A contextual definition of male sexual arousal

Sexual arousal is a construct without a widely shared definition. Historically, sexual arousal has usually referred to a central physiological state, but there has been much less agreement on its relation to motivation, emotion, and - for males - penile erection and ejaculation. Many behavioral and physiological measures have been used as operational definitions of sexual arousal, but the relation of the measure to arousal is often assumed rather than tested. For men, penile erection in the presence of erotic stimuli has been considered the most reliable and valid indicator of sexual arousal. The adoption of analogous criteria is recommended for research on other male mammals in order to establish a minimal basis for inferring that they are sexually aroused. That is, sexual arousal should be inferred only when penile erection is observed in a sexual context. A sexual context is provisionally defined as an environment that tends in most reproductively active males of the species to provoke further sexual stimulation, e.g., copulation or self-stimulation to ejaculation. Erection occurring outside of a sexual context, as during REM sleep or from injection of drugs, is not grounds for inferring arousal. Conversely, males engaging in behavior directed toward estrous females may be sexually motivated, but in the absence of erection, the males should not be assumed to be sexually aroused. Implications of other erection-context interactions are also considered. Adoption of these more conservative criteria for inferring sexual arousal may promote greater precision in identifying the physiological systems mediating this hypothetical construct.     

Immunohistochemical mapping of galanin-like neurons in the rat central nervous system

Using an antiserum generated in rabbits against synthetic galanin (GA) and the indirect immunofluorescence method, the distribution of GA-like immunoreactive cell bodies and nerve fibers was studied in the rat central nervous system (CNS) and a detailed stereotaxic atlas of GA-like neurons was prepared. GA-like immunoreactivity was widely distributed in the rat CNS. Appreciable numbers of GA-positive cell bodies were observed in the rostral cingulate and medial prefrontal cortex, the nucleus interstitialis striae terminalis, the caudate, medial preoptic, preoptic periventricular, and preoptic suprachiasmatic nuclei, the medial forebrain bundle, the supraoptic, the hypothalamic periventricular, the paraventricular, the arcuate, dorsomedial, perifornical, thalamic periventricular, anterior dorsal and lateral thalamic nuclei, medial and central amygdaloid nuclei, dorsal and ventral premamillary nuclei, at the base of the hypothalamus, in the central gray matter, the hippocampus, the dorsal and caudoventral raphe nuclei, the interpeduncular nucleus, the locus coeruleus, ventral parabrachial, solitarii and commissuralis nuclei, in the A1, C1 and A4 catechaolamine areas, the posterior area postrema and the trigeminal and dorsal root ganglia. Fibers were generally seen where cell bodies were observed. Very dense fiber bundles were noted in the septohypothalamic tract, the preoptic area, in the hypothalamus, the habenula and the thalamic periventricular nucleus, in the ventral hippocampus, parts of the reticular formation, in the locus coeruleus, the dorsal parabrachial area, the nucleus and tract of the spinal trigeminal area and the substantia gelatinosa, the superficial layers of the spinal cord and the posterior lobe of the pituitary. The localization of the GA-like immunoreactivity in the locus coeruleus suggests a partial coexistence with catecholaminergic neurons as well as a possible involvement of the GA-like peptide in a neuroregulatory role.     

Immunoreactive galanin-like material in magnocellular hypothalamoneurohypophysial neurones of the rat

Summary Immunoreactive galanin-like material was recently shown to co-exist with vasopressin in parvocellular and magnocellular perikarya of the paraventricular nucleus in the anterior hypothalamus of the rat (Melander et al. 1986). Since this distribution pattern differed from our observation of oxytocin-associated galanin-like immunoreactivity (LI) in the neurohypophysis, we compared in series of 0.5-m thick sections the localisation of galanin-LI with the localisation of oxytocin and vasopressin/dynorphin in the hypothalamus, the median eminence and the neurohypophysis. In the oxytocin system, galanin-LI was intense in oxytocin varicosities of the neurohypophysis. Oxytocin perikarya of the hypothalamic supraoptic and paraventricular nuclei exhibited galanin-LI only after intraventricular injection of colchicine and when sections were treated with trypsin prior to application of the antibody. In the vasopressin/dynorphin system galanin-LI was intense in hypothalamic perikarya after colchicine injection and in neurohypophysial varicosities after treatment of the sections with trypsin. In these neurones, galanin-LI was absent or weak in all elements when treatments with colchicine or trypsin were omitted. Galanin-LI in the neurohypophysis was not co-localised with the numerous fine endings showing GABA-LI. These observations indicate that galanin-like material coexists with vasopressin and oxytocin in the respective magnocellular neurones, although not always in an immunoreactive form.     

Distribution of galanin-like immunoreactivity in the pig, rat and human central nervous system

The distribution of galanin-like immunoreactivity in various regions of the central nervous system was assessed in three mammalian species, pig, rat, and human, by radioimmunoassay. Galanin concentrations were highest in the hypothalamus and pituitary region. In spinal cord, there was a rostrocaudal/dorsoventral gradient with highest levels observed in the sacral dorsal horn. Serial dilutions of porcine tissue extracts diluted parallel to the porcine standard curve, while the rat and human tissue extracts did not. In all tissues examined by high pressure liquid chromatography, the principal peak of immunoreactivity coeluted with the authentic porcine galanin standard and was decreased by trypsin cleavage. These results suggest a role for galanin in the central nervous system and support species differences in the structure of galanin.     

Subthalamic lesions eliminate sexual behavior in the male rat

Electrical stimulation of parts of the subthalamus and mesencephalon produces coordinated stepping movements, and for this reason these areas are sometimes referred to as the subthalamic and mesencephalic "locomotor" regions. In this study we contrast the sexual behavioral effect of electrolytic destruction of these two regions in the male rat. Lesions of the mesencephalic locomotor region had no significant effect on male sexual behavior. In contrast, subthalamic lesions centered on the caudal zona incerta just dorsal to the subthalamic nucleus eliminated sexual behavior in 6 of 15 males. The sexual behavior of the remaining males was affected to a lesser degree, for the most part in accord with the extent of destruction to this "critical zone." Subthalamic lesions produced no obvious impairment in locomotion, posture, limb use, muscle tone or sensorimotor orientation. Even so, the fact that electrical stimulation of the subthalamus elicits coordinated stepping suggests that the region is linked with systems directly concerned with movement and locomotion. These links could be particularly important in the process by which sexual motivation is translated into sexual behavior.     

The hypothalamic POMC mRNA expression is upregulated in prenatally undernourished male rat offspring under high-fat diet

Epidemiological studies demonstrated that adverse environmental factors leading to intrauterine growth retardation (IUGR) and low birth weight may predispose individuals to increased risk of metabolic syndrome. In rats, we previously demonstrated that adult male IUGR offspring from prenatal 70% food-restricted dams throughout gestation (FR30) were predisposed to energy balance dysfunctions such as impaired glucose intolerance, hyperleptinemia, hyperphagia and adiposity. We investigated whether postweaning moderate high-fat (HF) diet would amplify the phenotype focusing on the hypothalamus gene expression profile. Prenatally undernourished rat offspring were HF-fed from weaning until adulthood while body weight and food intake were measured. Tissue weights, glucose tolerance and plasma endocrine parameters levels were determined in 4-month-old rats. Hypothalamic gene expression profiling of adult FR30 rat was performed using Illumina microarray analysis and the RatRef-12 Expression BeadChip that contains 21,792 rat genes. Under HF diet, contrary to C animals, FR30 rats displayed increased body weight. However, most of the endocrine disorders observed in chow diet-fed adult FR30 were alleviated. We also observed very few gene expression changes in hypothalamus of FR30 rat. Amongst factors involved in hypothalamic energy homeostasis programming system, only the POMC and transthyretin mRNA expression levels were preferentially increased under HF diet. Both elevated gene expression levels may be seen as adaptive mechanisms counteracting against deleterious effects of HF feeding in FR30 animals. This study shows that the POMC gene expression is a key target of long-term developmental programming in prenatally undernourished male rat offspring, specifically within an obesogenic environment.     

Ultrastructural investigation of ACTH immunoreactivity in arcuate and supraoptic nuclei of the rat

Summary Fine structural localization of an ACTH-like substance was obtained in neurons of the rat arcuate nucleus using immuno-electron microscopy, whereas it could not be confirmed that ACTH-containing cell bodies are present in the supraoptic nucleus. The immunoreactive cells of the arcuate nucleus appeared to be more numerous than the unreactive neurons. Immunostaining was carried out before embedding in resin. Empty vesicles of irregular shape were found in dendrites of immunoreactive arcuate neurons, but their significance and nature remain enigmatic. The reaction product was distributed uniformly throughout the cytoplasm of the ACTH-positive cells, except that the mitochondria, rough endoplasmic reticulum and Golgi vesicles and cisternae were devoid of PAP molecules. This distribution differed from the localization reported in ACTH-secreting cells of the rat anterior pituitary, where the reaction product was found in the rough endoplasmic reticulum and Golgi complex as well as in secretory granules.     

Inhibition of testosterone-induced sexual behavior in the castrated male rat by aromatase blockers

The effect of some aromatase inhibitors (aminoglutethimide, 1,4,6-androstatrien-3, 17-dione, and 4-hydroxy-androstenedione) on testosterone propionate (TP)-induced copulatory behavior was tested in sexually inexperienced castrated male rats. A single injection of 6 mg of TP induced mounting in 48% and ejaculatory pattern in 19% of the rats within 120 hr. Treatment with the aromatase inhibitors (injections every 12 hr for 108 hr) suppressed ejaculation in all but one rat and significantly reduced the number of rats mounting and intromitting. Concurrent administration of estradiol benzoate (EB, 1 or 3 μg every 12 hr) prevented the inhibitory effect of aromatase blockers. No inhibitory effect of the aromatization blockers was observed in rats in which sexual behavior was induced by dihydrotestosterone (1 mg/day) and EB (2.5 μg/day) for 20 days. The results support the concept that aromatization is an essential step for the induction of male sexual behavior by androgen in the rat.     

Postnatal development of galanin-like peptide mRNA expression in rat hypothalamus

We examined the developmental change of GALP mRNA in male and female rat hypothalamus during postnatal day 1 to 60, using in situ hybridization histochemistry. Neuropeptide Y (NPY) and proopiomelanocortin (POMC) mRNA in the hypothalamus were also examined because they are important in the regulation of food intake. GALP mRNA was first detected in the arcuate nucleus (ARC) on day 8. GALP mRNA was gradually increased between day 8 and 14 and markedly increased between day 14 and 40, which is the weaning and pubertal period in rats. After day 40, there were no significant differences in GALP mRNA. In contrast to GALP, NPY and POMC mRNAs were detected in the ARC from day 1 and lasted to day 60. There was no sexual dimorphism in GALP, NPY and POMC mRNAs during postnatal development. Next, we examined the effect of the milk deprivation for 24 h on GALP, NPY and POMC mRNA in pups. GALP mRNA did not change by milk deprivation on day 9 and 15, while milk deprivation had a significant effect on NPY and POMC mRNA on day 15. These results suggest that the development of GALP may be associated with developmental changes such as weaning, feeding and maturation of reproductive functions. The regulatory mechanism of GALP mRNA is different from that of the NPY and POMC genes during postnatal development.     

Distribution of galanin-like immunoreactivity in the gastro-intestinal tract of several mammalian species

Summary The distribution of galanin-immunoreactive (GAL-IR) neurons was mapped in detail in the gastro-intestinal tract of the rat, mouse, guinea-pig and pig by use of the indirect immunofluorescence technique. GAL-IR cell bodies were found in both the submucous and the myenteric plexus, with considerably higher numbers in the former ganglia. The largest number of GAL-IR perikarya was seen in the duodenal submucous plexus of the pig. With some (single) exceptions, GAL-IR cell somata were not observed in the myenteric plexus of the pig and guinea-pig, and in the submucous plexus of the esophagus and the stomach of the guinea-pig.GAL-IR fibers ocurred in most parts of the gastro-intestinal tract. In the lamina propria a few non-varicose, weakly fluorescent fibers were noted in the mouse and rat, whereas in the pig and guinea-pig were large numbers of GAL-IR fibers with a varicose appearance was observed. These fibers were in all species most numerous in the distal portion of the intestinal tract. In the submucosa GAL-IR fibers were detected in all four species, and in the pig and guinea-pig some fibers surrounded blood vessels. A large number of GAL-IR fibers was generally seen in the circular smooth muscle layer, except in the guinea-pig, which only seemed to contain a few fibers. In the longitudinal muscle layer only single fibers could be detected. However, the gastric fundus region of the pig contained a moderate number of fibers in the longitudinally and obliquely oriented layers.In general, in the rat, mouse and pig, the submucous and myenteric plexus contained moderate or large numbers of GAL-IR fibers. In the guinea-pig, no or only single fibers were observed in the plexus of the upper gastro-intestinal tract and the rectum, while moderate numbers were seen in the ileum and colon.Thin adjacent sections stained for vasoactive intestinal polypeptide (VIP) and GAL revealed the coexistence of these two peptides in cell bodies of the myenteric plexus in the pig duodenum and guinea-pig colon. In these two species the GALand VIP-nerve fiber networks also exhibited marked similarities. However, in the rat and mouse VIPand GAL-distribution patterns were in general different.The present findings indicate the presence of yet another neuropeptide or peptide family in the gastro-intestinal tract of several rodents and the pig.     

Participation of galaninergic neurotransmission at the paraventricular hypothalamic nucleus in the suppression of baroreceptor reflex response by locus ceruleus in the rat

We evaluated the potential participation of galanin (GAL) at the paraventricular nucleus of hypothalamus (PVN) in the suppression of baroreceptor reflex (BRR) response by locus ceruleus (LC), using adult male Sprague-Dawley rats anesthetized with pentobarbital sodium. Microinjection of GAL (100 pmol) bilaterally into the PVN significantly depressed the BRR response. This suppressive effect was appreciably antagonized when GAL (100 pmol) and GAL antiserum (1:20) were coadministered into the bilateral PVN. Whereas bilateral microinjection of GAL antiserum into the PVN by itself elicited minimal effect, it nevertheless significantly attenuated the suppressive effect of either electrical or chemical activation of LC on the BRR response. Pretreatment with the same amount of normal rabbit serum (1:20), on the other hand, was ineffective. These results suggest that a galaninergic projection from the LC to PVN may participate in the suppression of BRR response by this dorsal pontine nucleus.     

Diurnal rhythm of galanin-like immunoreactivity in the paraventricular and suprachiasmatic nuclei and other hypothalamic areas

The peptide galanin (GAL), when injected into the rat hypothalamus, is known to stimulate feeding behavior and affect the secretion of various hormones, including insulin and the adrenal steroid, corticosterone. To determine whether endogenous peptide levels shift in relation to natural rhythms of feeding and circulating hormone levels, rats were sacrificed at different times of the light/dark cycle, and their GAL levels were measured, via radioimmunoassay, in medial hypothalamic dissections and micropunched hypothalamic areas. The results suggest the existence of two distinct diurnal rhythms for hypothalamic GAL. One rhythm, detected exclusively in the area of the SCN, is characterized by bimodal peaks of GAL, threefold higher than basal peptide levels, around the onset of the dark and light periods. The second rhythm shows a single peak of GAL towards the middle of the nocturnal feeding cycle, specifically between the third and sixth hour. This latter rhythm is evident in the dorsal region of the medial hypothalamus, localized specifically to the lateral portion of the PVN. Moreover, it is inversely related to circulating insulin but unrelated to the adrenal steroids, suggesting a possible association between this pancreatic hormone and GAL in the PVN.     

Proliferation of lamellar whorls in arcuate neurons of the hypothalamus of castrated and morphine-treated male rats

Summary A comprehensive ultrastructural examination of one cross-sectional level (middle 1/3) of the arcuate nucleus of the hypothalamus (AH) of male rats several weeks after castration or after two weeks of morphine treatment confirmed a marked increase in lamellar whorls of endoplasmic reticulum in AH neurons in each group. A comparable incidence of AH whorls was not detected in rats treated with lactose, those treated for only 1–3 days with morphine, or in those given testosterone plus morphine for 2 weeks. It is postulated that the testosterone deficiency following either castration or chronic morphine treatment stimulated the observed increase in AH whorls. A close correspondence was noted between the distribution within the AH of neurons containing whorls and those reported by others to contain luteinizing hormone releasing hormone (LH-RH). The possibility that whorls may be a marker for hypothalamic neurons which play a role in the LH-RH regulatory system warrants further consideration.Supported by Grants DA-00259 and NS-09156 from the United States Public Health ServiceResearch Scientist Development Award MH-38894Research Scientist Development Award MH-70180     

Catecholaminergic innervation of GRF-containing neurons in the rat hypothalamus revealed by electron-microscopic cytochemistry

Summary The Catecholaminergic innervation of neurons containing growth hormone-releasing factor (GRF) was examined by use of a method which combined either 5-hydroxydopamine (5-OHDA) uptake or autoradiography after intraventricular injection of ³H-noradrenaline with immunocytochemistry for GRF in the same tissue sections at the electron-microscopic level. In the ventrolateral part of the arcuate nucleus of the rat hypothalamus a large number of immunonegative axon terminals were found to make synaptic contact with GRF-like immunoreactive (GRF-LI) cell bodies and processes. ³H-noradrenaline autoradiography or 5-OHDA-labeling combined with GRF immunocytochemistry revealed that axon terminals labeled with ³H-noradrenaline or 5-OHDA make synaptic contact with the GRF-LI nerve cell bodies and processes. These findings indicate that catecholamine-containing neurons innervate GRF neurons to regulate GRF secretion via synapses in the rat arcuate nucleus.     

The endogenous androgen-regulated sialorphin modulates male rat sexual behavior

In sexually mature male rats, sialorphin is synthesized under androgenic control and its surge endocrine secretion is evoked in response to environmental acute stress. These findings led us to suggest that this signaling mediator might play a role in physiological and behavioral integration, especially reproduction. The present study investigates the effects induced by sialorphin on the male sexual behavior pattern. Intact male rats were treated in acute mode, with sialorphin at the 0.3, 1, and 3 microg/kg doses, before being paired with receptive female for 45 min. The data obtained show that sialorphin increased, in a dose-related manner, the occurrence of intromissions across the successive ejaculatory sequences. The rats treated with the highest 3 microg/kg dose significantly ejaculated less often compared to controls; however, 80% of them achieved up to three ejaculations. Further analyses of mount bouts for rats achieving three ejaculations reveal that there were significant stimulatory effects of sialorphin, at all doses, on the frequency of intromissions before ejaculation and on the propensity of males to engage in investigatory behavior directed to the female during the post-ejaculatory interval. Thus, sialorphin has the ability to modulate, at doses related to physiological circulating levels, the male rat mating pattern, that is, exerting a dual facilitative or inhibitory dose-dependent effect on the sexual performance, while stimulating the apparent sexual arousal or motivation. These findings led us to speculate that the endogenous androgen-regulated sialorphin helps modulate the adaptative balance between excitatory and inhibitory mechanisms serving appropriate male rat sexual response, depending on the context.     

Cyto- and synaptogenesis in the arcuate nucleus of the rat hypothalamus during fetal and early postnatal life

Summary Morphogenesis of the arcuate nucleus of the rat from the 15th fetal day to the 6th postnatal day was investigated light and electron microscopically. The arcuate neurons exhibit a gradual development after the 15th fetal day. All cytoplasmic constituents are present in these nerve cells already during the last days of gestation. Nevertheless, they are not fully differentiated at birth. The first synapse-like structures (presynapses) were observed in 17 day-old, the first synapses in 18 day-old fetuses. During the early postnatal period the number of presynapses decreases, but at the same time there is a gradual increase in the number of the relatively mature synapses. This process starts already during the last days of prenatal life. Although all structural elements of the arcuate nucleus of the adult rat appear to be present at birth, the extent of the neuropil area and the number of the presynapses indicate that the arcuate nucleus is still in a fairly undeveloped stage during the first postnatal days.     

Oxytocin in the medial preoptic area facilitates male sexual behavior in the rat

Oxytocin (OT) is a versatile neuropeptide that is involved in a variety of mammalian behaviors, and its role in reproductive function and behavior has been well established. The majority of pharmacological studies of the effects of OT on male sexual behavior have focused on the paraventricular nucleus (PVN), ventral tegmental area (VTA), hippocampus, and amygdala. Less attention has been given to the medial preoptic area (MPOA), a major integrative site for male sexual behavior. The present study investigated the effects of intra-MPOA administration of OT and (d(CH2)51, Tyr(Me)2, Thr4, Orn8, Tyr-NH29)-vasotocin, an OT antagonist (OTA), on copulation in the male rat. The relationship between OT receptor (OTR) binding levels in the MPOA and sexual efficiency was also explored. Microinjection of OT into the MPOA facilitated copulation in sexually experienced male rats, whereas similar injections of an OTA inhibited certain aspects of copulation but had no significant effect on locomotor activity in an open field. Contrary to expectation, sexually efficient males had lower levels of OTR binding in the rostral MPOA compared to inefficient animals. The present data suggest that OT activity in the MPOA is not necessary for the expression of male sexual behavior but is sufficient to facilitate copulatory behaviors and improve sexual efficiency in sexually experienced male rats. These data also suggest that OTR activity in the MPOA stimulates anogenital investigation, facilitates the initiation of copulation, and plays a role in the sensitization effect of the first ejaculation on subsequent ejaculations.