Castration decreases extracellular, but increases intracellular, dopamine in medial preoptic area of male rats

Dopamine (DA) is released in the medial preoptic area (MPOA) of male rats in the presence of a female, and it facilitates male sexual behavior. Castration blocks the DA response to a female and the male's ability to copulate. The present experiments examined the effects of castration on (1) basal levels of extracellular DA in the MPOA, using the no net flux microdialysis technique, (2) the response of extracellular DA to amphetamine, and (3) tissue levels of DA. Castrated rats had lower basal levels of extracellular DA in the MPOA, compared with gonadally intact rats; in vivo recovery, a measure of uptake, was not different. This suggests that castration decreases DA release in basal conditions, as well as in response to a female. However, systemic amphetamine injections, which induce DA release, resulted in greater DA release in castrates. Finally, tissue levels of DA were higher in the MPOA, the caudate–putamen and the bed nucleus of stria terminalis of castrates. These data suggest that DA synthesis and storage in the MPOA are normal, or even enhanced, in castrates, and uptake is not altered. The deficit in extracellular levels appears to be related to release, perhaps due to decreased nitric oxide.     

Microinjection of cis-flupenthixol, a dopamine antagonist, into the medial preoptic area impairs sexual behavior of male rats

Systemically administered dopamine agonists have been shown to facilitate copulation in male rats. Microinjection of the dopamine agonist apomorphine into the medial preoptic area has also been reported to facilitate sexual behavior. The present experiments investigated the effects of medial preoptic microinjections of the dopamine antagonist cis-flupenthixol on male rat copulatory behavior. Fewer males initiated copulation and fewer ejaculated following flupenthixol administration. Those males that did ejaculate following flupenthixol injections had fewer ejaculations and longer interintromission intervals. Flupenthixol also antagonized the facilitative effects of apomorphine injections into the medial preoptic area. Flupenthixol and apomorphine produced only minor alterations in noncopulatory behaviors. The results suggest that dopamine receptors within the medial preoptic area are important in the regulation of masculine sexual behavior in the rat.     

Microinjection of cis-flupenthixol, a dopamine antagonist, into the medial preoptic area impairs sexual behavior of male rats

Systematically administered dopamine agonists have been shown to facilitate copulation in male rats. Microinjection of the dopamine agonist apomorphine into the medial preoptic area has also been reported to facilitate sexual behavior. The present experiments investigated the effects of medial preoptic microinjections of the dopamine antagonist cis-flupenthixol on male rat copulatory behavior. Fewer males initiated copulation and fewer ejaculated following flupenthixol administration. Those males that did ejaculate following flupenthixol injections had fewer ejaculations and longer interintromission intervals. Flupenthixol also antagonized the facilitative effects of apomorphine injections into the medial preoptic area. Flupenthixol and apomorphine produced only minor alterations in non-copulatory behaviors. The results suggest that dopamine receptors within the medial preoptic area are important in the regulation of masculine sexual behavior in the rat.     

Microinjection of galanin-like peptide into the medial preoptic area stimulates food intake in adult male rats

Galanin-like peptide (GALP) is a neuropeptide implicated in the regulation of feeding behaviour, metabolism and reproduction. GALP is an endogenous ligand of the galanin receptors, which are widely expressed in the hypothalamus. GALP is predominantly expressed in arcuate nucleus (ARC) neurones, which project to the paraventricular nucleus (PVN) and medial preoptic area (mPOA). Intracerebroventricular or intraparaventricular (iPVN) injection of GALP acutely increases food intake in rats. The effect of GALP injection into the mPOA on feeding behaviour has not previously been studied. In the present study, intra-mPOA (imPOA) injection of GALP potently increased 0-1-h food intake in rats. The dose-response effect of imPOA GALP administration on food intake was similar to that previously observed following iPVN administration. The effects of GALP (1 nmol) or galanin (1 nmol) on food intake were then compared following injection into the PVN, mPOA, ARC, dorsal medial nucleus (DMN), lateral hypothalamus and rostral preoptic area (rPOA). GALP (1 nmol) increased food intake to a similar degree when injected into the imPOA or iPVN, but produced no significant effect when injected into the ARC, DMN, lateral hypothalamus or rPOA. Similarly, galanin (1 nmol) significantly increased food intake following injection imPOA and iPVN. However, the effect was significantly smaller than that following administration of GALP (1 nmol). Galanin also had no significant effect on food intake when administered into the ARC, DMN, lateral hypothalamus and rPOA. These data suggest that the mPOA and the PVN may have specific roles in mediating the orexigenic effect of GALP and galanin.     

Diurnal rhythm in cyclic GMP/nitric oxide efflux in the medial preoptic area of male rats

Since nitric oxide (NO) is implicated in the neuroendocrine control of luteinizing hormone-releasing hormone (LHRH) secretion and sexual behavior which show diurnal variations, we monitored cGMP levels (an index of NO activity) in the extracellular compartment of the medial preoptic area (MPOA) using microdialysis. It was observed that MPOA cGMP levels rose significantly in the afternoon in both castrated and intact male rats, thereby suggesting the existence of a diurnal rhythm in MPOA cGMP/NO efflux which may participate in eliciting the well-known diurnal variations in LHRH neuronal activity and male sexual behavior.     

Effects of hydroxyflutamide in the medial preoptic area or lateral septum on reproductive behaviors in male rats

We examined whether androgen receptors in the medial preoptic area (MPOA) and lateral septum (LS) are required for the expression of copulation and sexual motivation. Castrated males received testosterone-filled silastic capsules to restore behavior, and were implanted with the antiandrogen hydroxyflutamide (OHF) or blank cannulae. One group was implanted in either the anteroventral MPOA or LS (ANT group). Another group was implanted in the posterodorsal MPOA or LS (POST group). Copulation was tested on days 2, 6, 10, and 14 of OHF exposure; partner preference, a measure of sexual motivation, was tested on day 15. The results showed that sexual behavior was significantly suppressed by OHF in the MPOA of the ANT group, but not the POST group. However, sexual motivation was significantly reduced by OHF in the MPOA of the POST group, but not the ANT group. In the LS, OHF had no effect on sexual behavior and partner preference regardless of implant site. The data suggest site specificity within the MPOA for androgen receptor activation of male reproductive behaviors.     

Neostigmine influences the L-dopa-induced extracellular dopamine levels in the striatum

Using in vivo microdialysis in freely moving rats, we show that the addition to the dialysis perfusion fluid of the acetylcholinesterase inhibitor neostigmine influences the decarboxylation of levodopa (L-dopa). Continuous perfusion of neostigmine (10, 50 and 100 nM) in striatum attenuated the L-dopa-induced dopamine release in a dose-dependent manner. This effect suggests that changes in magnitude of drug responses may occur when an acetylcholinesterase inhibitor is included in the perfusion solution. Results obtained under these circumstances should be carefully interpreted concerning the pharmacological effects of other drugs when used concomitantly with neostigmine.     

Neostigmine influences the -dopa-induced extracellular dopamine levels in the striatum

Using in vivo microdialysis in freely moving rats, we show that the addition to the dialysis perfusion fluid of the acetylcholinesterase inhibitor neostigmine influences the decarboxylation of levodopa (-dopa). Continuous perfusion of neostigmine (10, 50 and 100 nM) in striatum attenuated the -dopa-induced dopamine release in a dose-dependent manner. This effect suggests that changes in magnitude of drug responses may occur when an acetylcholinesterase inhibitor is included in the perfusion solution. Results obtained under these circumstances should be carefully interpreted concerning the pharmacological effects of other drugs when used concomitantly with neostigmine.     

Stimulation of the medial amygdala enhances medial preoptic dopamine release: implications for male rat sexual behavior

Increased dopamine (DA) in the medial preoptic area (MPOA) facilitates male sexual behavior. A major source of innervation to the MPOA is the medial amygdala (MeA). We now report that chemical stimulation of the MeA enhanced levels of extracellular MPOA DA in anesthetized male rats. These results suggest that DA activity in the MPOA can be regulated by input from the MeA to the MPOA.     

Characteristics of exaggerated sexual behavior induced by electrical stimulation of the medial preoptic area in male rats.

Twenty-two sexually experienced male rats were implanted with lateral preoptic (LPO) and medial preoptic (MPO) electrodes. Following surgery, the 22 MPO and 18 LPO electrodes were screened for the induction of stimulation-bound copulation in the presence of estrus females. Stimulation through 5 of the MPO electrodes induced highly exaggerated, stimulation-bound sexual behaviou. Ten MPO electrodes produced escape behavior, 2 electrodes aggression and one electrode induced sexual behavior intermingled with aggression. The tips of all the electrodes which effected exaggerated copulation were located within a small preoptic region, less than 1 mm lateral to the midline. LPO electrodes had either inhibitory effects on mating behavior or none at all. The increase in copulatory activity by MPO electrodes' stimulation was expressed in a great enhancement in the number of ejaculations, and marked decreases in the latency to ejaculation, the post-ejaculatory refractory period, and in the number of intromissions preceding an ejaculation. It is concluded that the electrical stimulation affected both the sexual arousal mechanism and the intromission-ejaculation mechanism involved in mating behavior.     

GABA and glutamate in mating-activated cells in the preoptic area and medial amygdala of male gerbils

The posterodorsal medial amygdala (MeApd), the posterodorsal preoptic nucleus (PdPN), and the medial cell group of the sexually dimorphic preoptic area (mSDA) contain cells that are activated specifically at ejaculation as assessed by Fos expression. The mSDA also expresses Fos early in the mating context. Because little is known about the neurotransmitters of these activated cells, the possibility that they use gamma-aminobutyric acid (GABA) or glutamate was assessed. Putative glutamatergic cells were visualized with immunocytochemistry (ICC) for glutamate and its neuron-specific transporter. Their distributions were compared with those of GABAergic cells visualized with ICC for the 67-kDa form of glutamic acid decarboxylase (GAD(67)) and in situ hybridization for GAD(67) messenger RNA (mRNA). Colocalization of Fos and GAD(67) mRNA in recently mated males indicated that half of the activated cells in the PdPN, mSDA, and lateral MeApd are GABAergic. Colocalization of Fos and glutamate suggested that a quarter of the activated mSDA and lateral MeApd cells are glutamatergic. The PdPN does not appear to have glutamatergic cells. In the lateral MeApd, the percentage of activated cells that are GABAergic (45%) matches the percentage that project to the principal part of the bed nucleus of the stria terminalis (BST; 43%), and the percentage likely to be glutamatergic (27%) matches the percentage projecting to the mSDA (27%). The latter could help to trigger ejaculation. The distribution of GABAergic and putative glutamatergic cells in the caudal preoptic area, caudal BST, and medial amygdala of male gerbils is also described.     

Ibotenic acid-induced lesions of the medial preoptic area/anterior hypothalamus enhance the display of progesterone-facilitated lordosis in male rats

Electrical lesions of the medial preoptic area/anterior hypothalamus (MPOA/AH) have been reported to enhance the display of steroid-induced lordosis in castrated male rats. This study employed the cell body-specific neurotoxin, ibotenic acid, to ascertain whether neurons originating in this region (as opposed to axons of passage) tonically inhibit steroid-induced lordosis in adult male rats. Castrated, adult Long-Evans males received bilateral electrical lesions or injections of ibotenic acid or vehicle aimed at the MPOA/AH. Following administration of estradiol benzoate (EB) and progesterone, lordosis quotients (LQs) and lordosis ratings (LRs) were significantly higher in groups of rats with electrical lesions (LQ= 62.2 ± 15.1;LR= 1.22 ± 0.34) and ibotenic acid-induced lesions (LQ = 58.1 ± 12.2;LR= 0.99 ± 0.24) than in the control group (LQ= 12.8 ± 7.3;LR= 0.22 ± 0.13). To determine whether this enhancement of receptive behavior in MPOA/AH-lesioned males was an effect on estradiol-induced, as compared to progesterone-facilitated lordosis, groups of castrated rats in a second experiment received bilateral injections of ibotenic acid or vehicle aimed at the MPOA/AH and were tested for lordosis after administration of EB alone and again after injection of progesterone. Following treatment with EB alone, rats with ibotenic acid-induced MPOA/AH lesions tended to be slightly less receptive than control animals. However, following injections of progesterone, LQs and LRs were higher in the MPOA/AH-lesioned group than in the control animals, as had been observed in the first experiment. These data are consistent with the hypothesis that cell bodies, rather than axons of passage, originating in the MPOA/AH exert tonic inhibitory control over the display of progesterone-facilitated lordosis in adult male rats.     

Expression of mu-opioid receptor mRNA in the medial preoptic area of juvenile rats

Previous studies indicate that the latency to initiate parental behavior in both male and female rats increases with age; weanling (21 days old) rats display parental behavior 0-2 days after exposure to newborn pups, while older juveniles (30 days old) require 5-6 days of pup exposure before they express the behavior. Furthermore, activation of mu-opioid receptors inhibits parental behavior in juvenile and adult rats. We hypothesized that the age-related increase in behavioral latency could be modulated by the induction of mu-receptor expression in the medial preoptic area (MPOA), a region in which mu-receptors regulate parental behavior. In situ hybridization histochemistry was used to measure mu-receptor mRNA expression in the MPOA of male and female Sprague-Dawley rats that were 21, 24, 27, 30, or 33 days old. Using autoradiographic film analysis, we observed that neurons within part of the MPOA expressed very dense mu-receptor mRNA. Comparison of mRNA distribution with histological boundaries indicated that neurons within the medial preoptic nucleus (MPN), excluding the central part, exhibited the highest density of mu-receptor mRNA within the MPOA. High densities of mu-receptor mRNA extended dorsolaterally and caudally from the MPN toward the bed nucleus of the stria terminalis. MPN mu-receptor mRNA expression was not altered with age and no sex difference was observed. The dense presence of mu-receptor mRNA in the MPN suggests that ample substrate exists on which mu-receptor ligands could modulate the latency to begin parental behavior in juvenile rats, but such behavioral expression apparently is not mediated by a change in mu-receptor mRNA production.     

Effects of discrete lesions of the sexually dimorphic nucleus of the preoptic area or other medial preoptic regions on the sexual behavior of male rats

The sexually dimorphic nucleus of the rat medial preoptic area (SDN-POA) has a volume five times larger in the adult male compared with that of the adult female. In the present study, the effects of discrete electrolytic destruction of the SDN-POA or other specific medial preoptic (MPOA) regions on masculine sexual behavior were determined in adult, sexually experienced male rats. Small lesions encompassing the SDN-POA had no effect on the maintenance of copulatory behavior. Lesions of similar size placed within the ventral or anterio-dorsal MPOA also did not consistently affect the display of masculine sexual behavior. However, animals that received small lesions within their dorsal MPOA showed a substantial, long-term decrease in number of mounts, intromissions, and ejaculations compared to these parameters in sham-lesioned control rats, thus indicating a lesion-induced disruption of those neural mechanisms mediating these behaviors. Collectively these data suggest that the SDN-POA is not critical for a full expression of male sexual behavior and that the dorsal MPOA may be more important than other MPOA regions for copulatory behavior.     

Interrelationship of thermal and sleep-wakefulness changes elicited from the medial preoptic area in rats

The study investigated the possible interrelationship between changes in sleep-wakefulness and body temperature, primarily induced by manipulation of the noradrenergic system in the medial preoptic area. Saline, norepinephrine, and its alpha- and beta-blockers were injected in the medial preoptic area and in some control areas of rats, during their sleeping and active periods. 5-Hydroxytryptamine was injected in the medial preoptic area in another group of animals. Simultaneous changes in sleep-wakefulness and the body temperature were continuously recorded. Norepinephrine produced hypothermia and arousal, whereas alpha-adrenergic blockers induced hyperthermia and sleep. These changes in body temperature and in sleep-wakefulness did not follow an identical time course. 5-Hydroxytryptamine induced hyperthermia without affecting sleep-wakefulness. It is suggested that there are different neuronal mechanisms in the medial preoptic area that bring about the drug-induced changes in temperature and sleep-wakefulness.     

Alpha adrenergic system in medial preoptic area involved in sleep-wakefulness in rats

The study is aimed at investigating the possible involvement of adrenergic mechanisms in the medial preoptic area (mPOA) for modulation of sleep-wakefulness in rats. In this study, saline, norepinephrine (NE), phenoxybenzamine (PBZ) and propranolol (PROP) were injected in the mPOA in different groups of male rats during the day and night. NE and PBZ were injected, during the day and the night respectively, in some control areas adjoining the mPOA in two other groups of animals. Arousal was produced by NE, and sleep by PBZ when they were applied in the mPOA. All other procedures, including application of NE and PBZ in the control areas and beta blocker (PROP) in the mPOA, did not produce alterations in sleep-wakefulness. These findings provide support for a physiological role played by the alpha adrenergic system in the mPOA for arousal, and area specificity of action of this system.     

Lesions of the medial preoptic area/anterior hypothalamus (MPOA/AH) modify partner preference in male rats

When given the choice, male rats will interact with a receptive female while female rats will interact with a sexually active male. In the present experiment partner preference was tested in male and female rats before and after lesions of the medial preoptic area of the anterior hypothalamus (MPOA/HA). Subjects were gonadectomized, treated for 10 days with 5 μg/kg of estradiol benzoate (EB) and tested for male coital behavior with receptive females and for partner preference in a three compartment box with free access to either a sexually receptive female or a sexually active male. The same tests were repeated after 10 days of treatment with 5 mg/kg of testosterone propionate (TP). The subjects then received a bilateral electrolytic lesion aimed at the MPOA/AH. Two weeks after the lesion the hormonal treatments and behavioral tests were repeated in the same sequence. Prior to the lesion, females showed a clear preference to interact with the stimulus male while male subjects showed a preference to interact with the receptive female regardless of the hormonal treatment they received. After lesions the females preference for the opposite sex was not modified, they spent more time in the chamber with the stimulus male regardless of whether they had an extensive bilateral destruction of the MPOA/AH or a sham lesion. Males with bilateral destruction of the MPOA/AH changed their partner preference after the lesion. They spent significantly more time in the chamber with the stimulus male than in the chamber with the receptive female. As well, the coital behavior of males with bilateral destruction of the MPOA/AH was significantly reduced after the lesion. The change of preference was observed when the lesioned animals were treated either with EB or TP. The results of the present experiment further support the notion that the MPOA/AH is a crucial structure in the integration of sensory cues that determine partner preference.     

Glutamate microinjection at the medial preoptic area enhances slow wave sleep in rats

A large body of evidence has established the role of the medial preoptic area (mPOA) in regulation of slow wave sleep (SWS). Although the mPOA neurons contain excitatory neurotransmitter glutamate, its role in sleep-wakefulness is not known. In the present study microinjection of monosodium glutamate (40, 80 and 120 ng) into the mPOA augmented SWS. Earlier reports have shown enhancement of paradoxical sleep by glutamate in other brain areas.     

Changes in sleep-wakefulness in the medial preoptic area lesioned rats: role of thermal preference

Changes in sleep-wakefulness (S-W) were studied in adult male Wistar rats, along with body temperature (T(b)), locomotor activity (LMA) and thermal preference, after the lesion of the medial preoptic area (mPOA) with N-methyl-D-aspartic acid (NMDA). The sleep was decreased after the lesion of the mPOA, but there was recovery when the rats were given freedom to stay in an ambient temperature (T(amb)) which they preferred. When given a choice between three T(amb) (24, 27 and 30 degrees C), the rats preferred 27 degrees C before the mPOA lesion, and 24 degrees C during the initial days after the lesion. There was a shift in the thermal preference to 30 degrees C, on the fourth week after the lesion, which coincided with the considerable recovery of sleep. The preference for higher T(amb) probably helped to improve sleep, as T(amb) of 30 degrees C is known to promote sleep. When the lesioned rats were not given the freedom to select the T(amb), there was no recovery in sleep. The mPOA seems to be essential for increasing the durations of slow wave sleep (SWS) episodes, especially the light SWS (S1), as they remained shorter than the pre-lesion value, even when the rats were given freedom to stay in a preferred T(amb). The homeostatic recovery of sleep, especially the night time sleep, resulted in the disruption of circadian sleep rhythm. But, the LMA, T(b) and thermal preference maintained their diurnal variation. T(b) and LMA were elevated after the mPOA lesion and they remained so till the end of the study.     

Wakefulness-inducing area in the brainstem excites warm-sensitive and inhibits cold-sensitive neurons in the medial preoptic area in anesthetized rats

Sleep-wakefulness and body temperature are known to influence each other. The body temperature rises during wakefulness and falls during sleep. The midbrain reticular formation is one of the areas in the brainstem that induces wakefulness, while the preoptico-anterior hypothalamic area is the main thermoregulatory center in the brain. In order to understand the neural mechanism for simultaneous regulation of these functions we hypothesized that the wakefulness area in the brainstem is likely to have an opposite influence on warm- and cold-sensitive neurons in the preoptico-anterior hypothalamic area. Hence, first, the wakefulness-inducing area was identified in the brainstem by stimulating the site with high-frequency rectangular wave electrical pulses (100 Hz, 100 microA, 200 microsec for 5-8 sec) in freely behaving chronically prepared experimental rats. Then, single neuronal activity from the medial preoptico-anterior hypothalamic area was recorded and their thermosensitivity was established. Thereafter, the influence of such a confirmed wakefulness-inducing area in the brainstem on the responsiveness of the single neuronal activity of predetermined warm- and cold-sensitive neurons as well as on temperature-insensitive neurons was studied by overlapping stimulus (1 Hz, 500 microA, 200 microsec) bound responses. It was observed that the warm-sensitive neurons were excited and the cold-sensitive neurons were inhibited by stimulation of the wakefulness-inducing area in the brainstem. Most of the temperature-insensitive neurons remained unaffected. The results confirm our hypothesis and help in understanding the mechanism of simultaneous modulation of body temperature in association with changes in wakefulness at the single neuronal level.