Covergent effects of lordosis-relevant somatosensory and hypothalamic influences on central gray cells in the rat mesencephalon

Hormonal and circuit mechanisms controlling female sexual behavior

Sexual behavior is crucial for reproduction in many animals. In many vertebrates, females exhibit sexual behavior only during a brief period surrounding ovulation. Over the decades, studies have identified the roles of ovarian sex hormones, which peak in levels around the time of ovulation, and the critical brain regions involved in the regulation of female sexual behavior. Modern technical innovations have enabled a deeper understanding of the neural circuit mechanisms controlling this behavior. In this review, I summarize our current knowledge and discuss the neural circuit mechanisms by which female sexual behavior occurs in association with the ovulatory phase of their cycle.

Neural control of female sexual behaviors

Reproduction is the biological process by which new individuals are produced by their parents. It is the fundamental feature of all known life and is required for the existence of all species. All mammals reproduce sexually, a process that involves the union of two reproductive cells, one from a male and one from a female. Sexual behaviors are a series of actions leading to reproduction. They are composed of appetitive, action, and refractory phases, each supported by dedicated developmentally-wired neural circuits to ensure high reproduction success. In rodents, successful reproduction can only occur during female ovulation. Thus, female sexual behavior is tightly coupled with ovarian activity, namely the estrous cycle. This is achieved through the close interaction between the female sexual behavior circuit and the hypothalamic-pituitary-gonadal (HPG) axis. In this review, we will summarize our current understanding, learned mainly in rodents, regarding the neural circuits underlying each phase of the female sexual behaviors and their interaction with the HPG axis, highlighting the gaps in our knowledge that require future investigation.

Neurons expressing estrogen receptor α differentially innervate the periaqueductal gray matter of female rats

The periaqueductal gray matter (PAG) is a brainstem site involved in distinct autonomic and behavioral responses. Among them, the motor control of female sexual behavior, including lordosis, is well described. Lordosis reflex is highly dependent on increasing levels of estradiol that occur in the afternoon of the proestrus day in normally cycling females. This effect is thought to be mediated primarily via actions in the ventromedial nucleus of the hypothalamus (VMH). By binding to estrogen receptor α (ERα), estradiol changes the activity of VMH neurons that project to the PAG. Evidence also exists for the coordination of PAG outputs by estradiol-responsive neurons outside the VMH. However, a comprehensive analysis of these circuitries is not available. Using stereotaxic injection of the retrograde tracer Fluorogold in distinct columns of the PAG we performed a systematic mapping of neurons innervating the PAG and those coexpressing ERα immunoreactivity. We found that the forebrain projections to PAG columns are largely segregated and that most of the ERα expressing neurons preferentially target the lateral and the ventrolateral columns. Dual labeled neurons were mostly found in the intermediate subdivision of the lateral septal nucleus, the posterior aspect of the medial bed nucleus of the stria terminalis, the medial preoptic nucleus, the striohypothalamic nucleus and the ventrolateral VMH. Few dual labeled neurons were also observed in the arcuate nucleus, in the posterodorsal subdivision of the medial nucleus of the amygdala and in the ventral premammillary nucleus. Our findings indicate that ERα modulates sexual behavior in female rats via an integrated neural network that differentially innervate the columns of the PAG.

Identification of neural cells activated by mating stimulus in the periaqueductal gray in female rats

Induction of lordosis as typical female sexual behavior in rodents is dependent on a mount stimulus from males and blood levels of estrogen. Periaqueductal gray (PAG) efferent neurons have been suggested to be important for lordosis behavior; however, the neurochemical basis remains to be understood. In this study, we neuroanatomically examined (1) whether PAG neurons activated by mating stimulus project to the medullary reticular formation (MRF), which is also a required area for lordosis; and (2) whether these neurons are glutamatergic. Mating stimulus significantly increased the number of cFos-immunoreactive (ir) neurons in the PAG, particularly in its lateral region. Half of cFos-ir neurons in the lateral PAG were positive for a retrograde tracer (FluoroGold; FG) injected into the MRF. cFos-ir neurons also colocalized with mRNA of vesicular glutamate transporter 2 (vGLUT2), a molecular marker for glutamatergic neurons. Using retrograde tracing and in situ hybridization in conjunction with fluorescent microscopy, we also found FG and vGLUT2 mRNA double-positive neurons in the lateral PAG. These results suggest that glutamatergic neurons in the lateral PAG project to the MRF and are involved in lordosis behavior in female rats.

Sex differences in the anatomical and functional organization of the periaqueductal gray-rostral ventromedial medullary pathway in the rat: a potential circuit mediating the sexually dimorphic actions of morphine

Previous studies have demonstrated that morphine, administered systemically or directly into the periaqueductal gray (PAG), produces a significantly greater degree of antinociception in males in comparison with females. Because the midbrain PAG and its descending projections to the rostral ventromedial medulla (RVM) constitute an essential neural circuit for opioid-based analgesia, the present studies were conducted to determine whether sex differences in the anatomical organization of the PAG-RVM pathway, and its activation during persistent inflammatory pain, could account for sex-based differences in opioid analgesia. In the rat, retrograde tracing was combined with Fos immunocytochemistry to investigate sexual dimorphism in the organization of the PAG-RVM circuit and its activation by persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA). The ability of morphine to suppress the activation of the PAG-RVM circuit was also examined. Sexually dimorphic retrograde labeling was observed within the dorsomedial and lateral/ventrolateral PAG at all rostrocaudal levels, with females having significantly more PAG-RVM output neurons in comparison with males. While no sex differences were noted in the activation of the PAG by persistent inflammatory pain, significantly more PAG-RVM cells were activated in males in comparison with females. Systemic administration of morphine significantly suppressed CFA-induced Fos in the PAG in males only. The results of these studies demonstrate that both the anatomical organization and the functional activation of the PAG-RVM circuit are sexually dimorphic and may provide the anatomical substrate for sex-based differences in morphine analgesia.

GABA(A) receptor regulation of kyphotic nursing and female sexual behavior in the caudal ventrolateral periaqueductal gray of postpartum rats

Bilateral lesions of the ventrolateral caudal periaqueductal gray inhibit lordosis and kyphosis, the postures of female sexual receptivity and maternal nursing that are characterized respectively by dorsoflexion and ventroflexion of the spinal column. These lesions also inhibit the solicitation behaviors that accompany lordosis, but they do not impair retrieval or licking of pups. We tested the hypothesis that reproductive behaviors affected by these lesions are tonically inhibited by activity of the GABA(A) receptor via site-specific manipulations of receptor activity. Rats were bilaterally implanted during pregnancy with guide cannulae aimed at the caudal periaqueductal gray and ovariectomized on day 1 postpartum. Microinfusions (0.25 microl/side) of saline or drug took place on days 5 and 7 postpartum into the dorsolateral column and on days 9 and 11 into the ventrolateral column. Five minutes post-infusion dams were reunited with their pups and their maternal behavior was observed for 30 min. Feminine sexual behaviors were evaluated post-weaning after another set of microinfusions in the ventrolateral caudal periaqueductal gray. Potential facilitation of kyphosis and lordosis was tested with the GABA(A) antagonist bicuculline (15 ng/side) during sub-threshold conditions, i.e., non-suckling pups or sub-threshold ovarian hormone dosages; potential inhibition of these postures was tested with the GABA(A) agonist muscimol (125 ng/side) during optimal conditions, i.e., suckling pups or supra-threshold ovarian hormone treatments. Dorsolateral drug manipulations were ineffective. In the ventrolateral periaqueductal gray bicuculline significantly increased and muscimol significantly decreased kyphosis, lordosis, and sexual solicitations compared with saline. Retrieval and licking of pups were not altered by GABA(A) manipulations. These findings suggest that the reproductive postures of female rats, lordosis and kyphosis, as well as sexual solicitations, are tonically inhibited by the neurotransmitter GABA within the ventrolateral caudal periaqueductal gray in the midbrain. In contrast, retrieval and licking of pups appear to be under separate neurochemical or neuroanatomical control, or both. Further, this tonic inhibition is likely relieved by excitatory somatosensory inputs to this site, from mounting and suckling respectively.

Mapping of neural and signal transduction pathways for lordosis in the search for estrogen actions on the central nervous system

Estrogen can act on the brain to regulate various biological functions and behavior. In attempts to elucidate the estrogen action, the rodent female reproductive behavior, lordosis, was used as a model. Lordosis is an estrogen-dependent reflexive behavior and, hence, is mediated by discrete neural pathways that are modulated by estrogen. Therefore, a strategy of mapping the pathways, both neural and biochemical, and examining them for estrogen effect was used to localize and subsequently analyze the central action of estrogen. Using various experimental approaches, an 'inverted Y-shaped' neural pathway both sufficient and essential for mediating lordosis was defined. The top portion is a descending pathway conveying the permissive estrogen influence which originated from hypothalamic ventromedial nucleus relayed via midbrain periaqueductal grey down to medullary reticular formation, the top of the spino-bulbo-spinal reflex arc at the bottom. This estrogen influence alters the input-output relationship, shifting the output toward more excitation. With this shift in output, estrogen can enable the otherwise ineffective lordosis-triggering sensory stimuli to elicit lordosis. In the ventromedial nucleus, the origin of the estrogen influence, a multidisciplinary approach was used to map intracellular signaling pathways. A phosphoinositide pathway involving a specific G protein and the activation of protein kinase C was found to be involved in the mediation of lordosis as well as a probable target of the permissive estrogen action. The action of estrogen on this signal transduction pathway, a potentiation, is consistent with and, hence, may be an underlying mechanism for the estrogen influenced shift toward excitation. Thus, further investigation on this specific signal transduction pathway should be helpful in elucidating the action of estrogen on the brain.

The organization and function of endogenous antinociceptive systems

Much progress has been made the understanding of endogenous pain-controlling systems. Recently, new concepts and ideas which are derived from neurobiology, chaos research and from research on learning and memory have been introduced into pain research and shed further light on the organization and function of endogenous antinociception. These most recent developments will be reviewed here. Three principles of endogenous antinociception have been identified, as follows. (1) Supraspinal descending inhibition: the patterns of neuronal activity in diencephalon, brainstem and spinal cord during antinociceptive stimulation in midbrain periaqueductal gray (PAG) or medullary nucleus raphe magnus have now been mapped on the cellular level, using the c-Fos technique. Results demonstrate that characteristic activity patterns result within and outside the PAG when stimulating at its various subdivisions. The descending systems may not only depress mean discharge rates of nociceptive spinal dorsal horn neurons, but also may modify harmonic oscillations and nonlinear dynamics (dimensionality) of discharges. (2) Propriospinal, heterosegmental inhibition: antinociceptive, heterosegmental interneurons exist which may be activated by noxious stimulation or by supraspinal descending pathways. (3) Segmental spinal inhibition: a robust long-term depression of primary afferent neurotransmission in A delta fibers has been identified in superficial spinal dorsal horn which may underlie long-lasting antinociception by afferent stimulation, e.g. by physical therapy or acupuncture.

Distinct patterns of activated neurons throughout the rat midbrain periaqueductal gray induced by chemical stimulation within its subdivisions

This study provides a map of those neurons in the midbrain periaqueductal gray which are activated by chemical stimulation within different subdivisions of the periaqueductal gray. In pentobarbital anesthetized rats, the expression of the c-FOS protein was detected by immunocytochemistry and was used as a marker of neuronal activity. Microinjections of the gamma-aminobutyric acid (GABAA) receptor antagonist bicuculline (200 pmol in 50 nl) were used to increase selectively the firing rate of neurons originating from the injection site. The pattern of c-FOS immunoreactivity was highly specific for different injection sites. Dorsal injections were characterized by an extensive distribution of c-FOS immunoreactivity along the entire rostrocaudal extent of the periaqueductal gray, while ventral injections produced a much more restricted labeling. Following injection into the dorsal subdivision of the rostral periaqueductal gray, c-FOS immunoreactivity was present bilaterally in the dorsal and dorsolateral subdivisions of the rostral periaqueductal gray and was found in all subdivisions of the caudal periaqueductal gray. Dorsolateral injections at the level of the oculomotor nuclei produced strictly ipsilateral labeling in the dorsal and dorsolateral periaqueductal gray at the level of injection and throughout the ipsilateral half of the periaqueductal gray at more caudal levels. Stimulation in the ventrolateral periaqueductal gray induced FOS in the ventrolateral periaqueductal gray and the adjoining reticular formation. At rostral levels c-FOS immunoreactivity was also seen in the lateral periaqueductal gray but was absent caudal to the injection site. The identified patterns of activity in the periaqueductal gray provide a new basis for the interpretation of the diverse functional consequences of stimulation at periaqueductal gray sites.

Quantitative ultrastructural analysis of the periaqueductal gray in the rabbit

The fine structure of the periaqueductal gray (PAG) of the rabbit was examined using the transmission electron microscope. On the basis of synaptic polarity, vesicle size, and the nature of the pre- and post-synaptic elements, 10 essentially different synaptic types could be discerned (6 axo-dendritic, 2 axo-somatic, 1 axo-axonic, and 1 dendro-dendritic). Synaptic contacts on the soma of PAG neurons were small and covered, on average, only 1.6% of the soma surface. The most striking feature of the synaptic structure of the PAG was that more than 94.1% of all synapses were axo-dendritic. Of these, 83.5% were of the symmetrical type. Most of these contacts occurred on buttons of small to medium size, and contained either round vesicles of medium size or pleomorphic vesicles of medium size. Boutons containing only flattened vesicles were quite rare. Boutons contacting larger dendrites were generally small-to-medium in size, made asymmetric-type synaptic contacts, and contained pleomorphic vesicles of medium-to-large size. Medium-sized dendrites were contacted principally by small boutons exhibiting either symmetrical or asymmetrical junctions containing medium-sized pleomorphic vesicles, and in addition a few of these boutons contained both large, and small, round vesicles. Dendritic spines were generally provided with only one synaptic contact, stretching the entire width of the spinous process. Boutons and the spines on dendrites were approximately the same size. Synapses between two vesicle-containing structures (axo-axonic or dendro-dendritic synapses) were rare (1.4%). They were generally asymmetric and contained round vesicles of medium size. Complex synapses, where a glial sheet enclosed the synapse, were occasionally observed. Also seen were multiple synapses, with up to 11 contacts on a single dendritic profile. Large dense-core vesicle were seen in approximately 40% of all synapses, whereas small dense-core vesicles were only found in about 3%. Data is provided on how different synaptic features relate to ventral, lateral, dorsal, and medial PAG. Principally this is in relation to neuron size, glia cell content, axonal characterization, and vesicular type.

Midbrain tegmentum as an intermediate relay station of the periaqueductal-facial pathway in the cat

Iontophoretic injection of horseradish peroxidase (HRP) into the medial part of the facial nucleus resulted in retrograde labeling in the lateral midbrain tegmentum contralaterally which corresponded to the paralemniscal zone (PL). Further experiments in which HRP was injected iontophoretically into the PL revealed heavy retrograde labeling in the lateral portion of the ipsilateral periaqueductal gray (PAG). These light microscopic studies indicated the possibility of the pathway from the PAG to the facial nucleus via the midbrain PL. The electron microscopic observations were carried out on the lateral midbrain tegmentum containing the PL after kainic acid, and wheat germ agglutinin conjugated HRP injections were made into the PAG and the contralateral facial nucleus in the same animal, respectively. Although in the neuropil many degenerating PAG fibers and retrogradely labeled neurons were observed, it was of particular interest that the degenerating fibers made synaptic contacts with HRP-labeled somata and dendrites.

Reciprocal connections between the medial preoptic area and the midbrain periaqueductal gray in rat: A WGA-HRP and PHA-L study

The midbrain periaqueductal gray (PAG) participates in diverse functions such as analgesia, autonomic regulation, sexual behavior, and defense/escape responses. Anatomical studies of the circuits involved in such functions have largely focused on the connections of PAG with the medulla. Projections to PAG from forebrain structures are extensive, but their organization has received little attention. Previous anatomic studies indicate that the medial preoptic area (MPO), involved in a variety of physiological and behavioral functions, is a major source of afferent input to the periaqueductal gray. Here, we have examined the topography of reciprocal connections between these two structures in the rat by using wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and Phaseolus vulgaris leucoagglutinin (PHA-L). Multiple WGA-HRP injections at several rostrocaudal levels of PAG retrogradely labeled large numbers of neurons in the medial preoptic area; labeled cells were primarily located in the medial preoptic nucleus, the median preoptic nucleus, and the region lateral to the medial preoptic nucleus. The distribution of labeled cells shifted medially to laterally along the rostral to caudal axis of the medial preoptic area. Rostrally, there was selective retrograde labeling in the central and lateral divisions of medial preoptic nucleus, whereas caudally, labeled cells were primarily located only in the lateral subdivision of medial preoptic nucleus. Tracer injections in PAG also produced strong anterograde labeling in MPO. WGA-HRP and PHA-L injections in the medial preoptic area resulted in dense anterograde labeling along the entire rostrocaudal axis of PAG. The terminal labeling in PAG from the medial preoptic area was not uniformly distributed throughout PAG, however. Instead, this projection formed one or two rostrocaudally oriented longitudinal columns that terminated in different subregions of PAG along the entire rostrocaudal axis of this structure. Rostrally, inputs from the medial preoptic area project heavily to dorsomedial PAG, and at mid-PAG levels, the projection becomes distinctly bipartite with two discrete longitudinal terminal columns in dorsomedial and lateral PAG; caudally, the heaviest labeling is in ventrolateral PAG. The projection also exhibited a central to peripheral (radial) gradient; labelled fibers and terminals were heaviest near the aqueduct and much lower in the peripheral parts of PAG. WGA-HRP injections in MPO also produced retrograde labeling of neurons at all rostrocaudal levels of PAG; more neurons were labeled in the rostral than the caudal half of PAG. The majority of labeled cells were located in dorsomedial and ventral/ventrolateral parts of PAG; only a few neurons in the dorsal raphe region appear to project to MPO.(ABSTRACT TRUNCATED AT 400 WORDS)

Efferent projections of the periaqueductal gray in the rabbit

The efferent projections of the periaqueductal gray in the rabbit have been described by anterograde tract-tracing techniques following deposits of tritiated leucine, or horseradish peroxidase, into circumscribed sites within dorsal, lateral or ventral periaqueductal gray. No attempts were made to place labels in the fourth, extremely narrow (medial), region immediately surrounding the aqueduct whose size and disposition did not lend itself to confined placements of label within it. These anatomically distinct regions, defined in Nissl-stained sections, corresponded to the same regions into which deposits of horseradish peroxidase were made in order for us to describe afferent projections to the periaqueductal gray. In this present study distinct ascending and descending fibre projections were found throughout the brain. Terminal labelling was detected in more than 80 sites, depending somewhat upon which of the three regions of the periaqueductal gray received the deposit. Therefore, differential projections with respect to both afferent and efferent connections of these three regions of the periaqueductal gray have now been established. Ventral deposits disclosed a more impressive system of ramifying, efferent fibres than did dorsal or lateral placements of labels. With ventral deposits, ascending fibres were found to follow two major pathways from periaqueductal gray. The periventricular bundle bifurcates at the level of the posterior commissure to form hypothalamic and thalamic components which distribute to the anterior pretectal region, lateral habenulae, and nuclei of the posterior commissure, the majority of the intralaminar and midline thalamic nuclei, and to almost all of the hypothalamus. The other major ascending pathway from the periaqueductal gray takes a ventrolateral course from the deposit site through the reticular formation or, alternatively, through the deep and middle layers of the superior colliculus, to accumulate just medial to the medial geniculate body. This contingent of fibres travels more rostrally above the cerebral peduncle, distributing terminals to the substantia nigra, ventral tegmental area and parabigeminal nucleus before fanning out and turning rostrally to contribute terminals to ventral thalamus, subthalamus and zona incerta, then continuing on to supply amygdala, substantia innominata, lateral preoptic nucleus, the diagonal band of Broca and the lateral septal nucleus. Caudally directed fibres were also observed to follow two major routes. They either leave the periaqueductal gray dorsally and pass through the gray matter in the floor of the fourth ventricle towards the abducens nucleus and ventral medulla, or are directed ventrally after passing through either the inferior colliculus or parabrachial nucleus. These ventrally directed fibres merge just dorsal to the pons on the ventral surface of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitatory amino acid projections to the periaqueductal gray in the rat: a retrograde transport study utilizing D[3H]aspartate and [3H]GABA

The afferents to the periaqueductal gray utilizing excitatory amino acid transmitters have been described in rat brain by autoradiography following microinfusion and retrograde transport of D[3H]aspartate. Parallel experiments employing injections of [3H]GABA established that the retrograde labelling found with D[3H]aspartate was transmitter-selective. Following infusion of D[3H]aspartate, perikaryal labelling was found in nine subcortical areas, particularly infralimbic and cingulate cortices, with a predominance of ipsilateral labelled perikarya. Heaviest cortical labelling was localized in perirhinal cortex, in an extensive band of cells adjoining the rhinal sulcus. The hypothalamus contained the heaviest perikaryal labelling within brain: D[3H]aspartate labelled cells in 11 hypothalamic and mammillary nuclei. Intense bilateral labelling was obtained in ventromedial hypothalamus, although the number of perikarya was lower contralaterally. D[3H]Aspartate also produced heavy ipsilateral labelling of perikarya in posterior hypothalamus. Labelling patterns in cortex and hypothalamus were precise and topographic, and [3H]GABA never labelled cells in these regions. Other telencephalic and diencephalic areas containing prominent, retrogradely labelled cells were the lateral septum, amygdala, zona incerta and lateral habenula. The relative density of labelled cells in mesencephalic areas was much lower than that found in cortex and hypothalamus, although D[3H]aspartate labelled a moderate number of perikarya in the inferior colliculus and cuneiform nucleus. A smaller number of heavily labelled cells was found in the parabrachial nuclei, Kolliker-Fuse nucleus and laterodorsal tegmental nucleus. Only occasional labelled perikarya were observed in the myencephalon. Low densities of labelled cells were found after the injection of [3H]GABA into the periaqueductal gray, and the only regions in which a small number of perikarya were labelled by both [3H]GABA and D[3H]aspartate were the dorsal raphe and parabrachial nuclei. Overall, the retrograde transport of D[3H]aspartate revealed a complex topographic and convergent network of afferent pathways to the periaqueductal gray likely to utilize an excitatory amino acid transmitter. Our findings confirm the selectivity of this neurochemical mapping technique and provide evidence that hypothalamic, habenular, subthalamic and cuneiform afferents to the periaqueductal gray utilize an acidic amino acid as their transmitter. They also confirm that corticofugal afferents to periaqueductal gray utilize an excitatory amino acid.

Lateralized effects on hamster lordosis of unilateral hormonal and somatosensory stimuli

To better describe the hypothalamic efferents that mediate estrogenic effects on lordosis, each of 18 ovariectomized hamsters received a 30 gauge implant of estradiol aimed at the right or left ventromedial hypothalamus (VMN). Six days later, subjects were treated with progesterone and observed for their durations of lordosis during brief periods with sexually-active males. Subsequently, the male was removed, as light manual stimulation was applied to the flank ipsi- or contralateral to the implant in attempts to (a) maintain as yet unbroken lordosis responses, or (b) reinstate lordosis in females showing limb movements indicative of their emergence from the male-elicited posture. The results failed to show any difference in the behavioral effects of implants aimed at the right versus left VMN. However, implants did interact selectively with manual stimulation of the ipsi- and contralateral flanks. Specifically, lordosis responses were more likely, more rapid, and better maintained to stimulation of the contralateral flank. This lateralization of the priming effect derived from a VMN estradiol implant is impressive for its persistence, and for its consistency with the trajectories of VMN efferents to the midbrain and of midbrain afferents from the flanks. The consistency of these behavioral and neuroanatomical patterns suggests that lordosis depends on direct interactions of the VMN and dorsal midbrain. In turn, it implicates the dorsal midbrain in the mediation on VMN-initiated hormonal effects on lordosis, and in the eventual integration of hormonal and somatosensory influences on sexual receptivity.

Patterns of 2-deoxyglucose uptake reflect the neural processing of lordosis-inducing somatosensory stimuli in hamsters

Semi-quantitative [14C]2-deoxyglucose (2DG) autoradiography was used to map the neural responses of female hamsters to lordosis-inducing flank stimuli. Specifically, manual stimulation of one flank was used to maintain estrous females in lordosis for 20 min after an IV injection of 200 muCi/kg of 2DG. Hemispheric differences in 2DG uptake then were sought in brain nuclei implicated in the programming of lordosis, or in the mediation of somatosensory or hormonal influences on this response. The responses to lateralized flank stimulation included reliable contralateral elevations in 2DG uptake in the ventral posterior lateral nucleus of the thalamus (VPL), the dorsal mesencephalic central gray (dCG), and the tectum. Elevated activity on the part of the VPL may not be crucial for lordosis. However, the effects of flank stimulation on 2DG uptake by the dCG and tectum confirm and extend much previous evidence implicating the dorsal midbrain in the mediation of tactile and hormonal effects on sexual responses. For example, these results suggest that somatosensory influences on hamster lordosis are mediated by both the dCG and tectum. In addition, they suggest that these influences are strongly lateralized until at least this stage of sensory processing, leaving for some subsequent element of neural circuitry the task of translating these lateralized inputs into the bilaterally symmetric outputs ultimately required to program the normal, bilaterally symmetric, lordosis response.

An excitatory amino acid projection from ventromedial hypothalamus to periaqueductal gray in the rat: autoradiographic and electrophysiological evidence

The projection from ventromedial hypothalamus (VMH) to periaqueductal gray (PAG) has been implicated in various physiological processes in the rat, but the neurotransmitter mediating the excitatory response in PAG has not been identified. This pathway has been studied using a combination of anatomical and electrophysiological techniques. The retrograde labelling by D-[3H]aspartate in VMH, following injections into PAG, was particularly dense, suggesting that an excitatory amino acid may be the transmitter. This postulate is strengthened since VMH-evoked synaptic responses in PAG were reduced by microelectrophoretically administered antagonists of excitatory amino acids. Receptors of the N-methyl-D-aspartate and non-N-methyl-D-aspartate types appear to have a functional role in this projection.

Cytochrome oxidase histochemistry reveals regional subdivisions in the rat periaqueductal gray matter

The identification of different anatomical regions of the periaqueductal gray matter of rats was addressed in the present study by using the histochemical staining for the mitochondrial enzyme cytochrome oxidase. At caudal and middle levels, cytochrome oxidase histochemistry clearly demonstrates the existence of four subdivisions: dorsal, dorsolateral, ventrolateral and medial, whereas in sections from the rostral periaqueductal gray matter only two concentric bands are identifiable on the basis of the degree of cytochrome oxidase activity.

Dual progesterone action in diencephalon facilitates the induction of sexual receptivity in estrogen-primed golden hamsters

In order to identify and characterize the progesterone (P) sensitive neural system that regulates feminine sexual behavior, 28-gauge P-filled cannulae were implanted in the medial preoptic area (MPO), ventromedial hypothalamus (VMH), and central gray (CG) of ovariectomized estrogen-primed golden hamsters. Dual implants of P were placed either ipsilaterally or contralaterally in brain sites consisting of MPO-VMH, MPO-CG, or VMH-CG combinations. Tests for sexual receptivity commenced 44 hr after estrogen priming and consisted of a preimplantation test followed 4.5 to 5.5 hr later by a postimplantation test. In the preimplantation test, stimulus males were attacked when placed into the female's home cage which indicated that the subsequent display of sexual receptivity occurring in the postimplantation test was due to the action of P. Dual implants of P placed either ipsilaterally or contralaterally in MPO-VMH regions were significantly more effective in facilitating lordosis behavior than dual P implants placed in MPO-CG or VMH-CG regions. However, the duration of lordotic responsiveness produced by dual P implants in MPO and VMH regions appears to be shorter than the duration of lordosis typically observed in intact females on proestrus. Results suggest that MPO and VMH regions are sensitive to the lordosis facilitating actions of small dual implants of P.

Electrical stimulation of the midbrain central gray facilitates reticulospinal activation of axial muscle EMG

EMG responses were recorded from axial muscles transversospinalis, medial longissimus, and lateral longissimus in urethane-anesthetized rats during combined electrical stimulation of the reticular formation and midbrain central gray. Central gray stimulation facilitated reticular formation-evoked EMG activity in the back muscles of the rat. Electrical stimulation of the central gray lowered the threshold for reticulospinal activation of axial muscles and could maintain firing in these muscles after the end of a reticular formation train. Units were recruited in order of size from small to large. In only one case, central gray stimulation activated axial muscles directly without reticular formation stimulation. The central gray may be important in relaying hypothalamic influences to the reticular formation, which has direct access to the axial muscles responsible for lordosis behavior.

Afferent projections to the periaqueductal gray in the rabbit

The afferents to the periaqueductal gray in the rabbit have been described following hydraulic pressure injection of horseradish peroxidase at various sites throughout this structure. Every third section was reacted with tetramethylbenzidine, for the localization of afferent neurons. At the site of the deposit alternate sections were reacted with tetramethylbenzidine, Hanker-Yates reagent, or diaminobenzidine, for comparative assessment of the injection site. A large number of retrogradely labelled cells, assessed by bright- and dark-field microscopy, were observed in a wide range of areas throughout the brain. Major labelled areas within the telencephalon were cortical areas 5, 20, 21, 32 and 40. Within the diencephalon, the hypothalamus contained quantitatively by far the largest number of labelled cells. Of these nuclei, the dorsal pre-mammillary nucleus contained the largest number of labelled cells. Considerable labelling was also found within medial and lateral preoptic nuclei, anterior hypothalamic area, and ventromedial hypothalamic nucleus. Another diencephalic region containing a significant number of retrogradely labelled neurons was the zona incerta. At midbrain, pontine and medullary levels, additional labelled regions were: the substantia nigra, cuneiform nucleus, parabigeminal nucleus, raphe magnus, and reticular areas. Heavy labelling was seen within the periaqueductal gray itself, rostral and caudal to deposits placed within each subdivision. In addition, a large number of other areas labelled throughout the brain (Tables 2A-D). Not only were some differences noted in the pattern of labelled cells with deposits placed rostrally or caudally within periaqueductal gray, but certain topographical differences with respect to the degree of labelling within nuclei were also seen with injection sites ventral, lateral or dorsal to the aqueduct. In addition, a further difference was noted, in that over one third of the areas labelled with deposits in just one or other of the "divisions" within periaqueductal gray. The results therefore suggest that the periaqueductal gray might be divisible to some extent on the basis of connectivity with intrinsic subdivisions of the complex. It is hoped that, with time, it might prove possible to resolve any such differential input in functional terms. The wide variety of afferent input to the periaqueductal gray, and its strategic location, would seem to place it in a unique position for integrating and modifying a diversity of motor, autonomic, hormonal, sensory and limbic influences.(ABSTRACT TRUNCATED AT 400 WORDS)

Preoptic lesions increase the display of lordosis by male rats

Male rats do not normally show feminine patterns of sexual behavior even when injected with the ovarian hormones estrogen and progesterone. We find that brain lesions which damage the preoptic-anterior hypothalamic continuum augment the display of lordosis in hormone-treated male rats. The most effectively feminizing brain lesions are ones which bilaterally destroy a substantial portion of the medial preoptic area encompassing the sexually dimorphic nucleus of the preoptic area (SDN-POA). Males with particularly large preoptic lesions are receptive following estrogen treatment and show a progesterone facilitation of receptivity. In this respect, they cannot be behaviorally distinguished from females. Thus, axons originating in and/or passing through the preoptic area apparently inhibit the display of feminine sexual behaviors in males. Preoptic development and lordosis are each predictably affected by perinatal stimulation by testicular hormones, and hormone-stimulated preoptic development may form the neurological basis for some of the defeminizing effects of perinatal hormonal exposure. Our results raise the possibility that the site of this behavioral defeminization is the SDN-POA.

Dual estradiol action in diencephalon and the regulation of sociosexual behavior in female golden hamsters

Previously, we found that single implants of estradiol (E2) placed in the ventromedial hypothalamus (VMH) but not the anterior hypothalamus (AH) facilitated precopulatory, i.e. vaginal scent marking, and copulatory, i.e. lordosis, behavior following progesterone administration. However, the duration of lordosis was markedly attenuated in comparison to the duration shown by intact cycling females. This study was designed to examine whether dual implantation of E2 in the diencephalon would facilitate patterns of precopulatory and copulatory behaviors similar to those shown by intact cycling females. One E2 implant was placed in either the medial preoptic area (MPO) or AH and a second E2 implant was placed in the VMH. Control females were tested following E2 application at only the MPO or AH region in conjunction with a cholesterol implant in the VMH. An additional control group was tested with females implanted with cholesterol at both MPO-AH regions and the VMH. During a 2-day postimplantation test period, vaginal marking scores were elevated for both single and dual E2 implanted females and agonistic response patterns toward males declined significantly. In addition, a significant inverse relationship was found between the number of vaginal marks and bites exhibited by females with single E2 implants in the MPO, whereas these two response patterns were positively correlated in females with E2 stimulation occurring only in the AH region. No significant relationship was found between vaginal marking and biting attack for females receiving dual E2 stimulation. Systemic progesterone administration on the third postimplantation day facilitated sexual receptivity in the majority of females with dual E2 implants (greater than 90%). These receptive females displayed lordotic responsiveness that closely matches the full display of sexual receptivity shown by intact cycling females. In contrast, only one female with a single E2 implant in the AH region showed sexual responsiveness. The results demonstrate that: precopulatory vaginal marking and biting attack are mediated by E2 action in the MPO and AH but in a different manner; additional action of E2 in the VMH diminishes the distinctive precopulatory behavioral effects of E2 in the MPO and AH suggesting an influential role of the VMH in regulating sociosexual activities; and E2 action in either the MPO or AH region in conjunction with E2 action in the VMH may be necessary in order to facilitate the species-typical display of lordotic responsiveness.

Hyperprolactinemia: its electrophysiologic and pharmacologic effect on neurons of the ventromedial nucleus of the hypothalamus

Extracellular action potentials were recorded from 691 neurons in the ventromedial hypothalamus (VMH) of urethane anesthetized female rats under acute and chronic sham-operated and hyperprolactinemic conditions. Hyperprolactinemia was produced by transplanting pituitaries under the kidney capsules. Neuronal excitability was recorded and analyzed during spontaneous, baseline activity and following the iontophoretic application of prolactin, dopamine (DA) and luteinizing hormone-releasing hormone (LH-RH). No statistically significant changes were observed in the spontaneous electrical activity of VMH neurons under the conditions tested. Of the responsive neurons, approximately 90% of the neurons recorded and tested with prolactin displayed an increase in firing activity while DA produced a decrease, independent of the endogenous plasma prolactin levels (basal or elevated). However, the response to LH-RH was modified in the chronic hyperprolactinemic animal. The predominant response of VMH neurons to LH-RH in acute sham-operated and hyperprolactinemic as well as in chronic sham-operated animals was one of inhibition, while in the chronic hyperprolactinemic animal, the application of LH-RH initiated excitation rather than inhibition. These results provide evidence that chronic (long-term) exposure to elevated prolactin levels is a sufficient stimulus to modify the neuronal response pattern of VMH nerve cells to iontophoretically applied LH-RH but not to prolactin nor to DA.

Afferents to a midbrain periaqueductal grey region involved in the 'defence reaction' in the cat as revealed by horseradish peroxidase. I. The telencephalon

Horseradish peroxidase injections were made at sites, within the midcollicular portion of the midbrain periaqueductal grey region (PAG), at which both electrical stimulation and subsequent microinjections of excitatory amino acids elicited defensive behaviour. Since excitatory amino acids depolarize cell bodies and dendrites located in the vicinity of the injection site but not axons of passage, the injections were centred within a PAG region known to contain neurones whose excitation elicited defensive behaviour. The telencephalic afferents to these sites were then determined. Sixty percent of the labelled telencephalic neurones were found in the frontal cortex, specifically in the medial frontal cortex along the banks of the rostral two-thirds of the cruciate sulcus, primarily area 6 and area 4, and the medial frontal cortex ventral to area 6 (area 32). Twenty-five percent of the labelled telencephalic neurones were found in the orbito-insular cortex while 8% were found in the parietal cortex surrounding the anterior ectosylvian sulcus. Although the functional significance of these projections remains to be established, available data suggest that these projections to the PAG arise from frontal 'oculomotor' and motor cortices, a polysensory insular cortical region and somatosensory, visual and auditory parietal cortical areas.

Estrogenic maintenance of lordotic responsiveness: requirement for hypothalamic action potentials

Studies were conducted to determine the requirement for hypothalamic, sodium current-based action potentials in the performance of a stereotypic, estrogen-dependent reflex, the lordosis response. Intrahypothalamic infusion of local anesthetics (50% procaine or 0.5% bupivacaine) into conscious rats had no effect on lordotic responsiveness, and, in a separate group of urethane-anesthetized rats, depressed multiunit electrical activity temporarily. Intrahypothalamic infusion of tetrodotoxin into conscious rats, however, resulted in a dose-dependent, reversible decline in lordotic responsiveness. The first significant drop in lordotic responsiveness occurred 40 min after infusion; the minimum was reached 2-4 h after infusion. Recovery of lordotic responsiveness to preinfusion levels was complete by 12-24 h after infusion. Electrophysiological studies in a separate group of urethane-anesthetized rats revealed that intrahypothalamic tetrodotoxin infusion in most cases suppressed multiunit activity completely usually within 6 min, and this suppression lasted for at least several hours. These data indicate that large, prolonged decreases in electrical activity in the hypothalamus, where estrogenic action is necessary and sufficient to induce lordosis, result in a gradual, reversible decline in lordotic responsiveness. These data are consistent with a 'tonic' rather than a 'mount-by-mount' role of hypothalamic neurons in lordosis. Furthermore, since lordotic responsiveness declined only when hypothalamic electrical activity had been disrupted severely for at least 40 min, it is postulated that the neuroactive products released by lordosis-relevant, hypothalamic neurons may have a duration of action of at least several minutes.

Ultrastructural localization of LH-RH-immunoreactive synapses in the hamster accessory olfactory bulb

Electron microscopic immunocytochemistry was used to localize luteinizing hormone-releasing (LH-RH) immunoreactivity within the male golden hamster accessory olfactory bulb. Two LH-RH-immunoreactive fiber populations were identified in the accessory olfactory bulb. A superficial system of immunoreactive axons was localized to the vomeronasal nerve and glomerular layers, and a periventricular system appeared in granule cell and periventricular layers. LH-RH-immunoreactive varicosities were observed to contain large reactive vesicles (80-120 nm) as well as a variable degree of cytoplasmic reaction product. Additionally, small vesicles with unreactive lumens and mitochondria were often present. Intravaricose segments of immunoreactive fibers invariably displayed fewer reactive vesicles than did varicosities. Within both glomerular and periventricular layers, some LH-RH-immunoreactive varicosities were observed to form asymmetric contacts characterized by prominent postjunctional densities. In the glomerular layer, these junctions could be identified as synaptic by several features. The presence of LH-RH-immunoreactivity in presynaptic elements supports a neuromodulatory role for LH-RH. As the accessory olfactory system is critically involved in the initiation of mating behavior of the male golden hamster, LH-RH-immunoreactive synapses in the accessory olfactory bulb may function to regulate reproductive behavior.

Forebrain projections to the periaqueductal gray in the monkey, with observations in the cat and rat

There is considerable evidence that the midbrain periaqueductal gray (PAG) is involved in visceral, emotive, and sexual responses and in endogenous analgesic effects. To see which of the forebrain areas directly influence the PAG, small injections of horseradish peroxidase were made into the various regions of monkey, cat, and rat PAG. Despite the fact that regions of the PAG were injected in separate animals the majority of the forebrain areas labeled remained constant. Retrogradely filled pyramidal neurons in layer V were found in the frontal lobe in areas 6, 8, 9, 10, 13, and 24. Labeled neurons also appeared in the amygdala, preoptic area, and the anterior, dorsal, periventricular, ventromedial, periarcuate lateral, and posterior hypothalamic nuclei. The main route for the hypothalamic leads to PAG projection appeared to be via the periaqueductal bundle which immediately borders on the cerebral aqueduct. Labeled neurons were also observed in the zona incerta, mesencephalic reticular formation, deep layers of the superior colliculus, and the nucleus cuneiformis. Most labeling was ipsilateral to the injection site although a small but consistent contralateral labeling was present. Therefore a strict subdivision of the PAG based on each subnucleus having its own unique set of connections seems inappropriate. There were few striking differences found in the forebrain areas that project to the PAG in the three species examined. These results are discussed in terms of the possible contribution these forebrain areas have in regulating the PAG with regard to its functions as a visceral, nociceptive, and cognitive integrator.

The midbrain periaqueductal gray in the rat, cat, and monkey: a Nissl, Weil, and Golgi analysis

Anatomical staining methods including Nissl, Weil, Golgi, and horseradish peroxidase stain have been used to elucidate the cyto- and myeloarchitectural organization of the periaqueductal gray in monkey, cat, and rat. From these various staining methods it appears that the periaqueductal gray is composed of a tightly packed group of cells, which show a slight increase in soma size, dendritic diameter, and degree of myelinization from central to peripheral borders. This central gray region contains a wide variety of cell types including multipolar, fusiform, stellate, and pyramidal neurons. Clearly delineated subnuclei, distinguished on the basis of soma size, dendritic arborizations, pigmentation, or evidence of cytological individuality could not be discerned in this study. Together with the immunohistochemical and connectivity studies the present data suggest that the neuronal organization of the PAG could be described as a mosaic of clusters of functional related neurons rather than as three distinct subnuclei, each with its own unique cytoarchitecture and connectivity.

Hypothalamic, other diencephalic, and telencephalic neurons that project to the dorsal midbrain

Neurons in the hypothalamus, other diencephalic regions, and the telencephalon which project to the mesencephalic central gray (CG) and the region lateral to it were demonstrated, in the rat, by the horseradish peroxidase retrograde neuroanatomical tracing method with diaminobenzidine and tetramethyl benzidine visualization reactions. The greatest concentrations of neurons that project to the dorsal mesencephalon were found in the ventromedial nucleus, particularly the anterior and ventrolateral subdivisions, in the dorsal premammillary nucleus, and in the zona incerta. Neurons that project to or lateral to the CG were also found in the laterocaudal hypothalamus, the dorsomedial hypothalamus, regions of the anterior hypothalamic area, specific areas of the cerebral cortex (32, 29, 8, 8A, 13, 14), and the central nucleus of the amygdala. Some neurons that project were also found in the preoptic area, septum, bed nucleus of the stria terminals, and the habenula. More neurons in the mediocaudal quadrant of the hypothalamus project to the mesencephalon than do those in laterocaudal, mediorostral, or laterorostral quadrants. More neurons in the medial than the lateral half, and more in the caudal than the rostral half of the hypothalamus project to the mesencephalon. More neurons project to the central gray, or the region lateral to it, at the levels of the superior colliculus, or intercollicular region, than at the level of the inferior colliculus. These descending connections to the midbrain, particularly from the hypothalamus and zona incerta, are probably components of neural networks that regulate nociception, certain neuroendocrine functions, sexual and other behaviors, and certain autonomic functions.