Medial preoptic area involvement in norepinephrine-induced suppression of pulsatile luteinizing hormone release in ovariectomized rats

The role of oxytocin in male and female reproductive behavior

Oxytocin (OT) is a nonapeptide with an impressive variety of physiological functions. Among them, the 'prosocial' effects have been discussed in several recent reviews, but the direct effects on male and female sexual behavior did receive much less attention so far. As our contribution to honor the lifelong interest of Berend Olivier in the control mechanisms of sexual behavior, we decided to explore the role of OT in the present review. In the successive sections, some physiological mechanisms and the 'pair-bonding' effects of OT will be discussed, followed by sections about desire, female appetitive and copulatory behavior, including lordosis and orgasm. At the male side, the effects on erection and ejaculation are reviewed, followed by a section about 'premature ejaculation' and a possible role of OT in its treatment. In addition to OT, serotonin receives some attention as one of the main mechanisms controlling the effects of OT. In the succeeding sections, the importance of OT for 'the fruits of labor' is discussed, as it plays an important role in both maternal and paternal behavior. Finally, we pay attention to an intriguing brain area, the ventrolateral part of the ventromedial hypothalamic nucleus (VMHvl), apparently functioning in both sexual and aggressive behavior, which are at first view completely opposite behavioral systems.

Noradrenaline involvement in the negative-feedback effects of ovarian steroids on luteinising hormone secretion

Noradrenaline has been shown to modulate the ovarian-steroid feedback on luteinising-hormone (LH) release. However, despite the high amount of evidence accumulated over many years, the role of noradrenaline in LH regulation is still not clearly understood. The present study aimed to further investigate the involvement of noradrenaline in the negative-feedback effect of oestradiol and progesterone on basal LH secretion. In experiment 1, ovariectomised (OVX) rats received a single injection of oil, oestradiol, or progesterone at 09.00-10.00 h and were decapitated 30 or 60 min later. Levels of noradrenaline and its metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), were determined in microdissections of the preoptic area (POA) and medial basal hypothalamus-median eminence (MBH-ME) and correlated with LH secretion. Basal LH levels were decreased 30 and 60 min after oestradiol or progesterone injection, and this hormonal response was significantly correlated with a reduction in POA MHPG levels, which reflect noradrenaline release. In addition, noradrenaline levels in the POA were increased, whereas noradrenaline turnover (MHPG/noradrenaline ratio) was decreased 60 min after the injection of both hormones. No effect was found in the MBH-ME. In experiment 2, i.c.v. administration of noradrenaline (60 nmol), performed 15 min before oestradiol or progesterone injection in jugular vein-cannulated OVX rats, completely prevented the ovarian steroid-induced inhibition of LH secretion. The data obtained provide direct evidence that LH secretion in OVX rats is positively regulated by basal noradrenergic activity in the POA, and its reduction appears to play a role in the negative-feedback effect of ovarian steroids on LH secretion in vivo.

Norepinephrine suppresses gonadotropin-releasing hormone neuron excitability in the adult mouse

Norepinephrine (NE) is considered to exert an important modulatory influence upon the activity of GnRH neurons. In the present study, we used a transgenic GnRH-green fluorescent protein mouse model to examine the effects of NE on the electrical excitability of GnRH neurons in male and female mice. Gramicidin-perforated patch recordings demonstrated that NE (10-100 mum) exerted a robust membrane hyperpolarization, with associated suppression of firing, in more than 85% of male prepubertal and adult GnRH neurons (n = 25). The same hyperpolarizing action was observed in female GnRH neurons from diestrous (91%, n = 11), proestrous (50%, n = 14), estrous (77%, n = 13), and ovariectomized (82%, n = 11) mice. A subpopulation (<10%) of silent GnRH neurons in all groups responded to NE with hyperpolarization followed by the initiation of firing upon NE washout. The hyperpolarizing actions of NE were mimicked by alpha1-adrenergic (phenylephrine) and beta-adrenergic (isoproterenol) receptor agonists, but alpha2 receptor activation (guanabenz) had no effect. Approximately 75% of the NE-evoked hyperpolarization was blocked by the alpha1 receptor antagonist prazosin, and 75% of GnRH neurons responded to both phenylephrine and isoproterenol. These findings indicate that NE acts through both alpha1- and beta-adrenergic receptors located on the soma/dendrites of GnRH neurons to directly suppress their excitability throughout the estrous cycle and after ovariectomy. These data force a reanalysis of existing models explaining the effects of NE on gonadotropin secretion.

The role of the locus coeruleus in corticotropin-releasing hormone and stress-induced suppression of pulsatile luteinizing hormone secretion in the female rat

Despite a wealth of evidence for CRH mediating stress-induced suppression of the hypothalamic GnRH pulse generator, and hence reproductive dysfunction, the site and mechanism of action remains elusive. The locus coeruleus (LC), a prominent noradrenergic brain stem nucleus, is innervated by CRH neurons, mediates several behavioral stress responses, and is implicated in the control of pulsatile LH secretion. The aim of this study was to test the hypothesis that LC CRH has a critical role in mediating stress-induced suppression of pulsatile LH secretion in the rat. Ovariectomized rats with 17beta-estradiol or oil-filled s.c. capsules were implanted with bilateral LC and i.v. cannulae. Central administration of CRH (10 ng to 1 microg) resulted in a dose-dependent suppression of LH pulses, which was reversed by a CRH receptor antagonist (alpha-helical CRF(9-41), 1 microg). The induction of c-fos expression in glutamic acid decarboxylase67 immunostained neurons in the preoptic area suggests activation of the secretion of gamma-aminobutyric acid in response to intracoerulear administration of CRH; 17beta-estradiol further increased the percentage of glutamic acid decarboxylase67-positive neurons that expressed fos and augmented suppression of LH pulses. Furthermore, intracoerulear administration of alpha-helical CRF(9-41) completely blocked restraint stress-induced suppression of LH pulses, without affecting the inhibitory response to hypoglycemia. These results suggest that CRH innervation of the LC may play a pivotal, but differential, role in the normal physiological response of stress-induced suppression of the GnRH pulse generator and hence the reproductive system.

Estrogen uncouples noradrenergic activation of Fos expression in the female rat preoptic area

The preoptic area of the rat brain is a site at which gonadal steroids act to regulate sexual behaviour and gonadotrophin secretion. The expression of the immediate-early gene product, Fos, in the preoptic area was investigated in conscious ovariectomised, vehicle and estrogen-treated animals which had received an intracerebroventricular (i.c.v.) infusion of noradrenaline, and also in anaesthetised proestrous and ovariectomised rats following electrical stimulation of the brainstem A1 or A2 noradrenergic cell groups. In ovariectomised oil-treated rats, a third ventricular infusion of noradrenaline (45 micrograms) resulted in a significant (P < 0.05) increase in the numbers of Fos-immunoreactive cell nuclei throughout the preoptic area, compared to vehicle controls. In contrast, Fos expression in animals which had received estrogen replacement showed no change in response to i.c.v. noradrenaline compared with saline-treated controls. In anaesthetised, ovariectomised animals electrical stimulation of the A1 cell group resulted in a significant increase (P < 0.05) in Fos-like immunoreactivity compared with sham controls, specifically within the ventral preoptic area whilst stimulation of the A2 cell group had no significant effect. In anaesthetised, proestrous rats receiving electrical stimulation no significant changes in Fos-like immunoreactivity were detected within the preoptic area after either A1 or A2 stimulation compared with paired controls. These results show that noradrenaline-induced Fos expression in the preoptic area is dependent on estrogen status and suggest that the estrogenic regulation of reproductive functions may thus involve altered responses to noradrenaline in sub-populations of preoptic neurones.

Discrete-time stimulation of the oscillatory and excitable forms of a FitzHugh-Nagumo model applied to the pulsatile release of luteinizing hormone releasing hormone

A model for the pulsatile release of luteinizing hormone releasing hormone (LHRH) can be reduced to a FitzHugh-Nagumo model subject to regular and quasiregular (i.e., with slight random variation in the interstimulus interval), discrete-time stimulation. The relationship of output pulse frequency (OPF) to stimulus frequency is compared between the excitable and oscillatory forms of the model and discussed in the context of results from other pulse-driven model systems. Some examples of the changes in OPF caused by quasiregular and purely Poissonian stimuli are given for the excitable case. The unstimulated system frequently interacts with the stimulation in such a complex manner that the OPF bears little resemblance to the frequency of stimulation or of the unstimulated system. Furthermore, the inability of the oscillatory form of the model to allow complete suppression of output pulses for moderate stimulation frequencies suggests that the LHRH system can be more appropriately described by the excitable form of the model. (c) 1995 American Institute of Physics.

Modelling the luteinizing hormone-releasing hormone pulse generator

Pituitary hormones are released in pulses as a result of episodic patterns of electrical activity in neuroendocrine neurons. The mechanisms underlying such pulsatility have, however, been difficult to elucidate. For example, the luteinizing hormone-releasing hormone neurons regulating reproductive functioning have a sparse and scattered distribution within the hypothalamus which has made definitive electrophysiological investigation impracticable. Little is known not only of their electrical characteristics, but also of the critical neural components with which they interact to form the so-called "luteinizing hormone-releasing hormone pulse generator". We have used here a neural modelling approach, based on the FitzHugh-Nagumo model of a single neuron, to provide a simple dynamical network model of this neuroendocrine pulse generator. We have found that the minimal components required to generate pulsatile luteinizing hormone secretion arise from combining luteinizing hormone-releasing hormone neurons with reciprocally connected inhibitory interneurons and an external stimulatory input. Local GABA neurons and ascending noradrenergic and/or adrenergic inputs have been used as the biological basis for these respective components. The network displays a wide repertoire of behaviours comparable with experimental observations, including some thought previously to be paradoxical. The capacity of this model network to display complex behavioural features interpretable against experimental evidence suggests that this type of modelling may become a necessary adjunct to empirical studies of pulsatile neuroendocrine systems.

Analysis of brainstem A1 and A2 noradrenergic inputs to the preoptic area using microdialysis in the rat

Noradrenergic inputs to the preoptic area (POA) are involved in regulating a variety of homeostatic functions. However, the accurate measurement of endogenous noradrenaline (NA) release in the POA has been difficult to achieve and consequently little has been done to characterise the different noradrenergic pathways. By combining the technique of intracranial microdialysis with tissue pre-loading of [3H]NA we have developed a sensitive index of NA release in the POA [8]. Using this method we have now examined and compared the effects of electrical stimulation of the brainstem A1 and A2 cell groups on NA release in the POA. Anaesthetised proestrus rats were implanted with microdialysis probes either unilaterally or bilaterally in the POA and stimulating electrodes positioned in either the A1 or A2 regions. Electrical stimulation (10 Hz, 10s on/off for 20 min) of the A1 region resulted in repeatable, calcium-dependent increases in radioactivity outflow from the ipsilateral POA (P < 0.01). A1-evoked release was twice as large as that observed after equivalent 10 Hz electrical stimulation of the A2 region (P < 0.05). In experiments using bilateral POA microdialysis and A1 stimulation, a significant increase in release from the contralateral POA, amounting to approximately 80% of that observed in the ipsilateral POA, was observed. Experiments involving the blockade of A1-stimulated release in the ipsilateral POA by perfusion with a calcium-free medium demonstrated that increases in radioactivity measured in the contralateral POA were not originating from the ipsilateral POA.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of tritiated noradrenaline release from the rat preoptic area with microdialysis in vivo

Present techniques are unable to provide a sensitive and accurate index of noradrenergic activity in the rat preoptic area. In this study, we have examined the brainstem A1 noradrenergic input to the preoptic area using a new technique whereby [3H]noradrenaline is preloaded into the preoptic area and release of radioactivity from this region is measured subsequently using microdialysis in vivo. Electrical stimulation of the ipsilateral A1 area for 20 min at 5, 10, and 15 Hz evoked significant increases in dialysate radioactivity that were repeatable and frequency-dependent. After removal of calcium from the perfusion medium, basal release of radioactivity was markedly reduced and the effect of A1 stimulation abolished. Changing to a 100 mM K+ medium evoked an increase in the release of radioactivity that was sixfold greater than that seen after A1 stimulation. Separation of the dialysate with HPLC showed that 33% of the increase in measured radioactivity after A1 stimulation was directly attributable to [3H]noradrenaline and the remainder to the metabolites vanillylmandelic acid, 3,4-dihydroxymandelic acid, and 3,4-dihydroxyphenylglycol. In contrast, the increase in radioactivity after K+ depolarization was due almost completely to [3H]noradrenaline. Addition of 10 microM clonidine to the perfusion medium markedly reduced basal release of radioactivity, but had no effect on evoked release following A1 stimulation. Conversely, perfusion with 10 microM yohimbine had no effect on basal release, but significantly increased evoked release after A1 stimulation. These results now provide a characterization of noradrenergic activity in the preoptic area and indicate the importance of the A1 noradrenergic input to this region. The technique of measuring radioactivity with microdialysis after preloading with [3H]noradrenaline provides a relatively simple, sensitive index of noradrenergic activity in vivo with good temporal resolution.

Hypothalamic electrical activity that relates to the pulsatile release of luteinizing hormone exhibits diurnal variation in ovariectomized rats

The multiunit activity (MUA) was recorded in the arcuate nucleus-median eminence region in freely moving ovariectomized rats maintained under controlled lighting conditions (lights on 05.00-19.00 h) over a period of 24 h. The MUA of each rat had been confirmed to exhibit characteristic increases (volleys) in association with pulses of luteinizing hormone. In 6 MUAs examined, the frequency of MUA volleys was significantly greater in the dark period than in the light period, suggesting a diurnal variation in the release of luteinizing hormone-releasing hormone in the ovariectomized rat.

Effects on plasma luteinizing hormone levels of microinjection of noradrenaline and adrenaline into the septo-preoptic area of the brain of the ovariectomized ewe: changes with season and chronic oestrogen treatment

We have investigated the effects on luteinizing hormone (LH) secretion of noradrenaline (NA) and adrenaline (A) microinjected (1 μl) into the septo-preoptic area of ovariectomized (OVX) ewes with or without oestrogen (E) treatment and across the breeding and non-breeding seasons. Guide tubes (19 gauge) were placed into the septo-preoptic area of OVX ewes using lateral ventriculograms for localization of the target area. The sheep were tamed so that injections could be made into conscious animals during blood sampling procedures. Jugular venous blood was collected at 10-min intervals for 3 h, an injection of NA or A (10 μg) or saline was given and samples collected for a further 3 h. The plasma samples were assayed for LH. On completion of the experiments the brains were sectioned to locate the site of injection. In the non-breeding season of the first year, 9 ewes were used of which 3 had correct guide tube placement; in these 3 ewes NA and A had no effect in OVX ewes. In OVX ewes treated with 0.5 cm Silastic ®implants of E for 1 week, plasma LH levels were reduced from 9.1 ± 1.96 nglrnl before E treatment to 2.8±0.95 ng/ml after E treatment. In these E-treated ewes NA and A caused a robust increase in plasma LH levels. In the breeding season, 9 ewes were used of which 7 had correct guide tube placement; in these 7 ewes NA and A had no effect in OVX ewes. When OVX ewes were treated with 0.5cm E implants, NA or A injection decreased LH interpulse interval. In OVX ewes which received 1.0 cm E, NA caused a pronounced but transient suppression of plasma LH secretion due to an increase in interpulse interval. When 3.0 cm E implants were given to OVX ewes there was a strong suppression of plasma LH secretion with pulsatility abolished; NA injection had no effect in these sheep. In the second year, in the non breeding-season, 17 ewes were used of which 14 had correct guide tube placement although a number of injections were above the target region. NA injection had no effect on plasma LH levels in OVX ewes but had a variable effect on OVX ewes treated with 0.5 cm E implants, depending upon the degree of suppression of plasma LH secretion by E. When plasma LH was fully suppressed by E, injection of 1Opg NA provoked a profound and sustained increase in plasma LH levels. When plasma LH secretion was pulsatile after E treatment, NA injection decreased LH interpulse interval. Similar responses were obtained with 0.5cm E-treated sheep when injected with 1.Opg NA. When OVX ewes were given 3.0cm E implants a small and non-significant (P = 0.09) rise in plasma LH levels occurred, following 10 pg NA injection. These results provide further evidence of involvement of NA/A systems in the regulation of gonadotrophin-releasing hormone (GnRH) secretion at the level of the GnRH cell bodies in the septo-preoptic area, with clear influences of season and E status on this regulation. In the OVX sheep the GnRH pulse generation system is probably subserved by endogenous permissive NA/A input rendering exogenous input ineffective. In the breeding season NAlA can inhibit GnRH/LH secretion in the presence of physiological doses of E whereas in the non-breeding season, E profoundly suppresses GnRH/LH secretion, possibly by the removal of permissive NA inputs, which can be overcome by the injection of NAlA into the septo-preoptic area.

Role of medial preoptic GABA neurones in regulating luteinising hormone secretion in the ovariectomised rat

The role of GABA neurones in the medial preoptic area (MPOA) in regulating the activity of the luteinising hormone-releasing hormone (LHRH) neurones projecting to the median eminence was investigated in the conscious ovariectomised rat. Plasma luteinising hormone (LH) concentrations were measured while (1) endogenous GABA release from the MPOA was monitored with the technique of microdialysis, or (2) activity at the GABA receptor was modulated by local infusions into the MPOA. Microdialysis studies revealed a fluctuating level of GABA release in the MPOA which did not correlate with pulsatile LH secretion. Infusion of 10 microM GABA (n = 8) or bicuculline methiodide (BMI, n = 6) into the MPOA, at a rate of 1 microliter/30 min, significantly inhibited mean LH concentrations (P less than 0.05-0.001) and LH pulse frequency (P less than 0.05-0.001) compared with controls (n = 8). LH pulse amplitude was not significantly altered by infusion of GABA (P greater than 0.05) while too few pulses were found after BMI treatment to enable statistical analysis. Infusions of GABA into the ventral half of the MPOA had a more significant inhibitory effect upon LH secretion compared with dorsal infusions (P = 0.012). A similar relationship did not exist for BMI infusions. These results show that acute changes in preoptic GABA receptor occupancy result in disruption of pulsatile LH secretion in the ovariectomised rat. This suggests that GABA neurones provide a tonic input important for the functional integrity of the neural network controlling LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

LH release in ovariectomized rats is maintained without noradrenergic neurotransmission in the preoptic/anterior hypothalamic area: extreme functional plasticity of the GnRH pulse generator

Norepinephrine (NE) in the preoptic/anterior hypothalamic area (PO/AH) is known to be involved in the regulation of luteinizing hormone (LH) secretion. The effects of selective and complete depletion of NE in the PO/AH of ovariectomized (ovx) rats on LH secretion were studied. PO/AH concentrations of NE were reduced by 90% within 6 h and were undetectable (more than 98% depletion) 52 h after bilateral stereotaxic microinjections of 50 micrograms of 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine (5-ADMP). LH levels in the blood were significantly reduced within 60 min after NE depletion but remained low only for several hours. Despite continuously low preoptic NE concentrations episodic LH secretion reoccurred within 4-6 h such that normal blood LH levels were present 6 and 52 h after selective NE depletion. While the alpha 1-adrenoreceptor antagonist prazosin was inhibitory to LH secretion in control rats the drug was totally ineffective in the NE depleted animals. NE may be inhibitory to LH secretion via a beta-adrenergic receptor mechanism. It was therefore also tested whether 5-ADMP causes a massive NE release which might be inhibitory to LH secretion. Propranolol (PROP), a beta-adrenoreceptor blocking drug, was given 30 min prior to preoptic injection of 5-ADMP. Blockade of beta-receptors did not prevent the transient inhibition of LH release. These results indicate that under physiologic conditions the GnRH pulse generator functions only properly when NE is present in the PO/AH and that the stimulatory effect of NE is mediated via an alpha 1-adrenoreceptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Medial preoptic-anterior hypothalamic area involvement in the suppression of pulsatile LH release by a mu-opioid agonist in the ovariectomized rat

The objective of this study was to examine whether specific activation of mu-opioid receptors at the level of the medial preoptic-anterior hypothalamic area (MPOA-AHA) could suppress pulsatile LH release. The experiments were done using rats that had been ovariectomized (OVX) 24 hr before on diestrus 2, animals in which we have previously demonstrated an active endogenous opioid peptide suppression of pulsatile LH release (2). DAGO, DPDPE, or U50488H, specific agonists of mu-, delta- and kappa-opioid receptors, respectively, were continuously applied directly to the MPOA-AHA by means of push-pull perfusion. Perfusion of the MPOA-AHA with 0.5 micrograms DAGO/hr suppressed LH pulse amplitude. This effect of DAGO was not due to spread to the third ventricle and subsequent diffusion via the CSF to another CNS site, since push-pull perfusion with this dose of DAGO in the region just dorsal to or in the posterior hypothalamus was ineffective in altering LH pulse amplitude. The response to DAGO was dose-dependent since a higher dose (4.8 micrograms/hr) markedly suppressed both LH pulse amplitude and frequency. The same doses of DPDPE and U50488H (0.5 and 4.8 micrograms/hr) had no effect on pulsatile LH secretion, providing support for mu receptor involvement in the DAGO-induced suppressive action. These data demonstrate MPOA-AHA involvement in the suppression of pulsatile LH release by a mu-opioid agonist in the OVX rat.

A comparison of the effects of medial preoptic electrical versus electrochemical stimulation on luteinizing hormone-releasing hormone neuronal responsiveness to norepinephrine

Abstract Recently, we reported that luteinizing hormone-releasing hormone (LHRH) neurons of estrogen-treated, ovariectomized rats have only limited responsiveness to norepinephrine (NE). These conclusions were based upon observations that NE, when infused intracerebroventricularly, produced only minor increases in plasma luteinizing hormone (LH), whereas, similar infusions following preliminary medial preoptic area (MPOA) electrochemical stimulation (ECS) markedly amplified LH secretion. One difficulty with this approach is that ECS produces an irritative lesion and deposits iron within the tissue, whereas, electrical stimulation (ES) does not have such effects. Accordingly, in the present study, we compared the effects of MPOA-ECS versus -ES on LHRH neuronal responsiveness to NE. While equivalent peak LH concentrations occurred within 15 min after MPOA-ECS or -ES, in the ECS group, LH release was sustained, whereas, it abruptly ceased upon termination of ES (at 15 min). The intracerebroventricular pulse infusion of NE at the time of peak LH secretion (30 min) in MPOA-ECS animals markedly amplified LH release. In these animals, plasma LH remained significantly elevated for 75 min before a decline was observed. In contrast, an infusion of NE at the time of maximal LH release in ES rats (16 min) did not augment LH secretion. The second series of studies examined the effects of MPOA infusions of NE in animals receiving preoptic ES. A single infusion of NE 16 min after ES (i.e. one min after termination of ES) did not amplify LH release, but when two NE pulses were given at 5 and 16 min after beginning preoptic ES, peak plasma LH levels were maintained for an additional 30 min before a decline occurred. Pretreatment of rats with a yS-adrenoreceptor antagonist (propranolol) or a monoamine oxidase inhibitor did not affect peak LH responses obtained after either MPOA-ES alone or combined with two pulses of NE infused into the MPOA at 5 and 16 min. We conclude that following cessation of MPOA-ES, LHRH neurons rapidly lose their responsiveness to NE, whereas, rats which received MPOA-ECS retain such responsiveness possibly due to the stimulative properties of the iron deposited by the ECS. Presumably, for NE to trigger an LH surge requires prior removal of some intrinsic inhibitory control which regulates LHRH neuronal responsiveness to NE.

Arcuate Nucleus and Preoptic Area Involvement in Beta-Endorphin-lnduced Release of Prolactin in Conscious Ovariectomized Ratsdagger

Abstract The ability of ss-endorphin (ss-END) to release prolactin and the ability of naloxone to block prolactin's release when delivered to specific hypothalamic areas via push-pull perfusion was studied in unrestrained, conscious, Ovariectomized rats. Perfusion of either the arcuate nucleus or the preoptic area with ss-END for 15 to 30 min caused a large, brief increase in plasma prolactin levels. Perfusion for a longer time period (120 min) resulted in peak prolactin levels at 60 min, with a return to baseline by 120 min, suggesting that ss-END primarily acts to induce a sequence of events that culminates in prolactin release, but other factors are needed to maintain this release over long periods of time. Perfusion of the arcuate nucleus for two 15-min periods 90 min apart resulted in two surges of prolactin. When naloxone, the opiate receptor antagonist, was added to the perfusate, ss-END was not capable of stimulating prolactin release. These results provide a model to answer whether endogenous ss-END has a role in the neuroendocrine regulation of prolactin surges and what the location is of the opiate neurons involved in this neuronal pathway.

Limited responsiveness of LHRH neurons to norepinephrine may account for failure of locus coeruleus or medullary A1 electrical stimulation to increase plasma LH in estrogen-treated ovariectomized rats

We recently reported that electrical stimulation (ES) of the locus coeruleus (LC) or the medullary A1 noradrenergic cell groups markedly increased LH secretion. However, these amplifying effects occurred only in rats in which preliminary electrochemical stimulation (ECS) of the medial preoptic nucleus (MPN) was performed. In contrast, ES of either LC or A1 alone did not alter basal LH secretion. Possible explanations for this dichotomy in LH response include: (1) LHRH neurons in unanesthetized, estrogen-treated ovariectomized (OVX) rats are relatively unresponsive to NE, (2) the chloral hydrate anesthesia used in our brain stimulation studies elevates the threshold of excitability of LHRH neurons to norepinephrine (NE) and/or pituitary responsiveness to LHRH, (3) preliminary MPN-ECS reduces thresholds of responsiveness of LHRH neurons to NE, and (4) the LHRH secreted after MPN-ECS sensitizes the pituitary gland to the subsequent small amounts of LHRH released following LC- or A1-stimulation. To provide answers to these questions, 3 experiments were performed in estrogen-treated OVX rats into which had been inserted the third ventricle and jugular cannulae. In the first study, the effects of artificial cerebrospinal fluid (ACSF) or ACSF + NE (5, 20, or 45 micrograms) on plasma LH concentrations in unanesthetized, unrestrained rats were examined. The intracerebroventricular (ICV) infusion of 5 micrograms of NE increased plasma LH by 61.3% above basal levels within 10 min whereas 20 or 45 micrograms NE elevated LH values 166.9 and 182.8%, respectively. The next study examined the effects of anesthetic drugs on LH response produced by ICV infusions of 45 micrograms of NE. Regardless of whether rats were anesthetized with ether, chloral hydrate, urethane, Saffan (alphaxolone + alphadolone) or ketamine + acepromazine, peak LH responses to ICV NE were not significantly different from unanesthetized controls. In a second study we observed that ICV NE (45 micrograms) markedly amplified and prolonged the release of LH after MPN-ECS. Moreover, the peak LH responses in these animals were approximately 10 x greater than those obtained in rats which received ICV NE but not MPN-ECS. The third series of studies demonstrated that pituitary responsiveness to LHRH was not an important factor in dictating the LH response obtained after NE ICV infusions. These data suggest that LHRH neurons in estrogen-primed OVX rats are not particularly responsive to NE and that following MPN-ECS, LHRH neuronal responsiveness to this catecholamine markedly increases.

A1 noradrenergic action on medial preoptic-medial septal neurons: a neuropharmacological study

In an attempt to determine whether the excitatory and inhibitory orthodromic responses of single medial preoptic-medial septal (MPO-S) neurons to discrete electrical stimulation of the A1 noradrenergic region were mediated specifically by norepinephrine (NE) and involved different types of adrenoreceptors, a series of electrophysiological and neuropharmacological experiments was conducted. Extracellular single unit recording and local drug application techniques were used in female rats under urethane anesthesia. Chemical lesion of the catecholaminergic nerve terminal plexus in the medial preoptic area with 6-hydroxydopamine abolished both excitatory and inhibitory orthodromic effects of A1 region stimulation on MPO-S neurons, suggesting the noradrenergic nature of the effects. This conclusion was corroborated by the observation that the orthodromic effects were mimicked by locally applied exogenous NE. The excitatory effects were reliably mimicked by a low concentration of NE (0.5 mM; in-barrel concentration) and methoxamine (1.0 mM, an alpha-1 agonist), but not by either low or high concentrations (1 and 100 mM) of clonidine (an alpha-2 agonist) and isoproterenol (a beta agonist). The inhibitory orthodromic effects of A1 region stimulation were reliably mimicked by a high concentration of NE (50 mM), clonidine (100 mM) and isoproterenol (100 mM), but not by a low concentration of NE (0.5 mM), methoxamine (1 mM), clonidine (1 mM) or isoproterenol (1 mM). A high concentration (100 mM) of methoxamine mimicked the inhibitory effects less than 40% of the time. The low concentration (0.5 mM) NE-induced excitation that matched the excitatory orthodromic effect of A1 region stimulation was blocked by phentolamine (100 mM), an alpha blocker, but not by timolol (100 mM), a beta blocker. On the other hand, the high concentration (50 mM) NE-induced inhibition that matched the inhibitory orthodromic effect of A1 region stimulation was blocked by timolol, but not by phentolamine. Taken together, the present results are consistent with the hypotheses that the ascending noradrenergic projections from the A1 region affect the excitability of MPO-S neurons directly through NE and that the excitatory and inhibitory orthodromic effects involve different types of adrenoreceptors, i.e., alpha-1 and beta receptors, respectively.

Responses of ventromedial hypothalamic neurons in vitro to norepinephrine: dependence on dose and receptor type

Application of norepinephrine (NE) at 12.5 microM in the bath surrounding hypothalamic slices from ovariectomized rats could evoke excitation, inhibition, or biphasic inhibition-excitation from single neurons in the ventromedial nucleus. Whether the rats were treated with estrogen or not did not alter the distribution of the type of neuronal responses to NE in vitro. Altering the composition of the bathing solution to achieve synaptic blockade did not abolish or alter the type of responses, indicating that all these types of NE responses, including both phases of the biphasic response, were mediated by postsynaptic receptors. Experiments with varying doses of NE showed that the inhibitory response could be evoked at doses lower than those required to evoke the excitatory response. The effective dose for 50% of the responsive neurons (ED50) was lower than 1.25 microM for inhibitions and higher than 5 microM for excitations. Using specific adrenergic receptor agonists and antagonists, it was found that the excitation and the inhibition were mediated, primarily, by alpha 1- and alpha 2-receptors, respectively. beta-Receptors played only a minor role, but might be related to both excitation and inhibition. Study with adrenergic agents further revealed that different types of adrenergic receptors co-localized not only in neurons showing the biphasic response, but also in a major portion of neurons showing monophasic excitation or inhibition. Because of the co-localization and the differential sensitivities to NE, alteration of the dose of NE or the ratio of excitatory/inhibitor receptors co-localized on a neuron should be able to reverse the type of a neuronal response to NE.

Norepinephrine stimulates LH-RH secretion into the hypophysial portal blood of the rat

The present study was designed to investigate the effect of intracerebroventricular infusion of norepinephrine (NE) on the secretion of luteinizing hormone-releasing hormone (LH-RH) into the hypophysial portal blood of steroid-primed ovariectomized rats. Saline infusion into the third ventricle caused no significant change in LH-RH levels. NE infusion (20 micrograms) resulted in a significant release of LH-RH (p less than 0.05) into the portal blood 10-30 min later. This endogenous LH-RH was similar to synthetic LH-RH when characterized by thin-layer chromatography. LH secretion in similarly treated rats but with intact portal vessels, also was significantly elevated (p less than 0.05) at 20 and 40 min after the start of NE infusion. These results show that NE stimulated the secretion of LH-RH into the hypophysial portal blood and this correlated with an enhanced release of LH.