Central hyperglycaemic effect of adrenaline and carbachol

Localized glucoprivation of hindbrain sites elicits corticosterone and glucagon secretion

Glucose is required for brain energy metabolism. Decerebration, aqueduct occlusion, and cannula mapping studies have established that glucose-sensing cells capable of eliciting feeding and adrenal medullary responses to glucoprivation are localized in the hindbrain. Glucoprivation also evokes corticosterone and glucagon secretion, but the location of receptors mediating these responses is unknown. To determine whether glucoreceptive sites controlling these responses are present in the hindbrain, we administered the antiglycolytic agent, 5-d-thioglucose (5TG, 24 microg in 200 nl) into brain stem sites through implanted cannulas and examined plasma concentrations of corticosterone and glucagon. Both hindbrain and hypothalamic sites were tested. Blood was collected remotely from intra-atrial catheters at 0, 30, 60, 90, 120, 180, and 240 min after 5TG or control injection. Caudal hindbrain 5TG injections potently increased circulating corticosterone and glucagon concentrations. For corticosterone, the mean peak response (maximum concentration minus time 0 concentration) elicited at positive sites (23 of 40 sites) was 391 ng/ml (SE = 16). For glucagon, the mean peak response at positive sites (27 of 40 sites) was 46 pg/ml (SE = 6). Glucoprivically evoked glucagon secretion was abolished by the ganglionic blocker, hexamethonium, but not by adrenal denervation. Six of twenty-five hypothalamic sites were positive for corticosterone secretion, yielding plasma levels of 279 +/- 23 ng/ml, but none of the hypothalamic injection sites elevated glucagon concentrations. Results demonstrate that receptor cells responsive to glucose deficit and capable of increasing corticosterone and glucagon concentrations exist within the hindbrain, thus further delineating central glucoregulatory neural circuitry.

Alterations in the muscarinic M1 and M3 receptor gene expression in the brain stem during pancreatic regeneration and insulin secretion in weanling rats

Muscarinic M1 and M3 receptor changes in the brain stem during pancreatic regeneration were investigated. Brain stem acetylcholine esterase activity decreased at the time of regeneration. Sympathetic activity also decreased as indicated by the norepinephrine (NE) and epinephrine (EPI) content of adrenals and also in the plasma. Muscarinic M1 and M3 receptors showed reciprocal changes in the brain stem during regeneration. Muscarinic M1 receptor number decreased at time of regeneration without any change in the affinity. High affinity M3 receptors showed an increase in the number. The affinity did not show any change. The number of low affinity receptors decreased with decreased Kd at 72 hours after partial pancreatectomy. The Kd reversed to control value with a reversal of the number of receptors to near control value. Gene expression studies also showed a similar change in the mRNA level of M1 and M3 receptors. These alterations in the muscarinic receptors regulate sympathetic activity and maintain glucose level during pancreatic regeneration. Central muscarinic M1 and M3 receptor subtypes functional balance is suggested to regulate sympathetic and parasympathetic activity, which in turn control the islet cell proliferation and glucose homeostasis.

Adrenergic control of glycemia in the paraventricular nucleus of the hypothalamus in pigeons

The effects of local injections of adrenaline into the paraventricular nucleus (PVN) and into other anterior hypothalamic areas on blood glucose levels were investigated in conscious pigeons (Columba livia). Adrenaline evoked a reliable and potent increase in blood glucose levels when injected into the PVN; no glycemic change was observed after vehicle injections into the PVN or after adrenaline injections into the 3rd ventricle or hypothalamic structures adjacent to the PVN. Local pretreatment with propranolol, but not with phentolamine, abolished the adrenaline-induced hyperglycemia. The present results indicate that adrenergically mediated circuits into the avian PVN play an important role in central mechanisms of energy balance that may be equivalent to those observed in mammalian species.

Centrally injected atropine reduces hyperglycemia caused by 2-DG or immobilization stress in awake rats

The physiological significance of central cholinergic neurons was investigated by verifying the effect of previous intracerebroventricular administration of atropine on the hyperglycemia induced by 2-deoxyglucose (2-DG) or by immobilization stress in unrestrained, nonanesthetized rats. Intravenous 2-DG induced a marked increase in plasma glucose that was not affected by atropine injected intracerebroventricularly 30 min before. However, the hyperglycemia induced by intracerebroventricular 2-DG was significantly reduced by previous intracerebroventricular injection of atropine. Immobilization induced a rapid increase of plasma glucose levels that was reduced by about 50% by intracerebroventricular injection of atropine. The increase in plasma lactate induced by intravenous 2-DG, or immobilization, was not significantly affected by previous intracerebroventricular injection of atropine. The data suggest that central cholinergic neurons participate in the complex neural events responsible for the hyperglycemic response to neurocytoglucopenia and to stressful situations.

The metabolism of plasma glucose and catecholamines in Alzheimer's disease

Several lines of evidence suggest that the cholinergic system in the hippocampus plays a pivotal roll in regulating the peripheral metabolism of glucose and catecholamines. The injection of cholinergic stimulators including neostigmine, the acetylcholine esterase inhibitor, into the third ventricle or the hippocampus induces the elevation of glucose or catecholamines in plasma in rats. Under stress conditions, release of acetylcholine in the hippocampus increases, which coincides with the elevation of plasma glucose and catecholamines. Age-related reduction in responsivity of the cholinergic system in the hippocampus has been well documented. The intrahippocampal neostigmine injection induces significantly attenuated responses in plasma glucose and catecholamines in rats, the finding suggested that changes in cholinergic system activity in the hippocampus could result in alteration of the peripheral metabolism of glucose and catecholamines. In Alzheimer's disease (AD), the most common type of dementia, degeneration of the hippocampal cholinergic system is one of the most robust pathological features. Measurement of plasma catecholamines during a fasting state in the groups of AD subjects, vascular dementia subjects, and non-demented control subjects showed significantly lower plasma epinephrine levels in the AD subjects.

Low plasma epinephrine in elderly female subjects of dementia of Alzheimer type

One of the robust features of brain pathologies of dementia of Alzheimer type (DAT) is the impairment of the hippocampus, especially the cholinergic system. Several animal studies have suggested that the cholinergic system in the hippocampus is involved in the control of the plasma level of catecholamines and glucose. The stimulation of the hippocampal cholinergic system has resulted in the elevation of plasma catecholamines and glucose in rats. In the present study, we measured the plasma level of epinephrine, norepinephrine, dopamine, glucose, and insulin during a fasting state in the morning in hospitalized DAT (n=66), vascular dementia (VD) (n=28), or non-demented (ND) (n=21) females (mean age DAT=82. 49+/-4.98, VD=82.86+/-5.86, ND=82.95+/-7.77, respectively). Statistical analysis showed that the plasma level of epinephrine during a fasting state in DAT subjects was significantly lower than that of ND subjects; however, in VD subjects the level of epinephrine was not different from that of ND subjects. Other values did not differ significantly among the groups.

Role of central neural mechanisms in the regulation of hepatic glucose metabolism

Central monoamine neurotransmitters affect blood glucose homeostasis. Activation of central cholinergic, noradrenergic histaminergic, and serotonergic neurons rapidly increase hepatic glucose output via the sympathetic nervous system. Acute hyperglycemia is mediated by three distinct pathways: the action of epinephrine on the liver, the action of glucagon on the liver, and the direct innervation of the liver. The relative contribution of these factors to hyperglycemia can be altered by diet and the kinds of neurotransmitters evoked in the central nervous system, but the magnitude of epinephrine secretion is closely related to the magnitude of hyperglycemia. On the other hand, neuropharmacological stimulation of central cholinergic muscarinic receptors, histaminergic H1 receptors, and serotonergic 5-HT2 receptors increases hypothalamic noradrenergic neuronal activity, which is associated with hyperglycemia. In contrast, central GABAA receptors play an inhibitory role in the regulation of hepatic glucose metabolism. Thus, central monoaminergic neurons could be linked together, and play a homeostatic role in the regulation of hepatic glucose metabolism.

Central beta-adrenoceptor involvement in neural control of blood glucose in pigeons

The effects of ICV injections of adrenaline (30 nmol in 1 microL of saline) on blood glucose levels were investigated in conscious adult pigeons. This procedure increased blood glucose levels at 15-45 min after treatment. Previous ICV injection of propranolol (50 nmol) suppressed the increase observed at 15 min. The higher propranolol dose (100 nmol) was more effective than the lower dose (50 nmol) at blocking adrenaline-induced hyperglycemia. On the other hand, the ICV pretreatment with an alpha-adrenergic antagonist, phentolamine, slightly potentiated the hyperglycemia caused by ICV injection of adrenaline. The IP administration of propranolol (100 nmol) or phentolamine (100 nmol) before adrenaline ICV failed to induce change in the hyperglycemic response induced by this catecholamine. Both IP and ICV injections of these adrenergic blockers, before ICV injections of saline, evoked no changes in baseline glycemic levels. Therefore, elevation of blood glucose concentration by ICV adrenaline and blockade of the response by propranolol suggest the involvement of a central beta-adrenergic mechanism in the neural control of glycemia in pigeons.

Electrographic patterns of postprandial sleep after food deprivation or intraventricular adrenaline injections in pigeons

Intense postprandial sleep-like behavior was previously reported to occur, in a similar fashion, either after feeding evoked by intracerebroventricular adrenaline injections or after interruption of prolonged (96 h) fasting in conscious pigeons. These behavioral similarities were taken as indication of a physiological role for central adrenergic circuits in avian food intake regulation. In the present study we compared sleep-related electrographic signs (EEG, EMG, and EOG) that develop following both food intake-inducing situations to further examine these correspondences and their corollaries. Our data indicate that postprandial behaviors in the pigeon include dramatic increases in the incidence of slow-wave (SWS) and rapid eye movement sleep (REMS). The temporal distribution, total amount, and mean bout duration of these sleep states, as well as the latency for the first SWS episode, were statistically similar in both feeding behavior-inducing situations. Besides confirming early behavioral data, indicative of an engagement of adrenergic central components in food intake control, our results suggest that postprandial SWS could represent an important element of the feeding-related metabolic changes in pigeons, in a similar fashion to the observed in mammalian forms.

Hypothalamic cholinergic systems in mercuric chloride-induced hyperglycemia

The roles of hypothalamic neuron mechanisms in regulation of blood glucose were studied in rats. Microinjection of HgCl2 and neostigmine into the third ventricle under anesthesia caused marked hyperglycemia. To characterize the effective neurons in the hypothalamus, tissue contents of neurotransmitters and extracellular levels of metabolites in the ventromedial part of the hypothalamus were measured by microwave irradiation and in vivo brain microdialysis. HgCl2 increased both the content and the extracellular levels of choline. Neostigmine increased the acetylcholine content. In both instances, norepinephrine (NE) and dopamine (DA) content were decreased. Extracellular levels of DA metabolites were increased. On the other hand, serotonin content and extracellular serotonin metabolite level remained constant. In medulloadrenalectomized and atropine-coadministered rats, no marked hyperglycemia was induced by HgCl2 or neostigmine. These results show that the muscarinic cholinergic system participates in the HgCl2-induced central hyperglycemic effect through the function of the adrenal medulla. NE neurons may also be related to hypothalamic glycoregulation, and DA neurons might modulate the regulatory mechanisms.

Food deprivation- vs. intraventricular adrenaline-induced feeding and postprandial behaviors in the pigeon (Columba livia)

The present study was designed to investigate possible similarities between the feeding and postprandial behavioral profile observed after different periods of food deprivation and after intracerebroventricular (ICV) injections of adrenaline (A) (30 nmol/microliter) in satiated pigeons (Columba livia). The results indicate that the postprandial sleep-like behavior increases observed after A treatment are similar only to those observed after prolonged periods of fasting. These parallel behavioral effects are discussed as representing the product of similar levels of satiety signals, obtained after equivalent signaling of challenges to energy homeostasis, in both 96-h deprived and A-treated animals. Our data may also suggest that ICV A injections evoke a central state comparable to that of prolonged fasting, and that this neurotransmitter may participate as a chemical mediator in the regulation of food intake in the pigeon.

Neither adrenergic nor cholinergic antagonists in the central nervous system affect 2-deoxy-D-glucose(2-DG)-induced hyperglycemia

To investigate whether the brain adrenergic and cholinergic neurotransmitter systems are involved in the regulation of 2-deoxy-D-glucose (2-DG)-induced hyperglycemia, we studied the effects of adrenergic and cholinergic antagonists on 2-DG-induced secretion of epinephrine and glucagon, and hyperglycemia, in anesthetized fed rats. When 2-DG (10 mg/10 microliters) was injected into the third cerebral ventricle, hepatic venous plasma glucose, glucagon, and epinephrine concentrations were significantly increased. Co-administration of phentolamine, propranolol, atropine and hexamethonium (1 X 10(-7) mol) with 2-DG did not modify the hyperglycemia and hormonal responses normally observed after the administration of 2-DG alone. From this evidence we concluded that neither brain adrenoceptive nor cholinoceptive neurons are involved in the regulation of 2-DG-induced hyperglycemia.

Neostigmine-induced hyperglycemia is mediated by central muscarinic receptor in fed rats

We previously reported that neostigmine injected into the third cerebral ventricle stimulated adrenal secretion of epinephrine, secretion of glucagon from the pancreas, and direct neural innervation of the liver, resulting in hepatic venous plasma hyperglycemia in anesthetized fed rats. However, receptor type of these 3 mechanisms is not known. Therefore, we examined the effects of intraventricularly injected cholinergic or adrenergic antagonists on neostigmine-induced catecholamines in intact rats, glucagon secretion which is mediated by direct neural innervation of pancreas in bilateral adrenalectomized (ADX) rats, and hepatic venous hyperglycemia which is mediated by direct neural innervation of liver in ADX rats receiving constant infusion of somatostatin from femoral vein. Atropine injected into the third cerebral ventricle suppressed epinephrine secretion and dose-dependently inhibited hepatic venous hyperglycemia induced by neostigmine in intact rats. The neostigmine-induced glucagon secretion which occurs in ADX rats was suppressed by atropine. Atropine also prevented the neostigmine-induced hyperglycemia in ADX rats receiving constant somatostatin infusion through femoral vein (ADX-Somato rats). On the other hand, phentolamine, propranolol and hexamethonium showed no significant inhibitory effect on neostigmine-induced hyperglycemia, epinephrine and glucagon secretion in intact rats, glucagon secretion in ADX rats, or hyperglycemia in ADX-Somato rats. These results suggest that neostigmine-induced epinephrine and glucagon secretion and increased hepatic glucose output stimulated by direct neural innervation to liver is mediated by central muscarinic receptor in fed rats.

Regional cerebral blood flow in acute hypertension induced by adrenaline, noradrenaline and phenylephrine in the conscious rat

Hypertension was induced in conscious rats by intravenous infusion of phenylephrine (3, 6 or 12 micrograms kg-1 min-1), noradrenaline (3 micrograms min-1) or adrenaline (3 micrograms kg-1 min-1). Local cerebral blood flow was measured autoradiographically in 24 defined brain structures using [14C]iodoantipyrine as the diffusible tracer. The mean arterial pressure induced by adrenaline, noradrenaline and the two higher doses of phenylephrine was 158-168 mmHg with no significant differences between the groups. Only adrenaline significantly increased local cerebral blood flow in nine of the 24 structures studied. The smaller capacity for autoregulation after adrenaline compared with other drugs might be related to a beta-adrenoreceptor-stimulating effect.

Mechanism of central hyperglycemic effect of cholinergic agonists in fasted rats

The influence of cholinergic agonists on central nervous system (CNS) regulation of blood sugar homeostasis was studied in fasted rats. When carbachol, muscarine, bethanechol, methacholine, or neostigmine was injected into the third cerebral ventricle, it caused a dose-dependent increase in the hepatic venous plasma glucose concentration. However, in the case of 1,1-dimethylphenyl-4-piperazinium iodide (DMPP) or nicotine, the level of hepatic venous glucose did not differ from that of the saline-treated control rats. The increase in glucose level caused by neostigmine was dose-dependently suppressed by coadministration of atropine. These facts suggest that cholinergic activation of muscarinic receptors in the CNS plays a role in increasing hepatic glucose output. Injection of neostigmine (5 X 10(-8) mol), an inhibitor of cholinesterase, into the ventricle resulted in the increase of not only glucose, but also glucagon, epinephrine, and norepinephrine in the hepatic venous plasma. However, constant infusion of somatostatin through a femoral vein completely prevented the increase of glucagon after administration of neostigmine, although the increase of hepatic venous glucose and epinephrine levels were still observed. Neostigmine-induced increments in glucose did not occur in adrenalectomized rats. This suggests that the secreted epinephrine acts directly on the liver to increase hepatic glucose output.