Mónica Lemus

Arginine-vasopressin in nucleus of the tractus solitarius induces hyperglycemia and brain glucose retention

Hypothalamic arginine-vasopressin (AVP) plays an important role both as a neurotransmitter and hormone in the regulation of blood glucose and feeding behavior. AVP-containing axons from the parvocellular subdivision of paraventricular nucleus of the hypothalamus terminate in the nucleus of the tractus solitarius (NTS), but the function of this projection is not known. Interestingly, the NTS also receives afferent information from the carotid body and other peripheral receptors involved in glucose homeostasis. We have previously reported that stimulation of the carotid body receptors initiates a hyperglycemic reflex and increases brain glucose retention. Here we show that direct administration of micro-doses of AVP into the NTS of anesthetized or awake rats rapidly increased the levels of blood glucose concentration and brain arterio-venous (A-V) glucose difference. This effect was not observed when the same doses of AVP were injected in the brainstem outside NTS. Arginine-vasopressin antagonist microinjections alone produced a small but significant reduction in brain A-V glucose. Pre-administered VP1-receptor antagonist [beta-mercapto-beta,beta-cyclopentamethylene-propionyl(1),O-Me-Tyr(2),Arg(8)]vasopressin blocked the effects of AVP. These results indicate that AVP acting on its receptors locally within the NTS participates in glucose homeostasis, increasing both blood glucose concentration and brain A-V glucose differences. Hypothalamic AVP may facilitate hyperglycemic responses initiated by peripheral signals processed at the level of the NTS.

GabaB receptors activation in the NTS blocks the glycemic responses induced by carotid body receptor stimulation

The carotid body receptors participate in glucose regulation sensing glucose levels in blood entering the cephalic circulation. The carotid body receptors information, is initially processed within the nucleus tractus solitarius (NTS) and elicits changes in circulating glucose and brain glucose uptake. Previous work has shown that gamma-aminobutyric acid (GABA) in NTS modulates respiratory reflexes, but the role of GABA within NTS in glucose regulation remains unknown. Here we show that GABA(B) receptor agonist (baclofen) or antagonists (phaclofen and CGP55845A) locally injected into NTS modified arterial glucose levels and brain glucose retention. Control injections outside NTS did not elicit these responses. In contrast, GABA(A) agonist and antagonist (muscimol or bicuculline) produced no significant changes in blood glucose levels. When these GABAergic drugs were applied before carotid body receptors stimulation, again, only GABA(B) agonist or antagonist significantly affected glycemic responses; baclofen microinjection significantly reduced the hyperglycemic response and brain glucose retention observed after carotid body receptors stimulation, while phaclofen produced the opposite effect, increasing significantly hyperglycemia and brain glucose retention. These results indicate that activation of GABA(B), but not GABA(A), receptors in the NTS modulates the glycemic responses after anoxic stimulation of the carotid body receptors, and suggest the presence of a tonic inhibitory mechanism in the NTS to avoid hyperglycemia.

Chronic Exercise Increases Plasma Brain-Derived Neurotrophic Factor Levels, Pancreatic Islet Size, and Insulin Tolerance in a TrkB-Dependent Manner

Background Physical exercise improves glucose metabolism and insulin sensitivity. Brain-derived neurotrophic factor (BDNF) enhances insulin activity in diabetic rodents. Because physical exercise modifies BDNF production, this study aimed to investigate the effects of chronic exercise on plasma BDNF levels and the possible effects on insulin tolerance modification in healthy rats. Methods Wistar rats were divided into five groups: control (sedentary, C); moderate- intensity training (MIT); MIT plus K252A TrkB blocker (MITK); high-intensity training (HIT); and HIT plus K252a (HITK). Training comprised 8 weeks of treadmill running. Plasma BDNF levels (ELISA assay), glucose tolerance, insulin tolerance, and immunohistochemistry for insulin and the pancreatic islet area were evaluated in all groups. In addition, Bdnf mRNA expression in the skeletal muscle was measured. Principal Findings Chronic treadmill exercise significantly increased plasma BDNF levels and insulin tolerance, and both effects were attenuated by TrkB blocking. In the MIT and HIT groups, a significant TrkB-dependent pancreatic islet enlargement was observed. MIT rats exhibited increased liver glycogen levels following insulin administration in a TrkB-independent manner. Conclusions/Significance Chronic physical exercise exerted remarkable effects on insulin regulation by inducing significant increases in the pancreatic islet size and insulin sensitivity in a TrkB-dependent manner. A threshold for the induction of BNDF in response to physical exercise exists in certain muscle groups. To the best of our knowledge, these are the first results to reveal a role for TrkB in the chronic exercise-mediated insulin regulation in healthy rats.

Nitric oxide in the hypothalamus–pituitary axis mediates increases in brain glucose retention induced by carotid chemoreceptor stimulation with cyanide in rats

Neuronal nitric oxide synthase (nNOS), which catalyzes the generation of nitric oxide (NO), is expressed by neuron subpopulations in the CNS. Nitric oxide is involved in neurotransmission and central glucose homeostasis. Our prior studies have shown that carotid body receptors participate in brain glucose regulation in vivo, and suggest the presence of a NO tonic mechanism in the solitary tract nucleus (STn). However, the role of NO within STn in glucose control remains unknown. In this study, we explored the potential regulatory role of NO on brain glucose retention induced by carotid body chemoreceptor anoxic stimulation with sodium cyanide (NaCN) which inhibits oxidative metabolism. Intracisternal infusions of nitroxidergic drugs before carotid chemoreceptor stimulation in anesthetized rats, elicited changes in nitrite concentration in plasma and hypothalamus-pituitary (H-P) tissue, as well as in gene expression of neuronal and inducible isoforms (nNOS and iNOS) in H-P tissue. The changes observed in above variables modified brain glucose retention in an opposite direction. When the NO donor, sodium nitroprusside (SNP), was given before carotid stimulation, nitrite concentration in plasma and H-P tissue, and gene expression of nNOS and iNOS in H-P tissue increased, whereas brain glucose retention decreased. In contrast, when the NOS inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME) was infused immediately before carotid chemoreceptor stimulation, nitrite levels and nNOS expression decreased in plasma and H-P tissue, whereas brain glucose retention increased. Anoxic stimulation by itself induced an increase in the expression of both genes studied. All these results indicate that de novo expression of the nNOS gene in H-P tissue may be critically involved in central glucose changes observed after anoxic carotid chemoreceptor stimulation in conjunction with NO.

Induction of brain glucose uptake by a factor secreted into cerebrospinal fluid

It is well established that the carotid body receptors (CBR), at the bifurcation of the carotid artery, inform the brain of changes in the concentration of CO(2) and O(2) in arterial blood. More recent work suggests that these receptors are also extremely sensitive to blood glucose levels suggesting that they may play an important role as sensors of blood components important for brain energy metabolism. Much less is known about changes in brain glucose metabolism in response to CBR activation. Here we show that 2-8 min after local injection of sodium cyanide (NaCN) into the CBR or after electrical stimulation of the carotid sinus nerve in dogs and rats, brain glucose uptake increased fourfold. Cerebrospinal fluids (CSF) transferred from dogs, 2-8 min after CBR stimulation, into the cisterna magna of non-stimulated dogs or rats induced a similar increase in brain glucose uptake. CSF from stimulated dogs was also active when injected intravenously in anesthetized or awake rats. The activity was destroyed when the stimulated CSF was heated to 100 degrees C or treated with trypsin. We conclude that a peptide important for brain glucose regulation appears in the CSF shortly after CBR stimulation.

Brain-Derived Neurotrophic Factor in the Nucleus Tractus Solitarii Modulates Glucose Homeostasis After Carotid Chemoreceptor Stimulation in Rats

Neuronal systems, which regulate energy intake, energy expenditure and endogenous glucose production, sense and respond to input from hormonal related signals that convey information from body energy availability. Carotid chemoreceptors (CChr) function as sensors for circulating glucose levels and contribute to glycemic counterregulatory responses. Brain-derived neurotrophic factor (BDNF) that plays an important role in the endocrine system to regulate glucose metabolism could play a role in hyperglycemic glucose reflex with brain glucose retention (BGR) evoked by anoxic CChr stimulation. Infusing BDNF into the nucleus tractus solitarii (NTS) before CChr stimulation, showed that this neurotrophin increased arterial glucose and BGR. In contrast, BDNF receptor (TrkB) antagonist (K252a) infusions in NTS resulted in a decrease in both glucose variables.

Nitric Oxide in Brain Glucose Retention after Carotid Body Receptors Stimulation with Cyanide in Rats

In contrast to most other tissues, which exhibit considerable flexibility with respect to the nature of the substrates for their energy metabolism, the normal brain is restricted almost exclusively to glucose due to its distinguishing characteristics in vivo. Actual glucose utilization is 31 μmol/100 g tissue/min, in the normal, conscious human brain, indicating that glucose consumption is in excess for total oxygen consumption (Sokoloff, 1991). Although present in low concentration in brain (3.3 mmol/kg in rat), glycogen is a unique energy reserve for initiation of its metabolism. However, if glycogen concentration in the brain were the sole supply, normal energetic requirements would be maintained for less than 5 min (Sokoloff, 1991). While the brain contains insulin receptors, and insulin-responsive glucose transporters, the role of insulin in the regulation of brain glucose metabolism is controversial (Obici et al., 2002). The carotid body receptors (CBR) are sensitive to glucose (Alvarez-Buylla and Alvarez-Buylla, 1988, 1994, Pardal and Lopez Barneo, 2002) and play an important role in the insulin-induced counterregulatory response to mild hypoglycemia (Koyama et al., 2000). Local stimulation of CBR by cyanide (NaCN), or local low glucose levels in the isolated carotid sinus (CS), have been shown to promptly increase the activity in the carotid sinus nerve, that in turn trigger an enhancement in glucose retention by the brain (BGR) (Alvarez-Buylla et al., 1994). In contrast, this effect is not observed in animals with denervated carotid bodies (AlvarezBuylla and Alvarez-Buylla, 1988). The central mechanism that mediates the previously mentioned glycemic responses is unknown, but other studies from our laboratory suggest the participation of arginine-vasopressin (AVP), the endogenous ligand for the V1a vasopressin receptor, as the effector mediator in this response (Montero et al., 2003). AVP is widely synthesized in the brain, including the paraventricular, supraoptic and suprachiasmatic nuclei of the hypothalamus, and has been related to nitric oxide (NO) function in brain (Kadekaro et al., 1998). There are evidences that NO, an intercellular signaling

Nitric oxide in the commissural nucleus tractus solitarii regulates carotid chemoreception hyperglycemic reflex and c-Fos expression

Carotid body chemoreceptors function as glucose sensors and contribute to glucose homeostasis. The nucleus tractus solitarii (NTS) is the first central nervous system (CNS) nuclei for processing of information arising in the carotid body. Here, we microinjected a nitric oxide (NO) donor sodium nitroprusside (SNP), an NO-independent activator of the soluble guanylyl cyclase (sGC) (YC₁) or an NO-synthase (NOS) inhibitor Nω-nitro-l-arginine methyl ester (L-NAME) into the commissural NTS (cNTS) before carotid chemoreceptor anoxic stimulation and measured arterial glucose and the expression of Fos-like immunoreactivity (Fos-ir). Male Wistar rats (250-300 g) were anesthetized, and the carotid sinus was vascularly isolated. Either artificial cerebrospinal fluid (aCSF), SNP, YC₁ or L-NAME were stereotaxically injected into the cNTS. The SNP and YC₁ infused into the cNTS before carotid chemoreceptor stimulation (SNP-2 and YC₁-2 groups) similarly increased arterial glucose compared to the aCSF-2 group. By contrast, infusion of L-NAME into the cNTS before carotid chemoreceptor stimulation (L-NAME-2 group) decreased arterial glucose concentration. The number of cNTS Fos-ir neurons, determined in all the groups studied except for YC₁ groups, significantly increased in SNP-2 rat when compared to the aCSF-2 or SNP-2 groups. Our findings demonstrate that NO signaling, and the correlative activation of groups of cNTS neurons, plays key roles in the hyperglycemic reflex initiated by carotid chemoreceptor stimulation.

Nitric Oxide in the Solitary Tract Nucleus (STn) Modulates Glucose Homeostasis and FOS-ir Expression After Carotid Chemoreceptor Stimulation

We evaluate in rats the role of NO in the solitary tract nucleus (STn) after an anoxic stimulus to carotid body chemoreceptor cells (CChrc) with cyanide (NaCN), on the hyperglycemic reflex with glucose retention by the brain (BGR) and FOS expression (FOS-ir) in the STn. The results suggest that nitroxidergic pathways in the STn may play an important role in glucose homeostasis. A NO donor such as sodium nitroprusside (NPS) in the STn before CChrc stimulation increased arterial glucose level and significantly decreased BGR. NPS also induced a higher FOS-ir expression in STn neurons when compared to neurons in control rats that only received artificial cerebrospinal fluid (aCSF) before CChrc stimulation. In contrast, a selective NOS inhibitor such as Nomega-nitro-L-arginine methyl ester (L-NAME) in the STn before CChrc stimulation resulted in an increase of both, systemic glucose and BGR above control values. In this case, the number of FOS-ir positive neurons in the STn decreased when compared to control or to NPS experiments. FOS-ir expression in brainstem cells suggests that CChrc stimulation activates nitroxidergic pathways in the STn to regulate peripheral and central glucose homeostasis. The study of these functionally defined cells will be important to understand brain glucose homeostasis.

Effects of moderate- and high-intensity chronic exercise on brain-derived neurotrophic factor expression in fast and slow muscles

Introduction: Brain‐derived neurotrophic factor (BDNF) protein expression is sensitive to cellular activity. In the sedentary state, BDNF expression is affected by the muscle phenotype. Methods: Eighteen Wistar rats were divided into the following 3 groups: sedentary (S); moderate‐intensity training (MIT); and high‐intensity training (HIT). The training protocol lasted 8 weeks. Forty‐eight hours after training, total RNA and protein levels in the soleus and plantaris muscles were obtained. Results: In the plantaris, the BDNF protein level was lower in the HIT than in the S group (P < 0.05). A similar effect was found in the soleus (without significant difference). In the soleus, higher Bdnf mRNA levels were found in the HIT group (P < 0.001 vs. S and MIT groups). In the plantaris muscle, similar Bdnf mRNA levels were found in all groups. Conclusions: These results indicate that high‐intensity chronic exercise reduces BDNF protein level in fast muscles and increases Bdnf mRNA levels in slow muscles. Muscle Nerve 53: 446–451, 2016