Time-course analysis of changes in calcitonin gene-related peptide-and methionine-enkephalin-immunoreactivity in the female rat preoptic area after estrogen treatment

Estrogen effects on pain sensitivity and neuropeptide expression in rat sensory neurons

While a number of chronic pain conditions are much more prevalent in women than men, the role of estrogen in regulating nociception remains unclear. Estrogen receptors (ER) are known to be expressed in various parts of the nociceptive pathway, including in the small-sized primary sensory neurons of the dorsal root ganglion (DRG). This study evaluated the effects of long term estrogen replacement on pain sensitivity and neuropeptide expression in the DRG of female Sprague Dawley rats. The goal was to evaluate whether estrogen modulates nociceptive neuropeptides in the DRG in a manner consistent with its effects on pain sensitivity. Our results show that long term (28 days) ovariectomy (ovx) of adult rats induces a profound thermal and mechanical hyperalgesia of the hindpaw and tail compared to ovariectomized animals that were continuously estrogen-treated (ovx+E). Significant changes in the expression of two neuropeptides, substance P (SP) and calcitonin gene-related peptide (CGRP), were observed using immunocytochemistry and in situ hybridization (ISH) in the small lumbar DRG neurons which contain ER. CGRP and SP were differentially regulated by estrogen, with SP showing a significant downregulation at both the peptide and mRNA levels while CGRP and its mRNA were increased in the DRG of estrogen-treated animals. We also evaluated the development of mechanical allodynia after partial sciatic nerve injury and found that both ovx and ovx+E animals developed significant allodynia within a week of the partial nerve injury, which continued for at least one month. The estrogen-treated animals showed a partial amelioration of the extent of the allodynia at 2 weeks post injury. Overall, the results suggest that estrogen has significant anti-nociceptive actions that can be directly correlated with changes in expression of two peptides in the small nociceptive ERalpha expressing neurons of the DRG.

Estrogen increases calcitonin gene-related peptide-immunoreactive sensory innervation of rat mammary gland

Estrogen plays important roles in preparing mammary tissue for lactation. However, estrogen also influences innervation in some tissues. We examined the effect of estrogen on peripheral innervation of mammary tissues of ovariectomized adult virgin female rats. Seven days after ovariectomy, 17beta-estradiol or placebo pellets were implanted subcutaneously, and tissues were harvested 1 week later. Estrogen treatment decreased mammary gland mass and adipocyte content, while ductal content increased and vascular composition was unaffected. Estrogen increased total areas occupied by nerves in mammary gland sections immunostained for the pan-neuronal marker protein gene product 9.5, and this increase persisted after normalizing for treatment-induced differences in gland mass. Although a significant increase in tyrosine hydroxylase-immunoreactive sympathetic nerve area was observed, no difference was detected following correction for differences in gland size, implying a conserved number of sympathetic nerves in the face of reduced gland volume. Calcitonin gene-related peptide-immunoreactive sensory nerve sectional area was also increased, and corrected nerve area remained 88% greater, indicating nerve proliferation during estrogen treatment. Total, sensory, and sympathetic innervation of the nipple and adjacent dermal tissue were unaffected by estrogen. We conclude that chronic estrogen elevation induces selective proliferation of rat mammary gland calcitonin gene-related peptide-containing nerves, which are associated primarily with blood vessels and are probably nociceptors. Because they are likely to subserve a vasodilatory function, increased innervation may promote increased blood flow necessary for milk formation during suckling. Moreover, these findings may help explain abundant anecdotal reports of increased breast sensitivity in humans under high estrogen conditions.

Biphasic effects of orchidectomy on calcitonin gene-related peptide synthesis and release

We hypothesize that the decline of male gonadal hormones may play a role in age-related decrease of calcitonin gene-related peptide (CGRP) synthesis and release. Orchidectomized rats were raised with or without testosterone replacement and CGRP levels in serum and some tissues as well as the perfusate from the isolated mesenteric arterial bed (MAB) were measured at 1, 2 and 4 months after orchidectomy. CGRP levels of serum and tissues, and CGRP release from MAB were significantly elevated after 1 month and decreased after 4 months in orchidectomized rats. The changes were restored by testosterone replacement. Our results indicate that the age-related decline of testosterone might contribute to the age-related decrease of CGRP synthesis and release.

The distributions of apoptotic cells in the medial preoptic areas of male and female neonatal rats

The medial preoptic area (MPA) of the hypothalamus of the rat contains two sexually dimorphic nuclei, the periventricular preoptic nucleus (PVpo) and the medial preoptic nucleus (MPN). To examine the relationship between sexual dimorphism and neuronal death, we examined the number of apoptotic cells in the subdivisions of the MPA in neonatal rats of postnatal days 1 (P1), 4 (P4), 7 (P7) and 14 (P14). Apoptotic cells in these areas were classified according to their progression into three stages. P1 and P4 rats contained many apoptotic cells in the subfield along the third ventricle, including the PVpo, and their number was significantly larger in P1 males: in particular, the number of early-stage cells was larger in males than females. The number of apoptotic cells in the MPN was increased in P4 and P7 rats, although no significant sexual differences were seen in the total number or in the number of each progressive stage of apoptotic cells. In P14 rats, very few apoptotic cells were seen in the MPA. Our data revealed that the distribution of apoptotic cells in the MPA of developing rats depends on the sexuality, subdivision of the area and postnatal period.

Differential gene expression response to gonadal hormones by preoptic regulatory factors-1 and -2 in the female rat brain

Steroids and neuropeptides interact in the central nervous system (CNS) to regulate reproductive function and behavior. The preoptic regulatory factors, PORF-1 and PORF-2, are unique neuropeptides for which roles in gender-related brain development and function have been proposed. PORF-1 and PORF-2 expression in rat brain are age, region and gender dependent, and castration or hypophysectomy alter the metabolism of the PORF-1 and PORF-2 mRNAs in male rat brain and testes. If these two peptides have a role in gender-dependent brain function, then gonadal steroids might well affect their expression. The present study was designed to investigate the response of the PORF-1 and PORF-2 mRNAs to sex steroids in the female rat brain and to compare this response to that of two peptides whose roles in the neuroendocrinology of reproduction are well established, gonadotropin-releasing hormone (GnRH) and neuropeptide Y (NPY). Rats were ovariectomized and treated with placebo, estradiol (E2), progesterone (P4) or a combination of the two (E2/P4) and NPY, PORF-2, GnRH and PORF-1 mRNAs were quantified by nuclease protection assays. PORF-1, PORF-2 and GnRH mRNAs were also measured in intact rats during estrus and proestrus. Responses were compared in the preoptic anterior hypothalamus (POA), medial basal hypothalamus (MBH), cerebral cortex (CC) and hippocampus (HIPP). Expression of PORF-1 and PORF-2 was also confirmed in the female rat hypothalamus by in situ hybridization analysis. PORF-1 and PORF-2 mRNAs were detected in the adult female rat brain by both in situ hybridization and ribonuclease protection analyses. In situ hybridization analysis demonstrated that PORF-1 and PORF-2 mRNAs are expressed in hypothalamic neurons. RNase protection analysis showed that PORF-1, PORF-2 and NPY mRNAs were present in all four brain regions examined while GnRH expression was detected only in the MBH and POA. Estradiol alone upregulated expression of the PORF-1 and PORF-2 mRNAs in the ovariectomized rat in the POA and HIPP, and of NPY mRNA in the MBH and HIPP. Progesterone alone had a stimulatory effect on NPY mRNA in the MBH and HIPP. Treatment with a combination of E2/P4 downregulated PORF-2 mRNA in the POA as well as PORF-1, PORF-2 and NPY mRNAs in the CC. In contrast, E2/P4 upregulated the PORF-2 and NPY mRNAs in the HIPP and NPY mRNA in the MBH. In the cycling rat, PORF-1 mRNA levels were higher during proestrus than estrus in both the MBH and POA, while PORF-2 mRNA levels did not change. In contrast GnRH mRNA was lower in the POA and higher in the MBH during proestrus compared with estrus. Thus, intrinsic factors, most likely both ovarian and neuroendocrine, regulate PORF-1 and GnRH expression in the intact cycling rat CNS in a region-dependent manner. In the ovariectomized rat, PORF-1, PORF-2, NPY and GnRH mRNAs all respond in a region-specific manner to sex steroid treatment. These data support the role of PORF-1 and PORF-2 in gender-dependent brain function in the adult female rat.

The effects of oestrogen and progesterone on serotonin and its metabolite in the lateral septum, medial preoptic area and ventromedial hypothalamic nucleus of female rats

The effects of sex steroid hormones on serotonin and its metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA) in the lateral septal nucleus (LS), the medial preoptic area (MPA) and the ventromedial nucleus of the hypothalamus (VMH) of female rats were investigated, using immunohistochemistry and high-performance liquid chromatography (HPLC). Female rats were divided into three groups: ovariectomized rats (OVX group); OVX-rats treated with estradiol benzoate alone (E2 group); and OVX-rats treated with E2 plus progesterone (E2 + P group). We analysed the density of serotonin-immunoreactive fibres with a computer-assisted image analysis system, and measured the tissue concentrations of serotonin and 5-HIAA. Many serotonin-immunoreactive fibres were observed in the LS, MPA and VMH in all three groups. The density of serotonin-immunoreactive fibres in the MPA and VMH was significantly lower in the E2 and E2+P groups compared to the OVX group, whereas the LS showed no detectable differences among the three groups. In the HPLC study, the concentrations of serotonin in the MPA and VMH of the E2 and E2+P groups were significantly lower than that in the OVX group. There was no significant difference in the concentration of serotonin in the LS. The concentration of 5-HIAA and the ratio of 5-HIAA/serotonin in the LS, MPA and VMH showed no significant differences among the OVX, E2 and E2+P groups. The present results suggest that E2 priming for sexual behaviour can affect the serotonergic system by decreasing serotonin content, but not the turnover rate, in the MPA and VMH of female rats.

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.

Alteration of serotonergic innervation in the suprachiasmatic nucleus of the rat following removal of input fibers from retina and lateral geniculate nucleus

To examine the influence of afferent input to the suprachiasmatic nucleus (SCN) on the development of serotonergic fibers in the SCN, afferent fibers from the retina and lateral geniculate nucleus (LGN) were eliminated in neonatal rats. Eight weeks after lesion, the distribution pattern of serotonergic fibers in the SCN was examined immunohistochemically. Neither bilateral enucleation nor LGN ablation altered the serotonergic fiber distribution in the SCN as compared to the normal adult rat. However, following combined lesions of bilateral enucleation and bilateral LGN ablation, the density of serotonergic fibers decreased throughout the SCN. The present results indicate that both retino-hypothalamic and geniculo-hypothalamic fibers may play an important role in the development of serotonergic innervation in the SCN in vivo.

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.

Estrogen receptor-immunoreactive neurons contain calcitonin gene-related peptide, methionine-enkephalin or tyrosine hydroxylase in the female rat preoptic area

We have shown in our previous studies that estrogen treatment selectively influences calcitonin gene-related peptide (CGRP)-, methionine-enkephalin (Met-Enk)- and tyrosine hydroxylase (TH)-immunoreactive (IR) intensities in the neurons of the periventricular preoptic nucleus (PPN) and the medial preoptic area (MPA) of the female rat. In the present study, we examined whether estrogen receptor (ER)-IR neurons in the PPN and MPA contain CGRP, Met-Enk, or TH using a double-labeling immunohistochemical method and investigated changes in the number of double-labeling cells upon treatment with estrogen. Brain sections of ovariectomized rats and ovariectomized and estrogen-treated rat were stained using the avidin-biotin-peroxidase complex method followed by the peroxidase-anti-peroxidase method. The sections were first incubated with an anti-ER antibody in conjunction with nickel diaminobenzidine which produces a dark blue reaction product in the nucleus. Subsequently, CGRP, Met-Enk or TH antisera were applied to these sections and the resulting brown diaminobenzidine reaction product in the cytoplasm was examined. Neurons that were double-labeled for ER and CGRP, Met-Enk or TH were investigated in the PPN and MPA. The number of doubly labeled ER/CGRP- and ER/TH-IR neurons was large, whereas the number of ER/Met-Enk-IR neurons was small. These results suggest that ER in the PPN and MPA may be more closely related to the mechanism of changes in CGRP- and TH-IR intensities upon estrogen treatment than that in Met-Enk-IR intensity.

Region-specific changes of tyrosine hydroxylase-immunoreactivity by estrogen treatment in female rat hypothalamus

The effect of 17 beta-estradiol (E2) treatment for 28 days on tyrosine hydroxylase-immunoreactive (TH-IR) neurons in the periventricular preoptic nucleus (PPN) and medial preoptic area (MPA) of ovariectomized (OVX) rats was examined by morphometric analysis. The number of TH-IR neurons in the PPN of the E2-treated group was smaller than that of the OVX group, whereas the opposite result was found in the MPA; the number of TH-IR neurons in the MPA of the E2-treated group was larger than that of the OVX group. Numerous TH-IR neurons were found in the ventromedial portion of the MPA of the E2-treated group. In both the OVX and E2-treated groups, TH-IR neurons contained many short processes up to 40 microns in length. E2 treatment caused a significant decrement of the number of neurons containing the processes in the range of 10-40 microns length in the PPN, however it caused a significant increment of the number of neurons containing the processes in the range of 5-10 microns length in the MPA. These results suggested that immunoreactivity of TH in the PPN and MPA neuron are affected by E2 treatment and that E2 might modulate the production of TH in a region-specific pattern within the hypothalamus of the female rat.

Estrogen affects calcitonin gene-related peptide- and methionine-enkephalin-immunoreactive neuron in the female rat preoptic area

The effects of estrogen on the neurons of the medial preoptic area of the ovariectomized rat were immunohistochemically investigated using antisera to calcitonin gene-related peptide (CGRP) and methionine-enkephalin (Met-Enk). To visualize CGRP- and Met-Enk-immunoreactive (IR) cell somata, colchicine was injected into the cerebroventricle. CGRP- and Met-Enk-IR neurons were distributed in the medial preoptic nucleus (MPN) but few in the periventricular preoptic nucleus (PPN) in the ovariectomized rat. After estrogen treatment, CGRP immunoreactivity was markedly increased in the PPN and MPN, whereas Met-Enk immunoreactivity was increased in the MPN. These results, along with our previous data, suggest that estrogen accelerates CGRP- and Met-Enk expression in a different manner in the PPN and MPN neurons.