Modulation of plasma glucose by the medial preoptic area in freely moving rats

Intracerebroventricular physostigmine facilitates heat loss mechanisms in running rats

The aim of this study was to evaluate the participation of central cholinergic transmission in the regulation of metabolic rate, core temperature, and heat storage in untrained rats submitted to exercise on a treadmill (20 m/min, 5% inclination) until fatigue. The animals were separated into eight experimental groups, and core temperature or metabolic rate was measured in the rats while they were exercising or while they were at rest after injection of 2 microl of 5 x 10(-3) M physostigmine (Phy) or 0.15 M NaCl solution (Sal) into the lateral cerebral ventricle. Metabolic rate was determined by the indirect calorimetry system, and colonic temperature was recorded as an index of core temperature. In resting animals, Phy induced only a small increase in metabolic rate compared with Sal injection, without having any effect on core temperature. During exercise, the Phy-treated animals showed a lower core heating rate (0.022 +/- 0.003 degrees C/min Phy vs. 0.033 +/- 0.003 degrees C/min Sal; P < 0.02), lower heat storage (285 +/- 37 cal Phy vs. 436 +/- 34 cal Sal; P < 0.02) and lower core temperature at fatigue point than the Sal-treated group (38.5 +/- 0.1 degrees C Phy vs. 39.0 +/- 0.1 degrees C Sal; P < 0.05). However, despite the lower core heating rate, heat storage, and core temperature at fatigue, the Phy-treated rats showed a similar running time compared with the Sal-treated group. We conclude that the activation of the central cholinergic system during exercise increases heat dissipation and attenuates the exercise-induced increase in core temperature without affecting running performance.

Medial preoptic area adrenergic receptors modulate glycemia and insulinemia in freely moving rats

In order to investigate the role of medial preoptic area (MPOA) adrenoceptors in regulation of plasma glucose and insulin secretion, we injected 40 nmol of noradrenaline, clonidine or isoproterenol into the MPOA of freely moving Wistar rats. The animals were fitted with chronic jugular catheters for blood sampling and unilateral intracerebral cannulae placed into MPOA. The results showed that noradrenaline injection into MPOA produced a rapid increase in plasma glucose levels and insulin secretion, reaching a peak at 15 min post stimulus (25% over basal, P<0.01) for plasma glucose and at 30 min for insulin secretion (94% over basal, P<0.05). Injection of the alpha2-adrenergic agonist clonidine into MPOA produced a faster, more intense and longer-lasting hyperglycemic response (69% over basal, P<0.01). In contrast to the noradrenaline effect on insulin secretion, clonidine markedly decreased plasma insulin levels, reaching a maximal suppression at 10 min (72% below basal, P<0.01). On the other hand, the beta-adrenergic agonist isoproterenol only produced a small, transient increase in plasma glucose levels. When rats were pre-treated with guanethidine (10 mg/100 g, i.p.), despite reduced baseline of plasma glucose (35% smaller then control group, P<0.01) and increased plasma insulin baseline (300% higher then control group, P<0.01), they still showed a hyperglycemic response to noradrenaline injection into MPOA. We conclude that the activation of preoptic alpha2-adrenoceptors induced hyperglycemia and inhibit insulin secretion, probably by activation of the sympathoadrenal system that cannot be blocked by prior administration of guanethidine.

The suprachiasmatic nucleus generates the diurnal changes in plasma leptin levels

At present it is not clear which factors are responsible for the diurnal pattern of plasma leptin levels, although the timing of food intake and circulating hormones such as glucocorticoids and insulin have both been proposed as independent determinants. In this study we show that ablation of the biological clock by thermal lesions of the hypothalamic suprachiasmatic nucleus (SCN) completely eliminates the diurnal pattern of plasma leptin levels. By contrast, removal of the diurnal corticosterone signal by adrenalectomy and corticosterone replacement did not affect diurnal plasma leptin levels. More importantly, removal of the nocturnal feeding signal by submitting the animals to a regular feeding schedule of six meals per day did not abolish the diurnal plasma leptin levels. However, both SCN lesions and the regular feeding schedule did cause an increase in the 24-h mean plasma leptin levels. As neither rhythmic feeding, insulin, or corticosterone signals can completely explain the diurnal plasma leptin rhythm, we conclude that biological clock control of the sympathetic input to the adipocyte is essential for regulation of the daily rhythm in leptin release.

The medial preoptic area modulates the increase in plasma glucose and free fatty acid mobilization induced by acute cold exposure

The effect of cold exposure (0 degrees C, 1 h) on free fatty acid (FFA) mobilization and plasma glucose was studied in freely moving male rats injected with adrenergic blockers (phentolamine or propranolol) into the medial preoptic area (MPOA). The rats were implanted with chronic jugular catheters for blood sampling and with unilateral intracerebral cannulas placed just above the MPOA. Blood samples were taken 2 min before and 10, 20, 40 and 60 min after cold exposure. After cold exposure plasma glucose and plasma FFA levels rose rapidly, reaching a peak at 20 min post-stimulus. Previous administration of phentolamine (50 nmol), but not propranolol (100 nmol), into the MPOA blocked the glycemic response to cold exposure. On the other hand, previous administration of propranolol, but not phentolamine, into the MPOA blocked the increased FFA mobilization in response to cold exposure. On the basis of these results, we propose that MPOA alpha-adrenergic synapses relay impulses activating the sympathetic outflow expressed by neurally mediated hyperglycemia, and beta-adrenergic synapses relay impulses activating the sympathetic outflow to the adipose tissue, increasing FFA acid mobilization.

Effect of intracerebroventricular injection of atropine on metabolic responses during exercise in untrained rats

To investigate the role of the central cholinergic system in the regulation of metabolism during exercise, we injected atropine (5 x 10(-7) mol) into the lateral cerebral ventricle of normal and adrenodemedullated (ADM) untrained rats submitted to exercise on a treadmill (15 m min(-1), 5% grade) until exhaustion. Concentrations of blood glucose, plasma free fatty acids (FFA), and lactate were measured before and every 10 min after the start of exercise for a period of 60 min. Adrenomedullectomy had no effect on the maximal capacity of exercise (MCE), but atropine administered intracerebroventricularly (i.c.v.) reduced the maximal capacity of exercise of both normal and ADM rats. In normal rats, blood concentrations of glucose and plasma free fatty acids remained essentially unchanged compared to the levels at rest, whereas in ADM rats a rapid increase in plasma glucose and plasma free fatty acids levels occurred during exercise. These data indicate that adrenomedullectomy disrupted the accuracy of the feedback mechanism that regulates the mobilization of extramuscular fuels during exercise in normal rats. In addition, ADM rats showed an increased lipid mobilization as a source of energy during exercise, which might explain the increased plasma glucose by an inhibition of muscle glucose uptake. These results suggest that central cholinergic neurons might be involved in the control of energy substrate adjustment during exercise, thereby reducing the maximal capacity of exercise. In addition, the results of this study suggest that the adrenal glands are important for an accurate feedback mechanism during exercise.