Mediation of norepinephrine-stimulated cyclic AMP accumulation by adrenergic receptors in hypothalamic and preoptic area slices: effects of estradiol

Sex-influence of nicotine and nitric oxide on motor coordination and anxiety-related neurophysiological responses

RATIONALE

Nitric oxide (NO) is a messenger synthesized in both the neuronal and glial populations by nitric oxide synthase type 1 (NOS1). Nicotine regulates NO production in a sex-dependent manner, both molecules being involved in motor function.

OBJECTIVE

The present study evaluates sex differences in motor coordination, general movement, and anxiety-related responses resulting from both constant and continuous nicotine treatment and the genetic depletion of NOS1 activity.

METHODS

Male and female mice were analyzed with the open-field and the rotarod tests. To understand the role of NO, knockout mice for NOS1 (NOS1-/-) were analyzed. Nicotine was administered continuously at a dose of 24 mg/kg/day via osmotic mini-pumps over 14 days because the behavioral effects elicited are similar to those observed with discontinuous administration.

RESULTS

Data analyses revealed noteworthy sex differences derived from NOS1 depletion. Control NOS1-/- males exhibited an exacerbated anxiety-related response in relation to control NOS1-/- females and control wild-type (WT) males; these differences disappeared in the nicotine-administered NOS1-/- males. Additionally, nicotine administration differentially affected the horizontal movements of NOS1-/- females with respect to WT animals. NO depletion affected male but not female motor coordination improvement along the test days. However, the drug affected female motor coordination only at the end of the administration period.

CONCLUSIONS

We show for the first time that NO affects motor and anxiety behaviors in a sex-dependent manner. Moreover, the behavioral effects of constant nicotine administration are dimorphic and dependent on NO production.

Cyclic AMP relaxation of rat aortic smooth muscle is mediated in part by decrease of depletion of intracellular Ca(2+) stores and inhibition of capacitative calcium entry

Despite a large number of studies, the mechanism by which 3',5'-cyclic monophosphate (cAMP) induces vasorelaxation is not fully understood. The comparison between results obtained in different vessels or species has often been the source of conflicting reports. In order to shed more light onto this mechanism, we studied the effects of forskolin in phenylephrine-pre-contracted endothelium-denuded rat aorta and measured cAMP levels in rat aortic myocytes by enzyme-immunoassay. Nanomolar forskolin relaxed phenylephrine-induced contractions. This effect was mimicked by dibutyryl-cAMP and was potentiated by rolipram or a p38-mitogen-activated protein kinase (p38-MAPK) inhibitor (SB-203580). Nifedipine and verapamil partially relaxed phenylephrine-induced contractions, while further application of cAMP-elevating agents fully relaxed these contractions. In Ca(2+)-free extracellular solution, forskolin reduced phenylephrine-induced transient contractions and reduced the Ca(2+)-induced contraction after depletion of intracellular stores. Nanomolar concentrations of forskolin increased basal cAMP levels only in the presence of rolipram or phenylephrine, which did not modify intracellular levels of cAMP by themselves. In conclusion, relaxation by cAMP is mediated in part by decrease of depletion of intracellular Ca(2+) stores and inhibition of capacitative calcium entry. This study provides the first evidence that inhibition of PDE4 or p38-MAPK potentiates the vasodilator effect of cAMP-elevating agents in rat aortic myocytes.

Cold exposure increases the biosynthesis and proteolytic processing of prothyrotropin-releasing hormone in the hypothalamic paraventricular nucleus via beta-adrenoreceptors

Different physiological conditions affect the biosynthesis and processing of hypophysiotropic proTRH in the hypothalamic paraventricular nucleus, and consequently the output of TRH. Early studies suggest that norepinephrine (NE) mediates the cold-induced activation of the hypothalamic-pituitary-thyroid axis at a central level. However, the specific role of NE on the biosynthesis and processing of proTRH has not been fully investigated. In this study, we found that NE affects gene transcription, protein biosynthesis, and secretion in TRH neurons in vitro; these changes were coupled with an up-regulation of prohormone convertase enzymes (PC) 1/3 and PC2. In vivo, NE is the main mediator of the cold-induced activation of the hypothalamic-pituitary-thyroid axis at the hypothalamic level, in which it potently stimulates the biosynthesis and proteolytic processing of proTRH through a coordinated up-regulation of the PCs. This activation occurs via beta-adrenoreceptors and phosphorylated cAMP response element binding signaling. In contrast, alpha-adrenoreceptors regulate TRH secretion but not proTRH biosynthesis and processing. Therefore, this study provides novel information on the molecular mechanisms of control of hypophysiotropic TRH biosynthesis.

Preoptic alpha 1- and alpha 2-noradrenergic agonists induce, respectively, PGE2-independent and PGE2-dependent hyperthermic responses in guinea pigs

We have shown previously that norepinephrine (NE) microdialyzed into the preoptic area (POA) of conscious guinea pigs stimulates local PGE(2) release. To identify the cyclooxygenase (COX) isozyme that catalyzes the production of this PGE(2) and the adrenoceptor (AR) subtype that mediates this effect, we microdialyzed for 6 h NE, cirazoline (alpha(1)-AR agonist), and clonidine (alpha(2)-AR agonist) into the POA of conscious guinea pigs pretreated intrapreoptically (intra-POA) with SC-560 (COX-1 inhibitor) or nimesulide or MK-0663 (COX-2 inhibitors) and measured the animals' core temperature (T(c)) and intra-POA PGE(2) responses. Cirazoline induced T(c) rises promptly after the onset of its dialysis without altering PGE(2) levels. NE and clonidine caused early falls followed by late rises of T(c); intra-POA PGE(2) levels were closely correlated with this thermal course. COX-1 inhibition attenuated the clonidine-induced T(c) and PGE(2) falls but not the NE-elicited hyperthermia, but COX-2 inhibition suppressed both the clonidine- and NE-induced T(c) and PGE(2) rises. Coinfused cirazoline and clonidine reproduced the late T(c) rise of clonidine but not its early fall and also not the early rise produced by cirazoline; on the other hand, the PGE(2) responses were similar to those to NE. Prazosin (alpha(1)-AR antagonist) and yohimbine (alpha(2)-AR antagonist) blocked the effects of their respective agonists. These results indicate that alpha(1)- and alpha(2)-AR agonists microdialyzed into the POA of conscious guinea pigs evoke distinct T(c) responses: alpha(1)-AR activation produces quick, PGE(2)-independent T(c) rises, and alpha(2)-AR stimulation causes an early T(c) fall and a late, COX-2/PGE(2)-dependent T(c) rise.

Role of preoptic second messenger systems (cAMP and cGMP) in the febrile response

The present study aimed to test the hypothesis that a decrease in preoptic cAMP mediates fever. To this end, body core temperature (T(c)) of unanesthetized, freely moving rats was monitored by biotelemetry before and after pharmacological modulation of the cAMP pathway, and cAMP levels in the anteroventral third ventricular region (AV3V), where the preoptic region (POA) is located, were determined. We observed that intra-POA administration of the cAMP agonist dibutyryl-cAMP (Db-cAMP, 40 microg) reduced T(c). PGE(2) (the proximal mediator of fever, 200 ng) raised T(c) with a concomitant decrease in AV3V cAMP levels from 22.7+/-1.8 to 17.0+/-1.0 fmol/microg protein. Moreover, PGE(2)-induced fever was impaired by the phosphodiesterase inhibitor aminophylline. In order to verify the interaction between the cAMP- and cGMP-dependent pathways in the POA, we then co-injected Db-cAMP and 8-Br-cGMP into the POA. As a result, 8-Br-cGMP augmented the drop in T(c) evoked by Db-cAMP. Lastly, we observed that intra-POA co-microinjection of the protein kinase A inhibitor (Rp-cAMPS, 1 microg) with the protein kinase G inhibitor (Rp-cGMPS, 1 microg), mimicking the effects of reduced production of cAMP and cGMP, respectively, produced a fever-like response. In summary, the present data support that a decrease in the levels of cAMP and cGMP in the POA is associated with the genesis of fever.

Insulin-like growth factor-1 regulation of alpha(1)-adrenergic receptor signaling is estradiol dependent in the preoptic area and hypothalamus of female rats

Recently, we demonstrated that estradiol (E(2)) modulates cross-talk between protein tyrosine kinases and norepinephrine (NE) receptor signaling in the hypothalamus (HYP) and preoptic area (POA), brain areas that govern female reproductive function. We are now investigating the identity of protein tyrosine kinase(s) that modify NE receptor signaling in the HYP and POA. Incubation of POA and HYP slices with insulin-like growth factor I (IGF-I), which signals via a receptor (IGF-IR) with endogenous tyrosine kinase activity, enhances NE-stimulated cAMP accumulation only in tissue derived from ovariectomized, E(2)-primed animals. JB-1, an antagonist for IGF-IR, prevents the IGF-I enhancement of NE-stimulated cAMP accumulation in both POA and HYP slices. IGF-I enhances NE-stimulated cAMP accumulation via modulation of alpha(1)-adrenoceptor potentiation of adenylyl cyclase. Binding studies in membranes demonstrate that ovariectomized, E(2)-primed animals show a significant increase in the density of [(125)I]IGF-I-binding sites in both POA and HYP compared with ovariectomized control animals. Neither the IC(50) for [(125)I]IGF-I displacement by IGF-I nor the levels of IGF-I binding proteins in serum or brain tissue are affected by E(2). RIA results showed that E(2) does not modify serum or brain IGF-I levels. These results indicate that E(2) regulation of NE receptor function in the POA and HYP involves increased expression of IGF-IR, and that after E(2) treatment, IGF-IR activation augments alpha(1)-adrenoceptor signaling.

Estrogen modulates norepinephrine-induced accumulation of adenosine cyclic monophosphate in a subpopulation of immortalized luteinizing hormone-releasing hormone secreting neurons from the mouse hypothalamus

A subpopulation of luteinizing hormone-releasing hormone (LHRH)-producing cells that express the intermediate filament protein vimentin and the neuronal marker neurofilament 145, but not neurofilament 200 nor glial fibrillary acidic protein, has been isolated from GT1-7 cultures. These cells express the mRNA encoding estrogen receptor alpha (ERalpha) and respond to physiological concentrations of 17beta-estradiol (E2) by reducing the accumulation of cyclic adenosine monophosphate induced by norepinephrine, but not that induced by direct activation of adenylate cyclase. These results indicate that the activity of LHRH-producing neurons may be directly modulated by estrogen. In addition, they are suggestive of an estrogen-dependent desensitization of the beta-adrenoceptor in these cells.

The effect of diabetes on alpha2-adrenergic receptor activity in the reproductive centers of the female rat brain

OBJECTIVE

We hypothesized that known diabetes-induced deficits in female rat reproduction may result in part from decreased central alpha(2)-noradrenergic receptor density or affinity.

STUDY DESIGN

Female rats were oophorectomized and divided into 2 groups; one group received streptozocin during the operation to induce diabetes, and the other served as a nondiabetic control group. Random blood glucose levels were measured. Half the rats in each group were killed on postoperative day 10, and half were killed on postoperative day 14. Direct radioligand binding assays were performed on tissue prepared from the hypothalamus, preoptic area, and cortex of each rat. Analysis of variance was used to evaluate intergroup differences in receptor concentration or equilibrium constant.

RESULTS

We detected no significant difference in the mean receptor concentration or equilibrium constant between the groups with and without diabetes in the hypothalamus, the preoptic area, and the cortex on postoperative day 10 or 14.

CONCLUSION

Diabetes-induced impairments in female rat reproduction do not involve alterations in alpha(2)-receptor density or affinity in the hypothalamus, preoptic area, or cortex.

Tyrosine kinase effects on adrenoceptor-stimulated cyclic AMP accumulation in preoptic area and hypothalamus of female rats: modulation by estradiol

These studies examined the functional interactions between adrenergic G-protein coupled receptors and protein tyrosine kinases in the preoptic area and hypothalamus, brain regions that regulate reproductive function in female rats, and evaluated whether in vivo treatment with estradiol for 2 days modulates the cross-talk between these two signaling pathways. In hypothalamic slices genistein, a general tyrosine kinase inhibitor, enhances norepinephrine-stimulated cAMP synthesis independent of estradiol treatment. Genistein appears to act by increasing beta-adrenoceptor signaling. At high norepinephrine concentrations, estradiol potentiates genistein enhancement of the cAMP response in hypothalamic slices. This interaction between estradiol and genistein appears to involve modification of alpha(2)-adrenoceptor signaling mechanisms. In preoptic area slices, genistein enhancement of norepinephrine-stimulated cAMP synthesis is only observed in estradiol-treated rats. In this brain region, genistein enhances cAMP accumulation by modifying alpha(1)- and/or alpha(2)-adrenoceptor rather than beta-adrenoceptor signaling. Genistein amplification of norepinephrine-stimulated cAMP synthesis is not mediated by interactions with estrogen receptors, or by regulation of adenylyl cyclase or phosphodiesterase activities. At the concentration used, genistein inhibits tyrosine phosphorylation in slices from both brain regions. Daidzein, an inactive analogue of genistein, fails to enhance the norepinephrine-stimulated cAMP response in either brain region independent of hormone treatment. These results suggest that protein tyrosine kinases regulate adrenergic responses in the hypothalamus and preoptic area. Moreover, the functional interaction between adrenergic G-protein coupled receptor signaling and protein tyrosine kinases is modified in a brain region and receptor subtype specific manner by estradiol.

mu-Opioid agonist-stimulated [35S]GTPgammaS binding in guinea pig hypothalamus: effects of estrogen

mu-Opioid receptors play a critical role in the regulation of the female reproductive cycle, and estrogen modulates the coupling of mu-opioid receptors to a potassium channel in the basal hypothalamus (BH) of the female guinea pig. Therefore, we ascertained the distribution of mu-opioid receptors in the BH with autoradiography using the mu-opioid selective agonist [3H]DAMGO. In addition, we investigated the effects of estrogen on DAMGO- or the GABAB receptor agonist baclofen-stimulated [35S]GTPgammaS binding in the BH. Based on the high density of mu-opioid receptors, but the lack of effects of estrogen on [35S]GTPgammaS binding, we conclude that mu-opioid receptor interaction with its G-protein is not the target of estrogen's actions.

A thermodynamic analysis of estrogen regulation of alpha 2-adrenoceptor binding

We previously demonstrated that in vivo estradiol treatment markedly attenuates alpha 2-adrenoceptor function and coupling to G-proteins in the hypothalamus of female rats. Ligand binding studies indicated that 48 h exposure to estradiol decreases the number of alpha 2-adrenergic receptors in the agonist high affinity state. In the present studies, when [3H]RX821002 was used to label brain alpha 2-adrenoceptors, the density of binding sites significantly increased in the hypothalamus and preoptic area 48 h after estrogen treatment. Moreover, the thermodynamics of ligand binding to alpha 2-adrenergic receptors in membranes of female rat hypothalamus were modified by the same estradiol treatments that reduce alpha 2-adrenoceptor function. In hypothalamic membranes from ovariectomized control rats, antagonist (RX821002)-receptor binding was primarily entropy-driven while agonist (oxymetazoline) binding had a higher enthalpy component. In membranes from estradiol-exposed animals, the entropic contribution to both agonist and antagonist bindings was markedly increased, and the enthalpy component was reduced. Since the thermodynamic characteristics of ligand-receptor binding are strongly correlated with efficacy in activating signal transduction [36], these data raise the intriguing possibility that steroids regulate transmembrane signaling by stabilization of a receptor conformation with reduced intrinsic efficacy.

Estradiol plus progesterone promote glutamate-induced release of γ-aminobutyric acid from preoptic area synaptosomes

Treatment of ovariectomized rats with both estradiol and progesterone in vivo resulted in a marked enhancement of glutamate-induced release of newly synthesized [3H]gamma-aminobutyric acid (GABA) from synaptosomes of the preoptic area in vitro. With this treatment, as little as 0.01 nM glutamate, in vitro, enhanced release of GABA. In contrast, glutamate, in vitro, did not stimulate release of GABA from synaptosomes, obtained from rats treated with either estradiol or progesterone alone and only large concentrations of glutamate (1.0 and 10 mM) caused a modest release of GABA from synaptosomes from ovariectomized, vehicle-treated rats. Also, treatment with estradiol plus progesterone did not alter glutamate-induced release or exchange of [3H]glutamate. Glutamate-induced release of GABA was calcium-independent and attenuated by the putative chloride channel antagonist, 4,4'-diisothiocyanatostilbene-2,2'-DL-disulfonic acid. Thus, glutamate-induced, steroid-enhanced release of GABA may occur through a chloride-dependent carrier rather than by exocytosis. In addition to enhancement by glutamate, release of GABA was also enhanced by D-aspartate, an agent that is transported by the neuronal glutamate carrier. It is postulated that enhancement of glutamate-induced release of GABA, by estradiol plus progesterone in the preoptic area, represents one process by which these steroids modulate reproductive function in female rats.

Estrogen modulates stimulation of inositol phospholipid hydrolysis by norepinephrine in rat brain slices

The influence of estrogen on stimulation of inositol phospholipid hydrolysis by norepinephrine and carbamylcholine has been studied by measuring the accumulation of [3H]inositol-monophosphate ([3H]InsP) in cortical, hippocampal and striatal slices from ovariectomized rats. Repeated (but not a single) subcutaneous injections of estradiol benzoate (EB) (2 micrograms/animal once every 2 days for 10 days) markedly reduced stimulation of inositol phospholipid hydrolysis by norepinephrine in hippocampus and corpus striatum. Conversely, the efficacy of norepinephrine was increased in cortical slices. Estrogen treatment did not affect basal or carbamylcholine-stimulated [3H]InsP formation. In vitro addition of 17 beta-estradiol (1-100 nM) failed to modify norepinephrine- or carbamylcholine-induced [3H]InsP production in all regions examined. An increased density of alpha 1-adrenergic binding sites in cortical membranes paralleled the enhanced responsiveness of inositol phospholipid hydrolysis to norepinephrine induced by EB treatment in this area, whereas no significant changes in [3H]prazosin binding were found in membranes from hippocampus and corpus striatum. These results indicate that estrogen may affect inositol phospholipid hydrolysis in discrete brain areas, suggesting a complex role for estradiol in modulating noradrenergic receptor activity in the central nervous system.

Protein kinase C and phospholipase C mediate alpha 1- and beta-adrenoceptor intercommunication in rat hypothalamic slices

These experiments examined the mechanism by which phenylephrine enhances beta-adrenoceptor-stimulated cyclic AMP formation in rat hypothalamic and preoptic area slices. To this end we manipulated phospholipase C. phospholipase A2, and protein kinase C activity in slices and assessed the effects of these manipulations on phenylephrine augmentation of isoproterenol-stimulated cyclic AMP generation. Since previous work indicated that estrogen enhances the alpha 1-component of cyclic AMP formation, we examined slices from both gonadectomized and estrogen-treated animals. The alpha 1-antagonist prazosin eliminated phenylephrine augmentation of the beta-response, suggesting that alpha 1-adrenergic receptors mediate the potentiation of cyclic AMP formation. Inhibition of protein kinase C by H7 attenuated the alpha 1-augmentation of beta-stimulated cyclic AMP formation. Staurosporine, a more potent protein kinase C inhibitor, completely abolished the alpha 1-augmenting response. In addition, phenylephrine potentiation of the isoproterenol response was not observed if protein kinase C was first stimulated directly with a synthetic diacylglycerol (1-oleoyl-2-acetyl-sn-glycerol) or phorbol ester (phorbol 12,13-dibutyrate). Neomycin, an inhibitor of phospholipase C, decreased alpha 1-receptor enhancement of beta-stimulated cyclic AMP formation, whereas quinacrine, an inhibitor of phospholipase A2, did not. The data suggest that the postreceptor mechanism involved in alpha 1-adrenergic receptor potentiation of cyclic AMP generation in hypothalamic and preoptic area slices includes activation of phospholipase C and protein kinase C.

Intrahypothalamic implants of noradrenergic antagonists disrupt lordosis behavior in female rats

There is considerable experimental evidence that hormonal activation of lordosis in female rats involves norepinephrine (NE) neurotransmission. However, no clear picture has emerged regarding either: 1) the neural sites at which NE influences lordosis, or 2) the NE receptor subtype(s) mediating NE effects on lordosis. To address these two issues, the behavioral effects of antagonists with relative specificity for alpha 1, alpha 2, or beta adrenergic receptors were examined. Drugs were administered via bilateral crystalline implants directly into the ventromedial nucleus of the hypothalamus (VMN) or medial preoptic area (MPOA) of ovariectomized female rats primed for 48 hr with 3 micrograms of estradiol benzoate (EB) and given 200 micrograms of progesterone (P) 3.5-4 hr before testing. When applied to the VMN 1 hr before the P injection, the alpha 1 receptor antagonist prazosin reduced lordosis behavior in 86% of animals but in only 10% of rats when applied to the MPOA. However, prazosin did not inhibit lordosis when implanted into the VMN just prior to EB administration. Yohimbine, an alpha 2 receptor antagonist with low affinity for alpha 1 receptors, also suppressed lordosis in 41% of animals with VMN implants and in 37% of rats with MPOA implants. By contrast, the alpha 2 antagonist idazoxan, which has little activity at alpha 1 receptors, did not significantly affect estrous responding when implanted into either the VMN or MPOA. VMN implants of the beta receptor antagonists propranolol and pindolol reduced lordosis behavior in 50% and 86% of rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Ovarian steroid levels in migraine with and without aura

Radioimmunoassays were used to measure interictal levels of ovarian steroids (oestradiol, total oestrogens and progesterone) in migraine patients at the onset of menses and coincident with the luteinizing hormone surge preceding ovulation. Results of these verified biochemically-contrasting points of the ovarian cycle were used to compare 13 migraine patients without aura and 6 migraine patients with aura with 17 non-migraine women. No group differences were found for physiological basal levels of ovarian steroids measured at menses. Preceding ovulation elevation in oestradiol levels relative to normal was found in migraine patients with aura but not in migraine patients without aura. These results suggest that a variation in oestradiol levels is an important factor in the different clinical expressions of migraine.

Inhibition of cyclic AMP accumulation by alpha 2-adrenoceptors in the rat cerebral cortex

The effects of alpha 2-adrenoceptor agonist and antagonists on the accumulation of cyclic AMP were examined in rat cerebral cortex slices. Norepinephrine (10(-4) M) caused a 123 +/- 11% increase in the cyclic AMP concentration in the cortical slices, which was greater than the increase (89 +/- 7% increase) caused by isoproterenol (10(-4) M) alone. However, the cyclic AMP response to norepinephrine was completely inhibited by propranolol (10(-4) M), a beta-adrenoceptor antagonist. Yohimbine (10(-7)-10(-5) M), an alpha 2-adrenoceptor antagonist, intensified the cyclic AMP response to norepinephrine by 30%, whereas, clonidine, an alpha 2-adrenoceptor agonist, decreased the response. Treatment with reserpine (3.0 mg/kg) reduced the density of [3H]p-aminoclonidine binding sites (Bmax, 93.8 +/- 18.4 fmol/mg protein) compared to the density in non-treated rats (154.4 +/- 33.5 fmol/mg protein). The potentiating effect of yohimbine and the inhibitory effect of clonidine on the cyclic AMP response to norepinephrine were also reduced. These results suggest that alpha 2-adrenoceptors regulate the accumulation of cyclic AMP in the rat cerebral cortex in an inhibitory fashion. The results also suggest that the accumulation is mediated through beta-adrenoceptors and that this response is intensified by alpha 1-adrenoceptor stimulation.

Hormonal Activation of Female Sexual Behavior is Accompanied by Hypothalamic Norepinephrine Release

Abstract The present study employed the intracranial microdialysis technique to measure norepinephrine release in the ventrolateral dendritic fields of the ventromedial hypothalamus of freely-moving animals before and during ovarian steroid (estradiol and progesterone) activation of female sexual behavior (lordosis). One day after implantation of a dialysis probe, animals were injected with 3 mug of estradiol benzoate followed 44 h later by 200 mug of progesterone. Introduction of a male rat 4 h after progesterone treatment was correlated with dramatic increases in extracellular norepinephrine levels measured in dialysates of the ventrolateral ventromedial hypothalamus of female rats which displayed high levels of lordosis behavior. In contrast, female rats given the same steroid treatment but which did not show lordosis responses did not have elevated norepinephrine levels in their dialysates. Moreover, animals that received an estrogen antagonist concurrently with the estrogen treatment had neither an increase in ventromedial hypothalamic levels of norepinephrine during behavior testing nor did they display lordosis. These results indicate a close relationship among ovarian steroids, noradrenergic transmission in the ventromedial hypothalamus, and the expression of female sexual behavior.

Progesterone depression of norepinephrine-stimulated cAMP accumulation in hypothalamic slices

The present experiments examined the effects of progesterone on adrenergic receptor coupling to adenylate cyclase in hypothalamic and preoptic area slices by monitoring norepinephrine (NE)-stimulated increases in cAMP accumulation. Progesterone treatment of estrogen-primed rats decreased NE-induced slice cAMP accumulation. The reduced cAMP response was estrogen-dependent since it was not demonstrable in slices from rats exposed to progesterone without prior estrogen priming. Neither generalized increases in phosphodiesterase activity nor decreases in the catalytic activity of adenylate cyclase could account for the reduced ability of NE to stimulate cAMP accumulation in hypothalamic slices. Moreover, the cAMP response to two other activators of adenylate cyclase, adenosine and vasoactive intestinal peptide, was not decreased in slices from rats treated with estrogen plus progesterone. Selective adrenergic agonists and antagonists were employed to determine which adrenergic receptors mediate cAMP accumulation in progesterone-exposed slices. Slice cAMP levels were elevated by the beta receptor agonist isoproterenol but not by alpha 1 (phenylephrine) or alpha 2 (clonidine) agonists. However, clonidine potentiated the effect of isoproterenol on slice cAMP formation whereas phenylephrine did not. Likewise, NE-stimulated cAMP accumulation was completely antagonized only by a combination of both beta (propranolol) and alpha 2 (yohimbine) antagonists. The data suggest that in slices from estrogen plus progesterone-treated rats, alpha 2 receptors contribute significantly to NE stimulation of cAMP accumulation. The overall depression of the cAMP response to NE in progesterone-exposed slices may involve a decrease of alpha 1 receptor facilitation of cAMP synthesis.