The physiological control of eating: signals, neurons, and networks.

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Catecholaminergic projections into an interconnected forebrain network control the sensitivity of male rats to diet-induced obesity.

Hindbrain catecholamine neurons convey gut-derived metabolic signals to an interconnected neuronal network in the hypothalamus and adjacent forebrain. These neurons are critical for short-term glycemic control, glucocorticoid and glucoprivic feeding responses, and glucagon-like peptide 1 (GLP-1) signaling. Here we investigate whether these pathways also contribute to long-term energy homeostasis by controlling obesogenic sensitivity to a high-fat/high-sucrose choice (HFSC) diet. We ablated hindbrain-originating catecholaminergic projections by injecting anti-dopamine-ß-hydroxylase-conjugated saporin (DSAP) into the paraventricular nucleus of the hypothalamus (PVH) of male rats fed a chow diet for up to 12 wk or a HFSC diet for 8 wk. We measured the effects of DSAP lesions on food choices; visceral adiposity; plasma glucose, insulin, and leptin; and indicators of long-term ACTH and corticosterone secretion. We also determined lesion effects on the number of carbohydrate or fat calories required to increase visceral fat. Finally, we examined corticotropin-releasing hormone levels in the PVH and arcuate nucleus expression of neuropeptide Y ( Npy), agouti-related peptide ( Agrp), and proopiomelanocortin ( Pomc). DSAP-injected chow-fed rats slowly increase visceral adiposity but quickly develop mild insulin resistance and elevated blood glucose. DSAP-injected HFSC-fed rats, however, dramatically increase food intake, body weight, and visceral adiposity beyond the level in control HFSC-fed rats. These changes are concomitant with 1) a reduction in the number of carbohydrate calories required to generate visceral fat, 2) abnormal Npy, Agrp, and Pomc expression, and 3) aberrant control of insulin secretion and glucocorticoid negative feedback. Long-term metabolic adaptations to high-carbohydrate diets, therefore, require intact forebrain catecholamine projections. Without them, animals cannot alter forebrain mechanisms to restrain increased visceral adiposity.

Glucagon-like peptide-1 regulates brown adipose tissue thermogenesis via the gut-brain axis in rats.

Endogenous intestinal glucagon-like peptide-1 (GLP-1) controls satiation and glucose metabolism via vagal afferent neurons (VANs). Recently, VANs have received increasing attention for their role in brown adipose tissue (BAT) thermogenesis. It is, however, unclear whether VAN GLP-1 receptor (GLP-1R) signaling affects BAT thermogenesis and energy expenditure (EE) and whether this VAN mechanism contributes to energy balance. First, we tested the effect of the GLP-1R agonist exendin-4 (Ex4, 0.3 µg/kg ip) on EE and BAT thermogenesis and whether these effects require VAN GLP-1R signaling using a rat model with a selective Glp1r knockdown (kd) in VANs. Second, we examined the role of VAN GLP-1R in energy balance during chronic high-fat diet (HFD) feeding in VAN Glp1r kd rats. Finally, we used viral transsynaptic tracers to identify the possible neuronal substrates of such a gut-BAT interaction. VAN Glp1r kd attenuated the acute suppressive effects of Ex4 on EE and BAT thermogenesis. Consistent with this finding, the VAN Glp1r kd increased EE and BAT activity, diminished body weight gain, and improved insulin sensitivity compared with HFD-fed controls. Anterograde transsynaptic viral tracing of VANs infected major hypothalamic and hindbrain areas involved in BAT sympathetic regulation. Moreover, retrograde tracing from BAT combined with laser capture microdissection revealed that a population of VANs expressing Glp1r is synaptically connected to the BAT. Our findings reveal a novel role of VAN GLP-1R signaling in the regulation of EE and BAT thermogenesis and imply that through this gut-brain-BAT connection, intestinal GLP-1 plays a role in HFD-induced metabolic syndrome.

MAP kinases couple hindbrain-derived catecholamine signals to hypothalamic adrenocortical control mechanisms during glycemia-related challenges.

Physiological responses to hypoglycemia, hyperinsulinemia, and hyperglycemia include a critical adrenocortical component that is initiated by hypothalamic control of the anterior pituitary and adrenal cortex. These adrenocortical responses ensure appropriate long-term glucocorticoid-mediated modifications to metabolism. Despite the importance of these mechanisms to disease processes, how hypothalamic afferent pathways engage the intracellular mechanisms that initiate adrenocortical responses to glycemia-related challenges are unknown. This study explores these mechanisms using network- and cellular-level interventions in in vivo and ex vivo rat preparations. Results show that a hindbrain-originating catecholamine afferent system selectively engages a MAP kinase pathway in rat paraventricular hypothalamic CRH (corticotropin-releasing hormone) neuroendocrine neurons shortly after vascular insulin and 2-deoxyglucose challenges. In turn, this MAP kinase pathway can control both neuroendocrine neuronal firing rate and the state of CREB phosphorylation in a reduced ex vivo paraventricular hypothalamic preparation, making this signaling pathway an ideal candidate for coordinating CRH synthesis and release. These results establish the first clear structural and functional relationships linking neurons in known nutrient-sensing regions with intracellular mechanisms in hypothalamic CRH neuroendocrine neurons that initiate the adrenocortical response to various glycemia-related challenges.

The Medullary Targets of Neurally Conveyed Sensory Information from the Rat Hepatic Portal and Superior Mesenteric Veins.

Vagal and spinal sensory endings in the wall of the hepatic portal and superior mesenteric veins (PMV) provide the brain with chemosensory information important for energy balance and other functions. To determine their medullary neuronal targets, we injected the transsynaptic anterograde viral tracer HSV-1 H129-772 (H129) into the PMV wall or left nodose ganglion (LNG) of male rats, followed by immunohistochemistry (IHC) and high-resolution imaging. We also determined the chemical phenotype of H129-infected neurons, and potential vagal and spinal axon terminal appositions in the dorsal motor nucleus of the vagus (DMX) and the nucleus of the solitary tract (NTS). PMV wall injections generated H129-infected neurons in both nodose ganglia and in thoracic dorsal root ganglia (DRGs). In the medulla, cholinergic preganglionic parasympathetic neurons in the DMX were virtually the only targets of chemosensory information from the PMV wall. H129-infected terminal appositions were identified on H129-infected somata and dendrites in the DMX, and on H129-infected DMX dendrites that extend into the NTS. Sensory transmission via vagal and possibly spinal routes from the PMV wall therefore reaches DMX neurons via axo-somatic appositions in the DMX and axo-dendritic appositions in the NTS. However, the dearth of H129-infected NTS neurons indicates that sensory information from the PMV wall terminates on DMX neurons without engaging NTS neurons. These previously underappreciated direct sensory routes into the DMX enable a vago-vagal and possibly spino-vagal reflexes that can directly influence visceral function.

Catecholaminergic control of mitogen-activated protein kinase signaling in paraventricular neuroendocrine neurons in vivo and in vitro: a proposed role during glycemic challenges.

Paraventricular hypothalamic (PVH) corticotropin-releasing hormone (CRH) neuroendocrine neurons mount neurosecretory and transcriptional responses to glycemic challenges [intravenous 2-deoxyglucose (2-DG) or insulin]. Although these responses require signals from intact afferents originating from hindbrain CA (catecholaminergic) neurons, the identity of these signals and the mechanisms by which they are transduced by PVH neurons during glycemic challenge remain unclear. Here, we tested whether the prototypical catecholamine, norepinephrine (NE), can reproduce PVH neuroendocrine responses to glycemic challenge. Because these responses include phosphorylation of p44/42 mitogen-activated protein (MAP) kinases [extracellular signal-regulated kinases 1/2 (ERK1/2)], we also determined whether NE activates ERK1/2 in PVH neurons and, if so, by what mechanism. We show that systemic insulin and 2-DG, and PVH-targeted NE microinjections, rapidly elevated PVH phospho-ERK1/2 levels. NE increased Crh and c-fos expression, together with circulating ACTH/corticosterone. However, because injections also increased c-Fos mRNA in other brain regions, we used hypothalamic slices maintained in vitro to clarify whether NE activates PVH neurons without contribution of inputs from distal regions. In slices, bath-applied NE triggered robust phospho-ERK1/2 immunoreactivity in PVH (including CRH) neurons, which attenuated markedly in the presence of the alpha1 adrenoceptor antagonist, prazosin, or the MAP kinase kinase (MEK) inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene). Therefore, at a systems level, local PVH delivery of NE is sufficient to account for hindbrain activation of CRH neuroendocrine neurons during glycemic challenge. At a cellular level, these data provide the first demonstration that MAP kinase signaling cascades (MEK-->ERK) are intracellular transducers of noradrenergic signals in CRH neurons, and implicate this transduction mechanism as an important component of central neuroendocrine responses during glycemic challenge.

Loss of dorsomedial hypothalamic GLP-1 signaling reduces BAT thermogenesis and increases adiposity.

OBJECTIVE: Glucagon-like peptide-1 (GLP-1) neurons in the hindbrain densely innervate the dorsomedial hypothalamus (DMH), a nucleus strongly implicated in body weight regulation and the sympathetic control of brown adipose tissue (BAT) thermogenesis. Therefore, DMH GLP-1 receptors (GLP-1R) are well placed to regulate energy balance by controlling sympathetic outflow and BAT function. METHODS: We investigate this possibility in adult male rats by using direct administration of GLP-1 (0.5 ug) into the DMH, knocking down DMH GLP-1R mRNA with viral-mediated RNA interference, and by examining the neurochemical phenotype of GLP-1R expressing cells in the DMH using in situ hybridization. RESULTS: GLP-1 administered into the DMH increased BAT thermogenesis and hepatic triglyceride (TG) mobilization. On the other hand, Glp1r knockdown (KD) in the DMH increased body weight gain and adiposity, with a concomitant reduction in energy expenditure (EE), BAT temperature, and uncoupling protein 1 (UCP1) expression. Moreover, DMH Glp1r KD induced hepatic steatosis, increased plasma TG, and elevated liver specific de-novo lipogenesis, effects that collectively contributed to insulin resistance. Interestingly, DMH Glp1r KD increased neuropeptide Y (NPY) mRNA expression in the DMH. GLP-1R mRNA in the DMH, however, was found in GABAergic not NPY neurons, consistent with a GLP-1R-dependent inhibition of NPY neurons that is mediated by local GABAergic neurons. Finally, DMH Glp1r KD attenuated the anorexigenic effects of the GLP-1R agonist exendin-4, highlighting an important role of DMH GLP-1R signaling in GLP-1-based therapies. CONCLUSIONS: Collectively, our data show that DMH GLP-1R signaling plays a key role for BAT thermogenesis and adiposity.

A role for the forebrain in mediating time-of-day differences in glucocorticoid counterregulatory responses to hypoglycemia in rats.

The time of day influences the magnitude of ACTH and corticosterone responses to hypoglycemia. However, little is known about the mechanisms that impart these time-of-day differences on neuroendocrine CRH neurons in the hypothalamic paraventricular nucleus (PVH). Rats received 0-3 U/kg insulin (or 0.9% saline) to achieve a range of glucose nadir concentrations. Brains were processed to identify phosphorylated ERK1/2 (phospho-ERK1/2)-immunoreactive cells in the PVH and hindbrain and CRH heteronuclear RNA in the PVH. Hypoglycemia did not stimulate ACTH and corticosterone responses in animals unless a glucose concentration of approximately 3.15 mM or below was reached. Critically the glycemic thresholds required to stimulate ACTH and corticosterone release in the morning and night were indistinguishable. Yet glucose concentrations below the estimated glycemic threshold correlated with a greater increase in corticosterone, ACTH, and phospho-ERK1/2-immunoreactive neurons in the PVH at night, compared with morning. In these same animals, the number of phospho-ERK1/2-immunoreactive neurons in the medial part of the nucleus of the solitary tract was unchanged at both times of day. These data collectively support a model whereby changes in forebrain mechanisms alter the sensitivity of neuroendocrine CRH to the hypoglycemia-related information conveyed by ascending catecholaminergic afferents. Circadian clock-driven processes together with glucose-sensing elements in the forebrain would seem to be strong contenders for mediating these effects.

Rapid and preferential activation of Fos protein in hypocretin/orexin neurons following the reversal of dehydration-anorexia.

Dehydration (DE)-anorexia is stimulated by chronic consumption of hypertonic saline. Spontaneous nocturnal food intake is markedly reduced with this treatment but is rapidly reversed upon the return of drinking water. Here we examined the neurons in the lateral hypothalamic area (LHA) of chronically dehydrated rats for their peptidergic phenotype, colocalization, and activation profiles following the rapid reversal of anorexia. To do this, we used double-labeling combinations of Fos immunocytochemistry and radioisotopic- and digoxigenin-labeled in situ hybridization. We found that lateral hypothalamic corticotropin-releasing hormone (CRH) neurons show extensive coexpression with neurotensin mRNA, but they are distinct from hypocretin/orexin and melanin-concentrating hormone (MCH) neurons. Chronic dehydration increases Fos-ir in large numbers of neurons in dorsal regions of the LHA. Some of these LHA neurons also show increased CRH, but not hypocretin/orexin or MCH gene expression, as dehydration-anorexia develops. Furthermore, the behavioral sequence of eating and increased activity exhibited by DE animals in the minutes following water drinking is accompanied by a further increase in the number of Fos-ir nuclei in the LHA. Increased Fos activation occurs in a significant number of LHA hypocretin/orexin neurons, but not CRH or MCH neurons, in the LHA. Together these data implicate CRH but not hypocretin/orexin or MCH neurons in the LHA in the motor events associated with dehydration. However, when water is returned, contributions to the network controlling responses evidently come from hypocretin/orexin, but not CRH or MCH, neurons in the LHA.

Activation in neural networks controlling ingestive behaviors: what does it mean, and how do we map and measure it?

Over the past thirty years many of different methods have been developed that use markers to track or image the activity of the neurons within the central networks that control ingestive behaviors. The ultimate goal of these experiments is to identify the location of neurons that participate in the response to an identified stimulus, and more widely to define the structure and function of the networks that control specific aspects of ingestive behavior. Some of these markers depend upon the rapid accumulation of proteins, while others reflect altered energy metabolism as neurons change their firing rates. These methods are widely used in behavioral neuroscience, but the way results are interpreted within the context of defining neural networks is constrained by how we answer the following questions. How well can the structure of the behavior be documented? What do we know about the processes that lead to the accumulation of the marker? What is the function of the marker within the neuron? How closely in time does the marker accumulation track the stimulus? How long does the marker persist after the stimulus is removed? We will review how these questions can be addressed with regard to ingestive and related behaviors. We will also discuss the importance of plotting the location of labeled cells using standardized atlases to facilitate the presentation and comparison of data between experiments and laboratories. Finally, we emphasize the importance of comprehensive and accurate mapping for using newly emerging technologies in neuroinfomatics.

Interactions between heterotypic stressors and corticosterone reveal integrative mechanisms for controlling corticotropin-releasing hormone gene expression in the rat paraventricular nucleus.

Although the convergence of neural and humoral afferent information onto paraventricular neuroendocrine corticotropin-releasing hormone (CRH) neurons is a major determinant for adaptive stress responses, the underlying integrative mechanisms are poorly understood. To dissect the relative contributions made by neural afferents and corticosterone to these processes, we determined how the concurrent application of two heterotypic physiological stressors, chronic dehydration (produced by drinking hypertonic saline) and sustained hypovolemia (produced by subcutaneous injections of polyethylene glycol), is interpreted by the synthetic and secretory activity of CRH neurons using in situ hybridization and plasma ACTH measurements. These two stressors are encoded by relatively simple, distinct, and well defined sets of neural afferents to CRH neurons. Both increase plasma corticosterone, but they have opposing actions on CRH gene expression when applied separately. In the first experiment, we showed that chronic dehydration suppresses CRH gene transcription after hypovolemia, but not the preproenkephalin and c-fos mRNA responses or ACTH secretion. In the second, we showed that negative feedback actions of corticosterone do not suppress CRH gene activation after hypovolemia, but instead determine the prestress lower limit of a range within which the CRH gene then responds. Collectively, these data show that at least two processes are integrated to control how the CRH gene responds to multiple stimuli. First, the presence of corticosterone, which although permissive for appropriately activating the CRH gene during hypovolemia, does not mediate the suppressed gene response. Second, neural afferent-driven processes that encode dehydration play a central role in suppressing CRH activation.

Comparison of melanin-concentrating hormone and hypocretin/orexin mRNA expression patterns in a new parceling scheme of the lateral hypothalamic zone.

A high-resolution spatial distribution analysis of hypothalamic neurons expressing melanin-concentrating hormone or hypocretin/orexin was performed in adult male rats with in situ hybridization cytochemistry. For the analysis, a new parcellation of the lateral zone with some two-dozen regions was used, and distributions were plotted on 15 transverse reference levels through the hypothalamus. Qualitatively the results confirm earlier, much lower resolution mapping studies, although some discrepancies are clarified. Previous work indicates that each of these cell populations is far from homogeneous, and the present results should help establish a framework for clarifying more precisely how they are differentiated and organized in terms of axonal input-output relationships and gene expression patterns, and for defining precise relationships with other hypothalamic neuron populations.

Corticotropin-releasing hormone and arginine vasopressin gene transcription in the hypothalamic paraventricular nucleus of unstressed rats: daily rhythms and their interactions with corticosterone.

To drive the daily ACTH secretory rhythm from anterior pituitary corticotropes signals from the circadian clock schedule CRH and, to lesser extent, arginine vasopressin (AVP) release from neuroendocrine terminals. In turn, releasable pools of CRH and AVP in neuroendocrine terminals are sustained by synthetic mechanisms in the medial parvicellular paraventricular nucleus, a critical component of which involves transcribing primary (heteronuclear (hn)) RNA transcripts from their cognate genes. To determine the fundamental daily patterns of ACTH secretagogue gene transcription in unstressed rats, we measured CRH and AVP hnRNA levels at 1- to 4-h intervals throughout the day using in situ hybridization. Crh gene transcription is readily detectable throughout the day, and shows a pronounced rhythm that is temporally correlated with CRH mRNA levels, but is uncoupled from ACTH release. However, avp gene transcription is barely detectable and shows no discernable rhythm. We then performed similar experiments in adrenalectomized rats with or without corticosterone replacement. Corticosterone-dependent mechanisms regulate CRH hnRNA levels at the nadir and peak as well as the onset of nocturnal crh gene transcription. A prominent rhythm of avp hnRNA seen in adrenalectomized animals was dampened by corticosterone. This study shows, first, CRH synthesis in intact animals is maintained by a nocturnal episode of crh gene transcription, parameters of which are modulated by corticosterone-dependent mechanisms; second, circulating corticosterone is sufficient to completely inhibit a daily rhythm of avp gene transcription present in adrenalectomized rats; third, the neural systems that activate crh gene transcription can be uncoupled from those driving ACTH release.

Immunotoxin lesion of hypothalamically projecting norepinephrine and epinephrine neurons differentially affects circadian and stressor-stimulated corticosterone secretion.

Hindbrain norepinephrine (NE) and epinephrine (E) neurons play a pivotal role in the central distribution of sensory signals derived from the internal environment. Their projections influence the various secretory patterns of the hypothalamo-pituitary-adrenal axis and are essential for feeding and adrenal medullary responses to glucoprivation. NE and E terminals in the paraventricular nucleus of the hypothalamus (PVH) and associated hindbrain cell bodies can be virtually eliminated by PVH microinjection of a retrogradely transported conjugate of saporin (SAP, a ribosomal toxin) and a monoclonal antibody against dopamine beta-hydroxylase (dbetah), i.e. dbetah mouse monoclonal antibody conjugated to SAP (DSAP). To examine the effects of selective elimination of NE/E afferents on hypothalamo-pituitary-adrenal activation, we injected DSAP into the PVH and measured corticosterone secretion under basal circadian conditions and in response to two distinct challenges: glucoprivation and forced swim. DSAP lesions profoundly impaired glucoprivation-induced corticosterone secretion and induction of CRH heteronuclear RNA and Fos mRNA in the PVH, without impairing basal CRH mRNA expression, circadian corticosterone release, or the corticosterone response to swim stress. Thus, NE/E projections influence corticosterone secretion only in certain circumstances. They are required for the response to glucoprivation, but are dispensable for circadian activation and for the response to swim stress.

Salt-inducible kinase is involved in the regulation of corticotropin-releasing hormone transcription in hypothalamic neurons in rats.

Activation of CRH transcription requires phosphorylation of cAMP response element-binding protein (CREB) and translocation of the CREB coactivator, transducer of regulated CREB activity (TORC) from cytoplasm to nucleus. In basal conditions, transcription is low because TORC remains in the cytoplasm, inactivated by phosphorylation through Ser/Thr protein kinases of the AMP-dependent protein kinases (AMPK) family, including salt-inducible kinase (SIK). To determine which kinase is responsible for TORC phosphorylation in CRH neurons, we measured SIK1 and SIK2 mRNA in the hypothalamic paraventricular nucleus of rats by in situ hybridization. In basal conditions, low mRNA levels of the two kinases were found in the dorsomedial paraventricular nucleus, consistent with location in CRH neurons. One hour of restraint stress increased SIK1 mRNA levels, whereas SIK2 mRNA showed only minor increases. In 4B hypothalamic neurons, or primary cultures, SIK1 mRNA (but not SIK2 mRNA) was inducible by the cAMP stimulator, forskolin. Overexpression of either SIK1 or SIK2 in 4B cells reduced nuclear TORC2 levels (Western blot) and inhibited forskolin-stimulated CRH transcription (luciferase assay). Conversely, the nonselective SIK inhibitor, staurosporine, increased nuclear TORC2 content and stimulated CRH transcription in 4Bcells and primary neuronal cultures (heteronuclear RNA). Unexpectedly, in 4B cells specific short hairpin RNA knockdown of endogenous SIK2 but not SIK1 induced nuclear translocation of TORC2 and CRH transcription, suggesting that SIK2 mediates TORC inactivation in basal conditions, whereas induction of SIK1 limits transcriptional activation. The study provides evidence that SIK represses CRH transcription by inactivating TORC, providing a potential mechanism for rapid on/off control of CRH transcription.

The distribution of messenger RNAs encoding the three isoforms of the transducer of regulated cAMP responsive element binding protein activity in the rat forebrain.

Increasing evidence indicates that the cAMP responsive element binding protein (CREB)-dependent transcriptional activation of a number of genes requires the CREB co-activator: transducer of regulated CREB activity (TORC). Because of the central importance of CREB in many brain functions, we examined the topographic distribution of TORC1, 2, and 3 mRNAs in specific regions of the rat forebrain. In situ hybridisation analysis revealed that TORC1 is the most abundant isoform in most forebrain structures, followed by TORC2 and TORC3. All three TORC isoforms were found in a number of brain nuclei, the ventricular ependyma and pia mater. Although high levels of TORC1 were widely distributed in the forebrain, TORC2 was found in discrete nuclei and TORC3 mostly in the ependyma, and pia mater. The relative expression of TORC isoforms was confirmed by quantitative reverse transcriptase-polymerase chain reaction analysis in the hippocampus and hypothalamus. In the paraventricular nucleus of the hypothalamus, TORC1 and 2 mRNAs were abundant in the parvicellular and magnocellular neuroendocrine compartments, whereas TORC3 expression was low. All three isoform mRNAs were found elsewhere in the hypothalamus, with the most prominent expression of TORC1 in the ventromedial nucleus, TORC2 in the dorsomedial and arcuate nuclei, TORCs 1 and 2 in the supraoptic nucleus, and TORC2 in the suprachiasmatic nucleus. These differential distribution patterns are consistent with complex roles for all three TORC isoforms in diverse brain structures, and provide a foundation for further studies on the mechanisms of CREB/TORC signalling on brain function.

Hypoglycemic detection at the portal vein is mediated by capsaicin-sensitive primary sensory neurons.

To elucidate the type of spinal afferent involved in hypoglycemic detection at the portal vein, we considered the potential role of capsaicin-sensitive primary sensory neurons. Specifically, we examined the effect of capsaicin-induced ablation of portal vein afferents on the sympathoadrenal response to hypoglycemia. Under anesthesia, the portal vein was isolated in rats and either capsaicin (CAP) or the vehicle (CON) solution applied topically. During the same surgery, the carotid artery (sampling) and jugular vein (infusion) were cannulated. One week later, all animals underwent a hyperinsulinemic hypoglycemic clamp, with glucose (variable) and insulin (25 mU x kg(-1) x min(-1)) infused via the jugular vein. Systemic hypoglycemia (2.76 +/- 0.05 mM) was induced by minute 75 and sustained until minute 105. By design, no significant differences were observed in arterial glucose or insulin concentrations between groups. When hypoglycemia was induced in CON, the plasma epinephrine concentration increased from 0.67 +/- 0.05 nM at basal to 36.15 +/- 2.32 nM by minute 105. Compared with CON, CAP animals demonstrated an 80% suppression in epinephrine levels by minute 105, 7.11 +/- 0.55 nM (P < 0.001). A similar response to hypoglycemia was observed for norepinephrine, with CAP values suppressed by 48% compared with CON. Immunohistochemical analysis of the portal vein revealed an 85% decrease in the number of calcitonin gene-related peptide-reactive nerve fibers following capsaicin-induced ablation. That the suppression in the sympathoadrenal response was comparable to our previous findings for total denervation of the portal vein indicates that hypoglycemic detection at the portal vein is mediated by capsaicin-sensitive primary sensory neurons.