Brenda D. Shivers

Chemical Characterization of Neuroendocrine Targets for Progesterone in the Female Rat Brain and Pituitary

The secretory products of some of the cell types which respond directly to actions of progesterone in the female rat brain and pituitary were determined by combining immunocytochemistry with autoradiography following systemic administration of the synthetic progestin ligand [3H]-R5020. Four major findings are reported: (1) Approximately 90% of the tyrosine hydroxylase (TH)-immunoreactive neurons in the hypothalamic arcuate nucleus have progesterone receptors, while TH-immunoreactive neurons in other portions of the hypothalamus (e.g. the periventricular region and the zona incerta) do not. (2) Approximately 30% of the beta-endorphin neurons in the hypothalamus have progesterone receptors. (3) None of the luteinizing hormone-releasing hormone neurons examined have progesterone receptors. (4) Approximately 98% of the cells in the anterior pituitary that have progesterone receptors contain luteinizing hormone. Lactotrophs do not contain progesterone receptors. Many progestin targets in the brain remain to be characterized chemically. The implications for progesterone-inducible genes and neuroendocrine control systems are discussed.

Immunocytochemical Localization of Luteinizing Hormone-Releasing Hormone in Male and Female Rat Brains

The peroxidase-antiperoxidase immunocytochemical method was used to determine quantitatively the effects of gonadal steroids on the immunoreactive luteinizing hormone-releasing hormone (LHRH) content in the brain of male and female rats. In the male rat, gonadectomy decreased both the number of cell bodies and optical density of staining in cell bodies containing immunoreactive LHRH; decreased the percentage of area covered by LHRH fibers in the middle and caudal aspects of the median eminence (ME) but not the organum vasculosum lamina terminalis (OVLT), and decreased the number of LHRH fibers localized in the midbrain central gray (MCG). Since others have shown previously that gonadectomy increases the LHRH content of portal blood in the male rat, the results suggest that the decreased somal accumulation of LHRH and decreased LHRH content in fibers in the ME and MCG measured in the present study following loss of testicular steroids reflect increased LHRH release from the ME and MCG into the portal blood and brain, respectively. Estrogen replacement in the gonadectomized female rat increased the optical density of staining in cell bodies but not the number of cell bodies containing immunoreactive LHRH, increased the percentage of area containing LHRH fibers in the caudal aspect of the ME but not the OVLT, and decreased the number of fibers containing LHRH in the MCG. Since others have shown previously that estrogen replacement to gonadectomized female rats decreases LHRH release into portal blood, the present results suggest that the estrogen-induced increases in somal accumulation of LHRH and increases in LHRH content in fibers in the ME reflect a decreased LHRH release from the ME into the portal blood. By analogous reasoning, the decreased LHRH fiber content measured in the MCG following estrogen replacement to gonadectomized females reflects an increased LHRH release into the MCG. This inference is consistent with a postulated role for LHRH in this brain site in the facilitation of lordotic responsiveness.

Haloperidol increases proenkephalin mRNA levels in the caudate-putamen of the rat: a quantitative study at the cellular level using in situ hybridization

Previous immunocytochemical studies have shown that the opioid peptides, Met-enkephalin and Leu-enkephalin, are present in medium-sized, spiny projection neurons of the caudate-putamen. It has also been demonstrated that chronic treatment of rats with the dopamine receptor blocker, haloperidol, results in an increase in the levels of enkephalin peptides and proenkephalin mRNA in this brain region. To determine whether this increase in proenkephalin mRNA content is exhibited by all enkephalinergic neurons of the caudate-putamen or by only a subpopulation, we have used in situ nucleic acid hybridization to examine the haloperidol-induced increase in proenkephalin mRNA levels at the cellular level. Results of in situ hybridization suggest that all enkephalinergic neurons in the caudate-putamen can respond to haloperidol treatment with an increase in steady state levels of proenkephalin mRNA, and that the mean induction is an approximate 3-fold increase in the message levels. This suggests that dopamine exerts a tonic inhibitory effect on the expression of the proenkephalin gene in all of the enkephalinergic neurons of the caudate-putamen. Dot blot analysis indicated a 2.4-fold increase in the tissue levels of this mRNA. The agreement between the in situ hybridization results and dot blot analysis supports in situ hybridization as a reliable method for quantitative studies of alterations in neuropeptide precursor mRNAs in the brain.

Distribution and Partial Characterization of Immunoreactive Prolactin in the Rat Brain

Immunoreactive (IR) prolactin was localized immunocytochemically in cell bodies in the mediobasal hypothalamus and in fibers in many regions of the rat brain. The cell bodies were found in the arcuate nuclei and the adjacent areas ventral to the ventromedial nuclei. Fiber projections extended rostrally to and/or through the anterior hypothalamus, preoptic area, nucleus accumbens, septum, diagonal bands of Broca, caudate-putamen, frontal cortex and accessory olfactory bulb; laterally to the amygdala, especially the central nucleus and some parts of the medial nucleus; caudally to and/or through the midbrain central gray, reticular formation, parabrachial region, and several portions of the lower brain stem and spinal cord extending to sacral levels. The system appears to be essentially identical to that containing proopiomelanocortin (POMC) and its processed peptides, as shown by double immunocytochemistry. Preabsorption of the antiprolactin antiserum with either prolactin or the 16,000-dalton N-terminus of POMC eliminated immunoreactivity in the brain. Preabsorption with other POMC-derived peptides, including beta-lipotropic hormone, beta-endorphin, met-enkephalin, adrenocorticotrophic hormone (1-24), corticotropin-like intermediate lobe peptide, alpha- and gamma-melanocyte-stimulating hormones and an octapeptide region of the N-terminus of POMC bearing some homology with prolactin, did not eliminate immunoreactivity in the brain. Similarly, preabsorption with growth hormone, luteinizing hormone, follicle-stimulating hormone, motilin or fetuin did not eliminate immunoreactivity in the brain. The antiprolactin antiserum also recognized all cells in the intermediate lobe and a subset of cells in the anterior lobe of the Snell dwarf mouse pituitary. This immunoreactivity was eliminated by preabsorption of the antiserum with prolactin or with the 16,000-dalton N-terminus of POMC. These results suggest that IR prolactin in the brain may be related to the N-terminus of POMC. Additional results based on one- and two-dimensional gel electrophoresis and immunoblotting indicate that the antiprolactin antiserum used in the majority of the immunocytochemical studies recognized a number of proteins.

Inhibition of the lordosis reflex in rats by intrahypothalamic infusion of neural excitatory agents: Evidence that the hypothalamus contains separate inhibitory and facilitatory elements

In attempts to activate lordosis-facilitating neural mechanisms in the ventromedial hypothalamus (VMH), neural excitatory agents were infused into the medial hypothalamus, and the effects of the infusions on the lordosis reflex and on the electrical activity of VMH neurons were studied. Surprisingly, bilateral intrahypothalamic infusion of glutamate (10 mM, 1.0 microliter/side) into behaving, ovariectomized, estrogen-treated rats displaying moderate lordotic responsiveness did not facilitate lordosis, but instead, resulted in a rapid (within a few minutes) and transient (recovery in about 20 min) inhibition of lordosis. Further experiments showed that this lordosis-inhibiting effect of glutamate was dose-related, and was completely blocked by prior infusion of a local anesthetic, procaine. Infusion of KC1 (1.5 or 15%, 1.0 microliter/side) also induced a dose-related, rapid and transient inhibition of lordosis, that was essentially identical to that induced by glutamate. Kainic acid (0.25 micrograms/0.5 microliter/side) also caused a rapid inhibition of lordosis, but the effect was long-lasting (days). The inhibition of lordosis by these agents was dissociated in time course, presence, and/or severity from effects on non-lordotic behaviors. Electrophysiological studies showed that all three agents tested could excite multiunit activity of the VMH, and that the time courses of these excitations were closely comparable to those of the inhibition of lordosis induced by the respective agents. Altogether, these studies indicated that the excitation of certain medial hypothalamic neurons can inhibit the lordosis reflex. The implied lordosis-inhibiting neural mechanisms are separate from facilitatory mechanism(s), according to differences in latency, duration, and procaine-sensitivity of response.

A subset of neurons containing immunoreactive prolactin is a target for estrogen regulation of gene expression in rat hypothalamus.

Cells whose nuclei accumulated 3H-estradiol were identified autoradiographically in fixed, frozen sections of colchicine-treated rat hypothalamus (n = 3 animals). After autoradiogram development, these sections were subjected to immunocytochemistry using rabbit antirat prolactin antiserum and the avidin-biotinylated horseradish peroxidase method. In the hypothalamus, a substantial subset of the neurons containing immunoreactive prolactin accumulated 3H-estradiol in their nuclei: of 3, 642 immunoreactive cells examined, 1,216 had autoradiographically labeled nuclei, or about 33%. The immunoreactive prolactin neurons with autoradiographically labeled nuclei were located in the medial basal hypothalamus intermingled with immunoreactive prolactin neurons whose nuclei were not labeled autoradiographically. Since hypothalamic immunoreactive prolactin neurons have a rich and widely distributed fiber system, the present results suggest that estrogen, acting through a subset of these neurons, can modify directly the neuronal activity of several brain regions which regulate diverse aspects of the reproductive effort. Also, since immunoreactive prolactin and immunoreactive beta-endorphin exist in the same hypothalamic cell population, opioid peptides derived from pro-opiomelanocortin may mediate some effects of estrogen on the neural circuitry regulating reproduction.

Intrahypothalamic colchicine infusions disrupt lordotic responsiveness in estrogen-treated female rats

Since some estrogenic effects on lordotic responsiveness are mediated through hypothalamic protein synthesis, we conducted experiments to determine if axoplasmic transport in the hypothalamus is necessary for the induction and maintenance of this reflex by estrogen. Colchicine infusion into the hypothalamus, but not into the dorsal thalamus, of ovariectomized rats 24 h prior to administration of subcutaneous estrogen implants delayed the induction of lordotic responsiveness, as measured by the manual (cutaneous-pressure) method, by 2 days, as compared with vehicle-infused rats. In other experiments, colchicine infusion into the hypothalamus, but not into the dorsal thalamus, of conscious, ovariectomized, estrogen-implanted rats displaying maximal lordotic responsiveness resulted in a bimodal decline in lordotic responsiveness. An initial decline occurred 20-40 min after infusion, and was associated with general behavioral agitation and hyperactivity. A subsequent decline began 4 h after infusion and lasted for several days. Vehicle infusion did not decrease lordotic responsiveness. Colchicine infusion did not alter multiunit electrical activity recorded near hypothalamically directed cannulae tips over a period of several hours. Results suggest that axoplasmic transport within and/or from the hypothalamus is necessary for the estrogenic induction and maintenance of the lordosis reflex in rats.

Reversible disruption of lordosis via midbrain infusions of procaine and tetrodotoxin

Behavioral effects of bilateral intracranial infusions of tetrodotoxin (1, 3.3 or 10 ng/rat), 50% procaine (2 microliters/rat) or phosphate-buffered saline (PBS-2 microliters/rat) into the dorsal midbrain of conscious, lightly-restrained female rats were evaluated. High levels of lordotic responsiveness were induced in ovariectomized animals treated with estradiol (E2) capsules or subcutaneous injections of estradiol benzoate (EB) followed by progesterone (P). The effect of each of the 3 infusates on lordosis was determined using manual stimulation and lordosis quotient determinations. In addition, the vocalization by an animal during lordosis measurements, paw withdrawal to pinch, righting reflex latency and recognition of a platform edge were also monitored. Within 2 minutes following procaine or tetrodotoxin (TTX) infusions in E2 implanted rats, lordotic responsiveness declined sharply. Whereas procaine-treated animals returned to control levels of responsiveness within 20 minutes, TTX infusions induced a more prolonged depression of lordosis lasting up to 8 hours. Infusions of PBS had no effect on any of the behaviors. In a separate group of animals treated with either E2 or EB + P and infused with 10 ng TTX the time course of the decline in lordotic responsiveness was identical for both steroid treatments. Paw withdrawal was unaffected by TTX while all other measured behaviors were disrupted along the same time course as lordosis. Collectively the above results implicate the requirement of sodium-dependent neuronal activity within dorsal midbrain for the maintenance of the lordosis reflex, along with other behavioral responses influenced by this brain region.

Ultrastructural characterization of prolactin-like immunoreactivity in rat medial basal hypothalamus

Prolactin-like immunoreactivity has been reported in the medial basal hypothalamus at the light microscopic level, in hypophysectomized rats. Here, with preembedding immunocytochemistry at the electron microscopic level, we have observed prolactin-immunoreactive neurons and synapses in the hypothalamus. Reaction product was discovered in medial basal hypothalamic neurons, which had typical large nucleoli and received axosomatic synapses. In the cytoplasm, reaction product was distinctly granular. Immunoreactive neurons were usually surrounded by nonreactive cells. Reaction product was also seen in dendrites, some of which had spines. Some axons in the hypothalamus contained reaction product, usually surrounded by nonreactive axons, and immunopositive synapses were detected both in the hypothalamus and in the midbrain. In a small number of cases immunoreactive axons could be seen synapsing on immunoreactive dendrites.

Lordosis as a sexually dimorphic neural function.

Publisher Summary This chapter describes the sexually dimorphic function, the lordosis response displayed by sexually receptive female rats. In rats, lordosis is a stereotyped reflex consisting of dorsiflexion (extension) of the vertebral column, which produces an elevation of the head and the hind quarters region. There are several advantages of studying lordosis, such as the behavior is of vital importance to reproduction, and thus, plays a central role in the behavioral repertoire of this species. The lack of definitive evidence concerning the locations of sex differences in the neural circuitry controlling lordosis requires us to speculate about different possibilities. One possibility is that the sex difference in lordotic responsiveness relates to differences in the nuclear uptake of steroids. Another possible sex difference in lordosis-relevant hypothalamic cells might reside in their biochemical responses to steroids. Sex differences could also exist in the physiology of lordosis-relevant hypothalamic neurons projecting to the midbrain, and sex differences in the anatomical organization of lordosis-relevant cells might also exist. Cellular analysis of estrogenic action on neural circuitries producing lordosis is far from complete; however, the model that has been proposed may help to carry out a study of the putative sex differences in this system. Thus, future research into the effects of estrogen upon brain function may provide new insights into one of the most basic biological processes, the interactions between endocrine and nervous systems.