Noradrenergic innervation to the VMN or MPN is not necessary for lordosis

Estradiol Rapidly Attenuates ORL-1 Receptor-Mediated Inhibition of Proopiomelanocortin Neurons via Gq-Coupled, Membrane-Initiated Signaling

Estradiol rapidly regulates the activity of arcuate nucleus (ARH) proopiomelanocortin (POMC) neurons that project to the medial preoptic nucleus (MPN) to regulate lordosis. Orphanin FQ/nociceptin (OFQ/N) acts via opioid receptor-like (ORL)-1 receptors to inhibit these POMC neurons. Therefore, we tested the hypothesis that estradiol excites POMC neurons by rapidly attenuating inhibitory ORL-1 signaling in these cells. Hypothalamic slices through the ARH were prepared from ovariectomized rats injected with Fluorogold into the MPN. Electrophysiological recordings were generated in ARH neurons held at or near -60 mV, and neuronal phenotype was determined post hoc by immunohistofluorescence. OFQ/N application induced robust outward currents and hyperpolarizations via G protein-gated, inwardly rectifying K+ (GIRK) channels that were attenuated by pretreatment with either 17-β estradiol (E2) or E2 conjugated to bovine serum albumin. This was blocked by the estrogen receptor (ER) antagonist ICI 182,780 and mimicked by the Gq-coupled membrane ER (Gq-mER) ligand STX and the ERα agonist PPT. Inhibiting phosphatidylinositol-3-kinase (PI3K) blocked the estrogenic attenuation of ORL-1/GIRK currents. Antagonizing either phospholipase C (PLC), protein kinase C (PKC), protein kinase A (PKA) or neuronal nitric oxide synthase (nNOS) also abrogated E2 inhibition of ORL-1/GIRK currents, whereas activation of PKC, PKA, protein kinase B (Akt) and nNOS substrate L-arginine all attenuated the OFQ/N response. This was observed in 92 MPN-projecting, POMC-positive ARH neurons. Thus, ORL-1 receptor-mediated inhibition of POMC neurons is rapidly and negatively modulated by E2, an effect which is stereoselective and membrane initiated via Gq-mER and ERα activation that signals through PLC, PKC, PKA, PI3K and nNOS.

Spinal and supraspinal components of opioid antinociception in streptozotocin induced diabetic neuropathy in rats

This study investigated the antinociceptive effect of opioids given via intraperitoneal and intrathecal routes in a diabetes-induced neuropathic pain model in rats. Streptozotocin induced diabetes in 91% of juvenile male Wistar rats at the dose of 150 mg/kg (75 mg/kg intraperitoneal on 2 successive days). When compared with younger weight-matched saline treated rats, the diabetic rats developed hyperalgesia assessed by the paw pressure nociceptive test. Nociceptive thresholds and responses to fentanyl in all nociceptive tests in these younger normal rats were the same as those described previously for older normal rats. Fentanyl (10-100 microg/kg, i.p.) produced a dose-related antinociceptive effect in both neuropathic (n=6-8) and non-neuropathic (n=6-8) rats in electrical current, paw pressure and tail flick nociceptive tests. Higher doses of fentanyl were needed in neuropathic animals to achieve similar antinociceptive effects to those in non-neuropathic animals. Intrathecal injections of fentanyl (0.05-0.5 microg) in non-neuropathic rats, produced a spinally-mediated, dose-related antinociceptive effect assessed by all tests. In contrast, intrathecal administration of fentanyl that confined the drug action to the spinal cord produced little antinociceptive effect in neuropathic rats in all three tests. These experiments suggest that supraspinal mu opioid receptors are responsible for the antinociceptive effect of opioids in this model of neuropathic pain and that spinal cord opioid systems are in some way rendered ineffective for antinociception assessed with noxious heat, electrical and pressure stimuli.

Male faces and odours evoke differential patterns of neurochemical release in the mediobasal hypothalamus of the ewe during oestrus: an insight into sexual motivation?

During behavioural oestrus female sheep, like females of many species, become both attracted to and sexually receptive towards males, whereas at other times they will avoid them. The mediobasal hypothalamus is the main site for the feedback action of sex steroids to induce sexual behaviour in the sheep and in previous studies we have shown that noradrenaline and serotonin are released in this region during sexual interactions with males. The current study investigated whether such changes are specific to interactions with males and if visual or olfactory cues or somatosensory stimulation during mating are critical. In vivo microdialysis sampling was carried out in the mediobasal hypothalamus of ovariectomized ewes submitted to artificial oestrous cycles. Release of monoamines and amino acid transmitters was first measured in animals during and after oestrus when they were exposed to interactions with either males or females or presentation of food. Noradrenaline concentrations only increased significantly when the females were in oestrus and interacted with males irrespective of whether intromissions were permitted. Females were then exposed to visual (faces) or odour (a home pen) cues from males or to the males themselves. Slide images of male faces increased concentrations of amines, glutamate and GABA during early oestrus, when females spent most time looking at them. During late oestrus noradrenaline, glutamate and GABA concentrations also increased in response to the male faces but no transmitter changes were seen during the luteal phase or at any time where the females were exposed to female faces, or inverted male faces. Exposure to male odour produced a lower increase in noradrenaline concentrations when females were in early oestrus but marked increases 20 and 30 min after exposure to male odours in late oestrus. No other transmitters were affected. Exposure to a male and mating with him when females were in early or late oestrus produced increased noradrenaline concentrations similar to those seen with face stimuli alone although other neurotransmitters were unaffected. These results show that noradrenaline, and to a lesser extent dopamine, serotonin, glutamate and GABA release in the mediobasal hypothalamus, can be modulated specifically in the oestrous female by sensory information coming from the male during oestrus. The differential effects of male cues during early and late oestrus suggest their involvement in (i) proceptive or anticipatory sexual responses shown by the female to male cues, and (ii) receptive sexual responses, and suggest that the mediobasal hypothalamus plays a key role in the integration of hormonal action on sexual motivation and processing of sensory information during oestrus.

Effect of delequamine (RS15385) on female sexual behaviour in the rat

The role of alpha 2-adrenoceptors in mediating the noradrenergic control of female sexual behaviour was investigated employing a selective alpha 2-adrenoceptor antagonist, delequamine (RS15385). The drug was given in graded doses of 0.01-30 mg/kg p.o. to ovariectomised plus adrenalectomised rats primed with either 2 micrograms oestradiol benzoate which yielded mainly non-receptive animals or 5 micrograms oestradiol benzoate followed 48 h later by 0.5 mg progesterone, which stimulated a high level of receptivity. Doses between 0.1 and 30 mg/kg significantly increased lordotic activity (receptivity) with an ED50 of 0.32 mg/kg, but had no effect on ear-wiggling or hopping-and-darting (proceptivity). Delequamine had no inhibitory effect in animals displaying high levels of receptivity. Thus we have shown a selective alpha 2-adrenoceptor antagonist, given orally, can stimulate female receptivity in a dose-dependent manner. Bilateral administration into the ventromedial nucleus, but not medial preoptic area, of delequamine (10 micrograms/side/rat) stimulated receptivity and it is suggested that the alpha 2-adrenoceptor may exert its effect by enhancing endogenous noradrenaline release at its active sites.

Effects of neurochemical lesions of the preoptic area on male sexual behavior in the Japanese quail

Two experiments were carried out during which the noradrenergic neurotoxin, 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine (5-ADMP) was applied to the brain of quail in order to evaluate the role of the noradrenergic system in the control of male copulatory behavior. In the first experiment, the ICV injection of 5-ADMP slightly enhanced the sexual behavior observed in testosterone (T)-treated castrated male quail. This brings additional support to the notion that norepinephrine tonically inhibits male copulatory behavior in quail. In the second experiment, 5-ADMP implanted directly into the preoptic area disrupted the restoration by T of copulatory behavior in castrated quail and, at the same time, produced a brain lesion that partly destroyed the sexually dimorphic medial preoptic nucleus, a previously established site of T action on behavior. These lesions produced by a high (presumably too high) concentration of neurotoxin provided an independent confirmation of effects previously observed after electrolytic lesions. Correlation analyses also confirmed that the medial part of the POM just rostral to the anterior commissure is more closely associated with copulatory behavior and may, therefore, represent a key center for steroid action on this behavior.

α1-Adrenergic agonists act on the ventromedial hypothalamus to cause neuronal excitation and lordosis facilitation: electrophysiological and behavioral evidence

To see if activation of central alpha 1-adrenergic receptors can cause facilitation of lordosis in rats, the behavioral effects of centrally administered alpha 1-agonists, methoxamine (MA) and phenylephrine (PhE), and related agents were studied. In ovariectomized rats treated with estrogen, infusion of MA, PhE, or a beta-agonist isoproterenol, into the lateral ventricle, or bilateral infusions of MA or PhE into the ventromedial hypothalamus (VMH) facilitated lordosis. Conversely, intra-VMH infusion of the alpha 1-antagonist prazosin (PZ) inhibited lordosis. Intra-VMH infusion of isoproterenol or an alpha 2-agonist clonidine, had no effect. Neither was the intra-VMH infusion of MA effective if: (i) the rats were not primed with estrogen; (ii) the tips of the cannulae were outside the VMH; or (iii) it was preceded by an intra-VMH infusion of the alpha 1b-antagonist, chloroethylclonidine (CEC). These results not only verify implications from recent studies that alpha 1-receptors in the hypothalamus are important for lordosis facilitation, but further show that the adrenergic facilitatory effect are: (i) mediated specifically by alpha 1b-subtype of the alpha 1-receptor, (ii) estrogen-dependent, and (iii) site-specific to VMH. To investigate neural mechanisms potentially underlying the lordosis-facilitating effect of alpha 1-activation, the actions of MA and PhE on the electrical activity of single neurons of the ventromedial nucleus of the hypothalamus (VMN) in vitro were studied.(ABSTRACT TRUNCATED AT 250 WORDS)