Parvocellular subparafascicular thalamic nucleus in the rat: anatomical and functional compartmentalization

Neuropeptidergic control circuits in the spinal cord for male sexual behaviour: Oxytocin-gastrin-releasing peptide systems

The neuropeptidergic mechanisms controlling socio-sexual behaviours consist of complex neuronal circuitry systems in widely distributed areas of the brain and spinal cord. At the organismal level, it is now becoming clear that "hormonal regulations" play an important role, in addition to the activation of neuronal circuits. The gastrin-releasing peptide (GRP) system in the lumbosacral spinal cord is an important component of the neural circuits that control penile reflexes in rats, circuits that are commonly referred to as the "spinal ejaculation generator (SEG)." Oxytocin, long known as a neurohypophyseal hormone, is now known to be involved in the regulation of socio-sexual behaviors in mammals, ranging from social bonding to empathy. However, the functional interaction between the SEG neurons and the hypothalamo-spinal oxytocin system remains unclear. Oxytocin is known to be synthesised mainly in hypothalamic neurons and released from the posterior pituitary into the circulation. Oxytocin is also released from the dendrites of the neurons into the hypothalamus where they have important roles in social behaviours via non-synaptic volume transmission. Because the most familiar functions of oxytocin are to regulate female reproductive functions including parturition, milk ejection, and maternal behaviour, oxytocin is often thought of as a "feminine" hormone. However, there is evidence that a group of parvocellular oxytocin neurons project to the lower spinal cord and control male sexual function in rats. In this report, we review the functional interaction between the SEG neurons and the hypothalamo-spinal oxytocin system and effects of these neuropeptides on male sexual behaviour. Furthermore, we discuss the finding of a recently identified, localised "volume transmission" role of oxytocin in the spinal cord. Findings from our studies suggest that the newly discovered "oxytocin-mediated spinal control of male sexual function" may be useful in the treatment of erectile and ejaculatory dysfunction.

Behavioural actions of tuberoinfundibular peptide 39 (parathyroid hormone 2)

Tuberoinfundibular peptide of 39 residues (TIP39) acts via its endogenous class B G-protein coupled receptorthe parathyroid hormone 2 receptor (PTH2R). Hence, it is also known as parathyroid hormone 2. The peptide is expressed in the brain by a small number of neurons with a highly restricted distribution, which in turn project to a large number of brain regions that contain PTH2R. This peptide neuromodulator system has been extensively investigated over the past 20 years including its behavioural actions, such as its role in the control of nociception, fear and fear incubation, anxiety and depression-like behaviours, and maternal and social behaviours. It also influences thermoregulation and potentially auditory responses. TIP39 probably exerts direct effect on the neuronal networks controlling these behaviours based on the localization of PTH2R and local TIP39 actions. In addition, TIP39 also affects the secretion of several hypothalamic hormones providing the basis for indirect behavioural actions. Recently developed experimental tools have stimulated further behavioural investigations, and novel results obtained are discussed in this review.

A central alarm system that gates multi-sensory innate threat cues to the amygdala

Perception of threats is essential for survival. Previous findings suggest that parallel pathways independently relay innate threat signals from different sensory modalities to multiple brain areas, such as the midbrain and hypothalamus, for immediate avoidance. Yet little is known about whether and how multi-sensory innate threat cues are integrated and conveyed from each sensory modality to the amygdala, a critical brain area for threat perception and learning. Here, we report that neurons expressing calcitonin gene-related peptide (CGRP) in the parvocellular subparafascicular nucleus in the thalamus and external lateral parabrachial nucleus in the brainstem respond to multi-sensory threat cues from various sensory modalities and relay negative valence to the lateral and central amygdala, respectively. Both CGRP populations and their amygdala projections are required for multi-sensory threat perception and aversive memory formation. The identification of unified innate threat pathways may provide insights into developing therapeutic candidates for innate fear-related disorders.

Single-cell reconstruction reveals input patterns and pathways into corticotropin-releasing factor neurons in the central amygdala in mice

Corticotropin-releasing factor (CRF) neurons are one of the most densely distributed cell types in the central amygdala (CeA), and are involved in a wide range of behaviors including anxiety and learning. However, the fundamental input circuits and patterns of CeA-CRF neurons are still unclear. Here, we generate a monosynaptic-input map onto CeA-CRF neurons at single-cell resolution via a retrograde rabies-virus system. We find all inputs are located in 44 nested subregions that directly innervate CeA-CRF neurons; most of them are top-down convergent inputs expressing Ca²⁺/calmodulin-dependent protein kinase II, and are centralized in cortex, especially in the layer 4 of the somatosensory cortex, which may directly relay information from the thalamus. While the bottom-up divergent inputs have the highest proportion of glutamate decarboxylase expression. Finally, en passant structures of single input neuron are revealed by in-situ reconstruction in a modified 3D-reference atlas, represented by a Periaqueductal gray-Subparafascicular nucleus-Subthalamic nucleus-Globus pallidus-Caudoputamen-CeA pathway. Taken together, our findings provide morphological and connectivity properties of inputs onto CeA-CRF neurons, which may provide insights for future studies interrogating circuit mechanisms of CeA-CRF neurons in mediating various functions.

Viral retrograde tracing identifies input regions and patterns into the corticotropin releasing factor-expressing neurons in central amygdala, providing an important resource to disentangle the role of these cells in fear and anxiety.

Effects of ovariectomy on inputs from the medial preoptic area to the ventromedial nucleus of the hypothalamus of young adult rats

Puberty is an important phase of development when the neural circuit organization is transformed by sexual hormones, inducing sexual dimorphism in adult behavioural responses. The principal brain area responsible for the control of the receptive component of female sexual behaviour is the ventrolateral division of the ventromedial nucleus of the hypothalamus (VMHvl), which is known for its dependency on ovarian hormones. Inputs to the VMHvl originating from the medial preoptic nucleus (MPN) are responsible for conveying essential information that will trigger such behaviour. Here, we investigated the pattern of the projection of the MPN to the VMHvl in rats ovariectomized at the onset of puberty. Sprague Dawley rats were ovariectomized (OVX) at puberty and then subjected to iontophoretic injections of the neuronal anterograde tracer Phaseolus vulgaris leucoagglutinin into the MPN once they reached 90 days of age. This study analysed the connectivity pattern established between the MPN and the VMH that is involved in the neuronal circuit responsible for female sexual behaviour in control and OVX rats. The data show the changes in the organization of the connections observed in the OVX adult rats that displayed a reduced axonal length for the MPN fibres reaching the VMHvl, suggesting that peripubertal ovarian hormones are relevant to the organization of MPN connections with structures involved in the promotion of female sexual behaviour.

Stimulation of Posterior Thalamic Nuclei Induces Photophobic Behavior in Mice

OBJECTIVE

A hallmark of migraine is photophobia. In mice, photophobia-like behavior is induced by calcitonin gene-related peptide (CGRP), a neuropeptide known to be a key player in migraine. In this study, we sought to identify sites within the brain from which CGRP could induce photophobia.

DESIGN

We focused on the posterior thalamic region, which contains neurons responsive to both light and dural stimulation and has CGRP binding sites. We probed this area with both optogenetic stimulation and acute CGRP injections in wild-type mice. Since the light/dark assay has historically been used to investigate anxiety-like responses in animals, we measured anxiety in a light-independent open field assay and asked if stimulation of a brain region, the periaqueductal gray, that induces anxiety would yield similar results to posterior thalamic stimulation. The hippocampus was used as an anatomical control to ensure that light-aversive behaviors could not be induced by the stimulation of any brain region.

RESULTS

Optogenetic activation of neuronal cell bodies in the posterior thalamic nuclei elicited light aversion in both bright and dim light without an anxiety-like response in an open field assay. Injection of CGRP into the posterior thalamic region triggered similar light-aversive behavior without anxiety. In contrast to the posterior thalamic nuclei, optogenetic stimulation of dorsal periaqueductal gray cell bodies caused both light aversion and an anxiety-like response, while CGRP injection had no effect. In the dorsal hippocampus, neither optical stimulation nor CGRP injection affected light aversion or open field behaviors.

CONCLUSION

Stimulation of posterior thalamic nuclei is able to initiate light-aversive signals in mice that may be modulated by CGRP to cause photophobia in migraine.

CGRP-based Migraine Therapeutics: How Might They Work, Why So Safe, and What Next?

Migraine is a debilitating neurological condition that involves the neuropeptide calcitonin gene-related peptide (CGRP). An exciting development is the recent FDA approval of the first in an emerging class of CGRP-targeted drugs designed to prevent migraine. Yet despite this efficacy, there are some fundamental unanswered questions, such as where and how CGRP works in migraine. Preclinical data suggest that CGRP acts via both peripheral and central mechanisms. The relevance of peripheral sites is highlighted by the clinical efficacy of CGRP-blocking antibodies, even though they do not appreciably cross the blood-brain barrier. The most likely sites of action are within the dura and trigeminal ganglia. Furthermore, it would be foolish to ignore perivascular actions in the dura since CGRP is the most potent vasodilatory peptide. Ultimately, the consequence of blocking CGRP or its receptor is reduced peripheral neural sensitization. Underlying their efficacy is the question of why the antibodies have such an excellent safety profile so far. This may be due to the presence of a second CGRP receptor and vesicular release of a large bolus of peptides. Finally, despite the promise of these drugs, there are unmet gaps because they do not work for all patients; so what next? We can expect advances on several fronts, including CGRP receptor structures that may help development of centrally-acting antagonists, combinatorial treatments that integrate other therapies, and development of drugs that target other neuropeptides. This is truly an exciting time for CGRP and the migraine field with many more discoveries on the horizon.

Thalamic integration of social stimuli regulating parental behavior and the oxytocin system

Critically important components of the maternal neural circuit in the preoptic area robustly activated by suckling were recently identified. In turn, suckling also contributes to hormonal adaptations to motherhood, which includes oxytocin release and consequent milk ejection. Other reproductive or social stimuli can also trigger the release of oxytocin centrally, influencing parental or social behaviors. However, the neuronal pathways that transfer suckling and other somatosensory stimuli to the preoptic area and oxytocin neurons have been poorly characterized. Recently, a relay center of suckling was determined and characterized in the posterior intralaminar complex of the thalamus (PIL). Its neurons containing tuberoinfundibular peptide 39 project to both the preoptic area and oxytocin neurons in the hypothalamus. The present review argues that the PIL is a major relay nucleus conveying somatosensory information supporting maternal behavior and oxytocin release in mothers, and may be involved more generally in social cue evoked oxytocin release, too.

A Thalamo-Hypothalamic Pathway That Activates Oxytocin Neurons in Social Contexts in Female Rats

Oxytocin is released from neurons in the paraventricular hypothalamic nucleus (PVN) in mothers upon suckling and during adult social interactions. However, neuronal pathways that activate oxytocin neurons in social contexts are not yet established. Neurons in the posterior intralaminar complex of the thalamus (PIL), which contain tuberoinfundibular peptide 39 (TIP39) and are activated by pup exposure in lactating mothers, provide a candidate projection. Innervation of oxytocin neurons by TIP39 neurons was examined by double labeling in combination with electron microscopy and retrograde tract-tracing. Potential classic neurotransmitters in TIP39 neurons were investigated by in situ hybridization histochemistry. Neurons activated after encounter with a familiar conspecific female in a familiar environment were mapped with the c-Fos technique. PVN and the supraoptic nucleus oxytocin neurons were closely apposed by an average of 2.0 and 0.4 TIP39 terminals, respectively. Asymmetric (presumed excitatory) synapses were found between TIP39 terminals and cell bodies of oxytocin neurons. In lactating rats, PIL TIP39 neurons were retrogradely labeled from the PVN. TIP39 neurons expressed vesicular glutamate transporter 2 but not glutamic acid decarboxylase 67. PIL contained a markedly increased number of c-Fos-positive neurons in response to social encounter with a familiar conspecific female. Furthermore, the PIL received ascending input from the spinal cord and the inferior colliculus. Thus, TIP39 neurons in the PIL may receive sensory input in response to social interactions and project to the PVN to innervate and excite oxytocin neurons, suggesting that the PIL-PVN projection contributes to the activation of oxytocin neurons in social contexts.

Morphology and connectivity of parabrachial and cortical inputs to gustatory thalamus in rats

The ventroposterior medialis parvocellularis (VPMpc) nucleus of the thalamus, the thalamic relay nucleus for gustatory sensation, receives primary input from the parabrachial nucleus, and projects to the insular cortex. To reveal the unique properties of the gustatory thalamus in comparison with archetypical sensory relay nuclei, this study examines the morphology of synaptic circuitry in the VPMpc, focusing on parabrachiothalamic driver input and corticothalamic feedback. Anterogradely visualized parabrachiothalamic fibers in the VPMpc bear large swellings. At electron microscope resolution, parabrachiothalamic axons are myelinated and make large boutons, forming multiple asymmetric, adherent, and perforated synapses onto large-caliber dendrites and dendrite initial segments. Labeled boutons contain dense-core vesicles, and they resemble a population of terminals within the VPMpc containing calcitonin gene-related peptide. As is typical of primary inputs to other thalamic nuclei, parabrachiothalamic terminals are over five times larger than other inputs, while constituting only 2% of all synapses. Glomeruli and triadic arrangements, characteristic features of other sensory thalamic nuclei, are not encountered. As revealed by anterograde tracer injections into the insular cortex, corticothalamic projections in the VPMpc form a dense network of fine fibers bearing small boutons. Corticothalamic terminals within the VPMpc were also observed to synapse on cells that were retrogradely filled from the same injections. The results constitute an initial survey describing unique anatomical properties of the rodent gustatory thalamus.

Calcitonin gene-related peptide in migraine: intersection of peripheral inflammation and central modulation

Over the past two decades, a convergence of basic and clinical evidence has established the neuropeptide calcitonin-gene-related peptide (CGRP) as a key player in migraine. Although CGRP is a recognised neuromodulator of nociception, its mechanism of action in migraine remains elusive. In this review, we present evidence that led us to propose that CGRP is well poised to enhance neurotransmission in migraine by both peripheral and central mechanisms. In the periphery, it is thought that local release of CGRP from the nerve endings of meningeal nociceptors following their initial activation by cortical spreading depression is critical for the induction of vasodilation, plasma protein extravasation, neurogenic inflammation and the consequential sensitisation of meningeal nociceptors. Mechanistically, we propose that CGRP release can give rise to a positive-feedback loop involved in localised increased synthesis and release of CGRP from neurons and a CGRP-like peptide called procalcitonin from trigeminal ganglion glia. Within the brain, the wide distribution of CGRP and CGRP receptors provides numerous possible targets for CGRP to act as a neuromodulator.

Preferential networks of the mediodorsal nucleus and centromedian-parafascicular complex of the thalamus--a DTI tractography study

Distinct thalamic nuclei, like the mediodorsal (MD) nucleus and the centromedian/parafascicular complex (CM/Pf), are embedded in different basal ganglia-thalamocortical loops, which were shown to integrate cognitive and emotional aspects of human behavior. Despite well described connections on a microscopic scale, derived from tracing studies in animals, little is known about the intrinsic anatomical connections of these nuclei in humans. This lack of knowledge limits not only interpretation of functional imaging studies but also estimation of direct effects of deep brain stimulation which treats diseases as different as epilepsy or major depression. Therefore, non-invasive diffusion tensor imaging (DTI) studies are key to analyzing connectivity patterns and elaborate approaches to close this gap. For our study, we explored the structural connectivity of the MD thalamic nuclei and the CM/Pf complex towards five cortical and six subcortical regions by using a preferential fiber calculation. We found both thalamic nuclei to be preferentially associated to distinct networks: whereas the MD is preferentially connected to prefrontal and limbic cortical regions, the CM is linked to subcortical regions. The anterior insula was the only cortical region associated with the subcortical network of the CM and the cortical network of the MD comprised one subcortical hub, the caudate nucleus, suggesting an integrative role of these two regions. Adding to predescribed anatomical tract tracing connectivities in animal studies, our finding lends support to the existence of similar basal ganglia-thalamocortical circuits in humans and we could show a robust distinction of preferential connectivity for both thalamic nuclei.

Somatic genital reflexes in rats with a nod to humans: anatomy, physiology, and the role of the social neuropeptides

Somatic genital reflexes such as ejaculation and vaginocervical contractions are produced through the striated muscles associated with the genitalia. The coordination of these reflexes is surprisingly complex and involves a number of lumbosacral spinal and supraspinal systems. The rat model has been proven to be an excellent source of information regarding these mechanisms, and many parallels to research in humans can be drawn. An understanding of the spinal systems involving the lumbosacral spinal cord, both efferent and afferent, has been generated through decades of research. Spinal and supraspinal mechanisms of descending excitation, through a spinal ejaculation generator in the lumbar spinal cord and thalamus, and descending inhibition, through the ventrolateral medulla, have been identified and characterized both anatomically and physiologically. In addition, delineation of the neural circuits whereby ascending genitosensory information regarding the regulation of somatic genital reflexes is relayed supraspinally has also been the topic of recent investigation. Lastly, the importance of the "social neuropeptides" oxytocin and vasopressin in the regulation of somatic genital reflexes, and associated sociosexual behaviors, is emerging. This work not only has implications for understanding how nervous systems generate sexual behavior but also provides treatment targets for sexual dysfunction in people.

High field FMRI reveals thalamocortical integration of segregated cognitive and emotional processing in mediodorsal and intralaminar thalamic nuclei

Thalamocortical loops, connecting functionally segregated, higher order cortical regions, and basal ganglia, have been proposed not only for well described motor and sensory regions, but also for limbic and prefrontal areas relevant for affective and cognitive processes. These functions are, however, more specific to humans, rendering most invasive neuroanatomical approaches impossible and interspecies translations difficult. In contrast, non-invasive imaging of functional neuroanatomy using fMRI allows for the development of elaborate task paradigms capable of testing the specific functionalities proposed for these circuits. Until recently, spatial resolution largely limited the anatomical definition of functional clusters at the level of distinct thalamic nuclei. Since their anatomical distinction seems crucial not only for the segregation of cognitive and limbic loops but also for the detection of their functional interaction during cognitive–emotional integration, we applied high resolution fMRI on 7 Tesla. Using an event-related design, we could isolate thalamic effects for preceding attention as well as experience of erotic stimuli. We could demonstrate specific thalamic effects of general emotional arousal in mediodorsal nucleus and effects specific to preceding attention and expectancy in intralaminar centromedian/parafascicular complex. These thalamic effects were paralleled by specific coactivations in the head of caudate nucleus as well as segregated portions of rostral or caudal cingulate cortex and anterior insula supporting distinct thalamo–striato–cortical loops. In addition to predescribed effects of sexual arousal in hypothalamus and ventral striatum, high resolution fMRI could extent this network to paraventricular thalamus encompassing laterodorsal and parataenial nuclei. We could lend evidence to segregated subcortical loops which integrate cognitive and emotional aspects of basic human behavior such as sexual processing.

A direct neuronal connection between the subparafascicular and ventrolateral arcuate nuclei in non-lactating female rats. Could this pathway play a role in the suckling-induced prolactin release?

The neuronal pathways, through which prolactin secretion is regulated during lactation, have still not been fully explored. Studies indicate that the suckling stimulus travels through the spinal cord, the brain stem, and then reaches the hypothalamus. The focus of this present experiment is to further explore the neuronal connections between the brain stem and the arcuate nucleus that may be involved in suckling-induced prolactin release. Ante- and retrograde tracing techniques were used. To chemically characterize the explored neurons neuropeptide immunohistochemistry was applied. Previous studies have indicated that the peripeduncular nucleus is a relay of the suckling stimulus in the midbrain, conveying the information to the hypothalamus. In our experiments, we have found an additional cell group in the subparafascicular parvocellular nucleus located just behind the posterior thalamus that projects to the arcuate neurons. The injection of the retrograde tracer into the ventrolateral part of the arcuate nucleus labeled cells in the lateral subdivision of the subparafascicular parvocellular nucleus. Anterograde tracing from the subparafascicular parvocellular nucleus resulted in fiber labeling in the arcuate nucleus in close apposition with dynorphin immunopositive neurons. Double labeling revealed that a subpopulations of the subparafascicular parvocellular neurons projecting to the arcuate nucleus contained tuberoinfundibular peptide of 39 residues or calcitonin gene-related peptide. The presented findings suggest that the ascending fibers from the subparafascicular parvocellular nucleus might be in the pathway involved in the suckling-induced prolactin release.

The TIP39-PTH2 receptor system: unique peptidergic cell groups in the brainstem and their interactions with central regulatory mechanisms

Tuberoinfundibular peptide of 39 residues (TIP39) is the recently purified endogenous ligand of the previously orphan G-protein coupled parathyroid hormone 2 receptor (PTH2R). The TIP39-PTH2R system is a unique neuropeptide-receptor system whose localization and functions in the central nervous system are different from any other neuropeptides. TIP39 is expressed in two brain regions, the subparafascicular area in the posterior thalamus, and the medial paralemniscal nucleus in the lateral pons. Subparafascicular TIP39 neurons seem to divide into a medial and a lateral cell population in the periventricular gray of the thalamus, and in the posterior intralaminar complex of the thalamus, respectively. Periventricular thalamic TIP39 neurons project mostly to limbic brain regions, the posterior intralaminar thalamic TIP39 neurons to neuroendocrine brain areas, and the medial paralemniscal TIP39 neurons to auditory and other brainstem regions, and the spinal cord. The widely distributed axon terminals of TIP39 neurons have a similar distribution as the PTH2R-containing neurons, and their fibers, providing the anatomical basis of a neuromodulatory action of TIP39. Initial functional studies implicated the TIP39-PTH2R system in nociceptive information processing in the spinal cord, in the regulation of different hypophysiotropic neurons in the hypothalamus, and in the modulation of affective behaviors. Recently developed novel experimental tools including mice with targeted mutations of the TIP39-PTH2R system and specific antagonists of the PTH2R will further facilitate the identification of the specific roles of TIP39 and the PTH2R.

Neurons of the dopaminergic/calcitonin gene-related peptide A11 cell group modulate neuronal firing in the trigeminocervical complex: an electrophysiological and immunohistochemical study

Activation of spinal trigeminal afferents innervating the cranial vasculature is likely to play a role in migraine, although some parts of the clinical presentation may have a dopaminergic basis. The A11 nucleus, located in the posterior hypothalamus, provides the only known source of descending dopaminergic innervation for the spinal gray matter. Extracellular recordings were made in the trigeminocervical complex (TCC) in response to electrical stimulation of the dura mater. Receptive fields were characterized by mechanical noxious and innocuous stimulation of the ipsilateral ophthalmic dermatome. Stimulation of the A11 significantly inhibited peri-middle meningeal artery dural and noxious pinch evoked firing of neurons in the TCC. This inhibition was reversed by the D(2) receptor antagonist eticlopride. Lesioning of the A11 significantly facilitated dural and noxious pinch and innocuous brush evoked firing from the TCC. In previous work using immunohistofluorescence, it was shown that D(1) and D(2) receptors were found in the rat TCC, and here we report, in addition, that D(4) and D(5) dopamine receptors are also present, whereas D(3) receptors are not. No dopamine receptors were present in the A11 nucleus itself. However, the A11 does contain dopamine and calcitonin gene-related peptide (CGRP) and, by this combination, is distinct from the neighboring CGRPergic subparafascicular nucleus. Exploration of dopaminergic influences and mechanisms in migraine may open up an almost untapped opportunity to pursue potential new therapeutic options for the disorder.

Neural regulation of ejaculation

INTRODUCTION

Delineation of the underlying neurophysiological mechanisms of ejaculatory behavior is crucial for the treatment of male sexual dysfunction, including premature ejaculation. Recent studies provide compelling evidence that a population of lumbar spinothalamic (LSt) cells may play a role in the regulation of the ejaculatory response. Subsequent to ejaculation, LSt cells exhibit markers of activation that are not only highly correlated with ejaculatory behavior, but are also absent following the expression of other components of sexual behavior, such as mounts or intromissions. Similarly, targeted chemical lesion of LSt cells using substance P-saporin abolishes ejaculatory behavior explicitly. Early evidence suggests that pharmacological manipulation of LSt cells may offer additional evidence of crucial LSt cell involvement in the generation of ejaculation.

AIM

This review is intended to summarize what has currently been revealed regarding the role of LSt cells in the regulation and generation of ejaculatory behavior, and also to discuss the direction of future behavioral investigations.

METHODS

Information presented in this discussion was derived from analysis of numerous recent articles detailing the delineation of anatomical and physiological correlates of sexual behavior, as well as numerous literature searches using the National Library of Medicine PubMed Services.

RESULTS

A great deal of the work that has led to the implication of LSt cells in ejaculatory behavior is reviewed in the present article, including clinical data, as well as anatomical, physiological, and behavioral examinations. The rationale for ongoing pharmacological studies is also discussed.

CONCLUSION

LSt cells appear to play a vital role in the generation and regulation of ejaculatory behavior. Additional elucidation of this "ejaculation generator" could prove invaluable for the future treatment of male sexual dysfunction. Studies are currently in progress to further reveal the precise function of these cells and mechanisms of action through which they operate.

Sexual reward in male rats: effects of sexual experience on conditioned place preferences associated with ejaculation and intromissions

Various behavioral models and studies have provided evidence suggesting that male rat sexual behavior has rewarding and reinforcing properties. However, there is little information regarding the rewarding values of the different components of sexual behavior. Therefore, this study used a conditioned place preference (CPP) paradigm to address whether ejaculation and intromissions differ in their rewarding incentive values. We also addressed whether the differential rewarding values were dependent on prior sexual experience. Sexually naïve and experienced males received one pairing of either intromissions or ejaculation with one of the chambers in the CPP box. The amount of time spent in each chamber of the CPP apparatus after conditioning was then measured. Both sexually naïve and sexually experienced males formed a CPP for ejaculation, while only sexually naïve, and not sexually experienced, males formed a CPP for intromissions. Moreover, in sexually naïve males, multiple pairings of ejaculation with the designated chamber resulted in a CPP relative to the control chamber paired with display of intromissions. These data support the hypothesis that there is a hierarchy of rewarding sexual behavior, with ejaculation being the most rewarding component, and that the rewarding incentive value of other components of sexual behavior is dependent upon prior sexual experience.

Tuberoinfundibular peptide of 39 residues in the embryonic and early postnatal rat brain

Tuberoinfundibular peptide of 39 residues (TIP39) was identified as the endogenous ligand of parathyroid hormone 2 receptor. We have recently demonstrated that TIP39 expression in adult rat brain is confined to the subparafascicular area of the thalamus with a few cells extending laterally into the posterior intralaminar thalamic nucleus (PIL), and the medial paralemniscal nucleus (MPL) in the lateral pontomesencephalic tegmentum. During postnatal development, TIP39 expression increases until postnatal day 33 (PND-33), then decreases, and almost completely disappears by PND-125. Here, we report the expression of TIP39 during early brain development. TIP39-immunoreactive (TIP39-ir) neurons in the subparafascicular area first appeared at PND-1. In contrast, TIP39-ir neurons were detectable in the MPL at embryonic day 14.5 (ED-14.5), and the intensity of their labeling increased thereafter. We also identified TIP39-ir neurons between ED-16.5 and PND-5 in two additional brain areas, the PIL and the amygdalo-hippocampal transitional zone (AHi). We confirmed the specificity of TIP39 immunolabeling by demonstrating TIP39 mRNA using in situ hybridization histochemistry. In the PIL, TIP39 neurons are located medial to the CGRP group as demonstrated by double immunolabeling. All TIP39-ir neurons in the AHi and most TIP39-ir neurons in the PIL disappear during early postnatal development. The adult pattern of TIP39-ir fibers emerge during postnatal development. However, fibers emanating from PIL can be followed in the supraoptic decussations towards the hypothalamus at ED-18.5. These TIP39-ir fibers disappear by PND-1. The complex pattern of TIP39 expression during early brain development suggests the involvement of TIP39 in transient functions during ontogeny.

Topography of thalamic and parabrachial calcitonin gene-related peptide (CGRP) immunoreactive neurons projecting to subnuclei of the amygdala and extended amygdala

Injections of calcitonin gene-related peptide (CGRP) into the amygdala evoke fear-related behaviors and antinociceptive effects. In the present study we therefore characterized CGRP-containing amygdaloid afferents by injecting the retrograde tracer FluoroGold (FG) into subnuclei of the amygdala and adjacent divisions of the extended amygdala, namely, the lateral (LA) and central (CE) amygdaloid nuclei, interstitial nucleus of the posterior limb of the anterior commissure (IPAC), and the amygdalostriatal area (AStr). The distribution of retrogradely FG-labeled neurons and colocalization of CGRP-immunoreactivity with FG-labeling were mapped in the posterior paralaminar thalamic complex and parabrachial nuclei. The analysis of the posterior thalamus revealed that about 50% of CGRP-containing neurons projected to the AStr, the projections originating in the medial part of the medial geniculate body, posterior intralaminar nucleus, parvicellular subparafascicular nucleus, and peripeduncular nucleus. However, the percentage of CGRP-containing thalamic neurons projecting to the adjacent LA, medial part of the CE, and ventrocaudal part of the caudatoputamen rapidly dropped to 3-9%. There were no double-labeled cells after injections into the lateral and capsular parts of the CE and the IPAC. Thus, the AStr received the heaviest CGRP-containing projection from the posterior thalamus. CGRP-containing parabrachial neurons projected to the AStr and lateral, capsular, and medial parts of the CE, the projections originating in the external, crescent, and central parts of the lateral parabrachial nucleus and external part of the medial parabrachial nucleus. The results demonstrate a distinct projection pattern of CGRP-containing thalamic and parabrachial neurons to subnuclei of the amygdala and extended amygdala.

Effects of acute and chronic apomorphine on sex behavior and copulation-induced neural activation in the male rat

Apomorphine is a non-selective dopaminergic receptor agonist. Because of its pro-erectile effects, apomorphine is clinically used for treatment of erectile dysfunction. We investigated the effects of subcutaneous apomorphine administration (0.4 mg/kg rat) on sexual behavior and mating-induced Fos-expression following acute (day 1) or chronic apomorphine treatment (days 8 and 15) in sexually experienced male rats. Consistent facilitatory effects of apomorphine were observed in the reduced numbers of mounts and intromissions over time and an increased ejaculation frequency on day 1. The first post-ejaculatory interval, however, was lengthened, while other behavioral parameters were unaffected. Fos-immunoreactivity induced by acute apomorphine administration (barrel cortex, paraventricular hypothalamic nucleus, central amygdala and locus coeruleus) was strongly reduced after chronic administration. After mating, induction of Fos-immunoreactivity was observed in well-known areas like medial preoptic nucleus and the posterodorsal medial amygdaloid area. Apomorphine, however, reduced mating-induced Fos-immunoreactivity in the nucleus accumbens shell and prevented its occurrence in its core area. This remarkable apomorphine effect was not observed in any other brain area. We conclude that the behavioral (pro-erectile) effects of apomorphine are consistent over time, and that the diminished accumbens-Fos-immunoreactivity and the elongated post-ejaculatory interval may reflect a decreased response to remote cues from the estrus female.

Brain activation during human male ejaculation revisited

In a prior [O]-H2O positron emission tomographic study we reported brain regions involved in human male ejaculation. Here, we used another, more recently acquired data set to evaluate the methodological approach of this previous study, and discovered that part of the reported activation pattern was not related to ejaculation. With a new analysis of these ejaculation data, we now demonstrate ejaculation-related activations in the deep cerebellar nuclei (dentate nucleus), anterior vermis, pons, and ventrolateral thalamus, and, most importantly, ejaculation-related deactivations throughout the prefrontal cortex. This revision offers a new and more accurate insight into the brain regions involved in human male ejaculation.

Neurons containing tuberoinfundibular peptide of 39 residues are activated following male sexual behavior

Tuberoinfundibular peptide of 39 residues (TIP39)-immunoreactive (IR) neurons are present in the medial subdivision of the parvocellular subparafascicular thalamic nucleus (mSPFp) where ejaculation-specific Fos expression is localized. The mSPFp is reciprocally connected to the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BNST) and the medial nucleus of the amygdala (Me), all of which are critical for the regulation of male sexual behavior. The mSPFp also receives galanin and enkephalin containing projections from a region in the lumbar spinal cord, thought to be a central ejaculation center. Therefore, we hypothesized that TIP39 neurons in the mSPFp may be part of the neuronal circuitry activated by male sexual behavior. To test this hypothesis, we examined induction of Fos in TIP39 containing neurons in the mSPFp following male sexual behavior. Mating-induced Fos expression was evaluated in sexually experienced male rats under four experimental conditions: animals that (1) remained in their home cage without any interaction with females, (2) interacted with stimulus females and displayed intromission without ejaculation, (3) displayed one ejaculation, or (4) displayed 2 ejaculations. We found that Fos was induced in TIP39-IR neurons in the mSPFp in male rats following ejaculation but much less so following intromission without ejaculation. This suggests that TIP39-IR neurons in the mSPFp are part of the afferent circuits that process genital-somatosensory information related to ejaculation, and which contribute to mating and mating-induced changes in reproductive behavior.

Substance P and neurokinin-1 immunoreactivities in the neural circadian system of the Alaskan northern red-backed vole, Clethrionomys rutilus

The suprachiasmatic nucleus (SCN) of the hypothalamus houses the main mammalian circadian clock. This clock is reset by light-dark cues and stimuli that evoke arousal. Photic information is relayed directly to the SCN via the retinohypothalamic tract (RHT) and indirectly via the geniculohypothalamic tract, which originates from retinally innervated cells of the thalamic intergeniculate leaflet (IGL). In addition, pathways from the dorsal and median raphe (DR and MR) convey arousal state information to the IGL and SCN, respectively. The SCN regulates many physiological events in the body via a network of efferent connections to areas of the brain such as the habenula (Hb) in the epithalamus, subparaventricular zone (SPVZ) of the hypothalamus and locus coeruleus of the brainstem-areas of the brain associated with arousal and behavioral activation. Substance P (SP) and the neurokinin-1 (NK-1) receptor are present in the rat SCN and IGL, and SP acting via the NK-1 receptor alters SCN neuronal activity and resets the circadian clock in this species. However, the distribution and role of SP and NK-1 in the circadian system of other rodent species are largely unknown. Here we use immunohistochemical techniques to map the novel distribution of SP and NK-1 in the hypothalamus, thalamus and brainstem of the Alaskan northern red-backed vole, Clethrionomys rutilus, a species of rodent currently being used in circadian biology research. Interestingly, the pattern of immunoreactivity for SP in the red-backed vole SCN was very different from that seen in many other nocturnal and diurnal rodents.

Contact-call driven and tone-driven zenk expression in the nucleus ovoidalis of the budgerigar (Melopsittacus undulatus)

The effectiveness of species-typical contact calls and a 3-kHz pure tone to induce zenk gene protein expression in the primary thalamic auditory relay nucleus ovoidalis was compared in budgerigars (Melopsittacus undulatus), a parrot species capable of lifelong vocal learning. Ovoidalis consists of a core which projects topographically to field L of the telencephalon and a ventromedial shell containing many calcitonin-gene-related peptide neurons that project throughout field L as well as to an adjacent field receiving visual input. Tone-induced and call-induced zenk expression in the ovoidalis core were similar; however, call-induced zenk expression in ventromedial ovoidalis shell was significantly greater than tone-induced expression. These results support the idea that the ovoidalis shell may contain neurons specialized to process complex sounds including species-typical communication sounds.

Androgens regulate the sexually dimorphic production of co-contained galanin and cholecystokinin in lumbar laminae VII and X neurons

A population of rat lumbar laminae VII and X putative spinothalamic (STT) neurons that co-contain cholecystokinin-8 (CCK) and galanin (GAL) are sexually dimorphic. Males have a significantly greater number of these neurons, as well as having greater optical densities for both neuropeptides than females. Optical densities for GAL and CCK immunoreactivities in these lumbar neurons in rats that have the testicular feminization mutation (Tfm) are not significantly different from females; however, the number of these lumbar neurons in Tfm rats is significantly smaller than in females. These data suggest that androgens, as well as functional androgen receptors (that Tfm rats lack), are necessary for the establishment of these sexual dimorphisms. Functionally, these CCK- and GAL-containing neurons in the deep lumbar laminae may contribute to the establishment of known sex differences in the affective component of somatic and visceral nociception, as well as the sexually dimorphic nature of some pelvic diseases, e.g., irritable bowel syndrome or cystitis.

The role of the dorsolateral tegmentum in the control of male sexual behavior: A reevaluation

The medial preoptic area/anterior hypothalamus (MPOA/AH) plays a key role in the control of male sexual behavior. Independently of the type, MPOA/AH lesions permanently eliminate male sexual behavior in the rat. The MPOA/AH projects among other structures to the dorsolateral tegmentum (DLT). Bilateral electrolytic lesions of the DLT or the unilateral electrolytic destruction of the MPOA/HA combined with a contralateral electrolytic lesion of the DLT eliminate male sexual behavior. In the present experiment, we evaluated if neurotoxic lesions of the DLT produce the same behavioral deficits as those observed after electrolytic lesions. This would allow us to evaluate if neurons of the DLT or the fibers passing through this area are important in the control of male sexual behavior. To this aim, sexually experience male rats were tested for socio-sexual behavior, partner preference and motor execution in order to determine if the possible behavioral changes could be attributed to alterations in sexual motivation or motor execution. One week after the bilateral DLT lesions the animals were evaluated in the same behavioral tests. The lesions were identified by glial fibrillary acidic protein (GFAP) and neuronal nuclear protein (Neu-N) immunohistochemistry. No significant consistent effects upon sexual behavior were observed in any of the groups, including the group with clear bilateral damage of the DLT. A reduction in the percentage of males displaying ejaculation in the first post-lesion test was observed for all groups injected with quinolinic acid. No effects upon partner preference or motor coordination were observed after the lesion in any of the groups. The lack of effect of DLT neurotoxic lesions upon mating suggests that neurons of this structure are not involved in the control of male sexual behavior.

Afferent connections of the subparafascicular area in rat

The subparafascicular nucleus and the subparafascicular area are the major sites of synthesis of the recently discovered neuropeptide, tuberoinfundibular peptide of 39 residues (TIP39). Better knowledge of the neuronal inputs to the subparafascicular area and nucleus will facilitate investigation of the functions of TIP39. Thus, we have injected the retrograde tracer cholera toxin B subunit into the rostral, middle, and caudal parts of the rat subparafascicular nucleus. We report that the afferent projections to the subparafascicular nucleus and area include the medial prefrontal, insular, and ectorhinal cortex, the subiculum, the lateral septum, the anterior amygdaloid area, the medial amygdaloid nucleus, the caudal paralaminar area of the thalamus, the lateral preoptic area, the anterior, ventromedial, and posterior hypothalamic nuclei, the dorsal premamillary nucleus, the zona incerta and Forel's fields, the periaqueductal gray, the deep layers of the superior colliculus, cortical layers of the inferior colliculus, the cuneiform nucleus, the medial paralemniscal nucleus, and the parabrachial nuclei. Most of these regions project to all parts of the subparafascicular nucleus. However, the magnocellular subparafascicular neurons, which occupy the middle part of the subparafascicular nucleus, may not receive projections from the medial prefrontal and insular cortex, the medial amygdaloid nucleus, the lateral preoptic area, and the parabrachial nuclei. In addition, double labeling of cholera toxin B subunit and TIP39 revealed a remarkable similarity between input regions of the subparafascicular area and the brain TIP39 system. Neurons within regions that contain TIP39 cell bodies as well as regions that contain TIP39 fibers project to the subparafascicular area. Overall, the afferent connections of the subparafascicular nucleus and area suggest its involvement in central reproductive, visceral, nociceptive, and auditory regulation.

Do similar neural systems subserve aggressive and sexual behaviour in male rats? Insights from c-Fos and pharmacological studies

It is a common belief that male aggressive and sexual behaviour share many of the underlying neurobiological, neurological, pharmacological and neuroendocrine mechanisms. Therefore, we studied brain activation patterns in male rat after performance of aggressive and sexual behaviour and compared serotonergic pharmacology in the same paradigms to delineate possible similarities and differences. Patterns of Fos-immunoreactivity induced by aggressive and sexual encounters of Wild-type male Brown Norway rats were studied to localise the commonly activated (functionally shared) parts of the circuitry, and the specific (functionally different) parts of the neuronal circuitry. Some brain areas (caudal medial preoptic area and medial amygdala) were commonly activated, but other areas (e.g. posterodorsal parts of the medial amygdala, rostral preoptic and premammillary hypothalamus) showed remarkably specific differences in neural activation. 5-HT(1A) receptor agonists inhibit aggressive, but stimulate male sexual behaviour, whereas 5-HT(1B) receptor agonists inhibit both types of behaviour. Selective serotonin reuptake inhibitors share comparable inhibitory effects in aggression and sexual behaviour, although only at relatively high doses. We propose that separate hard-wired neural systems exist in the brain for aggressive and sexual behaviours, modulated via hierarchically 'higher-level' brain areas that are involved in the integration (gating) of the behavioural outcome of an organism.

Calcitonin gene-related peptide-containing pathways in the rat forebrain

The present study focuses on the topographical distribution of calcitonin gene-related peptide (CGRP)-containing cell bodies and fibers and their connections and pathways in the rat forebrain. We confirm previously reported CGRP projections from the perifornical area of the hypothalamus to the lateral septum, from the posterior thalamus to the caudate putamen and cerebral cortex, and from the parabrachial nuclei to the central extended amygdala, lateral hypothalamus, and ventromedial thalamus. Despite previous descriptions of CGRP in the central nervous system, important neuroanatomical aspects of the forebrain CGRP system remained obscure, which we addressed by using brain lesion techniques combined with modern immunohistology. We first report CGRP terminal fields in the olfactory-anterior septal region and also CGRP projections from the parabrachial nuclei to the olfactory-anterior septal region, the medial prefrontal cortex, the interstitial nucleus of the anterior commissure, the nucleus of the lateral olfactory tract, the anterior amygdaloid area, the posterolateral cortical amygdaloid nucleus, and the dorsolateral part of the lateral amygdaloid nucleus. In addition, we identified a CGRP cell group in the premamillary nuclei and showed that it projects to the medial CGRP layer of the lateral septum. CGRP fibers usually join other pathways rather than forming bundles. They run along the fornix from the hypothalamus, along the supraoptic decussations or the inferior thalamic peduncle-stria terminalis pathway from the posterior thalamus, and along the superior cerebellar peduncle, thalamic fasciculus, and ansa peduncularis from the parabrachial nuclei. This description of the forebrain CGRP system will facilitate investigation of its role in higher brain functions.

Neural control of ejaculation

Ejaculation is the most reinforcing component of sexual behavior. However, the neural substrates mediating ejaculation and processing ejaculation-related signals remain poorly understood. We review the current understanding of central control of ejaculation. Specifically, the recent identification of a candidate spinothalamic pathway involved in relay of ejaculation-specific signals is discussed. In addition, the discovery of a neural population of lumbar interneurons playing an pivotal role in expression of ejaculation is reviewed.

Thalamus and penile erection

Penile erection is a complex neurovascular event. The neuronal system involved is often divided into a spinal (generator) and supraspinal (controller) network. Little is known about the supraspinal control. The recent finding of changes in penile erection following deep brain stimulation of the thalamus in two patients has raised the question as to what extent the thalamus is involved in erectile function. The thalamus has generally been regarded as a group of relay nuclei that served as a 'gate' for sexual information from the spinal cord towards higher centres. Recent evidence, however, suggests a more integrated regulatory function. Our review of the literature from 1960 until 2003 revealed 13 reports describing original data (preclinical and clinical). Various thalamic regions, varying from the midline thalamus to the posterior thalamus, have been reported to be activated during erection. The majority of the reports, however, showed that mainly the mediodorsal (MD) nucleus and the centromedian-parafascicular nucleus (Cm-Pf complex) are involved in penile erection. MD is the second largest nuclear aggregation located within the medial part of the thalamus. Anatomically, the MD is closely related to the Cm-Pf complex. The Cm-Pf complex is one of the most important relay stations in which the anterolateral spinothalamic pathway is further processed. This pathway is thought to transmit peripheral sexual sensations. On the whole, the present data on the role of the thalamus in erection are far from complete and future experiments are required to delineate its involvement.

Central regulation of ejaculation

Ejaculation is a reflex mediated by a spinal control center, referred to as a spinal ejaculation generator. This spinal ejaculation generator coordinates sympathetic, parasympathetic and motor outflow to induce the two phases of ejaculation, i.e., emission and expulsion. In addition, the spinal ejaculation generator integrates this outflow with inputs that are related to the summation of sexual activity prior to ejaculation that are required to trigger ejaculation. Recently, a group of spinothalamic neurons in the lumbar spinal cord (LSt cells) were demonstrated to comprise an integral part of the spinal ejaculation generator. Specifically, lesions of LSt cells completely ablate ejaculatory function. Moreover, LSt cells are activated following ejaculation, but not following other components of sexual behavior. Furthermore, based on their relationship with autonomic nuclei, motoneurons and genital sensory inputs, LSt cells are also in the ideal anatomical position to integrate sensory inputs and autonomic and motor outflow. Additionally, the spinal ejaculation generator is under inhibitory and excitatory influence of supraspinal sites, including the nucleus paragigantocellularis (nPGi), the paraventricular nucleus of the hypothalamus (PVN) and the medial preoptic area (MPOA). Finally, sensory information related to ejaculation is processed in the spinal cord and brain, possibly contributing to the rewarding properties of ejaculation. One candidate pathway for relay of ejaculation-related cues consists of LSt cells and their projections to the parvocellular subparafascicular thalamic nucleus. Moreover, neural activation specifically related to ejaculation is observed in the brain and may reflect of processing of ejaculation-related sensory cues.

Projections from bed nuclei of the stria terminalis, posterior division: implications for cerebral hemisphere regulation of defensive and reproductive behaviors

The posterior division of the bed nuclei of the stria terminalis has three major nuclei: principal, interfascicular, and transverse, which receive topographically ordered inputs from the medial amygdalar nucleus. The overall pattern of axonal projections from each nucleus was determined in male rats with the Phaseolus vulgaris-leucoagglutinin method. Together, these nuclei project topographically back to the medial amygdalar nucleus, to the adjacent lateral septal nucleus, to the nucleus accumbens and substantia innominata, to hypothalamic parts of the behavior control column, and to the hypothalamic periventricular region, which controls patterned neuroendocrine and autonomic responses. The principal nucleus preferentially innervates septal and hypothalamic regions that control reproductive behavior and visceromotor responses, confirming a similar analysis by Gu et al. (J Comp Neurol [2003] 460:542-562). In contrast, the interfascicular and transverse nuclei differentially innervate septal and hypothalamic regions that control defensive as well as reproductive behaviors. In addition, the transverse nucleus projects significantly to midbrain parts of the behavior control column concerned with foraging/exploratory behavior. All three posterior division nuclei also project to thalamocortical feedback loops (by means of the nucleus reuniens and paraventricular nucleus). These structural data may be interpreted to suggest that the bed nuclei posterior division forms part (pallidal) of a corticostriatopallidal system involved in controlling two major classes of social (defensive and reproductive) behavior.

Afferent connections of the parvocellular subparafascicular thalamic nucleus in the rat: evidence for functional subdivisions

The parvocellular subparafascicular nucleus of the thalamus (SPFp) consists of separate subdivisions, i.e., a medial portion containing galanin-immunoreactive (-IR) axons and a lateral portion containing calcitonin gene related peptide (CGRP)-IR neurons and fibers. These subdivisions appear to have distinct functional roles. In particular, ejaculation-induced Fos expression is expressed in the medial SPFp. Hence, it was hypothesized that medial SPFp is involved in relay of copulation-related information. In contrast, lateral SPFp is involved in the processing of auditory and visual signals involved in fear-conditioned responses. Here we tested the hypothesis that medial and lateral subdivisions of SPFp receive different sets of afferents and that these differences contribute to the separate functional roles of the two subdivisions. Inputs to medial and lateral SPFp were identified following injections of FG restricted to either division in male rats. The medial SPFp receives unique inputs from lumbar spinothalamic cells and brain regions involved in processing of visceral stimuli, supporting the hypothesis that the medial SPFp is involved in the relay of genitosensory information critical for the expression of male sexual behavior. The afferents of the lateral SPFp include brain regions involved in processing of visual and auditory signals and support a role for this subdivision in relay of visual and auditory information. Thus, the two subdivisions of SPFp are anatomically and functionally distinctive.