Reciprocal changes in hypothalamic receptor binding and circulating leptin in anorectic tumor-bearing rats

Although reduced biological activity of the obese gene product, leptin, has been associated with obesity, little information is available concerning leptin alterations during anorexia. Therefore, we measured circulating leptin concentrations and hypothalamic leptin binding in anorectic tumor-bearing and pair-fed control rats. Plasma concentrations of leptin decreased in tumor-bearing rats early in the course of tumor growth, and fell to nearly non-detectable levels during severe anorexia. The pair-fed control rats that ate the same amount of food as did the anorectic tumor-bearing rats exhibited a 50% decrease in plasma leptin concentration. Concentrations of free fatty acids were elevated in both tumor-bearing and pair-fed groups, while circulating levels of triglycerides were increased only in anorectic tumor-bearing rats. Leptin receptor density was doubled in the hypothalamus of tumor bearing rats, while binding affinity was decreased by 50%. These results suggest that peripheral leptin production is down-regulated, perhaps due to increased lipolysis in tumor-bearing rats. It appears that hypothalamic leptin systems up-regulate receptor numbers in response to decreased blood leptin level, however, the decrease in binding affinity may compensate for these alterations. Therefore, the influence of leptin on hypothalamic neuropeptide Y feeding systems may be minimal in anorectic tumor-bearing rats.     

Anorexia following the intrahypothalamic administration of amylin.

The intrahypothalamic injection of rat amylin reduced feeding in schedule-fed rats for eight hours. Specificity of this anorectic response was indicated by an appropriate dose-response relationship and the absence of effect of human amylin. Amylin-induced anorexia was accompanied by alterations in neurotransmitter metabolism similar to those observed in anorectic tumor-bearing rats. These results indicate that amylin may inhibit feeding by acting directly on hypothalamic neurons to alter metabolism of neurotransmitter systems known to affect feeding 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.     

Intra-supraoptic nucleus sulpiride improves anorexia in tumor-bearing rats

Previous studies suggest that the dopaminergic system in the supraoptic nucleus (SON) is involved not only in the water balance control but also in the food intake regulation. Since we reported that an injection of the D2 receptor antagonist, sulpiride, into specific hypothalamic nuclei (e.g. the LHA, or the VMN) increases food intake in anorectic tumor-bearing rats, as well as in normal rats, we hypothesized that an injection of sulpiride into the SON would also improve cancer anorexia. Sulpiride injection (4 microg/0.5 microl) into bilateral SON of anorectic tumor-bearing male rats significantly improved food intake via increases in both meal size and meal number. These data suggest that pharmacological manipulation of the hypothalamic dopaminergic system is feasible in amelioration of cancer anorexia.     

The melanocortin agonist Melanotan-II reduces the orexigenic and adipogenic effects of neuropeptide Y (NPY) but does not affect the NPY-driven suppressive effects on the gonadotropic and somatotropic axes in the male rat

Neuropeptide Y (NPY) is a strong orexigenic neurotransmitter also known to modulate several neuroendocrine axes. alpha-Melanocyte-stimulating hormone (MSH) is an essential anorectic neuropeptide, acting on hypothalamic MC3/4 receptor subtypes. When given as an intracerebroventricular bolus injection, Melanotan-II (MT-II), a non selective MC receptor agonist, inhibits feeding, suppresses the NPY orexigenic action, and reduces basal insulinaemia. We evaluated the effects of a 7-day central infusion of MT-II (15 nmol/day) given either alone or in association with NPY (5 nmol/day) in male Sprague-Dawley rats. MT-II produced almost full anorexia for 1-2 days but then feeding gradually returned to normal despite continued MT-II infusion. When coinfused with NPY, MT-II also produced the same initial anorectic episode but then maintained feeding to upper normal levels, thus cancelling the hyperphagia driven by NPY. Whereas NPY infusion produced a doubling of fat pad weight, MT-II reduced adiposity by a factor of two compared to pair-fed rats, and vastly curtailed the NPY-driven increase in fat pad weight. MT-II infusion also significantly curtailed the NPY-induced rise in insulin and leptin secretions. NPY infusion significantly inhibited hypothalamic pro-opiomelanocortin mRNA expression, most likely cancelling the alpha-MSH anorectic activity. As expected from previous studies, chronic NPY infusion strongly inhibited both the gonadotropic and somatotropic axes, and coinfusion of MT-II did not reverse these NPY-driven effects, in sharp contrast with that seen for the metabolic data. MT-II infusion alone had little effect on these axes. In conclusion, chronic MT-II infusion generated a severe but transient reduction in feeding, suggesting an escape phenomenon, and clearly reduced fat pad size. When coinfused with NPY, MT-II was able to cancel most of the NPY effects on feeding, but not those on the neuroendocrine axes. It appears therefore that, as expected, NPY and alpha-MSH closely interact in the control of feeding, whereas the neural pathways by which NPY affects growth and reproduction are distinct and not sensitive to MC peptide modulation.     

Interaction between trophic action and electrical activity in spinal cord cultures

In crude rat brain membranes, apparent adenylate cyclase activity tested in the absence of phosphodiesterase inhibitors was dose-dependently inhibited by the adenosine ‘R-site’ agonist N⁶-phenylisopropyladenosine (N⁶-PIA). In membranes from adrenalectomized rats, however, N⁶-PIA induced, under the same conditions, an activation. However, in the presence of the phosphodiesterase inhibitor Ro-20-1724, brain adenylate cyclase responsiveness to N⁶-PIA resulted in a dose-dependent inhibition in both sham-operated and adrenalectomized rats. Thus, the low K_M cyclic AMP phosphodiesterase activity present in these brain membranes was investigated. Although this activity (tested in the presence of GTP) was unaltered by adrenalectomy, the dose-response curve of this enzyme to low concentrations of N⁶-PIA showed an activation in sham-operated and an inhibition in adrenalectomized rats, two effects which were suppressed by sodium (80 mM). These results showing that N⁶-PIA modulates both adenylate cyclase and cyclic AMP phosphodiesterase in the brain, provide an additional argument for a potential role of adenosine in the regulation of cyclic AMP metabolism in normal as well as in pathological brain.     

Anorectic Effect of Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) in Rats: Lack of Evidence for Involvement of Hypothalamic Neuropeptide Gene Expression

We investigated the effect of centrally administered pituitary adenylate cyclase activating polypeptide (PACAP) on feeding in rats, and the involvement of hypothalamic neuropeptide gene expression using in situ hybridization. lntracerebroventricular injection of PACAP (1000  pmol/rat) significantly decreased food intake in a dose-dependent manner. In PACAP-treated rats, neuropeptide Y (NPY) mRNA levels in the arcuate nucleus and galanin mRNA levels in the paraventricular nucleus increased, and corticotropin-releasing hormone (CRH) mRNA levels in the paraventricular nucleus decreased. In rats fasted for 72  h, NPY mRNA levels increased, and CRH mRNA levels decreased, but galanin mRNA levels were unchanged. These results indicate that the anorectic function of PACAP is not mediated by NPY or CRH, and that PACAP increases galanin synthesis.     

The effect of capsaicin on the adenylate cyclase activity of rat brain.

The effect of capsaicin on the adenylate cyclase activity in different regions of the rat brain (preoptic area of the hypothalamus, cerebral cortex and cerebellum) was investigated. Capsaicin added in vitro (10(-7)-10(-5) M) increased the adenylate cyclase activity of different brain regions. Following systemic capsaicin desensitization adenylate cyclase activity was significantly increased in the preoptic area. The enhanced adenylate cyclase activity in the preoptic area was inhibited by the vitro addition of capsaicin or 5-HT, whereas desensitization did not affect the in vitro activating effect of capsaicin in other brain regions (cerebral cortex, cerebellum). It is assumed that the pharmacological effect of capsaicin in the preoptic area is mediated through the activation of adenylate cyclase. Since capsaicin induces irreversible impairment of the function of warmsensitive hypothalamic neurons it is assumed that adenylate cyclase is involved in maintaining normal thermoregulatory functions.     

Somatostatin pretreatment desensitizes somatostatin receptors linked to adenylate cyclase and facilitates the stimulation of cyclic adenosine 3':5'-monophosphate accumulation in anterior pituitary tumor cells

Somatostatin-14 (SRIF) inhibits both hormone- and forskolin-stimulated cyclic adenosine 3':5'-monophosphate (cyclic AMP) formation in tumor cells of the mouse anterior pituitary (AtT-20/D16-16). However, long-term pretreatment of cells with SRIF modifies the responsiveness of this system in two ways: The response of adenylate cyclase to stimulatory agents is enhanced, whereas the ability of SRIF to inhibit stimulated cyclic AMP formation is reduced. The supersensitive adenylate cyclase response and the SRIF desensitization were dependent on the concentration and duration of SRIF pretreatment. Enhancement of forskolin-stimulated cyclic AMP formation occurred within 4 hr, whereas that of corticotropin-releasing-factor-, (-)-isoproterenol-, and vasoactive intestinal peptide-induced cyclic AMP accumulation required 16 hr of pretreatment. The elevated responses to each of these stimulants were due to increases in their maximal ability to stimulate cyclic AMP formation. Cycloheximide treatment blocked the enhanced cyclic AMP response induced by SRIF pretreatment, suggesting a requirement for protein synthesis. In membrane preparations, SRIF pretreatment facilitated activation of adenylate cyclase by forskolin, sodium fluoride, and guanosine 5'-(beta,tau-imido)-triphosphate without affecting basal activity. These results suggest that desensitization of an inhibitory input to adenylate cyclase is accompanied by a supersensitivity of adenylate cyclase to stimulatory agents through a process requiring protein synthesis.     

Histamine-activated adenylate cyclase in brain homogenates of several species

Histamine has been shown to activate cyclic AMP synthesis in brain slices and homogenates of certain species, although less is known about species differences in brain homogenates. Dutch Belted and New Zealand White rabbit brain homogenates contained a histamine-responsive adenylate cyclase similar to that of the guinea pig. In contrast, adenylate cyclase of gerbil and hamster brain exhibited little or no stimulation by histamine. Male rat hypothalamic homogenates contained adenylate cyclase, but also exhibited minimal stimulation by histamine, in disagreement with some recent reports. Detailed studies of the conditions of assay failed to resolve this discrepancy.     

Alterations in Cyclic AMP Concentration and Adenylate Cyclase Activity in Specific Brain Areas of Rats with Inherited Hypothalamic Diabetes Insipidus (Brattleboro Rats)

The concentration of cyclic AMP (cAMP) and the activity of sodium-fluoride-stimulated adenylate cyclase was measured in 29 microdissected brain areas of homozygous Brattleboro rats and their Long-Evans control rats. In ten of the investigated brain areas a decreased cAMP level was measured in Brattleboro rats. It was particularly decreased in the supraoptic nucleus, cingulate and parietal cortex, hippocampus, habenula and organum vasculosum laminae terminalis. Significantly lower cAMP levels were also found in the periventricular nucleus, bed nucleus of the stria terminalis, area postrema and locus coeruleus. An increased cAMP concentration was detected only in the subcommissural organ of Brattleboro rats. In most brain areas, where cAMP was decreased, sodium fluoride-stimulated adenylate cyclase activity was significantly increased (supraoptic nucleus, parietal cortex, periventricular nucleus, bed nucleus of the stria terminalis, locus coeruleus) or unchanged (hippocampus, habenula, organum vasculosum laminae terminalis). The coincidence of alterations in cAMP concentration and adenylate cyclase activity in brain areas of Brattleboro rats with relatively dense vasopressinergic innervation and/or vasopressin receptor population in control rats, suggests an influence of brain vasopressin on the cAMP-adenylate cyclase second messenger system.     

Lateral difference in responsiveness of norepinephrine-sensitive cyclic AMP-generating systems of rat cerebral cortex with iron-induced epileptic activity

Responsiveness of norepinephrine-sensitive cyclic AMP-generating systems was examined in slices of different cortical areas of rats showing electrographic spike and wave complexes after unilateral injection of ferrous chloride solution into the sensorimotor cortex. Accumulation of cyclic AMP elicited by norepinephrine was greater on the injection side of the cortex than on the other. Similar lateral differences were detected in cyclic AMP levels antagonized by phentolamine or propranolol, in which 8-phenyltheophylline almost completely inhibited the elicitation of cyclic AMP accumulation by a norepinephrine-propranolol combination but not by a norepinephrine-phentolamine combination. These results suggest that alterations in cyclic AMP generation through the beta-adrenoceptor-coupled adenylate cyclase system and through alpha-adrenergic activation of the adenosine receptor-coupled adenylate cyclase system are closely related to the electrographic activity of iron-induced epilepsy.     

N-phenylisopropyladenosine stimulates in normal and inhibits in adrenalectomized rats the low KM cyclic AMP phosphodiesterase in the brain

In crude rat brain membranes, apparent adenylate cyclase activity tested in the absence of phosphodiesterase inhibitors was dose-dependently inhibited by the adenosine ‘R-site’ agonist N⁶-phenylisopropyladenosine (N⁶-PIA). In membranes from adrenalectomized rats, however, N⁶-PIA induced, under the same conditions, an activation. However, in the presence of the phosphodiesterase inhibitor Ro-20-1724, brain adenylate cyclase responsiveness to N⁶-PIA resulted in a dose-dependent inhibition in both sham-operated and adrenalectomized rats. Thus, the low K_M cyclic AMP phosphodiesterase activity present in these brain membranes was investigated. Although this activity (tested in the presence of GTP) was unaltered by adrenalectomy, the dose-response curve of this enzyme to low concentrations of N⁶-PIA showed an activation in sham-operated and an inhibition in adrenalectomized rats, two effects which were suppressed by sodium (80 mM). These results showing that N⁶-PIA modulates both adenylate cyclase and cyclic AMP phosphodiesterase in the brain, provide an additional argument for a potential role of adenosine in the regulation of cyclic AMP metabolism in normal as well as in pathological brain.     

Lithium modulation of second messenger signal amplification in man: inhibition of phosphatidylinositol-specific phospholipase C and adenylate cyclase activity

The activity of phosphatidylinositol-specific phospholipase C was significantly reduced in platelets obtained from 20 euthymic manic-depressive patients on therapeutic lithium doses (mean blood level 0.85 mEq/l) compared to an age- and sex-matched group of 36 control subjects. The activities of prostaglandin E1-, aluminum/NaF-, and forskolin-stimulated platelet adenylate cyclase activity were also measured in a similar group of 16 lithium-treated and 22 control subjects. A marked reduction in both postreceptor (aluminum/NaF and forskolin) and receptor-stimulated (prostaglandin E1) platelet adenylate cyclase activity was observed in the lithium-treated group (mean blood level 0.81 mEq/l). These findings support the hypothesis that lithium's therapeutic mode of action in manic-depressive psychosis is mediated by the combined down-regulation of both principal second messenger systems, inositol phosphates and cyclic adenosine monophosphate, by reducing the activity of phosphatidylinositol-specific phospholipase C and adenylate cyclase.     

A general role for adaptations in G-proteins and the cyclic AMP system in mediating the chronic actions of morphine and cocaine on neuronal function.

Previous studies have shown that chronic morphine increases levels of the G-protein subunits Gia and Goa, adenylate cyclase, cyclic AMP-dependent protein kinase, and certain phosphoproteins in the rat locus coeruleus, but not in several other brain regions studied, and that chronic morphine decreases levels of Gia and increases levels of adenylate cyclase in dorsal root ganglion/spinal cord (DRG-SC) co-cultures. These findings led us to survey the effects of chronic morphine on the G-protein/cyclic AMP system in a large number of brain regions to determine how widespread such regulation might be. We found that while most regions showed no regulation in response to chronic morphine, nucleus accumbens (NAc) and amygdala did show increases in adenylate cyclase and cyclic AMP-dependent protein kinase activity, and thalamus showed an increase in cyclic AMP-dependent protein kinase activity only. An increase in cyclic AMP-dependent protein kinase activity was also observed in DRG-SC co-cultures. Morphine regulation of G-proteins was variable, with decreased levels of Gia seen in the NAc, increased levels of Gia and Goa in amygdala, and no change in thalamus or the other brain regions studied. Interestingly, chronic treatment of rats with cocaine, but not with several non-abused drugs, produced similar changes compared to morphine in G-proteins, adenylate cyclase, and cyclic AMP-dependent protein kinase in the NAc, but not in the other brain regions studied. These results indicate that regulation of the G-protein/cyclic AMP system represents a mechanism by which a number of opiate-sensitive neurons adapt to chronic morphine and thereby develop aspects of opiate tolerance and/or dependence. The findings that chronic morphine and cocaine produce similar adaptations in the NAc, a brain region important for the reinforcing actions of many types of abused substances, suggest further that common mechanisms may underlie psychological aspects of drug addiction mediated by this brain region.     

Adenylate cyclase and the search for new compounds with the clinical profile of lithium

It is possible to evaluate the beta-adrenergic receptor-adenylate cyclase complex in the human periphery by measuring the plasma cyclic AMP rise after adrenergic agonists. A clinical trial of the beta 2 adrenergic agonist salbutamol in depression provided an opportunity to test whether adrenergic receptor subsensitivity does occur during clinical antidepressant treatment. After 1 and 3 weeks of oral salbutamol treatment, depression scores declined significantly in 11 depressed patients, while the plasma cyclic AMP response to i.v. salbutamol declined over 60%. The results support the concept that receptor sensitivity changes occur during human antidepressant therapy. Data are presented that Li, too, markedly reduces activity of beta-adrenergic adenylate cyclase in humans. The effect was evaluated by studying the effect of Li at therapeutic serum concentrations on the plasma cyclic AMP response to subcutaneous epinephrine. The Li effect is specific, since the plasma cyclic AMP response to glucagon is not inhibited. In rat cortical slices Li inhibition of noradrenaline-induced cyclic AMP accumulation is clearly demonstrable only at concentrations close to 2 mM Li. However, fresh human brain slices from edges of surgically-removed tumors show Li inhibition at 1 mM Li concentrations. These results imply that in brain as well as periphery, human noradrenergic adenylate cyclase is inhibited by therapeutic concentrations of Li. Demeclocyclin, a tetracycline-derived antibiotic, was found to inhibit noradrenaline-sensitive adenylate cyclase in rat cortical slices and to inhibit amphetamine-induced hyperactivity in rats in an open field. Clinical trials should search for new compounds with the clinical profile of Li.     

Effects of prostaglandin E2, forskolin and cholera toxin on cAMP production and in vitro LH-RH release from the rat hypothalamus

The present study attempts to elucidate the possible role of adenosine 3′,5′-monophosphate (cAMP) and prostaglandin E₂ (PGE₂) in the function of the neural luteinizing hormone-releasing hormone (LH-RH) apparatus. To this end, in vitro LH-RH release from superfused hypothalamic fragments and cAMP production by hypothalamic P₂ membrane fractions were measured. Immature female rats (day 28) were ovariectomized and implanted with Silastic capsules containing estradiol (235 μg/ml). Two days later, animals were sacrificed and the mediobasal hypothalamic preoptic area (hypothalamic units or fragments) were removed. To examine in vitro LH-RH release from superfused hypothalamic fragments, effluents were collected into tubes on ice at 10-min intervals and LH-RH concentration was determined by radioimmunoassay (RIA). Following a 50-min control period, a step-wise increment in several doses of PGE₂ (each dose for a 50-min interval) evoked a dose-related increase in LH-RH release. PGE₂ induced significant (P<0.01) increments in LH-RH release at doses of 5.68 × 10⁻⁷, 5.68 × 10⁻⁶, and 5.68 × 10⁻⁵ M, respectively. When adenylate cyclase activators, such as forskolin and cholera toxin were infused in a step-wise manner (each dose for a 50-min interval) following a 50-min control period, a dose-related increase in LH-RH release was also obtained; forskolin and cholera toxin significantly (P<0.01) stimulated LH-RH release at doses of 1 × 10⁻⁴ and 5.4 × 10⁻¹⁰ M, respectively. These two substances were ineffective in stimulating LH-RH release when hypothalamic fragments were superfused in calcium-free plus EGTA (10 mM) containing medium. An intermittent infusion of dibutyryl cAMP (dbcAMP: 1 × 10⁻⁷ M, 10-min on, 20-min off) resulted in rhythmic LH-RH release from median eminences superfused in vitro. In separate experiments, to examine adenylate cyclase activity, P₂ membrane fractions from the mediobasal hypothalamus were preincubated with appropriate test agents. Adenylate cyclase reaction was initiated by adding adenosine triphosphate. After a 15-min incubation, the reaction was terminated by boiling, the supernatant recovered and subjected to cAMP determination by RIA. The following results were obtained: (1) in vivo E priming of ovariectomized animals significantly increased basal adenylate cyclase activity of P₂ membrane preparations as compared to those from unprimed rats; (2) known adenylate cyclase activators, such as forskolin and cholera toxin clearly produced a dose-related increase in cAMP production; and (3) PGE₂ at the concentration of 5.68 × 10⁻⁶ M stimulated cAMP production. It appears that cAMP and PGE₂ may be involved in the activation of the LH-RH neural apparatus. It is tempting to postulate that PGE₂ may stimulate adenylate cyclase to increase intracellular cAMP levels which, in turn, trigger the release of LH-RH from the median eminence nerve terminals.     

Peptidergic modulation of G-protein coupled cyclic-AMP accumulation in the rat caudate nucleus.

Somatostatin, substance P, and vasoactive intestinal polypeptide were incubated in an adenylate cyclase assay with a particulate fraction of caudate-putamen tissue of the rat in order to examine the effect of the neuropeptides on G-protein coupled adenylate cyclase in vitro. Somatostatin induced an enhancement of cyclic AMP formation in presence of guanine nucleotides and cholera toxin but inhibited pertussis toxin and forskolin enzyme stimulation. Pertussis toxin and cholera toxin also depressed forskolin-induced stimulation as described previously. Somatostatin was able to antagonize these inhibitory effects of both toxins. On the contrary, substance P reduced GTP and cholera toxin stimulated striatal adenylate cyclase, without affecting forskolin activation. In our preparation, VIP did not influence basal adenylate cyclase activity or the stimulation by guanine nucleotides, cholera toxin, and pertussis toxin. VIP potently inhibited the enhancement of cyclic AMP formation by forskolin and completely antagonized the inhibitory effect of cholera toxin on forskolin activation. These results suggest that neuromodulatory effects of somatostatin, substance P, and VIP are mediated by the inhibitory as well as stimulatory guanine nucleotide proteins G-i and G-s coupled to an adenylate cyclase system.     

Somatostatin inhibition of anterior pituitary adenylate cyclase activity: different sensitivity between male and female rats

Somatostatin (SRIF) is a potent inhibitor of growth hormone (GH) secretion. Although cyclic AMP (cAMP) has been suggested as intracellular mediator of SRIF action, a complete characterization of its effect and the different sensitivity between male and female animals, has not yet been carried out. In this study SRIF inhibited basal and GH-releasing factor (GRF) stimulated anterior pituitary adenylate cyclase activity with a greater effectiveness in male than in female glands. Similarly SRIF reduction of forskolin-stimulated anterior pituitary adenylate cyclase activity, was more pronounced in male than in female animals. By using pertussis toxin (PTX), which uncouples inhibitory receptors from adenylate cyclase catalytic subunit, SRIF inhibition of both basal and forskolin-stimulated adenylate cyclase activity was nearly abolished. These results show that anterior pituitary SRIF receptors are coupled in an inhibitory fashion with adenylate cyclase enzyme, and that male rat adenohypophyses are more responsive to SRIF inhibition.