMNvl) are believed to play an important role in mediating oestrogen's regulatory influence on reproductive behaviour. As somatostatin (SOM)-immunoreactive neurones are found exclusively within the ventrolateral division of VMN, this study has used double-labelling immunocytochemistry techniques to evaluate whether ER-immunoreactive cells in the VMNvl synthesize SOM in both the male and female rat. Rats gonadectomized 1 week earlier were perfused and brain sections through the mediobasal hypothalamus processed for ER and SOM immunoreactivity using the H222 monoclonal rat ER antibody and a polyclonal rabbit SOM antiserum. Within the VMN, cells immunoreactive for SOM were found predominantly in the rostral portion of the VMNvl while ER-immunoreactive cells were distributed throughout the VMNvl. Sequential double-labelling studies revealed that many ER-containing cells in the rostral VMNvl were also immunoreactive for SOM. A semi-quantitative analysis of double-labelled cells in the rostral VMNvl of male and female rats, respectively, estimated that 52 +/- 2% and 55 +/- 8% of SOM-IR cells possess ERs while 35 +/- 1% and 28 +/- 3% of ER-IR cells synthesise SOM in the rostral VMNvl. No sex differences were detected at this level. These results show that approximately half of the SOM-synthesizing neurones in the rostral VMN possess ERs and indicate that SOM should now be considered alongside enkephalin and Substance P as a putative mediator of oestrogen's regulatory influence on reproduction through the VMN of the rat.