Drop in the "atypical" beta-adrenergic response and modification of the beta/alpha 2-adrenoceptor balance in fat cells from aging rabbits.

Previous studies have shown a strong reduction of catecholamine-induced lipolysis in perirenal white fat cells in aging rabbits. The molecular basis of this observation was explored on scapular and perirenal adipocytes from 45 and 300- to 500-day-old rabbits. ACTH and forskolin were used to define the maximal lipolytic potencies of the adipocytes. beta-Adrenergic responsiveness was explored with isoproterenol and specific agonists of the "atypical" beta-adrenoceptor (beta-AR) (BRL37344 and (+/-)CGP12177); beta 1/beta 2-ARs were identified with [125I]cyanopindolol. alpha 2-adrenergic responses were evaluated with the full alpha 2-agonist, UK14304. The alpha 2 AR number was determined with the alpha 2-antagonist radioligand [3H]RX821002. Whatever the fat deposit, the relative order of lipolytic potency of the beta-agonists was: isoproterenol greater than BRL37344 greater than (+/-)CGP12177. As previously reported for catecholamines, the maximal lipolytic response initiated by isoproternol decreased with aging; the stronger reduction was observed in perirenal adipocytes compared to subscapular adipocytes. The most striking observation concerns the parallel and complete disappearance of the lipolytic responses induced by the atypical beta-agonists (BRL37344 and (+/-)CGP12177) and the preservation of a residual action of isoproterenol (30% of that described in young animals) which was attributed to the stimulation of beta 1/beta 2-ARs. The number of beta 1/beta 2-AR binding sites was practically equivalent whatever the fat deposite and the age of the animals. alpha 2-Adrenergic responsiveness and alpha 2-adrenergic receptor number were increased with aging in the various deposits but the stronger changes were observed in the perirenal adipocytes where epineprine initiated a biphasic effect on lipolysis (antilipolytic and then lipolytic). To conclude, the reduction of catecholamine-induced lipolysis observed in the rabbit fat cells with aging can be explained by changes in the atypical beta-AR/alpha 2-AR balance. First, a loss of responsiveness to the atypical beta-adrenergic agents was observed (it is impossible for the moment to distinguish between the loss of atypical beta-AR binding sites and their putative uncoupling from the adenylate cyclase system) whereas beta 1/beta 2-AR-mediated responses were maintained. Second, an increment of alpha 2-adrenergic responsiveness and of the alpha 2-AR binding sites accompanied aging and fattening. In the absence of, or after a strong reduction of the atypical beta-AR component of the lipolytic response in fat cells of aged rabbits, epinephrine exerts a biphasic effect on lipolysis, demonstrating the changes occurring in the atypical beta-AR/alpha 2-AR balance.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hamster adipocyte alpha 2-adrenoceptor changes during fat mass modifications are not directly dependent on adipose tissue norepinephrine content

It is questioned whether alterations of the sympathetic nervous system can explain the specific increment of the adipocyte alpha 2-adrenoceptor observed during white adipose tissue (WAT) enlargement and fat cell size increment. The impact of a 6-hydroxydopamine-induced sympathectomy, validated by determination of the WAT norepinephrine content, was tested on isolated white fat cell alpha 2-adrenoceptor function and binding [( 3H]RX 821002 and [3H]UK 14304 binding). Chemical sympathectomy of standard adult hamsters, neither alters the alpha 2-adrenergic-dependent antilipolysis nor the adipocyte alpha 2-adrenergic binding. A slight hypersensitivity of the beta 1-adrenergic lipolytic response was observed, without modification in beta-adrenergic binding [( 125I]cyanopindolol binding). Compared with controls, caloric restriction (10 days) of adult hamsters induced a large decrease in adipocyte alpha 2-adrenoceptor (P less than 0.001) with a parallel decrement in the mean fat cell size (P less than 0.001) and alpha 2-adrenergic responsiveness while there was no significant variation of the WAT norepinephrine content (on a per pad basis). 6-Hydroxydopamine treatment of the restricted animals (from day 6 to day 9) induced a higher preservation of adipocyte alpha 2-adrenoceptor density (P less than 0.001), no change in alpha 2-adrenergic responsiveness, and higher preservation of mean fat cell size (P less than 0.05) than in the restricted only animals. No significant modification in the beta-adrenergic binding was observed whatever the conditions. It was concluded that the specific increment in the adipocyte alpha 2-adrenoceptor during fat mass enlargement was not directly dependent on sympathetic nervous system alterations. Nevertheless, the sympathetic-dependent mobilization of the fat stores, after induction of caloric restriction, can indirectly modulate the alpha 2-adrenoceptor density in the adipocyte by promoting changes in white fat cell size.     

Temporal changes in gene expression in the arcuate nucleus precede seasonal responses in adiposity and reproduction

In anticipation of seasonal climate changes, Siberian hamsters display a strategy for survival that entails profound physiological adaptations driven by photoperiod. These include weight loss, reproductive quiescence, and pelage growth with shortening photoperiod and vice versa with lengthening photoperiod. This study reports gene expression changes in the hypothalamus of Siberian hamsters switched from short days (SD) to long days (LD), and also in photorefractory hamsters. Siberian hamsters were maintained in either LD or SD for 14 wk, conditions that generate physiological states of obesity under LD and leanness under SD. After 14 wk, SD lighting was switched to LD and gene expression investigated after 0, 2, 4, and 6 wk by in situ hybridization. Genes encoding nuclear receptors (RXR/RAR), retinoid binding proteins (CRBP1 and CRABP2), and histamine H3 receptor were photoperiodically regulated with significantly lower expression in SD, whereas VGF mRNA expression was significantly higher in SD, in the dorsomedial posterior arcuate nucleus. After a SD-to-LD switch, gene expression changes of CRABP2, RAR, H3R, and VGF occurred relatively rapidly toward LD control levels, ahead of body weight recovery and testicular recrudescence, whereas CRBP1 responded less robustly and rxrgamma did not respond at the mRNA level. In this brain nucleus in photorefractory animals, the CRABP2, RAR, H3R, and VGF mRNA returned toward LD levels, whereas CRBP1 and rxrgamma remained at the reduced SD level. Thus, genes described here are related to photoperiodic programming of the neuroendocrine hypothalamus through expression responses within a subdivision of the arcuate nucleus.     

Estradiol treatment decreases the lipolytic responses of hamster white adipocytes through a reduction in the activity of the adenylate cyclase catalytic subunit

After 5 days of daily administration of 10 micrograms estradiol to 6-week-old male hamsters, the in vitro maximal lipolytic and cAMP responses of white adipocytes to isoproterenol, epinephrine, ACTH, and theophylline were reduced by one half, with no change in the sensitivity of these responses. In contrast, the antilipolytic response to the alpha 2-adrenergic agonist clonidine was unimpaired. beta-Adrenergic receptor number and affinity, assessed in intact cells with [3H]CGP-12177 binding, showed no difference between control and estradiol-treated hamsters. In adipocyte membranes from estradiol-treated hamsters, maximal adenylate cyclase responses to Mn2+, GTP alone or in combination with isoproterenol, ACTH, or fluoride were all decreased by 30-40% below the values found in controls, but the sensitivity of these responses was unaltered. The maximum velocity (Vmax) of adenylate cyclase was reduced by one half in estrogen-treated animals, but the Michaelis-Menten constant (Km) of the enzyme for ATP was unchanged. Finally, complementation of adipocyte membranes with solubilized human erythrocyte Ns failed to restore to control values the maximal adenylate cyclase response to isoproterenol plus guanosine 5'-[beta, gamma-imido]-triphosphate in the estradiol-treated hamsters. These results indicate that a defect in the catalytic subunit of adenylate cyclase is one of the mechanisms through which estradiol treatment reduces the lipolytic response of hamster white adipocytes.     

Oestradiol and progesterone change beta3-adrenergic receptor affinity and density in brown adipocytes.

OBJECTIVE: To check if the oestradiol- and progesterone-driven reduction in noradrenaline responsiveness of brown adipocytes is due to a reduction in either the density or the affinity of beta3-adrenoceptors (beta3-AR). beta1/beta2-AR were also studied. DESIGN: Four groups of animals were considered. (i) control rats at thermoneutrality, (ii) cold-acclimated rats, to determine beta-AR under continuous sympathetic stimulation, which is known to decrease noradrenaline responsiveness, (iii) oestradiol- and (iv) progesterone-treated cold-acclimated rats to determine hormonal effects on beta-AR populations in thermogenically active brown adipocytes. METHODS: Oestradiol and progesterone were chronically elevated by means of s.c. Silastic implants. Densities and affinities of beta-AR populations were determined by binding studies using [3H]CGP-12177 as radioligand. RESULTS: Two populations of low and high binding affinities (K(d) 1.6 and 27.3 nmol/l) corresponding to beta3- and beta1/beta2-AR respectively were found at thermoneutrality. beta3-AR density was higher than that of beta1/beta2-AR (B(max) 419 and 143 fmol/mg protein respectively). Cold-acclimated rats showed a reduction of beta3-AR binding capacity (B(max) 308 fmol/mg protein). Oestradiol and progesterone reduced the density of beta3-AR to 167 and 185 fmol/mg protein respectively, while increasing their affinity for [3H]CGP-12177 (K(d) 9.5 and 4.0 nmol/l vs 16 nmol/l in cold-acclimated untreated rats). The density of beta1/beta2-AR was also reduced after oestradiol treatment (B(max) 51 fmol/mg protein). CONCLUSIONS: Both oestradiol and progesterone reduce the density of beta3-AR in brown adipose tissue (BAT) while increasing their affinity for [3H]CGP-12177. Oestradiol also reduces the density of beta1/beta2-AR whereas cold-acclimation reduces the density of beta3-AR.     

Effects of photoperiod and temperature on the binding of follicle-stimulating hormone (FSH) to testicular preparations and plasma FSH concentration in the Djungarian hamster, Phodopus sungorus

The effects of artificial photoperiod and temperature on testicular FSH binding and plasma FSH levels were studied in adult male Djungarian hamsters. In Exp I, hamsters were transferred to long day (LD) photoperiods (16-h light, 8-h dark) after 8 weeks of adaptation in short day (SD) photoperiods (8-h light, 16-h dark), but the ambient temperature was maintained at 25 C throughout the experiments. A marked increase in the total FSH binding per two testes occurred between 10 and 47 days after transfer to LD, a change that was accompanied by testicular growth. Binding of FSH per testicular weight decreased during the same period. Scatchard plot analyses of the parameters indicated that LD decreased both the concentration of FSH binding sites and the equilibrium dissociation constant (Kd). Plasma FSH levels increased between 10 and 47 days after transfer to LD. In contrast, when hamsters reared under LD for 12 weeks were transferred to SD (Exp II), FSH binding per unit weight basis increased 19 weeks after transfer to SD, but the total binding per two testes decreased markedly. In Exp III, sexually mature male hamsters were subjected to different ambient temperature and photoperiods. There were no significant effects of different ambient temperatures on the testicular weight and testicular FSH binding in animals exposed for 8 weeks to either LD or SD. However, plasma FSH levels of hamsters maintained at 25 C under LD was significantly higher than FSH levels at 7 C under LD. A similar effect of temperature on plasma FSH levels was observed in hamsters under SD. The present study indicates that photoperiod is a more important environmental factor than temperature for the regulation of FSH receptor in the Djungarian hamster.     

Identification and functional studies of a specific peptide YY-preferring receptor in dog adipocytes.

Specific binding sites for peptide YY (PYY) and neuropeptide Y (NPY) as well as functional responses were identified in dog adipocytes. Studies were carried out using the radioligand [125I-Tyr1]monoiodo-PYY on crude adipocyte membranes. [125I]PYY bound to dog adipocyte membranes with a high affinity (156 +/- 24 pM) and binding capacity of 314 +/- 48 fmol/mg protein. Competition studies revealed a higher affinity of the binding sites for PYY than NPY (inhibition constants were 118 +/- 17 pM and 300 +/- 53 pM, respectively, P < or = 0.001). NPY analogs displaced [125I]PYY specific binding with the following order of potency: NPY-(13-36) > NPY-(18-36) > NPY-(22-36) > [Leu31-Pro34]NPY. Neither adrenergic nor adenosine agents (activating or inhibiting other antilipolytic systems) interacted with [125I]PYY binding sites. So [125I]PYY binding was specific, saturable, and reversible. Lipolysis experiments performed with PYY, NPY, and NPY analogs confirm the relative order of potency found in competition experiments. The data agree with the definition of PYY-preferring receptor which resembles a Y2 receptor subtype since NPY-(13-36), a specific Y2 receptor agonist, inhibited binding and lipolysis in a similar way to PYY, whereas [Leu31-Pro34]NPY did not. No difference was observed in the antilipolytic response between IC50 values measured on omental, perirenal, and subcutaneous fat deposits. Moreover, PYY and NPY (10(-6) M) significantly attenuated forskolin-stimulated cAMP levels, involving inhibition of adenylyl cyclase as a transmembrane signaling mechanism. Cross-linking of bound [125I]PYY to membranes indicated that the mol wt of the receptor was 62K. The relative importance of such a receptor on fat cells alongside another powerful antilipolytic receptor--the alpha 2-adrenoceptor--is discussed.     

Functional uncoupling of the platelet alpha 2-adrenergic receptor-adenylate cyclase complex in the elderly

Elderly humans demonstrate decreased responsiveness in several hormone-receptor systems, including adrenergic receptors. Studies of the beta-adrenergic receptor (beta-AR) system have shown that reduced beta-adrenergic sensitivity in the elderly may be due to reduced beta-AR affinity for agonists. To determine the mechanisms underlying altered alpha-adrenergic sensitivity in the elderly, we assessed the relationships between age and platelet membrane alpha 2-adrenergic receptor (alpha 2-AR)-binding properties, receptor-linked adenylate cyclase (AC) activity, and the affinity of the alpha 2-AR-AC complex for agonists in 18 young (mean age, 24 yr; range 19-34) and 13 elderly (mean age, 69 yr; range, 63-85) normal subjects. In platelet membrane preparations from elderly compared to young subjects, we found similar antagonist-binding properties and similar activity of the catalytic unit of platelet AC, as indicated by the cAMP response to sodium fluoride stimulation. However, mean epinephrine-mediated inhibition of sodium fluoride-stimulated platelet AC activity was less in the elderly [20 +/- 4% (+/- SEM) vs. 31 +/- 2% inhibition; P less than 0.005). In addition, platelet alpha 2-AR affinity for agonist was lower in the elderly, as indicated by the higher concentration of epinephrine needed to inhibit 50% of specific [3H]yohimbine binding (IC50, 3.2 +/- 0.6 vs. 1.4 +/- 0.3 microM; P less than 0.02). These data provide evidence that platelet membranes from elderly humans have decreased responsiveness to alpha-adrenergic stimulation, which can be attributed to reduced alpha 2-AR-AC affinity for agonists. Similarly to reported age-related alterations in beta-adrenergic receptor function, these results suggest that there is also functional uncoupling of the alpha 2-AR-AC complex in elderly humans.     

alpha-Adrenergic receptors in human adipocyte membranes: direct determination by [3H]yohimbine binding

The presence of alpha 2-adrenergic receptors in membranes derived from human sc adipose tissue was directly demonstrated with a new alpha 2-selective ligand, [3H]yohimbine. Binding of this radiolabeled antagonist to adipocyte membranes was of high affinity (Kd = 3.9 +/- 2.4 nM) and saturable. Computer modelling of [3H]yohimbine saturation curves demonstrated that it binds to a homogeneous class of sites with a density of 145.0 +/- 33.8 fmol/mg protein. Adrenergic agonists competed with [3H]yohimbine in the order expected of alpha-receptors, and their binding was strongly influenced by guanine nucleotides. Competition of alpha-antagonists with this radioligand demonstrated yohimbine to be more potent than prazosin, indicative of alpha 2-receptors. Antagonist binding was unaffected by guanine nucleotides. Paired saturation curves in these adipocyte membranes with the alpha 2-selective [3H]yohimbine and the nonsubtype selective alpha-antagonist [3H]dihydroergocryptine demonstrated similar receptor concentrations. [3H]Dihydroergocryptine has been previously shown to label both alpha 3- and alpha 2-receptors with equal affinity. Therefore, these data indicate that the vast majority of alpha-receptors in human sc fat are of the alpha 2-subtype. [3H]Yohimbine with its alpha 2-selectivity and high specific binding will provide an excellent tool for the clinical investigation of human adipocyte alpha-receptor mechanisms in both normal and pathological states.     

Alpha2A-adrenoceptor mediated tachypnea in awake goats

To further elucidate the role of alpha2-adrenoceptors (alpha2-ARs) in the control of respiratory rhythm we examined the ventilatory effects of guanfacine (a preferentially selective alpha2A-AR agonist) and clonidine (a non-selective alpha2-AR agonist) in awake adult goats. Systemic administration of guanfacine in cumulative doses (20 microg/kg; 140-180 microg/kg total cumulative dose) increased breathing in all animals in a dose-dependent manner. The excitatory effect was entirely mediated by increases in respiratory frequency. The magnitude of the guanfacine-induced tachypnea was similar to that produced by systemic administration of cumulative doses of clonidine (1-2 microg/kg; 4-10 microg/kg total cumulative dose) in the same animals studied on a separate day. Both guanfacine- and clonidine-induced tachypnea was reversed by the preferentially selective alpha2A-AR antagonist RX821002 (2-6 microg/kg IV). Unlike clonidine however, guanfacine administration did not produce slow arrhythmic breathing episodes (irregular TE intervals and central apneas) that are characteristic of alpha2-AR stimulation with alpha2-AR agonists in the awake goat. The results suggest that alpha2-AR agonist-induced ventilatory excitation (tachypnea) requires the activation of alpha2A-ARs whereas clonidine-induced ventilatory depression (arrhythmic breathing) requires the activation of an alternate alpha2-AR subtype (presumably alpha2C-ARs). The results further demonstrate that alpha2-AR pathways exert an important influence on respiratory rhythm in the awake goat.     

Agonist versus antagonist binding to alpha-adrenergic receptors.

The binding properties of two alpha-adrenergic radioligands, [3H]epinephrine (an agonist) and [3H]dihydroergocryptine (an antagonist), were compared in two model systems--membranes derived from human platelets and membranes from rat liver. The platelet contains exclusively alpha 2 and the liver mostly (approximately 80%) alpha 1 receptors. Agonists induce the formation of a guanine nucleotide-sensitive high-affinity state of alpha 2 but not alpha 1 receptors. [3H]Dihydroergocryptine labels all the alpha receptors, whereas [3H]epinephrine at low concentrations labels predominantly the high-affinity form of the alpha 2 receptor in both platelet and liver. However, in the liver, alpha-adrenergic effects such as glycogen phosphorylase activation are shown to be mediated via alpha 1 receptors. Thus, in liver membranes the endogenous "physiological" agonist may not label the physiologically relevant alpha 1 receptors in typical radioligand binding assays using low concentrations of [3H]epinephrine.     

Adrenergic regulation of lipolysis in human adipocytes: findings in hyper- and hypothyroidism

In isolated sc adipocytes removed from hyperthyroid patients, the specific binding of [3H]dihydroalprenolol and [125I]iodocyanopindolol was greater than that in adipocytes from normal subjects. Based on Scatchard analysis of the [125I] iodocyanopindolol data, this difference was due to a significant (P less than 0.01) increase in adrenoceptor number, which was 1.72 +/- 0.18 (+/- SEM) pmol/10(7) cells in the hyperthyroid patients and 0.94 +/- 0.16 pmol/10(7) cells in the normal subjects. When the patients were restudied when they were euthyroid, a significant decrease in the specific binding of the two radioligands was found. In hyperthyroidism, the lipolytic responsiveness (maximum effect) to norepinephrine was increased 5-fold, and that to isopropylnorepinephrine was increased 2-fold. No changes in either the binding of [3H]yohimbine or the antilipolytic effect of clonidine were found. In isolated adipocytes from hypothyroid patients, the specific binding of [3H]dihydroalprenolol and [125I]iodocyanopindolol did not differ from that in the normal subjects. The basal rate of lipolysis (P less than 0.025) and the lipolytic responsiveness to isopropylnorepinephrine (P less than 0.025) were significantly lower than normal, and the response to norepinephrine was almost completely abolished in the hypothyroid state. The sensitivity and responsiveness to clonidine were comparable in the adipocytes of the hypothyroid patients and normal subjects. There was no difference between hypothyroid patients and normal subjects in the binding of [3H]yohimbine. We conclude that the sc adipocytes in hyperthyroidism have beta-adrenergic, but not alpha 2-adrenergic abnormalities. Although there was a moderate increase in the beta-adrenoceptor density in hyperthyroidism, the most important abnormality, namely the increased responsiveness to the catecholamines, seems to be located beyond the receptor level. On the other hand, in hypothyroidism, there was no evidence of changes in either the alpha 2- or the beta-adrenoceptors. The chief abnormality in hypothyroidism, decreased responsiveness to beta-adrenergic agonists, also would appear to be localized beyond the adrenoceptor level.     

125I]Aminobenzyladenosine, a new radioligand with improved specific binding to adenosine receptors in heart

The density of adenosine receptors in membranes derived from rat hearts in 25 times lower than the density of receptors in rat brain membranes. Consequently, adenosine radioligands which are useful in brain such as l-[3H]phenylisopropyladenosine, [3H]cyclohexyladenosine, [3H]-2-chloroadenosine and l-[125I]hydroxyphenylisopropyladenosine are of limited usefulness in heart, due to a high ratio of nonspecific to specific binding. We have synthesized a new radioligand, [125I]-N6-4-aminobenzyladenosine, which binds to rat heart membranes with one-sixth the nonspecific binding of the other radioligands. [125I]-N6-4-aminobenzyladenosine bound to rat ventricle membranes with a KD equivalent to that of l-[125I]hydroxyphenylisopropyladenosine and a Bmax of 15.2 fmol/mg protein. [125I]-N6-4-aminobenzyladenosine bound with a higher affinity to brain (KD = 1.93 nM) than to heart membranes (KD = 11.6 nM). At the radioligand KD, 60% of the total [125I]-N6-4-aminobenzyladenosine bound to heart membranes was specifically bound. Iodination of aminobenzyladenosine increased its affinity for the adenosine receptor by 22-fold, possibly due to a steric or hydrophobic effect of iodine. The new ligand was found to be a full adenosine agonist based on its ability to inhibit cyclic adenosinemonophosphate accumulation in isolated embryonic chick heart cells and rat adipocytes. [125I]-N6-4-Aminobenzyladenosine bound to a single affinity site and was displaced from cardiac and brain adenosine receptors by other adenosine analogues with a potency order of l-phenylisopropyladenosine greater than 5'-N-ethylcarboxamide adenosine. These characteristics suggest that the radioligand binds to an Ri adenosine receptor.     

Failure of Prazosin to Mimic the Effects of Melatonin Under In Vivo and In Vitro Conditions

The gonads of male hamsters exposed to short photoperiod (LD, 10:14) or treated with melatonin in the late afternoon under long photoperiod (LD, 14:10) had undergone complete regression by the end of the treatments (8 weeks). Animals treated for the same period of time with prazosin (a putative melatonin analogue) under the same conditions failed to show a difference in their gonadal status as compared to the long photoperiod controls. Prazosin was unable to prevent melatonin-induced gonadal atrophy when injected either in the morning or 30 min before melatonin. Moreover, prazosin was without any effect on (and unable to prevent the melatonin-stimulated) progesterone production by rat adrenals under in vitro conditions. These data demonstrate that prazosin, which reportedly inhibits 2-[125I]iodomelatonin binding in the hamster brain in an affinity-related manner, does not possess properties of a biological melatonin analogue under conditions of two different model systems in two species.     

Prostaglandin E2 receptor binding and action in human fat cells

The prostaglandin E2 (PGE2) receptor in human adipocytes was identified by the use of [3H]PGE2. The receptor binding at physiological temperature and pH was specific, saturable, and slowly reversible. Half-maximal displacement for [3H]PGE2 binding occurred with 2.5 nmol/liter. Half-maximal inhibition of isoproterenol-induced lipolysis was achieved at a concentration of PGE2 of 3.8 nmol/liter and half-maximal inhibition of basal lipolysis was achieved at a concentration of PGE2 of 0.9 nmol/liter. The order of potency for prostaglandin inhibition of receptor binding and antilipolytic effect was the same, with PGE2 greater than PGF2 alpha much greater than arachidonic acid. Scatchard analysis of the binding data revealed a nonlinear plot indicating the existence of two or more binding sites with different affinities. The binding sites of high affinity had an equilibrium constant (Kd) of 2 nmol/liter and a total binding capacity of 58 fmol/10(6) adipocytes which corresponds to about 33,000 binding sites per adipocyte. The binding sites of low affinity had a Kd of 56 nmol/liter and a total binding capacity of 700 fmol/10(6) adipocytes. We conclude that [3H]PGE2 binds to receptors in isolated human adipocytes and that their antilipolytic effects are mediated by this binding.     

Naloxone-induced secretion of LH in the male Syrian hamster: modulation by photoperiod and gonadal steroids

The role of endogenous opiates in the regulation of photoperiodically induced testicular regression was studied in the male Syrian hamster. In reproductively active hamsters exposed to a long photoperiod (LD; 16 h light: 8 h darkness) or to short days (SD; 8 h light: 16 h darkness) for 20 weeks or to SD after pinealectomy, administration of naloxone, a competitive opiate receptor antagonist, at doses of 2.5-20 mg/kg, significantly increased serum LH concentrations. In marked contrast, these doses of naloxone did not produce any change in LH levels in reproductively quiescent hamsters exposed to SD for 8 weeks. The influence of gonadal steroids on the LH response to naloxone was studied in hamsters castrated or castrated and implanted s.c with a capsule containing testosterone. Naloxone did not induce LH release in castrated hamsters maintained in LD or in SD, but this response was restored in LD but not SD when serum testosterone concentrations were maintained at levels similar to those observed in intact reproductively active hamsters. These results show that inhibition of reproduction by the photoperiod prevents naloxone-induced LH release in the male hamster. This lack of response to naloxone is not due, however, to the lower testosterone titres present in these animals compared with reproductively active animals. Responsiveness to naloxone can be restored when the animal is rendered insensitive to the inhibitory photoperiod either by removal of the pineal gland or by induction of photorefractoriness by extended exposure to SD.