Dendritic arborization and axon trajectory of neurons in the hypothalamic arcuate nucleus of the rat--updated

Orphanin FQ in the mediobasal hypothalamus facilitates sexual receptivity through the deactivation of medial preoptic nucleus mu-opioid receptors

Sexual receptivity, lordosis, can be induced by sequential estradiol and progesterone or extended exposure to high levels of estradiol in the female rat. In both cases estradiol initially inhibits lordosis through activation of β-endorphin (β-END) neurons of the arcuate nucleus of the hypothalamus (ARH) that activate μ-opioid receptors (MOP) in the medial preoptic nucleus (MPN). Subsequent progesterone or extended estradiol exposure deactivates MPN MOP to facilitate lordosis. Opioid receptor-like receptor-1 (ORL-1) is expressed in ARH and ventromedial hypothalamus (VMH). Infusions of its endogenous ligand, orphanin FQ (OFQ/N, aka nociceptin), into VMH-ARH region facilitate lordosis. Whether OFQ/N acts in ARH and/or VMH and whether OFQ/N is necessary for steroid facilitation of lordosis are unclear. In Exp I, OFQ/N infusions in VMH and ARH that facilitated lordosis also deactivated MPN MOP indicating that OFQ/N facilitation of lordosis requires deactivation of ascending ARH-MPN projections by directly inhibiting ARH β-END neurons and/or through inhibition of excitatory VMH-ARH pathways to proopiomelanocortin neurons. It is unclear whether OFQ/N activates the VMH output motor pathways directly or via the deactivation of MPN MOP. In Exp II we tested whether ORL-1 activation is necessary for estradiol-only or estradiol+progesterone lordosis facilitation. Blocking ORL-1 with UFP-101 inhibited estradiol-only lordosis and MPN MOP deactivation but had no effect on estradiol+progesterone facilitation of lordosis and MOP deactivation. In conclusion, steroid facilitation of lordosis inhibits ARH β-END neurons to deactivate MPN MOP, but estradiol-only and estradiol+progesterone treatments appear to use different neurotransmitter systems to inhibit ARH-MPN signaling.

The caudal end of the rat spinal cord: transformation to and ultrastructure of the filum terminale

Contrary to the current belief, the spinal cord of the rat does not terminate with the conus terminalis (CT), but its basic components (central canal, gray matter, white matter) continue in the filum terminale (FT). Proceeding caudally in the conus terminalis, first the motoneuron cell column discontinues in the ventral horn. More caudally the dorsal horns separate from the intermediate zone, and discontinue. The ensuing filum terminale consists of the slit-like central canal lined by ciliated ependymal cells, the periventricular gray matter and the peripheral white matter. Uniform small size neurons and glial cells populate the gray matter. Ultrastructural analysis revealed various types of axodendritic and axosomatic synapses as well as fine unmyelinated axons. The white matter consists mainly of myelinated nerve fibers. The neuronal components of the filum terminale, if they occur also in the human spinal cord, should be involved in the diagnosis and treatment of various diseases, e.g. tethered spinal cord syndrome, vascular malformations and disraphysm.

Effect of intracerebroventricular injection of lipopolysaccharide on the tuberoinfundibular dopaminergic system of the rat

Numerous studies indicate that monoaminergic systems are sensitive to both peripheral and central inflammatory stimuli, and in particular dopaminergic neurons in the nigrostriatal system degenerate after local injection of lipopolysaccharide (LPS). However, data about the response of other dopaminergic groups to local inflammation are very sparse. In this study, we have examined the effect of i.c.v. injection of LPS on the tuberoinfundibular dopaminergic (TIDA) system by using biochemical and morphological parameters. Our results show that 6 h after i.c.v. injection of LPS, in parallel to a transient and intense immunoreaction to interleukin-1beta in arcuate microglial cells, there is a decrease in tyrosine hydroxylase (TH) activity in the median eminence and in the number of TH- and TH mRNA-positive cells in the arcuate nucleus, and at 12 h, an increase of prolactin levels in serum. Posterior changes were found in the TH mRNA labeling pattern, mostly in the ventrolateral region of the arcuate nucleus, but they were not accompanied by any changes in TH activity and immunoreactivity and TH-cell count. This suggests that the TIDA system is functionally susceptible to local inflammation, but the effects are transient and do not induce neurodegeneration.

Testosterone modulates the dendritic architecture of arcuate neuroendocrine neurons in adult male rats

Recent studies have demonstrated that gonadectomy of adult male rats induces dendritic growth of neuroendocrine neurons in the arcuate nucleus. We have hypothesized that these changes are secondary to the loss of testosterone negative feedback. In the present study, we examined the effects of testosterone replacement on the dendritic morphology of arcuate neuroendocrine neurons in castrated rats. Rats were orchidectomized and implanted with silastic capsules designed to produce physiological levels of plasma testosterone (n=9) or empty silastic capsules (n=9) for 2 months. Retrograde labeling with systemically injected Fluoro-Gold, followed by intracellular injection of labeled neurons in a fixed slice preparation, were used to visualize arcuate neuroendocrine neurons. Quantitative analysis of dendritic morphology was performed using three-dimensional computer reconstruction. Serum levels of LH (luteinizing hormone) and testosterone were measured by radioimmunoassay. Treatment of castrated rats with physiological levels of testosterone significantly reduced dendritic length, volume and terminal branch number relative to the castrated rats receiving empty silastic capsules. Dendritic spine density was also greater in the testosterone-treated animals, although the total numbers of spines per dendrite was not significantly different between the two groups. In addition, testosterone replacement was effective in reducing serum LH to levels found in intact rats. These studies demonstrate that testosterone replacement suppresses the dendritic outgrowth of arcuate neuroendocrine neurons that occurs in response to castration. The parallel changes in dendritic arbor and serum LH after castration and hormone replacement suggests that the suppressive effects of testosterone are related to steroid negative feedback.

Stereologic study of the hypothalamic infundibular nucleus in young and older women

Aging in women is associated with dramatic changes in neuronal morphology and neuropeptide gene expression in the medial basal hypothalamus. There is hypertrophy of neurons expressing substance P and neurokinin B gene transcripts in the infundibular (arcuate) nucleus, accompanied by increased tachykinin gene expression. In addition, gonadotropin-releasing hormone (GnRH) gene expression is increased in a separate subpopulation of neurons within the medial basal hypothalamus. In contrast, the number of neurons expressing proopiomelanocortin (POMC) mRNA in the infundibular nucleus of older women is decreased. To determine whether neuronal degeneration contributes to these phenomena, unbiased stereologic methods were used to compare the total number of infundibular neurons between groups of young (premenopausal) and older (postmenopausal) women. There was no significant difference in the total number of infundibular neurons between young (520,000 +/- 42,000 neurons, mean +/- SEM) and older women (505,000 +/- 51,000 neurons, mean +/- SEM). The mean volume of neuronal somata, however, was increased by 40% in the older women (young, 1,860 +/- 180 microm(3) vs. older, 2,610 +/- 230 microm(3), mean +/- SEM, P < 0.05). These data demonstrate that neuronal hypertrophy in older women is not accompanied by degeneration of the infundibular nucleus. We conclude that the loss of menstrual cyclicity in middle-aged women cannot be explained by loss of neurons within the hypothalamic control center for reproduction.

Effects of prolactin on expression of Fos-related antigens in tyrosine hydroxylase-immunoreactive neurons in subdivisions of the arcuate nucleus

Dual immunohistochemistry was employed to determine the effects of prolactin on expression of Fos and its related antigens (FRA) in tuberoinfundibular dopamine (TIDA) neurons located in the dorsomedial (DM) and ventrolateral (VL) subdivisions of the arcuate nucleus (ARC) in the male rat. Systemic administration of the DA receptor antagonist haloperidol caused a sustained (up to 12 h) increase in plasma prolactin concentrations that was accompanied by a transient increase (at 3 h) in the percentage of tyrosine hydroxylase (TH)-immunoreactive (IR) neurons containing FRA-IR nuclei in the DM-ARC. In contrast, haloperidol caused a prolonged (1. 5 to 12 h) decrease in the percentage of TH-IR neurons with FRA-IR nuclei in the VL-ARC. Haloperidol had no effect, however, on the overall number of TH-IR neurons in either of these regions. Co-administration of prolactin antisera (PRL-AB) blocked haloperidol-induced increases in both plasma prolactin concentrations and the percentage of TH-IR neurons expressing FRA in the DM-ARC, but had no effect on haloperidol-induced inhibition of FRA expression in TH-IR neurons in the VL-ARC. Intracerebroventricular (i.c.v.) administration of prolactin also increased the percentage of TH-IR neurons containing FRA-IR nuclei in the DM-ARC, but this effect was of longer duration (up to 6 h) than that of haloperidol in all but the most caudal portion of the DM-ARC. In the VL-ARC, prolactin caused a transient increase (at 1.5 h) in the percentage of TH-IR containing FRA-IR nuclei. These results demonstrate that prolactin regulates immediate early gene expression in TIDA neurons in male rats, and reveal that there are temporal differences in the responsiveness of discrete subpopulations of these neurons to prolactin. Prolactin causes a short-lived increase in FRA expression in TIDA neurons in the VL-ARC which is followed by a more prolonged activation of FRA expression in TIDA neurons in the DM-ARC.

Role of gonadal steroids in determining sexual differences in expression of Fos-related antigens in tyrosine hydroxylase-immunoreactive neurons in subdivisions of the hypothalamic arcuate nucleus

Dual immunohistochemistry was employed to examine the role of gonadal steroids in determining sexual differences in the expression of Fos and its related antigens (FRA) in tuberoinfundibular dopaminergic (TIDA) neurons located in the dorsomedial (DM-) and ventrolateral (VL-) subdivisions of the arcuate nucleus (ARC). In the DM-ARC, there was no sexual difference in the number of tyrosine hydroxylase (TH)-immunoreactive (-IR) perikarya, but the number of these containing FRA-IR was greater in females than in males in all but the most caudal region. In the VL-ARC, there were more TH-IR perikarya in males than in females, but there was no sexual difference in the numbers of those containing FRA-IR throughout the entire rostrocaudal extent of this nucleus. Ovariectomy decreased the number of TH-IR perikarya containing FRA-IR in the DM-ARC, but not in the VL-ARC, whereas orchidectomy increased the number of TH-IR perikayra containing FRA-IR in both the DM-ARC and VL-ARC. These gonadectomy-induced effects were reversed by estrogen and testosterone, respectively. These results reveal gonadal steroid-dependent sexual differences in the regulation of immediate early gene expression in anatomically discrete subpopulations of TIDA neurons. In females, estrogen stimulates FRA expression in TIDA neurons in the DM-ARC, whereas in males, testosterone inhibits FRA expression in TIDA neurons in both the DM-ARC and the VL-ARC.

Effects of an acute immobilization stress upon proopiomelanocortin (POMC) mRNA levels in the mediobasal hypothalamus: a quantitative in situ hybridization study

The aim of this study was to examine by quantitative in situ hybridization the effects of an acute stress on the expression of the POMC gene in the mediobasal hypothalamus (MBH) of the rat. In control animals, the highest levels of POMC mRNA were observed in the posterior periventricular region of the MBH. Lower levels were found in the anterior and posterior arcuate nucleus. At the end of a one hour immobilization, a small decrease (-8%) was observed in the periventricular region only. Four hours after the end of immobilization, increases in POMC mRNA levels were detected in the anterior part (7%), in the posterior part (25%) and in the periventricular region (13%) of the MBH. These results suggest that MBH POMC-derived peptides might be an important component in the central response to stress.

Progesterone Receptor-Containing Neurons in the Guinea-Pig Mediobasal Hypothalamus have Axonal Projections to the Medial Preoptic Area

The location and number of progesterone receptor-containing neurons in the mediobasal hypothalamus that project to the medial preoptic area were determined by combining retrograde fluorescent tract tracing with progesterone receptor immunocytochemistry. Injections of the retrograde tract tracer Fluoro-gold were made in the preoptic area of female guinea-pigs ovariectomized and primed with estradiol. After 5 days survival to allow for retrograde transport, tissue sections were incubated with monoclonal antibodies to the progesterone receptor to detect the presence of progesterone receptor-immunoreactive neurons. Cell bodies were labelled with Fluoro-gold throughout the arcuate nucleus. These neurons were not concentrated in any particular area of the nucleus but were diffusely distributed bilaterally. Retrogradely-labelled neurons were also observed in the ventrolateral and ventromedial nuclei mainly contralateral to the injection site. Progesterone receptor immunofluorescence labelled a subpopulation (7% to 10%) of these retrogradely-labelled cells particularly in the arcuate nucleus, including the median eminence. The double-labelled cells were more numerous in the anterior two-thirds of the arcuate nucleus. Although our estimates of the proportion of hypothalamic progesterone receptor-immunoreactive neurons that sent axons directly to the medial preoptic area were low, (about 0.35%), these neurons may be part of a neural circuit involved in the regulation of reproductive processes.

A small subpopulation of progesterone receptor-containing neurons in the guinea pig arcuate nucleus projects to the median eminence

In female guinea pigs, a combination of retrograde tracing and immunofluorescence for progesterone receptors (PR) was applied to determine if PR-immunoreactive (PR-IR) neurons in the arcuate nucleus (AR) send their axons directly to the median eminence (ME). Axonal projections to the ME were studied by different techniques using fluorescent dyes. From 31 adult animals, ovariectomized and primed by estradiol, small deposits of Lucifer Yellow (LY) were made on the cut surface of the ME, either by direct application of LY crystals or by iontophoresis. These techniques were carried out on excised mediobasal hypothalamus maintained in vitro and allowed visualization of AR perikarya projecting to the ME after dye diffusion in the severed axons. In another group of ten immature animals primed by estradiol, Granular Blue (GB) was injected in the jugular vein. Blood-borne GB was taken up in the ME by intact nerve endings and retrogradely transported to the perikarya of origin. PR-IR neurons and perikarya filled with LY or retrogradely labeled by GB were intermingled with each other throughout the rostrocaudal extent of the AR. Double-labeled cells, displaying PR immunoreactivity and dye labeling, were observed consistently, but their number was small. This result demonstrates that some AR neurons sending axonal projections to the ME are target cells for progesterone. As the majority of PR-IR neurons in the AR do not project to the ME, it is suggested that most PR-IR neurons present in this nucleus form local circuit projections or project to distant areas of the central nervous system.

Substance P-like immunoreactive neurons in the arcuate nucleus project to the median eminence in rat

Substance P-containing nerve fibers in the median eminence of rat arise in cells located in the arcuate nucleus. Two days following surgical lesioning of the median eminence immunoreactive substance P accumulated in neuronal perikarya in the middle part (rostrocaudally) of the arcuate nucleus, mainly in its ventromedial portion. Substance P-immunostained cells appeared nowhere else in the hypothalamus following surgical lesion of the median eminence while they were found in several hypothalamic and extrahypothalamic cell groups after colchicine treatment.