Orexin signaling is necessary for hypoglycemia-induced prevention of conditioned place preference

While the neural control of glucoregulatory responses to insulin-induced hypoglycemia is beginning to be elucidated, brain sites responsible for behavioral responses to hypoglycemia are relatively poorly understood. To help elucidate central control mechanisms associated with hypoglycemia unawareness, we first evaluated the effect of recurrent hypoglycemia on a simple behavioral measure, the robust feeding response to hypoglycemia, in rats. First, food intake was significantly, and similarly, increased above baseline saline-induced intake (1.1 ± 0.2 g; n = 8) in rats experiencing a first (4.4 ± 0.3; n = 8) or third daily episode of recurrent insulin-induced hypoglycemia (IIH, 3.7 ± 0.3 g; n = 9; P < 0.05). Because food intake was not impaired as a result of prior IIH, we next developed an alternative animal model of hypoglycemia-induced behavioral arousal using a conditioned place preference (CPP) model. We found that hypoglycemia severely blunted previously acquired CPP in rats and that recurrent hypoglycemia prevented this blunting. Pretreatment with a brain penetrant, selective orexin receptor-1 antagonist, SB-334867A, blocked hypoglycemia-induced blunting of CPP. Recurrently hypoglycemic rats also showed decreased preproorexin expression in the perifornical hypothalamus (50%) but not in the adjacent lateral hypothalamus. Pretreatment with sertraline, previously shown to prevent hypoglycemia-associated glucoregulatory failure, did not prevent blunting of hypoglycemia-induced CPP prevention by recurrent hypoglycemia. This work describes the first behavioral model of hypoglycemia unawareness and suggests a role for orexin neurons in mediating behavioral responses to hypoglycemia.

FAT/CD36: a major regulator of neuronal fatty acid sensing and energy homeostasis in rats and mice

Hypothalamic "metabolic-sensing" neurons sense glucose and fatty acids (FAs) and play an integral role in the regulation of glucose, energy homeostasis, and the development of obesity and diabetes. Using pharmacologic agents, we previously found that ~50% of these neurons responded to oleic acid (OA) by using the FA translocator/receptor FAT/CD36 (CD36). For further elucidation of the role of CD36 in neuronal FA sensing, ventromedial hypothalamus (VMH) CD36 was depleted using adeno-associated viral (AAV) vector expressing CD36 short hairpin RNA (shRNA) in rats. Whereas their neuronal glucosensing was unaffected by CD36 depletion, the percent of neurons that responded to OA was decreased specifically in glucosensing neurons. A similar effect was seen in total-body CD36-knockout mice. Next, weanling rats were injected in the VMH with CD36 AAV shRNA. Despite significant VMH CD36 depletion, there was no effect on food intake, body weight gain, or total carcass adiposity on chow or 45% fat diets. However, VMH CD36-depleted rats did have increased plasma leptin and subcutaneous fat deposition and markedly abnormal glucose tolerance. These results demonstrate that CD36 is a critical factor in both VMH neuronal FA sensing and the regulation of energy and glucose homeostasis.

Long-term, intermittent, insulin-induced hypoglycemia produces marked obesity without hyperphagia or insulin resistance: a model for weight gain with intensive insulin therapy

A major side effect of insulin treatment of diabetes is weight gain, which limits patient compliance and may pose additional health risks. Although the mechanisms responsible for this weight gain are poorly understood, it has been suggested that there may be a link to the incidence of recurrent episodes of hypoglycemia. Here we present a rodent model of marked weight gain associated with weekly insulin-induced hypoglycemic episodes in the absence of diabetes. Insulin treatment caused a significant increase in both body weight and fat mass, accompanied by reduced motor activity, lowered thermogenesis in response to a cold challenge, and reduced brown fat uncoupling protein mRNA. However, there was no effect of insulin treatment on total food intake nor on hypothalamic neuropeptide Y or proopiomelanocortin mRNA expression, and insulin-treated animals did not become insulin-resistant. Our results suggest that repeated iatrogenic hypoglycemia leads to weight gain, and that such weight gain is associated with a multifaceted deficit in metabolic regulation rather than to a chronic increase in caloric intake.

Chronic alterations in rat brain α-adrenoceptors following traumatic brain injury

Norepinephrine (NE) has been implicated in cerebral plasticity and recovery of function after brain injury. To examine the status of noradrenergic mechanisms in the brain following traumatic brain injury (TBI), male Sprague-Dawley rats underwent right sensorimotor cortex contusions and were observed for the next 30 days for recovery of motor function by measurement of the time taken to perform a modified beam walking task! At 30 days, their brains were assayed by receptor autoradiography for αr- and α2-adrenoceptor binding with 1 nM [3H]prazosin and 1 nM [3H]paraminoclonidine, respectively. One day after contusion, TBI rats took 60% longer to run the beam than sham-lesioned controls. Run times were directly proportional (r = 0.784; P = 0.012) to lesion volume determined at 30 days. The motor deficit persisted for 8 days, after which TBI and control rats had similar run times, largely due to increased run times in sham rats. At 30 days, TBI rats had a generalized, bilateral decrease in [3H]prazosin binding across all brain areas read (F[l,13] = 9.23; P = 0.009) with specific 12%-21% decreases in the cortex contralateral to the lesion and bilaterally in the dorsomedial hypothalamic and three thalamic nuclei. On the other hand, [3H]paraminoclonidine binding did not differ from sham lesion controls in any brain area of TBI rats. Thus, unilateral TBI is followed by widespread, bilateral changes in α1-adrenoceptor binding which would leave the animal vulnerable to any factors which reduced the access of NE to its postsynaptic adrenoceptors. This is compatible with the observation that α1-antagonists and α2-agonists can transiently reinstate the motor deficit after recovery has occurred.

Hypothalamic Fkbp51 is induced by fasting, and elevated hypothalamic expression promotes obese phenotypes

To discover hypothalamic genes that might play a role in regulating energy balance, we carried out a microarray screen for genes induced by a 48-h fast in male C57Bl/6J mouse hypothalamus. One such gene was Fkbp51 (FK506 binding protein 5; Locus NP_034350). The product of this gene is of interest because it blocks glucocorticoid action, suggesting that fasting-induced elevation of this gene in the hypothalamus may reduce glucocorticoid negative feedback, leading to elevated glucocorticoid levels, thus promoting obese phenotypes. Subsequent analysis demonstrated that a 48-h fast induces Fkbp51 in ventromedial, paraventricular, and arcuate hypothalamic nuclei of mice and rats. To assess if hypothalamic Fkbp51 promotes obesity, the gene was transferred to the hypothalamus via an adeno-associated virus vector. Within 2 wk following Fkbp51 overexpression, mice on a high-fat diet exhibited elevated body weight, without hyperphagia, relative to mice receiving the control mCherry vector. Body weight remained elevated for more than 8 wk and was associated with elevated corticosterone and impaired glucose tolerance. These studies suggest that elevated hypothalamic Fkbp51 promotes obese phenotypes.

Metabolic sensing and the brain: who, what, where, and how?

Unique subpopulations of specialized metabolic sensing neurons reside in a distributed network throughout the brain and respond to alterations in ambient levels of various metabolic substrates by altering their activity. Variations in local brain substrate levels reflect their transport across the blood- and cerebrospinal-brain barriers as well as local production by astrocytes. There are a number of mechanisms by which such metabolic sensing neurons alter their activity in response to changes in substrate levels, but it is clear that these neurons cannot be considered in isolation. They are heavily dependent on astrocyte and probably tanycyte metabolism and function but also respond to hormones (e.g. leptin and insulin) and cytokines that cross the blood-brain barrier from the periphery as well as hard-wired neural inputs from metabolic sensors in peripheral sites such as the hepatic portal vein, gastrointestinal tract, and carotid body. Thus, these specialized neurons are capable of monitoring and integrating multiple signals from the periphery as a means of regulating peripheral energy homeostasis.

Effects of maternal genotype and diet on offspring glucose and fatty acid-sensing ventromedial hypothalamic nucleus neurons

Maternal obesity accentuates offspring obesity in dams bred to develop diet-induced obesity (DIO) on a 31% fat, high-sucrose, high-energy (HE) diet but has no effect on offspring of diet-resistant (DR) dams. Also, only DIO dams become obese when they and DR dams are fed HE diet throughout gestation and lactation. We assessed glucose and oleic acid (OA) sensitivity of dissociated ventromedial hypothalamic nucleus (VMN) neurons from 3- to 4-wk old offspring of DIO and DR dams fed chow or HE diet using fura-2 calcium imaging to monitor intracellular calcium fluctuations as an index of neuronal activity. Offspring of DIO dams fed chow had approximately 2-fold more glucose-inhibited (GI) neurons than did DR offspring. This difference was eliminated in offspring of DIO dams fed HE diet. At 2.5 mM glucose, offspring of chow-fed DIO dams had more GI neurons that were either excited or inhibited by OA than did DR offspring. Maternal HE diet intake generally increased the percentage of neurons that were excited and decreased the percentage that were inhibited by OA in both DIO and DR offspring. However, this effect was more pronounced in DIO offspring. These data, as well as concentration-dependent differences in OA sensitivity, suggest that genotype, maternal obesity, and dietary content can all affect the sensitivity of offspring VMN neurons to glucose and long-chain fatty acids. Such altered sensitivities may underlie the propensity of DIO offspring to become obese when fed high-fat, high-sucrose diets.

Effects of leptin on rat ventromedial hypothalamic neurons

Neurons in the ventromedial and arcuate hypothalamic nuclei (VMN and ARC, respectively) mediate many of leptin's effects on energy homeostasis. Some are also glucosensing, whereby they use glucose as a signaling molecule to regulate their firing rate. We used fura-2 calcium (Ca2+) imaging to determine the interactions between these two important mediators of peripheral metabolism on individual VMN neurons and the mechanisms by which leptin regulates neuronal activity in vitro. Leptin excited 24%, inhibited 20%, and had a biphasic response in 10% of VMN neurons. Excitation occurred with a EC50 of 5.2 fmol/liter and inhibition with a IC50 of 4.2 fmol/liter. These effects were independent of the ambient glucose levels, and both glucosensing and non-glucosensing neurons were affected by leptin. In contrast, the ARC showed a very different distribution of leptin-responsive neurons, with 40% leptin excited, 10% leptin inhibited, and 2% having a biphasic response (chi2=60.2; P<0.0001). Using pharmacological manipulations we found that leptin inhibits VMN neurons via activation of phosphoinositol-3 kinase and activation of the ATP-sensitive K+ channel. In addition, leptin inhibition was antagonized by 5'-AMP-activated protein kinase activation in 39% of neurons but was unaffected by 5'-AMP-activated protein kinase inhibition. No mechanism was delineated for leptin-induced excitation. Thus, within the physiological range of brain glucose levels, leptin has a differential effect on VMN vs. ARC neurons, and acts on both glucosensing and non-glucosensing VMN neurons in a glucose-independent fashion with inhibition primarily dependent upon activation of the ATP-sensitive K+ channel.

Reduced anorexic effects of insulin in obesity-prone rats fed a moderate-fat diet

Rats prone to develop diet-induced obesity (DIO) have reduced central sensitivity to many metabolic and hormonal signals involved in energy homeostasis. High-fat diets produce similar defects in diet-resistant (DR) rats. To test the hypothesis that genotype and diet exposure would similarly affect central insulin signaling, we assessed the anorectic effects of 8 mU third ventricular (iv3t) insulin before and after 4 wk intake of a 31% fat, high-energy (HE) diet intake in outbred (OutB) rats. Rats were retrospectively designated as DR or DIO by their low or high weight gains on HE diet. Before the HE diet, iv3t insulin reduced 4-h and 24-h chow intake by 53% and 69% in DR rats but by only 17% and 27% in DIO rats, respectively. Also, the anorectic response to iv3t insulin in OutB rats was inversely correlated (r = 0.72, P = 0.002) with subsequent 4-wk weight gain on the HE diet. Similarly, in selectively bred (SB) chow-fed DR rats, 8 mU iv3t insulin reduced 4-h and 24-h intake by 21% and 22%, respectively, but had no significant effect in SB DIO rats. Four-week HE diet intake reduced 4-h and 24-h insulin-induced anorexia by 45% in OutB DR rats and completely abolished it in SB DR rats. Reduced insulin responsiveness was unassociated with differences in arcuate nucleus insulin receptor mRNA expression between DIO and DR rats or between rats fed chow or HE diet. These data suggest that DIO rats have a preexisting reduction in central insulin signaling, which might contribute to their becoming obese on the HE diet. However, since the HE diet reduced central insulin sensitivity in DR rats but did not make them obese, it is likely that other brain areas are involved in insulin's anorectic action or that other pathways contribute to the development and maintenance of obesity.

Reduced brain CRH and GR mRNA expression precedes obesity in juvenile rats bred for diet-induced obesity

To assess the role of endogenous peptides involved in stress responsivity in the development of diet-induced obesity (DIO), selectively bred DIO and diet-resistant (DR) male were weaned onto a low fat (4.5%) chow diet at 3 weeks of age and then fed either chow or a 31% fat by energy content (high energy (HE)) diet for 9 days beginning at 4 weeks of age. Regardless of diet, DIO rats gained more weight than DR rats but did not show the selective DIO weight gain trait characteristic of older DIO rats fed HE diet. At this early age, both DR and DIO rats on HE diet ate more and had higher leptin levels but gained less body weight and had lower feed efficiency (body weight gain (g)/food intake (kcal)) than their chow-fed controls. HE diet also prevented the decline in 24h urine corticosterone levels from the third to fifth week observed in chow-fed rats. Terminally, DIO rats had lower hippocampal glucocorticoid receptor (GR) and amygdalar central nucleus corticotrophin-releasing hormone (CRH) mRNA than DR rats, regardless of their diets. Taken together with prior studies in these rats, there appears to be a critical period between 3 and 5 weeks of age when DIO and DR rats are not phenotypically different and hypothalamo-pituitary-adrenal (HPA) function is rapidly changing. The reduced expression of brain GR and CRH expression at the end of this period might contribute to the propensity of DIO rats to become obese selectively on HE diet after 5 weeks of age.

Widespread and lateralization effects of acute traumatic brain injury on norepinephrine turnover in the rat brain

Norepinephrine (NE) has been implicated in recovery of function following traumatic brain injury (TBI). While bilateral decrease in brain NE turnover occur at 6-24 h after TBI, it is unknown what effects unilateral TBI might have on brain NE turnover the first few minutes after injury. Her male Sprague-Dawley rats had unilateral confusions of either the right or left somatosensory cortex produced by an air between piston. At 30 min after TBI, brain NE turnover was assessed by measuring the ratio of 3-methoxy-4 hydroxyphenylglycol (MHPG) to NE levels in various brain regions. Both right and left TBI produced 32-103% increases in NE turnover at the injury site and in the ipsilateral cerebral cortex surrounding, rostral and caudal to the injury as compared to the contralateral, uninjured site or to the homologous sites in uninjured controls. NE turnover was also altered selectively in some brain areas not affected by right TBI. Left TBI decreased NE turnover by 29% in the frontal cortex contralateral to the injury and by 24% bilaterally in the hypothalamus while increasing locus coeruleus NE turnover by 72% compared to uninjured controls. Thus, unilateral cortical TBI produced predominantly ipsilateral increases in cortical NE turnover but variable, bilateral changes in NE turnover in subcortical areas which were dependent upon the side of injury. These subcortical differences may explain some of the lateralized effects of cortical injury on post-injury behavior.

Focal traumatic brain injury causes widespread reductions in rat brain norepinephrine turnover from 6 to 24 h

The effect of right sensorimotor traumatic brain injury (TBI) in male Sprague-Dawley rats on brain norepinephrine (NE) turnover was assessed by measuring the decline of endogenous NE levels following tyrosine hydroxylase inhibition produced with alpha-methyl-p-tyrosine. Right sensorimotor cortex contusions were produced by a pneumatically driven piston which depressed the dural surface by 2 mm at 3.2 m/s. TBI rats were compared to uninjured, anesthetized controls at 6 h and 24 h after surgery. While NE turnover was not affected at the lesion site at 6 h after TBI, it was either abolished or decreased by 33-75% bilaterally in the hypothalamus and in the cerebral cortex surrounding and rostral to the lesion site. In the cortex caudal to the lesion site, NE turnover was completely abolished. NE turnover in cerebral cortex opposite the lesion site and in the contralateral cerebellum was decreased by 51 and 43%, respectively, at 6 h. At 24 h, NE turnover was either abolished or decreased bilaterally by 45-92% in all cortical areas, in the hypothalamus, cerebellum, locus coeruleus and medulla. Thus, right sensorimotor cortex contusion causes a marked, early and widespread depression of brain NE turnover. Since amphetamine increases NE turnover, this may explain the dramatic improvement in behavioral deficits which occurs following amphetamine administration at 24 h after such lesions.

Regulation of growth-associated protein 43 (GAP-43) messenger RNA associated with plastic change in the adult rat barrel receptor complex

Plastic change occurs in the adult rat barrel receptor complex following peripheral deafferentation by removal of facial vibrissae (vibrissectomy) and can be prevented by prior depletion of brain norepinephrine. Growth-associated protein (GAP-43, B50, F1, pp46), a marker for synaptic reorganization, increases in the barrel cortex of adult rats following both peripheral and central deafferentation. Here we followed changes in GAP-43 mRNA expression in the barrel receptor system following vibrissectomy. Adult rats had unilateral total vibrissectomy with sparing of the central (C3) vibrissa. By in situ hybridization, GAP-43 mRNA first increased at 24h (9%, P < 0.05) in the ipsilateral trigeminal complex. Levels remained elevated (up to 25% of the unlesioned side) over the next 6 days, decreased to 88% at 7 days and returned to control levels at 14 days. Contralateral barrel cortex levels of GAP-43 mRNA increased by 14% at 4-5 days remained elevated through 7 days and returned to control levels by 14 days. Increased GAP-43 mRNA levels 6 days after vibrissectomy were reproduced by complete transection of the infraorbital nerve and were blocked by depletion of brain norepinephrine. No change occurred in ventrobasal thalamus GAP-43 mRNA at any time. Dot blot and Northern blot hybridizations of GAP-43 mRNA after vibrissectomy showed a 43% increase in the ipsilateral trigeminal complex and a 16% increase in the contralateral barrel cortex at 3 days and an 84% increase in ipsilateral trigeminal and 50% increase in contralateral barrel cortex GAP-43 mRNA at 6 days, respectively. Thus, deafferentation-induced plasticity in the barrel pathway depends upon norepinephrine and is associated with increase in both GAP-43 mRNA and protein suggesting that this may involve a structural change.

Adult rat barrel cortex plasticity occurs at 1 week but not at 1 day after vibrissectomy as demonstrated by the 2-deoxyglucose method

Stimulation of a single facial vibrissa in rats receiving [14C]2-deoxyglucose leads to increased local cerebral glucose utilization in the corresponding contralateral barrel of lamina IV of the first somatosensory cortex (SmI). In the adult rat, the metabolic representation of such a barrel enlarges 2 months after removal of all other vibrissal follicles but enlargement is prevented by prior removal of SmI norepinephrine. Here, the early time course of such enlargement and how this was affected by cortical norepinephrine manipulations were examined in adult rats. One day after total vibrissal follicle removal with sparing of the central (C3) vibrissa, neither the areal extent nor absolute glucose utilization in the stimulated, spared C3 cortical barrel were changed. However, 7 days after follicle removal, the spared C3 barrel was enlarged by 41%, although absolute glucose utilization remained constant. This delayed onset of enlargement is compatible with either a structural or neurochemical change in barrel circuitry following vibrissal deafferentation. With ipsilateral locus coeruleus lesions but intact vibrissae, there was progressive enlargement of stimulated C3 barrel areas with increasing cortical norepinephrine depletion (r = 0.864) suggesting a suppressive effect of norepinephrine on activity spread in barrels with intact vibrissal afferents. Previously shown blockade of chronic (2 month) vibrissectomy-induced barrel enlargement by norephinephrine depletion suggested an additional effect on plasticity. Even though acute (1 day) follicle removal here produced no change in spared C3 barrel area, addition of norepinephrine depletion produced a surprising 40% decrease in barrel area. Thus, barrel plasticity assessed by 2-deoxyglucose reflects a complex interaction between barrel metabolic activity and the extent of vibrissal and noradrenergic afferent input.

Noradrenergic innervation does not affect chronic regulation of [125I]pindolol receptors in fetal rat brain transplants or host neocortex

Fetal (E15-16) somatosensory cortex (n = 15) or cerebellum (n = 9) were placed into the somatosensory cortex (SmI) of adult rat hosts to study the relative importance of tissue origin versus host milieu on graft beta-adrenoceptor regulation. Autoradiographic studies of [125I]pindolol ([125I]pin) binding in the presence of 3 microM serotonin were performed as an index of beta-receptor binding in both intact hosts and those with ipsilateral locus coeruleus (LC) lesions and/or ipsilateral superior cervical ganglionectomy. [125I]pin binding within fetal grafts was highly variable with areas of highest specific binding in cortical grafts (Kd = 209 +/- 30 pM, Bmax = 106 +/- 7 (fmol/mg protein) being comparable to host cortex (Kd = 211 +/- 41 pM, Bmax = 111 +/- 9 fmol/mg protein). Average total binding in whole cortical grafts was 73% and in cerebellar grafts was 60% of that in comparable adult cortex. Host cortex had 66-73% and cerebellum had 4-8% beta 1-receptors while cortical grafts had 59% and cerebellar grafts had 43% beta 1-receptors as determined by competitive binding with ICI 89406 and 118551. Noradrenergic fibers derived from both the host LC and superior cervical ganglion grew into fetal cortical grafts. Binding to high affinity uptake sites ([3H]desmethylimipramine, [3H]DMI) on noradrenergic terminals in cerebellar grafts was 28% higher than that in cortical grafts; superior cervical ganglionectomy decreased [3H]DMI binding in cortical grafts by 37% but had no effect on cerebellar grafts. Neither ganglionectomy nor LC lesions affected total specific binding or binding to beta-receptor subtypes in the grafts or host cortex 3-6 months after removal. Therefore, anatomic site of origin appeared to be the predominant factor in determining the development of beta-adrenoceptors in fetal cortical tissue. In ectopically placed cerebellar grafts, beta-receptor subtypes did not develop comparably to host cerebellar receptors suggesting that host milieu may be of critical importance in receptor development in this tissue.

Functional integration of fetal cortical grafts into the afferent pathway of the rat somatosensory cortex (SmI)

The somatosensory area of fetal (E15-16) Sprague-Dawley rat neocortex was implanted into the barrel receptor area of adult first somatosensory cortex (SmI) to determine if it could be integrated into the host vibrissa-cortical pathway. Functional integration was tested 3-6 months later using the 2-deoxyglucose [44,45] (2-dg) method for estimation of glucose utilization. A variety of control grafts (cerebellum in SmI cortex, neocortex outside the host SmI cortex and appropriately placed neocortex in non-stimulated hosts) all had uniform 2-dg uptake (70 +/- 10 mumol/100 g/min) throughout each graft which averaged 42% less than non-stimulated host SmI cortex (121 +/- 6 mumol/100 g/min; p less than 0.001) but was 3-fold greater than white matter (24.8 +/- 3.1 mumol/100 g/min; p less than 0.001). In appropriately placed neocortical grafts, vibrissal stimulation produced 125% greater average 2-dg uptake (157 +/- 19 mumol/100 g/min) than control grafts (p less than 0.01). Such appropriately placed and stimulated neocortical grafts also contained focal areas of increased 2-dg uptake which were 43% greater than average graft uptake. These data suggest that fetal neocortical grafts were functionally integrated into the physiologically relevant afferent pathways of adult host brains.