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

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.

Coordinated regulation of circadian rhythms and homeostasis by the suprachiasmatic nucleus

We have demonstrated that in rats activities of various enzymes related to gluconeogenesis and amino acid metabolism show circadian rhythms. Based on these results, we have explored the molecular mechanisms underlying circadian oscillation and phase response to light of the master clock located in the dorsomedial subdivision of the suprachiasmatic nucleus (SCN) and found various proteins closely related to phase response such as BIT/SHPS-1 and those of circadian oscillation, some of which are involved in protein-tyrosine phosphorylation.On the other hand, we have presented several lines of evidence that the ventrolateral subdivision of the SCN includes not only the control center of energy supply to the brain, but also that of homeostasis such as blood glucose, blood pressure, water balance, and body temperature. We have also shown that besides these functions, the latter subdivision is involved in the regulations of hormone secretions such as insulin, glucagon, corticosterone and vasopressin. It has been also shown by electrophysiological means that light exposure to rat eye enhances sympathetic nerve activity, whereas it depresses parasympathetic nerve activity. Thus, environmental light is implicated not only in the phase-shift through the retinohypthalamic tract (RHT), but also control of autonomic nerve activities through the RHT, It is also discussed in this review how the two divisions are interconnected and how environmental light is involved in this interconnection.

The suprachiasmatic nucleus participates in food entrainment: a lesion study

Daily feeding schedules entrain temporal patterns of behavior, metabolism, neuronal activity and clock gene expression in several brain areas and periphery while the suprachiasmatic nucleus (SCN), the biological clock, remains coupled to the light/dark cycle. Because bilateral lesions of the SCN do not abolish food entrained behavioral and hormonal rhythms it is suggested that food entrained and light entrained systems are independent of each other. Special circumstances indicate a possible interaction between the light and the food entrained systems and indicate modulation of SCN activity by restricted feeding. This study explores the influence of the SCN on food entrained rhythms. Food entrained temporal profiles of behavior, core temperature, corticosterone and glucose, as well as Fos and PER1 immunoreactivity in the hypothalamus and corticolimbic structures were explored in rats bearing bilateral SCN lesions (SCNX). In SCNX rats food anticipatory activity and the food entrained temperature and corticosterone increase were expressed with earlier onset and higher values than in intact controls. Glucose levels were lower in SCNX rats in all time points and SCNX rats anticipation to a meal induced higher c-Fos positive neurons in the hypothalamus, while a decreased c-Fos response was observed in corticolimbic structures. SCNX rats also exhibited an upregulation of the PER1 peak in hypothalamic structures, especially in the dorsomedial hypothalamic nucleus (DMH), while in some limbic structures PER1 rhythmicity was dampened. The present results indicate that the SCN participates actively during food entrainment modulating the response of hypothalamic and corticolimbic structures, resulting in an increased anticipatory response.

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.

Immunohistochemical characterization of the suprachiasmatic nucleus and the intergeniculate leaflet in the hereditary bilaterally microphthalmic rat

Immunohistochemical observation was performed in the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) of hereditary bilaterally microphthalmic rats without the optic nerve on both sides. In the microphthalmic rats, volume of the SCN reduced to ca. 70% of the normal and numbers of the vasoactive intestinal polypeptide (VIP)-like immunoreactive (lir) neurons were significantly decreased. Although the arginine vasopressin (aVP)- and the VIP-lir neurons distributed in the dorsomedial and the ventrolateral part of the SCN, respectively, as reported in the normal one, somatostatin-lir neurons, localizing mainly in a border area between the dorsomedial and the ventrolateral region of the normal SCN, were shifted to the ventral part of the SCN in the microphthalmic rats. The ventral part of the SCN was covered with neuropeptide Y (NPY)-lir fibers in both normal and mutant rats. The IGL was hardly delineated cytologically in the lateral geniculate nucleus (LGN) of the mutant rats. NPY-lir neurons were found in the dorsal part of the ventral LGN, in contrast to their even distribution in the normal IGL. These findings suggest that the IGL-SCN tract remains in the hereditary microphthalmic rats without the retinal projections.

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.

Effects of light stimulation on the activity of the autonomic nerves in anesthetized rats

The existence of the retino-hypothalamic pathway suggests that light stimulation may influence the activity of the autonomic outflows. Efferent activities of the pancreatic, hepatic, and gastric branches of the vagus nerve and those of pancreatic, hepatic, splenic, adrenal, and renal branch of the splanchnic nerve were recorded. Light stimulation with 2000 1x for 10 min to the left eye increased the splanchnic (sympathetic) outflows and suppressed the vagal outflows. The effects lasted for several hours. The minimal effective stimulation was 20 1x for 1 min or 200 1x for 0.1 min. These responses were observed in the light period as well as dark period. However, in the suprachiasmatic nucleus (SCN) lesioned rat, the changes in autonomic outflows following light stimulation were absent. The observations suggest that light stimulation modulates visceral functions through changes in the autonomic nervous system activities via the SCN.

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.