Glucose, insulin and sympathoadrenal activation

Selective activation of the two components of the sympatho-adrenal system (SAS) was produced by infusing male Sprague-Dawley rats with glucose or insulin. Intravenous glucose (0.5, 1 or 2 g/kg) raised plasma levels of glucose and insulin associated with a predominant activation of the sympathetic nervous system (SNS) as indicated by increased plasma norepinephrine (NE) levels. Glucose-induced release of NE was still seen in hypoinsulinemic, streptozotocin-treated rats but was absent after ganglionic blockade with chlorisondamine or following mannitol or saline infusions which failed to alter glucose or insulin levels. Insulin-induced (10 or 20 U/kg) hyperinsulinemia with its associated hypoglycemia produced predominant activation of the adrenal medulla producing increased plasma epinephrine (E) levels. But significant SNS activation also occurred with a concomitant increase in NE levels (14-22% of E) which appeared to be primarily from sympathetic nerves since increases still occurred following adrenal demedullectomy. Therefore, changes in plasma glucose levels alone and possibly altered intracellular concentrations or rates of glucose utilization can act as an important determinant of the pattern SAS activation although this may require pathological changes in plasma glucose levels to occur.     

Effect of insulin excess and deficiency on norepinephrine turnover in rats

Summary To examine whether insulin enhances norepinephrine (NE) turnover, an index of sympathetic nerve activity, the effects of excess insulin and streptozotocin (STZ) induced insulin deficiency were examined in Sprague-Dawley rats. Exogenous insulin caused hyperphagia and elevated (approximately 300%) urinary epinephrine excretion, but did not alter cardiac NE content or turnover. STZ-induced insulin deficiency caused hyperglycemia and hyperphagia, but also did not alter cardiac NE content or turnover. Insulin deficiency reduced hepatic NE content 18%, but did not affect NE turnover or content of kidney or spleen. These data do not support the hypothesis that insulin influences cardiac sympathetic nerve activity in rats.     

Endogenous opioids modulate the effect of cholecystokinin on insulin release in dogs

Recently we have demonstrated in dogs and man that endogenous opioids participate in the regulation of pancreatic endocrine function following the ingestion of a meal. Since intestinal hormones such as cholecystokinin (CCK) are also released by the presence of nutrients in the gastrointestinal tract and participate in the postprandial stimulation of pancreatic endocrine function, an interaction between CCK and endogenous opioids seems possible. The present study was designed to examine this further. In a group of 8 conscious dogs the octapeptide of CCK was infused intravenously in its sulfated (CCK-8S) or nonsulfated (CCK-8NS) form and in addition the tetrapeptide of CCK (CCK-4) was given at increasing infusion rates of 50, 200 and 500 pmol/kg . h, respectively. The experiments were performed during a background infusion of saline to assess the effect on basal insulin and during a background infusion of glucose (0.2 g/min) to determine the effects on stimulated insulin release. The effect of endogenous opioids was examined by addition of the opiate-receptor antagonist naloxone. The studies demonstrate that in the basal state CCK-8S has no stimulatory effect on insulin secretion unless naloxone is added indicating that endogenous opioids help to prevent insulin secretion in the absence of elevated glucose levels. During i.v. glucose naloxone reduced the stimulatory effect of CCK-8S at 50 and 200 pmol/kg . h and that of CCK-4 at 50 pmol/kg . h. Infusion of CCK-8S and CCK-4 at 500 pmol/kg . h had no effect on glucose-stimulated insulin levels, however, the addition of naloxone elicited a significant stimulatory effect. These data demonstrate stimulatory as well as inhibitory effects of endogenous opioids depending on the dose of CCK-8 and -4. CCK-8NS reduced glucose-stimulated insulin release already at the lowest dose of 50 pmol/kg . h. This was reversed to a stimulatory effect with the addition of naloxone. These data demonstrate that the interaction between CCK-8 and -4 and endogenous opioids on prestimulated insulin secretion is much more dependent on the dose of CCK - low doses induce stimulatory and high doses inhibitory mechanisms via endogenous opioids. In view of previous in vitro and in vivo studies with exogenously infused opiate-active compounds it might be speculated that increasing doses of CCK elicit a parellel increase in the release of endogenous opioids which might be responsible for some but certainly not all of the effects observed recently for the action of naloxone in the post-prandial state.     

New role of nerve growth factor--an inhibitory neuromodulator of adrenergic transmission.

In the present study, we investigated the effects of nerve growth factor (NGF) on norepinephrine (NE) release from peripheral sympathetic nerve endings of rat mesenteric artery. We made isolated mesenteric artery-intestinal loop preparations, by the modified method of Castelluci et al., from 4- and 8-week-old Wistar rats. NGF produced a dose-dependent inhibition of NE overflow from sympathetic nerve endings evoked by electrical nerve stimulation in the range of 0.1-10 ng/ml. Inhibition of NE overflow also occurred in the presence of a neuronal uptake blocker, desipramine (5 x 10(-8) M). NGF showed no effect on pressor response to exogenous NE (1 micrograms). These results suggest that NGF inhibits NE release from sympathetic nerve endings, in other words, NGF acts as an inhibitory neuromodulator of adrenergic transmission. This function of NGF might be considered as an inhibitory feedback mechanism against catecholamine-stimulated NGF synthesis.     

Cardiovascular effects and modulation of noradrenergic neurotransmission following central and peripheral administration of neuropeptide Y

Experiments have been conducted to evaluate the effect of neuropeptide Y (NPY) administered at three distinct levels of the nervous system: 1) the posterior hypothalamic nucleus, 2) the spinal cord, and 3) the vascular noradrenergic neuroeffector junction. It was observed that NPY produced varying cardiovascular effects at these three distinct sites of the nervous system. Microinjections into the posterior hypothalamic nucleus resulted in an increase in blood pressure, which was reduced by prior microinjection of a muscarinic or H1-histamine antagonist but not an H2-histamine antagonist. In addition to the involvement of histaminergic and cholinergic pathways, the pressor effect of NPY appears to result from an increase in sympathetic outflow. NPY was also seen to decrease the potassium-induced release of norepinephrine (NE) from slices obtained from the posterior hypothalamic nucleus. In contrast to what was observed in the hypothalamus, the intrathecal injection of NPY at a level of T4 or T10 in anesthetized or T10 in unanesthetized rats resulted in a depressor effect as well as a decrease in heart rate. Both an alpha 2- and beta-adrenoceptor antagonist reduced the NPY effect. The depressor effect of intrathecal NPY was attenuated in rats pretreated with reserpine as well as in Spontaneously Hypertensive rats (SHR). These data suggest that the effects of NPY are closely associated with sympathetic preganglionic neurons in the spinal cord. At the vascular noradrenergic neuroeffector junction, NPY decreased the nerve stimulation-induced release of NE while potentiating the contractile response. Moreover, NPY potentiated the increase in perfusion pressure of the perfused mesenteric arterial bed in response to angiotensin, vasopressin, or phenylephrine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Blunted responsiveness of posterior hypothalamic norepinephrine to quinpirole in DOCA/NaCl hypertensive rats

Our previous studies have demonstrated that the specific dopamine D2 receptor agonist, quinpirole (LY171555), has a pressor effect in conscious normotensive rats and that this is accompanied by a centrally mediated increase in sympathetic activity and arginine vasopressin release. This pressor response to quinpirole is blunted in the DOCA/NaCl hypertensive rat. To examine the hypothesis that the responsiveness of the central noradrenergic and serotonergic systems to quinpirole treatment is altered in DOCA/NaCl rats, the norepinephrine (NE), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents of hypothalamic and brainstem areas were measured in 4-week DOCA/NaCl hypertensive and H2O control rats 15 minutes after the intravenous administration of quinpirole (1 mg/kg). The results demonstrate that quinpirole selectively reduced (26%) posterior hypothalamic NE content in control rats, but not in DOCA/NaCl hypertensive rats. The NE content in the spinal cord and 5-HIAA content in the pons were greater in DOCA/NaCl rats than in normotensive controls in both saline and quinpirole treated groups. Our data suggest that the specific D2 agonist may effect its central pressor response by stimulating NE release from posterior hypothalamic area, a "pressor" region of hypothalamus, and that this D2 agonist induced pressor mechanism may be blunted in DOCA/NaCl hypertension.     

Ghrelin stimulates insulin secretion from the pancreas of normal and diabetic rats

Ghrelin is a novel 28-amino acid gut-brain peptide, which was first isolated in the rat stomach. This study examined the effect of ghrelin on insulin secretion from the isolated pancreas of normal and diabetic rats. Diabetes was induced by a single dose of streptozotocin. Four weeks after the induction of diabetes, pancreatic tissue fragments of normal and diabetic rats were treated with different concentrations (10(-12), 10(-9) and 10(-6) M) of ghrelin. Ghrelin evoked large and significant increases in insulin secretion from the pancreas of both normal and diabetic rats. In the pancreas of normal rats, diltiazem (calcium channel antagonist) or a combination of atropine (muscarinic cholinergic receptor antagonist), propranolol (beta-adrenergic receptor antagonist) and yohimbine (alpha2-adrenergic receptor antagonist) significantly reduced the stimulatory effect of ghrelin on insulin secretion. Diltiazem and yohimbine failed to inhibit ghrelin-evoked insulin release in diabetic rat pancreas. Ghrelin-immunoreactivity cells was observed in 2.6% and 3.8% of the total cell population in the islet of Langerhans of normal and diabetic rats, respectively.     

Comparison of cyclocytidine and sympathetic nerve induced changes in norepinephrine and adrenoceptors in salivary glands.

Norepinephrine (NE) concentration of parotid and submandibular glands of young rats was reduced 51% and 39%, respectively at 1 h, and 60% and 47% at 2 h after i.p. administration of a single dose (500 mg/kg body weight) of the anti-tumor agent, cyclocytidine (CC). For adult rats, the reductions were 44% and 46%, respectively, at 1 h and 54% and 49% at 2 h. This decrease from controls was generally similar to the decrease induced following 1 and 2 h of electrical stimulation (square wave pulses of 4 V, 5 ms duration, and frequency of 16 Hz) of the sympathetic innervation to these glands (young rats, 59% and 58% at 1 h; 66% and 63% at 2 h; for adult rats, 51% and 55% at 1 h and 69% and 53% at 2 h for parotid and submandibular, respectively). The changes in density of beta-adrenoceptors induced by direct nerve stimulation also corresponded to the changes induced by CC (CC induced a decrease in parotid of 12%, compared with a decrease of 11% with electrical stimulation; a 15% and 18% reduction in number of beta-adrenoceptors of submandibular gland was found at 1 h after CC and electrical stimulation, respectively). Compelling evidence for the mechanism of CC action was thus established, showing that CC mimics effects of sympathetic nerve stimulation (inducing reduction in NE concentration and transient change in beta-adrenoceptor density) by causing release of NE from sympathetic nerve endings.     

Morphine but not Naloxone Enhances Luteinizing Hormone-Releasing Hormone Neuronal Responsiveness to Norepinephrine

Some axon terminals of hypothalamic opiate neurons directly synapse on luteinizing hormone-releasing hormone (LHRH) neurons. To determine whether such synaptic connections affect LHRH neuronal activity, we have examined the profiles and concentrations of LH released in response to intracerebroventricular (icv) norepinephrine (NE, 45 μg) infusions alone or following medial preoptic area (MPOA) electrochemical stimulation (ECS) in estrogen-treated ovariectomized rats. Similar studies were performed in rats treated with naloxone (5 mg/kg ip) or morphine (20 mg/kg sc) given 15 min prior to MPOA-ECS or 30 min prior to icv NE. Naloxone neither augmented nor suppressed the LH response obtained with NE alone. MPOA-ECS evoked a significant increase in plasma LH. When the preoptic area was stimulated (0 min) and NE was infused at 30 min, a significant amplification of LH release occurred. Prior treatment of rats (-15 min) with naloxone had no effect on LH responses elicited by either preoptic stimulation alone or combined with icv NE. In the second study, morphine was given sc and had no effect on basal plasma LH levels. However, when morphine was given (-15 min) and icv NE infusions were made (30 min), the rise in plasma LH induced by NE was significantly enhanced. Preoptic ECS (0 min) evoked a rise in plasma LH and this response was also enhanced by morphine pretreatment. The major effect on LH release occurred when sc morphine injections (-15 min) were combined with MPOA-ECS (0 min) followed by icv NE (30 min). In these rats, a remarkable and highly significant release of LH occurred which reached peak levels even greater than those observed during spontaneous LH surges (2,392 versus 16 to 1,800 ng/ml). Since morphine has profound effects on the serotonergic system, in the third series of studies, morphine was infused into the dorsal raphe nucleus (DRN) and LH responses to MPOA-ECS or icv NE alone or following combined ECS + NE were examined. DRN morphine did not affect basal LH release but it produced a rapid and highly significant rise in plasma prolactin. When DRN morphine was given (-15 min) and NE was infused icv (30 min), there was marked amplification in LH release compared to those values observed after only NE. However, there were no appreciable differences in LH values obtained after sc versus DRN morphine injections in response to NE. Similarly, the amplification of LH release which occurred following DRN morphine (-15 min) + MPOA-ECS (0 min) was not different from that obtained after sc morphine. In the final group of rats, DRN morphine was given (-15 min), the preoptic area was stimulated (0 min) and NE was infused (30 min). Following this treatment, plasma LH release was also markedly enhanced and did not differ appreciably (except at 60 and 120 min) from the levels of LH released after sc morphine. Prolactin concentrations rose slowly after icv NE to reach peak levels 75 min after treatment. Combinations of morphine + MPOA-ECS without or with NE neither augmented nor suppressed the high prolactin concentrations achieved after only DRN morphine infusions. We conclude from these data that: 1) those opiate neuronal terminals which synapse directly on LHRH neurons do not affect LHRH neuronal responsiveness to either NE, to MPOA-ECS or to combined preoptic stimulation+ NE, and 2) morphine has profound effects on LHRH neuronal responsiveness to both NE, to MPOA-ECS and, in particular, to combined ECS + NE. Since amplification of LH release occurs after treatment of rats with morphine either by sc injections or DRN infusions, the augmented LH and prolactin responses observed are most likely due to the morphine-induced release of serotonin and not to direct morphine effects on LHRH neurons.     

Diabetes-induced neuroendocrine changes in rats: role of brain monoamines,insulin and leptin

Diabetes is characterized by hyperphagia, polydypsia and activation of the HPA axis. However, the mechanisms by which diabetes produces these effects are not clear. This study was conducted to examine the effects of diabetes on the neuroendocrine system and to see if treatment with insulin and/or leptin is capable of reversing these effects. Streptozotocin-induced diabetic adult male rats were subjected to the following treatments: vehicle, insulin (2 U/day, s.c.), leptin (100 microg/kg BW) or leptin+insulin every day for 2 weeks. Food intake, water intake, and body weight were monitored daily. We measured changes in monoamine concentrations in discrete nuclei of the hypothalamus at the end of treatment. Diabetes produced a marked increase in food intake and water intake and this effect was completely reversed by insulin treatment and partially reversed by leptin treatment (P<0.05). Diabetes caused an increase in norepinephrine (NE) concentrations in the paraventricular nucleus with a concurrent increase in serum corticosterone. Treatment with insulin and leptin completely reversed these effects. Induction of diabetes also increased the concentrations of NE, dopamine and serotonin in the arcuate nucleus and NE concentrations in the lateral hypothalamus, ventromedial hypothalamus (VMH) and suprachiasmatic nucleus (P<0.05). Although insulin treatment was capable of reversing all these changes, leptin treatment was unable to decrease diabetes-induced increase in NE concentrations in the VMH. These data provide evidence that hypothalamic monoamines could mediate the neuroendocrine effects of diabetes and that insulin and leptin act as important signals in this process.     

Sympathetic activity following paraventricular injections of glucose and insulin

The effect of microinjections of glucose and insulin into the paraventricular nucleus (PVN) on sympathetic firing rate has been examined in rats. Following the injection of insulin 144 pmoles, there was a 30% reduction in sympathetic firing rate with a minimum reached 2 minutes following the injection, and recovery to baseline within 4 minutes. Lower doses of insulin were without effect. Following the injection of glucose 138 nmoles. there was a brief 20% increase in sympathetic firing rate which peaked at 1 minute and had returned to control levels by the second minute. This effect of glucose, like that of insulin, was dose-related. When compared with injections of insulin and glucose into the ventromedial hypothalamus, injection of either substance into the PVN showed a smaller and more attenuated response, raising the possibility that the effect of these nutrients injected into the PVN may be through the small quantities which could reach the ventromedial nucleus. These studies are consistent with the hypothesis that the paraventricular nucleus is not the principal modulator of the sympathetic firing rate.     

Facilitation of ACTH secretion by morphine is mediated by activation of CRF releasing neurons and sympathetic neuronal pathways

Exogenously applied opioid agonists have a stimulatory effect on adrenocorticotropic hormone (ACTH) secretion. The present experiments were designed to examine the mechanisms involved in the stimulatory effect of the mu-receptor agonist morphine on ACTH release in chronically cannulated, freely moving, non-stressed rats. Morphine (7.5 mg/kg, i.v.) treatment was followed by a significant increase in plasma levels of ACTH. Pretreatment with the peripheral ganglionic blocker chlorisondamine (3 mg/kg, i.p.) attenuated the response to morphine. The morphine stimulatory effect was also partially inhibited if the rats were pretreated with a specific antiserum to corticotropin-releasing factor (CRF). In rats given both CRF antiserum and chlorisondamine, the plasma ACTH levels remained unchanged after morphine application. These findings indicate that morphine stimulates the release of ACTH by activating both CRF-secretion and peripheral sympathetic neuronal pathways.     

Ab-Induced Insulin Resistance and the Effects of Insulin on the Cholesterol Synthesis Pathway and Ab Secretion in Neural Cells

Alzheimer's disease(AD) is characterized by amyloid-b(Ab) toxicity,tau pathology,insulin resistance,neuroinflammation,and dysregulation of cholesterol homeostasis,all of which play roles in neurodegeneration.Insulin has polytrophic effects on neurons and may be at the center of these pathophysiological changes.In this study,we investigated possible relationships among insulin signaling and cholesterol biosynthesis,along with the effects of Ab42 on these pathways in vitro.We found that neuroblastoma 2a(N2a) cells transfected with the human gene encoding amyloid-b protein precursor(Ab PP)(N2aAb PP) produced Ab and exhibited insulin resistance by reduced p-Akt and a suppressed cholesterol-synthesis pathway following insulin treatment,and by increased phosphorylation of insulin receptor subunit-1 at serine 612(p-IRS-S612) as compared to parental N2 a cells.Treatment of human neuroblastoma SH-SY5 Y cells with Ab42 also increased p-IRS-S612,suggesting that Ab42 is responsible for insulin resistance.The insulin resistance was alleviated when N2a-Ab PP cells were treated with higher insulin concentrations.Insulin increased Ab release from N2 aAb PP cells,by which it may promote Ab clearance.Insulin increased cholesterol-synthesis gene expression in SHSY5 Y and N2 a cells,including 24-dehydrocholesterol reductase(DHCR24) and 3-hydroxy-3-methyl-glutaryl-Co A reductase(HMGCR) through sterol-regulatory element-binding protein-2(SREBP2).While Ab42-treated SH-SY5 Y cells exhibited increased HMGCR expression and c-Jun phosphorylation as pro-inflammatory responses,they also showed down-regulation of neuro-protective/antiinflammatory DHCR24.These results suggest that Ab42 may cause insulin resistance,activate JNK for c-Jun phosphorylation,and lead to dysregulation of cholesterol homeostasis,and that enhancing insulin signaling may relieve the insulin-resistant phenotype and the dysregulated cholesterol-synthesis pathway to promote Ab release for clearance from neural cells.     

Adrenergic and Noradrenergic Regulation of Pulsatile Luteinizing Hormone Release*

The object of this study was to further define the roles of both norepinephrine (NE) and epinephrine (EPIN) in regulating pulsatile luteinizing hormone (LH) release in 4-day ovariectomized rats, in particular to examine the effect of decreasing NE synthesis on pulsatile LH secretion in animals with already greatly depleted levels of brain EPIN. Rats were injected ip with vehicle or drug at -27, -20, -5 and - 3 h relative to the onset of a 3-h blood sampling period. Hypothalamic-preoptic area (HPOA) levels of NE and EPIN were determined by high-performance liquid chromatography. Compared to controls, FLA-63 (25 mg/kg, a dopamine-ß- hydroxylase inhibitor), given at - 3 h, produced 50% and 22% declines in HPOA-NE and EPIN, respectively, and reductions in pulse amplitude and frequency. LY134046 (50 mg/kg, a phenylethanolamine N-methyltransferase inhibitor), given at - 27, - 20 and - 5 h, or -27, -20, -5 and -3 h, produced no change in NE, 88% and 86% declines in EPIN, respectively, and reductions in pulse frequency only. Each LY134046 treatment protocol produced the same decline in EPIN and pulse frequency. Thus, EPIN levels were maximally decreased by three LY134046 injections. When rats were given LY134046 at -27, -20 and -5 h, and FLA-63 at -3 h, compared to rats treated with LY134046 alone, there was no further decrease in HPOA-EPIN (82% decline), a 46% decline in NE, a further reduction in pulse frequency and a reduction in pulse amplitude. This further suppression of LH release must be due to a reduction in HPOA-NE levels since no further decrease in EPIN levels occurred. These data demonstrate within the same animal that NE and EPIN are both stimulatory to pulsatile LH release. NE stimulates the amplitude and frequency, and EPIN stimulates the frequency of pulsatile LH secretion.     

Effect of secretion of splenocytes after superior ovarian nerve section on the ovarian steroidogenesis.

It is known that ovary and spleen are innervated extensively by afferent and efferent noradrenergic sympathetic nerve fibers from the celiac ganglion. Furthermore, immune cells located in the ovary influence the ovarian physiology. However, the peripheral interaction between the immune and neuroendocrine system is poorly understood. This work was undertaken to study the effect of superior ovarian nerve (SON) transection, in adult rats, on the number of splenocyte beta-adrenergic receptors and their possible relation to ovarian steroidogenesis, measuring the effect of secretions of those splenocytes on progesterone and estradiol release from the ovary. Seven days after SON transection, the splenocytes were isolated and then cultured for 48 h. Their number of beta-adrenergic receptors, measured using [125I]-cyanopindolol as ligand, increased, and their culture media, used to stimulate ovaries from 60-day-old intact (neither SON-transected nor sham-operated) rats in vitro on diestrous day 2 showed a decrease in progesterone release and an increase in estradiol release in relation to splenocyte culture media of control rats (sham-operated; p < 0.001, respectively). The effects of in vivo SON transection were simulated by an in vitro system modulating the splenocyte beta-adrenergic receptor number. The splenocytes from SON-transectioned rats were preincubated with and without norepinephrine (NE) 10(-6) M for 48 h, a low and high number of beta-adrenergic receptors respectively, and then were stimulated with NE 10(-6) M for 24 h. After that, the culture medium from splenocytes with a low number of beta-adrenergic receptors induced progesterone release from the ovaries of intact rats (p < 0. 001), but produced no change in estradiol release. The data suggest that splenocyte secretions, which participate in the ovarian steroidogenic response, particularly in progesterone release, might be controlled by adrenergic influences since the number of splenocyte beta-adrenergic receptors changes through SON-celiac ganglion-noradrenergic postganglionic innervation of the spleen. In estradiol release, probably other neurotransmitters than norepinephrine (NE) are involved when the SON is sectioned. In this paper we also show functional evidence for modulation of immune function by the sympathetic nervous system and its principal neurotransmitter, NE.     

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)     

Bilateral carotid occlusion and the sympathetic regulation of insulin secretion

After a 2-min bilateral carotid arterial occlusion (BCO) in puppies, a centrally originating, sympathetic discharge takes place which increases heart rate and blood pressure. We examined the specificity of this sympathetic neural outflow by determining whether it also caused a direct neurally mediated inhibition of insulin release from the pancreas. The effects of this BCO on portal venous insulin concentrations, as well as on heart rate and blood pressure, were examined during i.v. glucose infusions of 0 (saline), 5 and 15 mg/kg X min-1. To determine changes in splanchnic blood flow and to more closely estimate pancreatic insulin secretion rates, a major vein draining the pancreas, the gastroduodenal, was catheterized. Blood flows and the amount of insulin traversing this vein per min (insulin flow rate) were followed before, during and after BCO. BCO decreased portal vein insulin concentrations during i.v. glucose infusions of 5 and 15 mg/kg X min-1, but not when saline was infused. Since bilateral splanchnicotomy altered this result little and since BCO increased blood flow and the insulin flow rate in the gastroduodenal vein, it appears that the lower portal venous insulin concentrations during BCO are secondary, not to sympathetically induced decrease in insulin secretion rates but, to dilution of pancreatic effluent blood. We conclude that while BCO causes appropriate changes in heart rate and blood pressure, this central stimulus to the sympathetic system does not provide a direct neuroendocrine reflex change in insulin secretion. BCO alters portal venous insulin concentration indirectly, and the alteration depends on the plasma glucose concentration and an enhancement in the splanchnic blood flow.     

Peripubertal development of noradrenergic stimulation of luteinizing hormone-releasing hormone neurosecretion in vitro

The effect of age on norepinephrine (NE) stimulation of luteinizing hormone-releasing hormone (LH-RH) secretion from preoptic area-mediobasal hypothalamic (POA-MBH) explants was examined in the present study. Explants were obtained from juvenile (9-day-old), prepubertal (29-day-old) and adult female rats. Following decapitation and surgical isolation, POA-MBH explants were individually perifused with culture medium which was collected for radioimmunoassay of LH-RH. Explants were exposed to two pulses of medium containing NE (5 x 10(-4) M, peak concentration) and a terminal pulse of medium containing KCl (45 mM, peak concentration) for assessment of viability. POA-MBH explants obtained from prepubertal female rats exhibited increased LH-RH release in response to the two pulses of NE and subsequent KCl pulse (P less than 0.05). NE was without effect in stimulating LH-RH neurosecretion from POA-MBH explants obtained from 9-day-old female rats although these explants were responsive to KCl (P less than 0.01). Two-day pretreatment of 9-day-old, and prepubertal rats with estradiol benzoate (EB) did not alter the LH-RH response to this dose of NE or KCl (no group or interaction effects) in prepubertal female rat explants and did not render the explants from 9-day-old rats responsive to NE. Furthermore, NE was equally effective in stimulating LH-RH release from explants obtained from estradiol-treated or control ovariectomized adult rats. These observations demonstrate a peripubertal activation of the stimulatory effect of NE on LH-RH release from POA-MBH explants in vitro. Although these data also suggest that estrogen is not obligatory for NE stimulation of LH-RH release from POA-MBH explants, further investigation is required to determine the developmental time course and estrogen dependency of the noradrenergic stimulation of LH-RH neurosecretion and to evaluate whether estrogen alters the sensitivity of POA-MBH explants to lower concentrations of NE as has been previously reported for median eminence fragments.     

Chronic intracerebroventricular insulin attenuates the leptin-mediated but not alpha melanocyte stimulating hormone increase in sympathetic and cardiovascular responses

The co-existence of hyperinsulinemia and hyperleptinemia of obesity is well established. Additionally, both insulin resistance and leptin resistance are also characteristic of these states. Possible central nervous system (CNS) mechanisms could mediate these responses in that leptin receptors are located on hypothalamic neurons that coexpress neuropeptide-Y (NPY) or proopiomelanocortin (POMC) and both peptides that have been implicated as mediators of the CNS action of leptin. Leptin has been demonstrated to decrease or down regulate NPY expression and increase POMC expression. Insulin also has been demonstrated to decrease NPY and insulin insufficiency is associated with an increased POMC. Since both leptin and insulin share and modulate the same effector systems, we investigated the effect of CNS-induced hyperinsulinemia on the subsequent cardiovascular and sympathetic nervous response to leptin. Normal rats were implanted with intracerebroventricular (i.c.v.) cannula and allowed to recover. They were treated with insulin via i.c.v. cannula for 3 days. Following treatment, they were instrumented for the recording of cardiovascular and sympathetic nervous responses. Intracerebroventricular leptin administration in normal animals result in a progressive increase in both lumbar sympathetic nerve activity and mean arterial pressure. However, in animals pretreated with insulin for 3 days the leptin-induced response was completely attenuated. However, insulin treatment did not affect the POMC peptide product, alpha-melanocyte stimulating hormone (alphaMSH), mediated sympathetic nervous and cardiovascular responses. From these studies we conclude that CNS hyperinsulinemia can act to attenuate the leptin-induced increases in sympathetic nervous and cardiovascular system activity. The decreased responsiveness is not due to decreased sensitivity of the melanocortin, alphaMSH, mediated pathway.We suggest that the hyperinsulinemia of obesity may play a role in the obesity-induced leptin resistance.     

Cerebral insulin increases brain response to glucose

The hypothalamus participates in the regulation of carbohydrate metabolism involving a feedback loop between the brain and the periphery in which glucose-sensitive hypothalamic areas appear to be involved. We have previously shown that a glucose injection (9 mg/kg) in the carotid artery toward the brain, in an amount that did not modify glycaemia, caused a rapid and transient increase in plasma insulin concentrations. To determine whether central insulin could influence this response, we investigated the change in central glucose-induced insulin secretion in intracerebroventricular (i.c.v) insulin-injected rats and in hyperinsulinaemic obese Zucker rats. Central glucose-induced insulin secretion was increased by 50% in i.c.v. insulin-injected rats compared to control rats. When a similar test was performed at a lower dose of glucose (3 mg/kg), a significant insulin secretion was observed only in rats submitted to a prior central insulin injection. These data indicate an increase in the brain response to glucose after insulin treatment. Using an identical lower glucose dose, we also demonstrated an enhanced brain glucose sensitivity in hyperinsulinaemic and insulin-resistant obese Zucker rats. Together, these results indicate that acute i.c.v. insulin or pathological hyperinsulinaemic state (i.e. obese Zucker rat) modulates the nervous control of insulin secretion by increasing the brain response to glucose.