Temporary suppression of the hyperglycemic response to 2-deoxy-D-glucose by blinding

Suprachiasmatic Nucleus Neuropeptides and Their Control of Endogenous Glucose Production

Defective control of endogenous glucose production is an important factor responsible for hyperglycaemia in the diabetic individual. During the past decade, progressively more evidence has appeared indicating a strong and potentially causal relationship between disturbances of the circadian system and defects of metabolic regulation, including glucose metabolism. The detrimental effects of disturbed circadian rhythms may have their origin in disturbances of the molecular clock mechanisms in peripheral organs, such as the pancreas and liver, or in the central brain clock in the hypothalamic suprachiasmatic nuclei (SCN). To assess the role of SCN output per se on glucose metabolism, we investigated (i) the effect of several SCN neurotransmitters on endogenous glucose production and (ii) the effect of SCN neuronal activity on hepatic and systemic insulin sensitivity. We show that silencing of SCN neuronal activity results in decreased hepatic insulin sensitivity and increased peripheral insulin sensitivity. Furthermore, both oxytocin neurones in the paraventricular nucleus of the hypothalamus (PVN) and orexin neurones in the lateral hypothalamus may be important targets for the SCN control of glucose metabolism. These data further highlight the role of the central clock in the pathophysiology of insulin resistance.

[Role of VIP-neurons in the hypothalamic suprachiasmatic nucleus in the control of blood glucose]

The hypothalamic suprachiasmatic nucleus (SCN), a master circadian oscillator in mammals, contains VIP-neurons. In our study on the mechanism of the central regulation of glucose metabolism in rats, we obtained following results: 1) intracranial injection of either 2-deoxy-D-glucose (2DG) or VIP elicited hyperglycemia by enhancing neural activities of the sympathetic nerves and by the suppression of the insulin secretion and enhances of secretions of adrenaline and glucagon; 2) bilateral lesions of the SCN eliminated the hyperglycemia and sympathetic excitation induced by intracranial injection of 2DG, and intracranial administration of VIP restored the 2DG-hyperglycemia; 3) infusion of VIP-antisense oligo in the SCN reduced the VIP content in the SCN and abolished the 2DG-hyperglycemia, and intracranial injection of VIP restored the 2DG-hyperglycemia in rats infused the VIP-antisense oligo; 4) intrapancreatic injection of pseudorabies virus (PRV, Bartha), which is retrogradedly transported, caused the transfer of PRV to VIP-neurons in the SCN, and denervations of both the sympathetic and parasympathetic nerves innervating the pancreas eliminated the retrograde transport of PRV to VIP-neurons in the SCN. These findings suggest that VIP-neurons in the SCN regulate the blood glucose level through the enhancement of the sympathetic activity.

Changes in L1 and NCAM expression in the rat suprachiasmatic nucleus during growth and after orbital enucleation

Mammals possess a master circadian clock in the hypothalamic suprachiasmatic nucleus (SCN). In order to clarify the roles of the L1 adhesion molecule (L1) and neural cell adhesion molecule (NCAM), both members of the immunoglobulin superfamily, in the organization of the clock core, changes in the expression of these molecules in the SCN during the growth of rats were examined by immunohistochemistry. On postnatal day 7, L1 and NCAM were chiefly expressed in the region surrounding the SCN, but not in the SCN itself. In subsequent weeks, however, expression of both molecules shifted predominately to the SCN. This change seemed to parallel immunoreactivity increases in the SCN of synaptotagmin, a synapse marker, and of phosphotyrosine, a possible factor in the photic entrainment of the SCN clock. To further elucidate the roles of the L1 and NCAM adhesion molecules in the formation and maintenance of retinal neural projection into the SCN, the effects of orbital enucleation on their expression in the SCN were examined. L1 expression decreased on days 1 and 2 after the operation, in parallel with reductions in the tyrosine phosphorylation of several proteins, but recovered to the control level by the second week. In contrast, the expression of NCAM showed little change following orbital enucleation. These results suggest that L1 and NCAM are involved in the morphological organization of the SCN during the developmental stage, and that expression of L1 also contributes to the formation of the SCN network in a manner that is dependent on the retinal neural input to it.

Effect of bilateral lesions of the suprachiasmatic nucleus on hyperglycemia caused by 2-deoxy-D-glucose and vasoactive intestinal peptide in rats

In mammals, the brain usually uses glucose as a sole energy source. Thus, under a central glucopenic condition after intracranial injection of 2-deoxy-D-glucose (2DG), an inhibitor of glucose utilization, it has been shown that rats elevate their blood glucose level through excitation of the sympathetic nerves. Experiments were conducted with rats to examine the role of the hypothalamic suprachiasmatic nucleus (SCN) in the hyperglycemic response to intracerebroventricular injection of either 2DG or vasoactive intestinal peptide (VIP). It was observed that, (1) intracerebroventricular injection of a VIP-antagonist inhibited the hyperglycemic and hyperglucagonemic responses to the intracranial injection of 2DG; (2) bilateral electrolytic lesioning of the SCN suppressed the hyperglycemic and hyperglucagonemic responses to intracranial injection of 2DG, and intracerebroventricular injection of VIP restored these responses to 2DG; and (3) bilateral electrolytic lesioning of the SCN also suppressed the hyperglycemic and hyperglucagonemic responses to the VIP injection, and additional intracerebroventricular injection of 2DG caused hyperglycemia. These findings indicate that in rats with bilateral lesions of the SCN intracranial injection of 2DG is able to elicit hyperglycemia when VIP was administered intracranially, and suggest that neurons containing VIP-like immunoreactive substance (VIP-neurons) in the SCN have an important role in the mechanism of hyperglycemia elicitation following intracranial injection of 2DG. Moreover, these findings show that 2DG and VIP are able to realize their functions through acting on the brain sites outside the SCN.

Morphological changes in the hypothalamic suprachiasmatic nucleus and circadian rhythm of locomotor activity in hereditary microphthalmic rats

Analysis of circadian locomotor activity, Golgi-Cox impregnation, and immunohistochemistry were studied on the hereditary microphthalmic rat which congenitally lacked the optic nerve. These blind rats showed free-running circadian rhythms in their locomotor activities. Both the normal and microphthalmic rats had similar ultradian rhythms in addition to circadian rhythms. The neuronal cell population and volume of the hypothalamic suprachiasmatic nucleus (SCN) of the microphthalmic rats were 66% and 71% of those in normal rats, respectively. The number of SCN neurons containing vasoactive intestinal peptide-like immunoreactive substance was dramatically decreased to 35% of that in normal rats. Golgi-Cox impregnation revealed that three types of neurons in the SCN of the microphthalmic rats were consistently distinguished as observed in normal rats. Although there were no changes in the numbers of primary dendrites of the SCN neurons in the microphthalmic and normal rats, the number of secondary and tertiary dendrites in the SCN of the microphthalmic rats was smaller than that of normal rats. These observations suggest that the retinal input may be important for normal morphological formation of the SCN during development, but not for the generation of circadian rhythms and ultradian rhythms.

Bilateral lesions of the hypothalamic suprachiasmatic nucleus eliminated sympathetic response to intracranial injection of 2-deoxy-D-glucose and VIP rescued this response

We previously found that bilateral lesions of the suprachiasmatic nucleus abolished hyperglycemic response to intracranial injection of 2-deoxy-D-glucose in rats. Because the hyperglycemia due to 2-deoxy-D-glucose was shown to be dependent on the functions of the adrenal medulla and sympathetic nervous system, the effect of bilateral lesions of the suprachiasmatic nucleus on changes in the nervous activity of sympathetic efferents to the adrenal after intracranial injection of 2-deoxy-D-glucose was examined in rats. It was found that bilateral lesions of the nucleus eliminated the increase in neural activity of the sympathetic efferent that occurred after the injection of 2-deoxy-D-glucose. Because the suprachiasmatic nucleus possesses neurons containing a vasoactive intestinal polypeptide-like substance, the effects of vasoactive intestinal polypeptide and 2-deoxy-D-glucose, administered alone or in combination, on the sympathetic activity were examined in intact control rats and in rats with bilateral lesions of the suprachiasmatic nucleus. It was found that in the normal control rats, vasoactive intestinal polypeptide alone increased the sympathetic activity, whereas it dramatically enhanced the sympathetic response to 2-deoxy-D-glucose. However, in rats with bilateral lesions of the suprachiasmatic nucleus, vasoactive intestinal polypeptide alone elicited no increase in the nervous activity of the sympathetic efferents to the adrenal, but combined administration of vasoactive intestinal polypeptide and 2-deoxy-D-glucose caused an increase in the nervous activity of sympathetic efferents to the adrenal. These findings suggest that the suprachiasmatic nucleus is involved in the enhancement of sympathetic activity caused by intracranial injection of 2-deoxy-D-glucose, and that neurons containing a vasoactive intestinal polypeptide-like substance in the suprachiasmatic nucleus play an important role in the sympathetic enhancement that occurs after intracranial injection of 2-deoxy-D-glucose. This role might be a permissive and facilitative one.

Circadian rhythms and energy metabolism with special reference to the suprachiasmatic nucleus

The role of the hypothalamic suprachiasmatic nucleus (SCN) was examined in rats and obtained following results: (a) The time-dependent (light > dark) hyperglycemic response to intracranial injection of 2-deoxy-D-glucose (2DG) disappeared in rats with bilateral lesions of the SCN, in rats on weeks 4-6 after surgical blinding, and in congenitally blind (hereditary microphthalmic) rats; (b) The hyperglycemia induced by electrical stimulation of the SCN was not observed in weeks 4-8 after surgical blinding; (c) Change in the blood glucose concentration after insulin injection into the SCN was eliminated by SCN lesions; (d) Alterations in activity of autonomic efferents to peripheral organs on light exposure disappeared after SCN lesions; (e) SCN lesions decreased the blood glucagon level and increased the blood insulin level; (f) SCN lesions decreased protein intake, and glucagon increased it; (g) Increases in the plasma renin activity and vasopressin concentration after water-deprivation were suppressed in hereditary microphthalmic rats with abnormal SCN. These findings suggest that the SCN is involved in the mechanism of blood glucose and body fluid intake as well as that of circadian rhythm.

Permissive effect of VIP on the hyperglycemic response induced by 2-deoxy-D-glucose

We previously obtained evidence that in rats the neurons in the suprachiasmatic nucleus (SCN) receiving retinal neural inputs may be involved in the regulation of glucose metabolism. In this study we examined whether the SCN neurons containing vasoactive intestinal peptide (VIP)-like immunoreactive substance (VIP neurons) are involved in its regulation. We found that the hyperglycemic and hyperglucagonemic responses to intracranial injection of 2-deoxy-D-glucose (2DG) were synergistically enhanced by intracranial administration of VIP, and that these responses were significantly suppressed by treatment with anti-VIP antibody. These results suggest that VIP has a permissive effect on the hyperglycemic and hyperglucagonemic responses to 2DG, and thus that the VIP neurons in the SCN are probably involved in the regulation of glucose metabolism.

Reduced increase in plasma renin activity on water-deprivation in blind hereditary microphthalmic rats

We compared the plasma renin activity (PRA) before and after 24-h water-deprivation in blind hereditary microphthalmic rats and Donryu rats. In the congenitally blind rats with a morphologically abnormal suprachiasmatic nucleus (SCN), hypovolemia induced significantly less elevation of the PRA and significantly more increase in the hematocrit value than in normal rats. The changes after water-deprivation in the blind rats were quite similar to those reported in rats with SCN lesions. However, the free-running circadian rhythms persisted in these blind rats, whereas those in rats with SCN lesions were completely eliminated. Thus, it is likely that SCN cells are involved in regulation of the PRA, and that if this is the case these cells are different from those containing the circadian pacemaker.

Effect of orbital enucleation on glucose homeostasis and morphology of the suprachiasmatic nucleus

In rats there is a direct neural connection called the retinohypothalamic tract (RHT) from retinal ganglion cells to the ventrolateral part of the suprachiasmatic nucleus (SCN), which has neurons containing vasoactive intestinal polypeptide (VIP)-like substance. Previously, we observed that bilateral orbital enucleation (blinding) caused temporary suppression of the hyperglycemic response to intracranial injection of 2-deoxy-D-glucose (2DG) from week 4 to 6 after blinding. Moreover, bilateral lesions of the SCN had a similar effect. From these findings, we supposed that the neurons responsible for the hyperglycemic response to 2DG were present in the SCN, that after blinding these neurons temporarily lost their activity, and that this functional change was reflected in the morphology of the SCN. To investigate this possibility, we examined the morphological changes of the SCN by Nissl staining and immunohistochemical studies with anti-VIP and anti-peptide histidine isoleucine (PHI) antibodies in blinded rats, and the relationship between these morphological changes and the hyperglycemic response to 2DG. After surgical blinding, we observed following changes. (1) The optic chiasm became thinner. (2) The SCN became displaced rostrally. (3) The density of neurons in the middle to caudal part of the SCN, where the retinal ganglion cells projected, decreased markedly without change in cell number during the period when the hyperglycemic response to intracranial injection of 2DG was temporarily suppressed after blinding. The first and second changes seemed to reflect reduction of fibers and axon terminals of retinal ganglion cells and their innervation, respectively. As the third change was parallel with suppression of the hyperglycemic response to 2DG injection, it may reflect functional change of the neurons in the SCN that are responsible for the hyperglycemia due to 2DG.

Light enhances sympathetic and suppresses vagal outflows and lesions including the suprachiasmatic nucleus eliminate these changes in rats

Neurons in the suprachiasmatic nucleus (SCN) are suggested to be involved in the mechanism of glucose homeostasis. This mechanism was examined by studies on the effect of illumination on the activity of autonomic efferents to the adrenals, pancreas and liver. Exposure of one eye of anesthetized rats to light enhanced the efferent activity of the adrenal nerve and suppressed that of vagal pancreatic and hepatic nerves. No change in efferent activities of these nerves was observed on light-stimulation of rats with lesions that included the bilateral SCN. These findings indicate that light signals modulate visceral functions including metabolic processes through the retinohypothalamic tract probably via the SCN to autonomic efferent pathways innervating visceral organs.

Effect of glucagon in macronutrient self-selection: glucagon-enhanced protein intake

The effect of glucagon on macronutrient selection was studied using rats. Continuous infusion of glucagon (5 ng/microliters/h) into the lateral cerebral ventricle increased total caloric intake and protein selection, and decreased carbohydrate selection. Continuous infusion of glucagon subcutaneously induced similar changes. Since a hyperglycemic response to the intracranial injection of 2-deoxy-D-glucose (2DG) disappeared in rats either with bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN) (17) or with acquired (21) and congenital (10) blindness, and bilateral lesions of the SCN eliminated the hyperglucagonemic response to the 2DG-injection (19), changes in the plasma glucagon concentration after 2DG injection were examined in acquired and congenital blind rats. Consequently, it was found that the hyperglucagonemic response to 2DG was not observed in those blind rats which lacked the hyperglycemic response. In those SCN-lesioned and blind rats lacking the hyperglucagonemic response to 2DG, the protein selection was lower, and carbohydrate selection tended to be higher, than those selections found in the control rats. Considering the neural connection between the retina and the SCN, these findings suggest that glucagon may have a stimulatory effect on protein intake and a suppressive one for carbohydrate intake; and that the SCN may be involved in such a regulatory mechanism of feeding behavior through controlling the blood glucagon level.

Dual 24-hour feeding response to 2DG in rats: daytime increase and nighttime decrease

Thirty-six rats were injected IP with 2DG (0, 250, or 500 mg/kg) at 7-day intervals, once at light onset (7 a.m.) and once at dark onset (7 p.m.), and postinjection food intake was monitored for 24 hours. Five hundred mg/kg 2DG caused food intake to rise above control levels during the first 6 hours of daylight, regardless of whether the injection had occurred that morning or the previous evening, whereas intake during the first 6 hours of darkness was consistently below control levels. In a second study, 24 rats were injected first at 7 a.m. (500 mg/kg 2DG or saline), and 7 days later at 7 p.m. (opposite drug), and food was withheld 12 hours until the light:dark period had changed. For 12 hours after food was returned, 2DG again decreased nighttime food intake (Injection 1) and increased daytime intake (Injection 2). 2DG's dual long-term effects cannot be accounted for either by malaise or by an initial action that later is compensated by its opposite. Rather, 2DG (500 mg/kg) appears to exert two independent, opposite alimentary effects which persist 18-24 hours and which change direction with phase changes in the light:dark cycle.

Little or no induction of hyperglycemia by 2-deoxy-D-glucose in hereditary blind microphthalmic rats

Studies were made on whether hereditary microphthalmic rats (1), which are congenitally blind, showed a hyperglycemic response to intracerebroventricular injection of 2-deoxy-D-glucose (2DG) in their subjective light period. In contrast to previous findings in normal rats in which 2DG injection caused light-cycle dependent hyperglycemia (2) and bilateral lesion of the suprachiasmatic nucleus (SCN) completely abolished this hyperglycemia (3), 2DG injection caused no and only slight hyperglycemia in male and female rats with hereditary microphthalmia, respectively. Gross and histological examinations indicated that these rats had no optic nerve or retinohypothalamic tract and that their SCN had an abnormal structure. Locomotive activity recordings showed that all the blind rats had a free-running circadian activity rhythm. These findings suggest that the projection sites of the retinohypothalamic tract to the SCN are involved in the mechanism of the hyperglycemic response to 2DG, but that neural cells, which may be responsible for the generation of circadian rhythms, are not. We have reported that when adult rats were blinded by orbital enucleation, their hyperglycemic response to 2DG was suppressed temporarily 3-5 weeks after the operation, but that their plasma insulin level was basically higher and increased further after 2DG injection during this period (4). In congenitally blind rats, however, the basal plasma insulin level was not higher and the level did not change after 2DG treatment. This difference is discussed from the view point of the role of the premature SCN in regulation of the plasma insulin concentration.