Changes in preproenkephalin messenger RNA level in the rat ventromedial hypothalamus during the estrous cycle

Estrogen affects levels of Bcl-2 protein and mRNA in medial amygdala of ovariectomized rats

The survival factor Bcl-2 is a cyclic AMP response element-binding protein (CREB) gene product implicated in mediating some of estrogen's effects on neuroprotection. Previously, we showed an effect of estradiol benzoate (E) on numbers of neuron-specific protein (NeuN)- and phosphorylated CREB (pCREB)-positive cells in medial (MeA), but not central (CeA), amygdala of ovariectomized rats. To determine whether these effects are accompanied by an E-induced increase in Bcl-2, we examined the effects of E on levels of Bcl-2 protein and mRNA in MeA and CeA of ovariectomized rats treated with E regimens resulting in moderate (2.5 microg E for 4 or 14 days) or high (10 microg E for 14 days) plasma estradiol levels. As a physiological control, we showed that all E treatments increased uterine wet weight relative to vehicle; 10 microg E for 14 days also increased uterine weight compared with that seen with lower E levels. Western blot analysis revealed that all E groups displayed an increase in uterine Bcl-2 protein levels compared with vehicle. We found that 2.5 microg and 10 microg E for 14 days increased levels of Bcl-2 gold immunolabeling compared with vehicle and 2.5 microg E for 4 days in MeA, but not CeA. We measured Bcl-2 mRNA levels in vehicle and 2.5 microg E-treated 14-day groups. There was a significant increase in Bcl-2 mRNA levels in MeA, but not CeA, of E-treated ovariectomized rats compared with vehicle controls. The E-induced increase in protein and mRNA levels of Bcl-2 in MeA may be important for neuroprotection in this region.

Hormone effects on specific and global brain functions

The first demonstration of how biochemical changes in neurons in specific parts of the brain direct a complete mammalian behavior derived from the effects of estrogens in hypothalamic neurons that facilitate lordosis behavior, the primary reproductive behavior of female quadrupeds (Pfaff. Estrogens and Brain Function. 1980; Pfaff. Drive: Neurobiological and Molecular Mechanisms of Sexual Motivation. 1999). Sex behaviors depend on sexual arousal that in turn depends on a primitive function: generalized CNS arousal (Pfaff. Brain Arousal and Information Theory. 2006). Here we summarize one of the ways in which a generalized arousal transmitter, norepinephrine, can influence the electrical excitability of ventromedial hypothalamic cells in a way that will foster female sex behavior.

Variations in opioid peptide levels during the estrous cycle in Sprague-Dawley rats

The estrous cycle, with its various hormonal conditions, may provide us with the means of understanding how endocrine states relate to opioid mechanisms. There has been increasing experimental support for interaction between sex steroids and opioid peptides in the central nervous system. Here, we describe fluctuations in endogenous brain immunoreactive (ir) peptide levels during various phases of the estrous cycle in the female Sprague-Dawley rat. Ir levels of dynorphin A, dynorphin B, Leu-enkephalin-Arg(6), Met-enkephalin-Arg(6)Phe(7) and nociceptin/orphanin FQ were measured in the pituitary gland and in 10 areas of the brain during the diestrus, proestrus and estrus phase. In several areas of the brain, basal levels of endogenous opioid peptides showed variation during the course of the estrous cycle. Significant differences were found between the diestrus state and the proestrus and/or estrus conditions, particularly in the nucleus accumbens, caudate putamen and the substantia nigra. The ir levels of the endogenous peptide nociceptin/orphanin FQ became altered in only one of the areas measured, indicating less variance during the estrous cycle. Correlation analyses revealed that significant associations between dynorphin A or dynorphin B and Leu-enkephalin-Arg(6) were found more often during estrus than during the diestrus and proestrus conditions. The ratio between the ir levels of Leu-enkephalin-Arg(6), a cleavage product of the enzymatic conversion of dynorphin peptides into shorter peptides in vivo, and dynorphin peptides was calculated. The significantly lower ratio between Leu-enkephalin-Arg(6) and dynorphin B in diestrus than in proestrus and estrus also indicates cyclic fluctuations in the enzymatic cleavage of dynorphin. These findings are discussed in relation to the possible role of interactions between sex steroids and opioid peptide mechanisms during the normal estrous cycle.

Estrogen decreases levels of calcineurin in rat amygdala and hippocampus

We examined the effects of estradiol benzoate (E2) on the protein expression of calcineurin in amygdaloid and hippocampal structures of ovariectomized (OVX) rats. Significant decreases in levels of calcineurin immunolabeling were seen in the medial and basomedial, but not central or basolateral, amygdala. Estrogen also reduced calcineurin immunoreactivity in the CA1 region of the hippocampus, but not in the CA3 region, hilus or ventral or dorsal dentate gyrus structures of hippocampus. These results indicate that E2 acts on calcineurin in a neuroanatomically specific manner and may be involved in estrogen-mediated regulation of gene expression.

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.

Effects of estrogen treatment on expression of brain-derived neurotrophic factor and cAMP response element-binding protein expression and phosphorylation in rat amygdaloid and hippocampal structures

Clinical studies indicate an effect of estrogen (E2) on affect and cognition, which may be mediated by the cAMP response element-binding protein (CREB) pathway and CREB-related gene target brain-derived neurotrophic factor (BDNF). We investigated the effect of E2 on CREB expression and phosphorylation and BDNF expression in the amygdala and hippocampus, areas involved in emotional processing. Ovariectomized rats were given 10 microg 17beta-estradiol or vehicle for 14 days and expression of components of the CREB signaling pathway, i.e., CREB, phosphorylated CREB (pCREB), and BDNF in amygdala and hippocampus were investigated using immunogold labeling. Levels of BDNF mRNA were determined by in situ reverse-transcriptase polymerase chain reaction. We also examined the effect of E2 on calcium/calmodulin kinase (CaMK IV) immunolabeling in the hippocampus. E2 increased immunolabeling and mRNA levels of BDNF in the medial and basomedial amygdala and CA1 and CA3 regions of the hippocampus, but not in any other amygdaloid or hippocampal regions examined. E2 increased immunolabeling of CREB and pCREB in the medial and basomedial, but not central or basolateral amygdala. E2 also increased CaMK IV and pCREB immunolabeling in the CA1 and CA3 regions, but not CA2 region or dentate gyrus, of the hippocampus. There was no change in immunolabeling of CREB in any hippocampal region. These data identify a signaling pathway through which E2 increases BDNF expression that may underlie some actions of E2 on affective behavior and indicate neuroanatomical heterogeneity in the E2 effect within the amygdala and hippocampus.

Estrogen and thyroid hormone receptor interactions: physiological flexibility by molecular specificity

The influence of thyroid hormone on estrogen actions has been demonstrated both in vivo and in vitro. In transient transfection assays, the effects of liganded thyroid hormone receptors (TR) on transcriptional facilitation by estrogens bound to estrogen receptors (ER) display specificity according to the following: 1) ER isoform, 2) TR isoform, 3) the promoter through which transcriptional facilitation occurs, and 4) cell type. Some of these molecular phenomena may be related to thyroid hormone signaling of seasonal limitations upon reproduction. The various combinations of these molecular interactions provide multiple and flexible opportunities for relations between two major hormonal systems important for neuroendocrine feedbacks and reproductive behaviors.

Differential regulation of pituitary adenylate cyclase-activating peptide receptor variants in the rat suprachiasmatic nucleus

Pituitary adenylate cyclase-activating peptide is densely distributed in the suprachiasmatic nucleus, which functions as the circadian pacemaker. A receptor for pituitary adenylate cyclase-activating peptide, denoted as PAC(1), exists in six variant forms. We used reverse transcriptase-polymerase chain reaction to identify the PAC(1) variants that are expressed in the suprachiasmatic nucleus. Dominant variant forms of PAC(1) in the suprachiasmatic nucleus were PAC(1)short, PAC(1)hip, and PAC(1)hop1. By in situ hybridization, we examined 24-h profiles of mRNAs for the identified receptor variants in the suprachiasmatic nucleus in constant darkness and during the light-dark cycle. In constant darkness there were clear circadian rhythms in PAC(1)short mRNA with a peak at circadian time 4 but no rhythmicity was observed in PAC(1)hip mRNA or PAC(1)hop1 mRNA. In light-dark cycles, on the other hand, PAC(1)hip mRNA displayed a bimodal rhythm with troughs at zeitgeber time 4 and 16 but PAC(1)hop1 mRNA stayed constant during the day. These results suggest that PAC(1) splice variants are differentially regulated in the rat suprachiasmatic nucleus.

Effect of estrogen on the expression of Cry1 and Cry2 mRNAs in the suprachiasmatic nucleus of female rats

To determine whether estrogen has an effect on clock genes in the suprachiasmatic nucleus (SCN), we examined the effect of estrogen on the expression of Cry1 and Cry2 mRNAs in the SCN of female rats. Ovariectomized rats were injected with 20 microg 17beta-estradiol at zeitgeber time (ZT) 6 and ZT 18 and killed 24 h after the treatment. Northern blot revealed that the expression of Cry2 mRNA was significantly decreased in the SCN of estrogen-treated rats at both time points [correction]. But estrogen did not affect Cry1 mRNA levels in the SCN at any ZT. These results suggested that Cry1 and Cry2 mRNAs in the SCN were differently regulated by estrogen.

Circadian and photic regulation of cryptochrome mRNAs in the rat pineal gland

Expressions of Cry1 and Cry2 mRNA in the rat pineal gland were examined by Northern blot. The levels of Cry1 and Cry2 transcript had a marked circadian rhythm with peaks at circadian time (CT) 20 in constant darkness. But the amplitude of the Cry1 rhythm was higher than that of the Cry2 rhythm. Furthermore, a significant increase in Cry1 mRNA levels was caused by light pulse given at CT 16 but not at CT 4, but the expression of Cry2 was not significantly induced by light pulses given at either CT 4 or CT 16. These results suggest that Cry1 in the pineal gland is regulated by photic and circadian information but Cry2 is only regulated by circadian information.

Effects of estrogen and progesterone on the expression of connexin-36 mRNA in the suprachiasmatic nucleus of female rats

To determine the effects of ovarian steroid hormones on gap junction communication in the suprachiasmatic nucleus (SCN), we examined the effects of estrogen and/or progesterone on the expression of connexin-36 mRNA in the SCN and cerebral cortex (CX) of female rats. Ovariectomized adult rats were injected with 20 microg 17beta-Estradiol or sesame oil 48 h before sacrifice and further injected with 1.5 mg progesterone or sesame oil 24 h before sacrifice. Northern blot revealed that estrogen significantly increased the expression of connexin-36 mRNA in the SCN and this increase was inhibited by progesterone. On the other hand, the connexin-36 mRNA level in the CX was not affected by estrogen or progesterone. These results suggest that the gap junction with connexin-36 in the SCN is specifically regulated by ovarian steroid hormones of female rats.

Molecular analysis of estrogen induction of preproenkephalin gene expression and its modulation by thyroid hormones

Estrogen receptors (ER) and thyroid hormone receptors (TR) are ligand-dependent nuclear transcription factors. Estrogen-induced preproenkephalin (PPE) gene expression in the hypothalamus is directly related to estrogen-induced lordosis behavior in the rat. In the present study, we showed that the PPE mRNA level in the ventromedial hypothalamus of female rats was significantly decreased by ovariectomy. This decrease was reversed by estrogen replacement in a dose- and time-dependent manner. Using transient transfection and electrophoretic mobility shift assays (EMSA), functional estrogen response elements (ERE) were identified between -437 and -145 base pairs (bp) of the rat PPE gene promoter region. Two ERE-like elements are present between -405 and -364 of the rat PPE gene promoter, which bind ERalpha as demonstrated by EMSA. Estrogen produced a dose-dependent increase in CAT activity in cotransfection assays with ERalpha expression vector and a 437PPE-CAT reporter construct containing 437 bp of the rat PPE gene promoter and the CAT reporter gene. This estrogen-induced PPE promoter activity was inhibited by liganded-TR in transient cotransfection assays. Analysis of DNA-protein interactions by EMSA revealed that both ERalpha and TR (alpha1 and beta1) could bind to the EREs in the rat PPE gene promoter. Furthermore, estrogen induction of PPE mRNA in the ventromedial hypothalamus of the ovariectomized female rat was significantly attenuated by concomitant administration of triiodothyronine. These results suggest that estrogen regulation of the hypothalamic PPE gene expression is mediated through an estrogen-receptor complex directly interacting with the functional EREs in its promoter region; and that this estrogen effect can be modified by thyroid hormones.

Influence of restraint stress on the expression and the serine/threonine phosphatase activity of calcineurin in the rat brain

To investigate the role of calcineurin (CaN) in the pathogenesis of stress-related psychiatric disorders, we examined the expression of CaN A mRNA and the serine/threonine phosphatase activity of CaN in the rat brain following various restraint stress paradigms. Northern blot analysis revealed no significant changes in the levels of CaN A mRNA expression, in either the frontal cortex or the hippocampus, in response to a single restraint stress for either 15, 45, or 90 min. In addition, no significant change in the levels of CaN A mRNA was found 3 or 6 h after a single restraint stress for 90 min. In situ hybridization analysis revealed that a single restraint stress for 45 min had no influence on CaN A mRNA expression in the CA1 and CA3 pyramidal cell layers or in the dentate gyrus granule cell layer of the hippocampus. However, serine/threonine phosphatase activity was significantly increased in both regions in response to a single restraint stress for either 45 or 90 min. These results demonstrate that the single restraint stress paradigms used in this study increase the activity of CaN without any changes in the expression of CaN, suggesting that the activation of calmodulin and the increased levels of heat shock proteins in response to the restraint stress may upregulate the activity of CaN in the rat brain.

Effects of a kappa agonist, spiradoline mesylate (U62,066E), on activation and vaginocervical-stimulation produced analgesia in rats

Previous research has demonstrated increased pain threshold during copulation, gestation, and parturition in animals. In the laboratory, mechanostimulation of the vaginocervical region in many animals, as well as humans, can increase responsiveness to noxious but not to innocuous stimuli. This increased pain inhibition to vaginocervical stimulation, which mimics natural parturition, is mediated by spinal and supraspinal neuropeptides, including the opiates. The present research was designed to ascertain the possible effects of a kappa opioid agonist on vaginocervical-stimulated analgesia in rats. Initially, the novel kappa-selective agonist, spiradoline mesylate (U62,066E; 0, 0.1, 1.0, 10.0 mg/kg, i.p.), was injected intraperitoneally and general behavioral arousal in an open field apparatus was recorded. Results from this experiment indicate that stimulation with the kappa-selective drug caused significant decreases in behavioral activity at the high dose as compared to saline and the medium and low doses. Next, the effects of U62,066E (0, 0.1, 1.0, 10.0 mg/kg, i.p.) on the analgesia associated with vaginocervical stimulation were determined in a tail flick apparatus. The kappa drug significantly increased antinociceptive thresholds prior to and during vaginocervical stimulation at the 0.1 and 1.0 mg/kg doses. By contrast, the high dose (10.0 mg/kg) of U62,066E decreased vaginocervical stimulation-produced analgesia. Results are discussed in terms of the potential of nonaddictive kappa-selective opioid compounds being utilized in reproductive pain.

Ultrastructural evidence for enkephalin mediated disinhibition in the ventromedial nucleus of the hypothalamus

The ventromedial nucleus of the hypothalamus (VMN) regulates the estrogen-dependent appearance of female mating behavior, lordosis. Accumulating evidence suggests that estrogen might exert its control over lordosis by acting, in part, on neurons that contain enkephalin in the VMN. The expression of the enkephalin precursor gene is robustly stimulated by estrogen and is correlated with the later appearance of lordosis. GABA has also been implicated as an important neurotransmitter for the appearance of lordosis. Because enkephalin is thought to act in several brain areas to modulate the activity of GABAergic neurons, we studied the ultrastructural morphology and relationship between neurons containing these neurochemicals using dual-labeling immunocytochemistry in ovariectornized rats, half of which received estrogen replacement. Immunolabeling for enkephalin was almost always detected within axon terminals (695 axonal profiles sampled), while GABA immunoreactivity was more often localized to cell bodies and dendrites (191 profiles), than to axons (63 profiles). Axon terminals containing enkephalin immunolabeling provided a major innervation to soma or dendrites containing GABA. That is, over one third (94/245) of the axon terminals in contact with GABA-immunoreactive dendrites contained enkephalin. Furthermore, these GABA-immunoreactive dendrites accounted for a fifth of the somatodendritic processes associated with enkephalin-containing axon terminals. These findings support the hypothesis that enkephalin may act in the VMN by inhibiting GABAergic neurons, which could result in the disinhibition of neural circuits relevant for lordosis.

A membrane-associated progesterone-binding protein, 25-Dx, is regulated by progesterone in brain regions involved in female reproductive behaviors

The ventromedial hypothalamus (VMH) plays a central role in the regulation of the female reproductive behavior lordosis, a behavior dependent upon the sequential activation of receptors for the ovarian steroid hormones estradiol (E) and progesterone (P). These receptors function as transcription factors to alter the expression of target genes. To discover behaviorally relevant genes targeted by E and P in the VMH, we used the differential display PCR to identify messenger RNAs that are differentially expressed in the hypothalamus of ovariectomized (ovx) rats treated with E alone compared with ovariectomized rats treated with E and P. We show here that one interesting mRNA within the hypothalamus that is repressed by P after E priming encodes the protein 25-Dx, the rat homolog of the human membrane-associated P-binding protein Hpr6.6. Neurons in the brain containing the highest levels of 25-Dx are located in several nuclei of the basal forebrain, including the VMH. 25-Dx expression is also higher in the hypothalamus of female P receptor "knockout" mice than in their wild-type littermates. These findings suggest a mechanism in which the activation of nuclear P receptor represses expression of a membrane P receptor, 25-Dx, during lordosis facilitation.

Changes in estrogenic regulation of estrogen receptor alpha mRNA and progesterone receptor mRNA in the female rat hypothalamus during aging: an in situ hybridization study

We examined two molecular responses to estrogen, reduction in estrogen receptor alpha (ER alpha) mRNA and increase in progesterone receptor (PR) mRNA, in the hypothalamus of 3- (young) and 10-month-old (middle-aged) cycling, and 15-month-old (old) acyclic, Fischer 344 female rats. The rats were ovariectomized and then given silastic capsules containing 5% 17beta-estradiol. or empty implants, and killed 4 days after implantation. By means of in situ hybridization, we found that, in young rats, estrogen reduced ER alpha mRNA in both the ventromedial hypothalamus (VMH) and arcuate nucleus (ARC) but not in the preoptic area (POA). In contrast, the effect of estrogen on ER alpha mRNA in the VMH and ARC of middle-aged and old rats was not statistically significant. On the other hand in all regions the induction of PR mRNA by estrogen was at least as strong in middle-aged and old as in young rats. The present study revealed that the induction of PR mRNA by estrogen in the hypothalamus was not impaired with age but ER alpha mRNA in the VMH and ARC was significantly impaired with age, but not in the POA.

Effects of estrogen on the expression of connexin32 and connexin43 mRNAs in the suprachiasmatic nucleus of female rats

The suprachiasmatic nucleus of the hypothalamus (SCN) is the site of the endogenous biological clock that regulates circadian rhythms in mammals. It has been shown that the gap junction couples neurons in the rat SCN. To determine the effects of estrogen on gap junction communication in the SCN, we examined the effects of estrogen on the expression of connexin32 and connexin43 mRNAs in the SCN and cerebral cortex (CX) of female rats. Ovarectomized adult rats were injected with 20 microg of 17beta-estradiol and killed 24 h after the treatment. Northern blot revealed that the expression of connexin32 mRNA was significantly increased in the SCN but not in the CX. On the other hand, estrogen increased the connexin43 mRNA level in the CX but not in the SCN. These results suggest that the gap junction with connexin32 is specifically regulated by estrogen in the SCN of female rats.

Preproenkephalin mRNA levels are regulated by acute stress and estrogen stimulation

Enkephalins facilitate female reproductive behavior. Within the limbic system and hypothalamus, estrogen induced the expression of preproenkephalin (PPE) mRNA. Estrogen injection caused a biphasic increase in the PPE mRNA levels within the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. The first peak of PPE mRNA levels occurred within an hour, and the second 24 to 48 h after subcutaneous injection of estrogen. The present studies indicated that the rapid first peak of PPE mRNA expression was stress induced, whereas the second peak was estrogen induced. In the posterodorsal medial amygdala but not in the ventromedial hypothalamic nucleus, the antiestrogen, tamoxifen, did not inhibit the first peak, but blocked the second peak of PPE mRNA expression. Subcutaneous oil injection induced a 1-h peak of PPE mRNA levels but not a 24-h peak. Peak levels of plasma corticosterone were coincident with peak PPE mRNA levels. Adrenalectomy plus a constant, low level of corticosterone eliminated the injection-induced increase of corticosterone levels and the subsequent increase in PPE mRNA expression in the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. The present results indicate that both stress steroids and estrogen positively regulate PPE mRNA levels in the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. These results are consistent with the hypothesis that acute, mild stress may contribute to the activation of circuits that facilitate reproductive behavior in the female.

Estrogen increases arginine-vasopressin V1a receptor mRNA in the preoptic area of young but not of middle-aged female rats

We determined whether estrogen regulates the expression of arginine-vasopressin (AVP) receptor mRNA in the preoptic area (POA) of female rats. By reverse transcription-polymerase chain reaction (PCR), we found that all three types of the AVP receptor mRNA, V1a, V1b and V2, were expressed in the POA, though the amount of PCR products was apparently different among them. In situ hybridization indicated that AVP V1a receptor mRNA was densely expressed in the POA, especially in the anteroventral periventricular nucleus of the POA; in contrast, AVP V1b and V2 receptor mRNAs were not abundant in this area. Finally, we demonstrated by Northern blot that estrogen significantly increased the expression of AVP V1a receptor mRNA in the POA of young ovariectomized rats. However, this regulation by estrogen was lost in middle-aged rats, indicating an age-related impairment in the regulation of AVP V1a receptor mRNA by estrogen in the POA.

Progesterone receptor isoforms expression pattern in the rat brain during the estrous cycle

Progesterone receptor (PR) isoforms expression was determined in the hypothalamus, the preoptic area, the hippocampus and the frontal cerebral cortex of the rat at 12:00 h on each day of the estrous cycle by using reverse transcription coupled to polymerase chain reaction. Rats under a 14:10 h light-dark cycle, with lights on at 06:00 h were used. We found that PR-B isoform was predominant in the hypothalamus, the preoptic area and the frontal cerebral cortex. Both PR isoforms were similarly expressed in the hippocampus. The highest PR-B expression was found on proestrus day in the hypothalamus; on metestrus in the preoptic area; and on diestrus in the frontal cortex. We observed no changes in PR isoforms expression in the hippocampus during the estrous cycle. These results indicate that PR isoforms expression is differentially regulated during the estrous cycle in distinct brain regions and that PR-B may be involved in progesterone actions upon the hypothalamus, the preoptic area and the frontal cortex of the rat.

Estrogen-induced alterations of spinal cord enkephalin gene expression

Enkephalin-synthesizing neurons in the superficial laminae of the spinal and trigeminal dorsal horn are critical components of the endogenous pain-modulatory system. We have previously demonstrated that these neurons display intracellular estrogen receptors, suggesting that estrogen can potentially influence their enkephalin expression. By using Northern blot, we now show that a bolus injection of estrogen results in a rapid increase in spinal cord enkephalin mRNA levels in ovariectomized female rats. Thus, 4 h after estrogen administration the enkephalin mRNA-expression in the lumbar spinal cord was on average 68% higher (P<0.05) than in control animals injected with vehicle only. A small increase in the amount of enkephalin mRNA was also seen after 8 h (P<0.05), whereas no difference between estrogen-injected and control animals was found after 24 h or at time periods shorter than 4 h. Taken together with the previous anatomical data, the present findings imply that estrogen has an acute effect on spinal opioid levels in areas involved in the transmission of nociceptive information.

Transcription of actin, cyclophilin and glyceraldehyde phosphate dehydrogenase genes: tissue- and treatment-specificity

Studies involving RNA transcription, in varying biological systems, usually necessitate a term of transcriptional reference. Traditionally, the transcription of the gene of interest was compared to a constitutively expressed 'control' gene. Run-on transcription analysis was undertaken to evaluate and compare the transcription of three frequently used 'control genes' (beta-actin, cyclophilin and glyceraldehyde-3-phosphate dehydrogenase), in nine rat tissues. Similarities, but also clear and highly significant differences, were found in the transcription profiles of these three genes. There was significantly greater transcription for uterine glyceraldehyde-phosphate dehydrogenase compared to all other tissues tested, while both cyclophilin and glyceraldehyde-phosphate dehydrogenase were significantly elevated in the adrenal cortex. Upon cholinergic agonist treatment, both beta-actin and glyceraldehyde-phosphate dehydrogenase RNA expression were greatly induced in the adrenal medulla (41- and 94-fold, respectively), while cyclophilin transcription was not altered. In another treatment paradigm, surgical ovariectomy, only uterine glyceraldehyde-phosphate dehydrogenase transcription was significantly reduced. While, all three of these genes are assumed to be constitutively expressed throughout the body and hence used as normalization controls, the current study questions these accepted terms of reference. As cyclophilin transcription was not affected in both treatment paradigms, it should be considered more seriously as a RNA normalization control.

Temporal profiles of vasoactive intestinal polypeptide precursor mRNA and its receptor mRNA in the rat suprachiasmatic nucleus

Many vasoactive intestinal polypeptide (VIP) neurons are distributed in the suprachiasmatic nucleus (SCN) and most of them arborize throughout the nuclei. We determined temporal patterns of VIP receptor (VPAC2 receptor) mRNA in the rat SCN in constant darkness and light-dark cycles, by means of in situ hybridization histochemistry. Amounts of VIP precursor mRNA in the SCN were also quantified in adjacent sections to compare its temporal profile with that of VPAC2 receptor mRNA. In constant darkness no circadian rhythms in VIP precursor mRNA or VPAC2 receptor mRNA in the SCN were detected. In light-dark cycles VIP precursor mRNA and VPAC2 receptor mRNA in the SCN showed robust but different daily rhythms. VPAC2 receptor mRNA decreased over the course of the light period and then gradually recovered during the dark period. VIP precursor mRNA was also reduced in response to light but the reduction was transient. This difference in the nature of light responsiveness between VIP mRNA and VPAC2 receptor mRNA suggests that pre- and postsynaptic events of the VIP transduction system represent different stages of light information processing in the SCN.

Changes in neurotensin mRNA by estrogen in the female rat preoptic area during aging: an in situ hybridization histochemistry study

The present study examined changes in the response of neurotensin mRNA to estrogen during aging at the single cell level. Ten days after ovariectomy, 3-, 10-, and 15-month-old female rats were implanted with estrogen or cholesterol and sacrificed 4 days later. An in situ hybridization study revealed that estrogen significantly increased the number of cells expressing neurotensin mRNA in the preoptic area of all age groups. Furthermore, frequency analysis indicated that estrogen significantly increased the proportion of heavily labeled cells in older rats but not in younger rats. Distributions of the grains/cells between cholesterol- and estrogen-treated rats suggested that older rats were at least as responsive to estrogen as young rats and possibly even more responsive. The result suggests that, at least as reflected by neurotensin mRNA, reproductive senescence in rats is not due to a general decrease in sensitivity to estrogen. Indeed, there is evidence of an increased responsiveness to estrogen with age.

Cellular localization of mRNA and protein: in situ hybridization histochemistry and in situ ligand binding

Powerful methods for the detection of mRNA and proteins in cells and tissue sections have been developed since the mid-1980s. This chapter discusses the applications of in situ hybridization histochemistry and in situ ligand binding to cells in culture and tissue sections. In situ hybridization takes advantage of paired nucleotide interactions between a labeled probe (antisense strand) and the endogenous mRNA (sense strand). Following processing, the mRNA is localized through detection of the disintegration pattern of the radiolabeled probe. Protein-protein interaction is detected in a similar fashion. Proteins are radiolabeled and incubated with tissues that contain target-binding proteins or receptors. On processing, the interaction sites are localized through detection of the radiolabeled probe. The methods are rapid, sensitive, specific, and provide important information regarding the sites of mRNA synthesis, abundance of protein, and the ability of the ligand to interact with the receptor in restricted cellular populations. Application of these techniques to cells in culture allows in vitro manipulation of endogenous mRNA or protein with various hormones or growth factors and a method to detect the results.

Sample and probe: a novel approach for identifying development-specific cis-elements of the enkephalin gene

We have developed a novel 'sample and probe' approach as a means to identifying specific DNA elements of the enkephalin gene that control differentiation of the enkephalinergic phenotype during neurodevelopment. The approach is a systematic spatiotemporal analysis of protein-DNA interactions; soluble nuclear proteins ('samples') prepared from microdissected regions of the developing brain are 'probed' with radiolabeled DNA fragments representing various regulatory regions of the enkephalin gene. The resulting spatiotemporal 'molecular maps', i.e. characteristic patterns of protein-DNA complexes showed DNA regions that harbor potential cis-elements regulating differentiation of the enkephalin phenotype at various stages of neurodevelopment. DNase I footprint analysis of such a DNA region identified a binding site (GACGGGAGATCGCTCGT) which is similar to the motif for a lymphoid-specific, developmentally regulated transcription factor, Ikaros, suggesting that the developing brain expresses Ikaros-like transcription factor(s) in a spatiotemporally defined manner. In summary, our approach offers a unique view into the chronology of coordinated protein-DNA interactions and will greatly facilitate identifying DNA elements and isolating development-specific transcription factors.

Effects of an intrahypothalamic injection of antisense oligonucleotides for preproenkephalin mRNA in female rats: evidence for opioid involvement in lordosis reflex

Previous studies in female rats have shown that estrogen increases preproenkephalin (PPE) mRNA levels in the ventrolateral part of the ventromedial nucleus of the hypothalamus (VMHVL), an area implicated in the modulation of sexual behavior. In order to assess the physiological role of hypothalamic opioid expression in lordosis reflex 16-mer oligodeoxynucleotide (ODN) directed towards the PPE mRNA were acutely microinjected above the VMH of estradiol-primed ovariectomized rats. Estradiol-induced lordosis behavior was observed in response to a stud male 2 days thereafter. Antisense (without or with 4 mismatches) ODN injections near the VMHVL resulted in a significant reduction in lordosis quotient compared to control (reverse sense) ODN treatment or to antisense ODN injections targeted anterior or posterior to the VMHVL. In contrast, locomotor activity of these animals in the open-field test was not affected by ODN treatments. Enkephalin immunoreactive levels were determined by radioimmunoassay in the preoptic area, a major terminal field of the VMHVL. Estradiol-induced enkephalin levels were greatly reduced in antisense-treated groups. Using the in situ hybridization technique, PPE mRNA levels in the VMHVL were also determined. A 1.5-2-fold increase in PPE mRNA levels was observed in estradiol-treated rats compared to ovariectomized rats as previously described. This increase in PPE mRNA levels was not affected by ODN treatment, suggesting that the reduction of enkephalin expression was mainly due to physical blockade of PPE mRNA translation and not to its degradation. Taken together, these data further support the behavioral role of PPE expressing VMHVL neurons. They also highlight the in vivo potency of acute administration of antisense phosphorothioate ODNs in blocking neuronal target gene expression.

Roles of steroid hormones and their receptors in structural organization in the nervous system

Due to their chemical properties, steroid hormones cross the blood-brain barrier where they have profound effects on neuronal development and reorganization both in invertebrates and vertebrates, including humans mediated through their receptors. Steroids play a crucial role in the organizational actions of cellular differentiation representing sexual dimorphism and apoptosis, and in the activational effects of phenotypic changes in association with structural plasticity. Their sites of action are primarily the genes themselves but some are coupled with membrane-bound receptor/ion channels. The effects of steroid hormones on gene transcription are not direct, and other cellular components interfere with their receptors through cross-talk and convergence of the signaling pathways in neurons. These genomic and non-genomic actions account for the divergent effects of steroid hormones on brain function as well as on their structure. This review looks again at and updates the tremendous advances made in recent decades on the study of the role of steroid (gonadal and adrenal) hormones and their receptors on developmental processes and plastic changes in the nervous system.