Effects of intraperitoneally administered L-histidine on food intake, taste, and visceral sensation in rats

To evaluate relative factors for anorectic effects of L-histidine, we performed behavioral experiments for measuring food and fluid intake, conditioned taste aversion (CTA), taste disturbance, and c-Fos immunoreactive (Fos-ir) cells before and after i.p. injection with L-histidine in rats. Animals were injected with saline (9 ml/kg, i.p.) for a control group, and saline (9 ml/kg, i.p.) containing L-histidine (0.75, 1.5, 2.0 g/kg) for a L-histidine group. Injection of L-histidine decreased the average value of food intake, and statistically significant anorectic effects were found in animals injected with 1.5 or 2.0 g/kg L-histidine but not with 0.75 g/kg L-histidine. Taste abnormalities were not detected in any of the groups. Animals injected with 2.0 g/kg L-histidine were revealed to present with nausea by the measurement of CTA. In this group, a significant increase in the number of Fos-ir cells was detected both in the area postrema and the nucleus tractus solitarius (NTS). In the 0.75 g/kg L-histidine group, a significant increase in the number of Fos-ir cells was detected only in the NTS. When the ventral gastric branch vagotomy was performed, recovery from anorexia became faster than the sham-operated group, however, vagotomized rats injected with 2.0 g/kg L-histidine still acquired CTA. These data indicate that acute anorectic effects induced by highly concentrated L-histidine are partly caused by induction of nausea and/or visceral discomfort accompanied by neuronal activities in the NTS and the area postrema. We suggest that acute and potent effects of L-histidine on food intake require substantial amount of L-histidine in the diet.

Movement-related cortical magnetic fields associated with self-paced tongue protrusion in humans

Sophisticated tongue movements are coordinated finely via cortical control. We elucidated the cortical processes associated with voluntary tongue movement. Movement-related cortical fields were investigated during self-paced repetitive tongue protrusion. Surface tongue electromyograms were recorded to determine movement onset. To identify the location of the primary somatosensory cortex (S1), tongue somatosensory evoked fields were measured. The readiness fields (RFs) over both hemispheres began prior to movement onset and culminated in the motor fields (MFs) around movement onset. These signals were followed by transient movement evoked fields (MEFs) after movement onset. The MF and MEF peak latencies and magnitudes were not different between the hemispheres. The MF current sources were located in the precentral gyrus, suggesting they were located in the primary motor cortex (M1); this was contrary to the MEF sources, which were located in S1. We conclude that the RFs and MFs mainly reflect the cortical processes for the preparation and execution of tongue movement in the bilateral M1, without hemispheric dominance. Moreover, the MEFs may represent proprioceptive feedback from the tongue to bilateral S1. Such cortical processing related to the efferent and afferent information may aid in the coordination of sophisticated tongue movements.

CCK stimulation of GLP-1 neurons involves α1-adrenoceptor-mediated increase in glutamatergic synaptic inputs

OBJECTIVE

Glucagon-like peptide 1 (GLP-1) is involved in the central regulation of food intake. It is produced within the brain by preproglucagon (PPG) neurons, which are located primarily within the brain stem. These neurons project widely throughout the brain, including to the appetite centers in the hypothalamus, and are believed to convey signals related to satiety. Previous work demonstrated that they are directly activated by leptin and electrical activity of the afferent vagus. Another satiety hormone, cholecystokinin (CCK), has also been linked to activation of brain stem neurons, suggesting that it might act partially via centrally projecting neurons from the nucleus tractus solitarius (NTS). The aim of this study was to investigate the neuronal circuitry linking CCK to the population of NTS-PPG neurons.

RESEARCH DESIGN AND METHODS

Transgenic mice expressing yellow fluorescent protein (Venus) under the control of the PPG promoter were used to identify PPG neurons in vitro and to record their electrical and pharmacological profile.

RESULTS

PPG neurons in the NTS were excited by CCK and epinephrine, but not by the melanocortin receptor agonist melanotan II. Both CCK and epinephrine acted to increase glutamatergic transmission to the PPG neurons, and this involved activation of α(1)-adrenergic receptors. Inhibition of adrenergic signaling abolished the excitatory action of CCK.

CONCLUSIONS

CCK activates NTS-PPG cells by a circuit involving adrenergic and glutamatergic neurons. NTS-PPG neurons integrate a variety of peripheral signals that indicate both long-term energy balance and short-term nutritional and digestional status to produce an output signal to feeding and autonomic circuits.

Leptin directly depolarizes preproglucagon neurons in the nucleus tractus solitarius: electrical properties of glucagon-like Peptide 1 neurons

OBJECTIVE

Glucagon-like peptide (GLP)-1 inhibits food intake, acting both in the periphery and within the central nervous system. It is unclear if gut-derived GLP-1 can enter the brain, or whether GLP-1 from preproglucagon (PPG) cells in the lower brainstem is required to activate central GLP-1 receptors. Brainstem PPG neurons, however, have been poorly characterized, due to the difficulties in identifying these cells while viable. This study provides data on the electrical properties of brainstem PPG cells and their regulation by orexigenic and anorexigenic peptides.

RESEARCH DESIGN AND METHODS

Transgenic mice expressing Venus under control of the PPG promoter were used to identify PPG neurons in vitro in brainstem slice preparations for electrophysiological recordings. RESULTS The majority of PPG neurons were spontaneously active. Further electrical and molecular characterization revealed that GLP-1 receptor activation had no pre- or postsynaptic effect and that PPG neurons lack GLP-1 receptors. Similarly, they were unresponsive to PYY and ghrelin. In contrast, leptin rapidly and reversibly depolarized these neurons. Responses to electrical stimulation of the solitary tract suggest that PPG cells are mostly second-order neurons, receiving direct input from vagal afferent fibers. Both evoked and spontaneous excitatory postsynaptic currents were predominantly glutamatergic.

CONCLUSIONS

The study introduces PPG-promoter-Venus transgenic mice as a viable and important tool to study brainstem PPG cells. PPG neuron activity is directly modulated by leptin but was unaffected by other satiety or hunger peptides. Direct synaptic input from the solitary tract suggests that peripheral signals (including GLP-1) could modulate PPG cells via vagal afferents.

5-HT inhibition of rat insulin 2 promoter Cre recombinase transgene and proopiomelanocortin neuron excitability in the mouse arcuate nucleus

A number of anti-obesity agents have been developed that enhance hypothalamic 5-HT transmission. Various studies have demonstrated that arcuate neurons, which express proopiomelanocortin peptides (POMC neurons), and neuropeptide Y with agouti-related protein (NPY/AgRP) neurons, are components of the hypothalamic circuits responsible for energy homeostasis. An additional arcuate neuron population, rat insulin 2 promoter Cre recombinase transgene (RIPCre) neurons, has recently been implicated in hypothalamic melanocortin circuits involved in energy balance. It is currently unclear how 5-HT modifies neuron excitability in these local arcuate neuronal circuits. We show that 5-HT alters the excitability of the majority of mouse arcuate RIPCre neurons, by either hyperpolarization and inhibition or depolarization and excitation. RIPCre neurons sensitive to 5-HT, predominantly exhibit hyperpolarization and pharmacological studies indicate that inhibition of neuronal firing is likely to be through 5-HT_1F receptors increasing current through a voltage-dependent potassium conductance. Indeed, 5-HT_1F receptor immunoreactivity co-localizes with RIPCre green fluorescent protein expression. A minority population of POMC neurons also respond to 5-HT by hyperpolarization, and this appears to be mediated by the same receptor-channel mechanism. As neither POMC nor RIPCre neuronal populations display a common electrical response to 5-HT, this may indicate that sub-divisions of POMC and RIPCre neurons exist, perhaps serving different outputs.

Melanocortins and agouti-related protein modulate the excitability of two arcuate nucleus neuron populations by alteration of resting potassium conductances

The hypothalamic melanocortin system is crucial for the control of appetite and body weight. Two of the five melanocortin receptors, MC3R and MC4R are involved in hypothalamic control of energy homeostasis, with the MC4R having the major influence. It is generally thought that the main impact of the melanocortin system on hypothalamic circuits is external to the arcuate nucleus, and that any effect locally in the arcuate nucleus is inhibitory on proopiomelanocortin-expressing (POMC) neurons. In contrast, using current- and voltage-clamp recordings from identified neurons, we demonstrate that MC3R and MC4R agonists depolarize arcuate POMC neurons and a separate arcuate neuronal population identified by the rat insulin 2 promoter (RIPCre) transgene expression. Furthermore, the endogenous MC3R and MC4R antagonist, agouti-related protein (AgRP), hyperpolarizes POMC and RIPCre neurons in the absence of melanocortin agonist, consistent with inverse agonism at the MC4R. A decreased transient outward (I(A)) potassium conductance, and to a lesser extent the inward rectifier (K(IR)) conductance, underlies neuronal depolarization, whereas an increase in I(A) mediates AgRP-induced hyperpolarization. Accordingly, POMC and RIPCre neurons may be targets for peptide transmitters that are possibly released locally from AgRP-expressing and POMC neurons in the arcuate nucleus, adding further previously unappreciated complexity to the arcuate system.

The role of insulin receptor substrate 2 in hypothalamic and beta cell function

Insulin receptor substrate 2 (Irs2) plays complex roles in energy homeostasis. We generated mice lacking Irs2 in beta cells and a population of hypothalamic neurons (RIPCreIrs2KO), in all neurons (NesCreIrs2KO), and in proopiomelanocortin neurons (POMCCreIrs2KO) to determine the role of Irs2 in the CNS and beta cell. RIPCreIrs2KO mice displayed impaired glucose tolerance and reduced beta cell mass. Overt diabetes did not ensue, because beta cells escaping Cre-mediated recombination progressively populated islets. RIPCreIrs2KO and NesCreIrs2KO mice displayed hyperphagia, obesity, and increased body length, which suggests altered melanocortin action. POMCCreIrs2KO mice did not display this phenotype. RIPCreIrs2KO and NesCreIrs2KO mice retained leptin sensitivity, which suggests that CNS Irs2 pathways are not required for leptin action. NesCreIrs2KO and POMCCreIrs2KO mice did not display reduced beta cell mass, but NesCreIrs2KO mice displayed mild abnormalities of glucose homeostasis. RIPCre neurons did not express POMC or neuropeptide Y. Insulin and a melanocortin agonist depolarized RIPCre neurons, whereas leptin was ineffective. Insulin hyperpolarized and leptin depolarized POMC neurons. Our findings demonstrate a critical role for IRS2 in beta cell and hypothalamic function and provide insights into the role of RIPCre neurons, a distinct hypothalamic neuronal population, in growth and energy homeostasis.

Superoxide anion impairs contractility in cultured aortic smooth muscle cells

We examined the effects of superoxide anion (O) generated by xanthine plus xanthine oxidase (X/XO) on the intracellular Ca(2+) concentration ([Ca(2+)](i)) and muscle contractility in cultured bovine aortic smooth muscle cells (BASMC). Cells were grown on collagen-coated dish for the measurement of [Ca(2+)](i). Pretreatment with X/XO inhibited ATP-induced Ca(2+) transient and Ca(2+) release-activated Ca(2+) entry (CRAC) after thapsigargin-induced store depletion, both of which were reversed by superoxide dismutase (SOD). In contrast, Ca(2+) transients induced by high-K(+) solution and Ca(2+) ionophore A-23187 were not affected by X/XO. BASMC-embedded collagen gel lattice, which was pretreated with xanthine alone, showed contraction in response to ATP, thapsigargin, high-K(+) solution, and A-23187. Pretreatment of the gel with X/XO impaired gel contraction not only by ATP and thapsigargin, but also by high-K(+) solution and A-23187. The X/XO-treated gel showed normal contraction; however, when SOD was present during the pretreatment period. These results indicate that O(2)(-) attenuates smooth muscle contraction by impairing CRAC, ATP-induced Ca(2+) transient, and Ca(2+) sensitivity in BASMC.

Volume-regulated anion channels serve as an auto/paracrine nucleotide release pathway in aortic endothelial cells

Mechanical stress induces auto/paracrine ATP release from various cell types, but the mechanisms underlying this release are not well understood. Here we show that the release of ATP induced by hypotonic stress (HTS) in bovine aortic endothelial cells (BAECs) occurs through volume-regulated anion channels (VRAC). Various VRAC inhibitors, such as glibenclamide, verapamil, tamoxifen, and fluoxetine, suppressed the HTS-induced release of ATP, as well as the concomitant Ca(2+) oscillations and NO production. They did not, however, affect Ca(2+) oscillations and NO production induced by exogenously applied ATP. Extracellular ATP inhibited VRAC currents in a voltage-dependent manner: block was absent at negative potentials and was manifest at positive potentials, but decreased at highly depolarized potentials. This phenomenon could be described with a "permeating blocker model," in which ATP binds with an affinity of 1.0 +/- 0.5 mM at 0 mV to a site at an electrical distance of 0.41 inside the channel. Bound ATP occludes the channel at moderate positive potentials, but permeates into the cytosol at more depolarized potentials. The triphosphate nucleotides UTP, GTP, and CTP, and the adenine nucleotide ADP, exerted a similar voltage-dependent inhibition of VRAC currents at submillimolar concentrations, which could also be described with this model. However, inhibition by ADP was less voltage sensitive, whereas adenosine did not affect VRAC currents, suggesting that the negative charges of the nucleotides are essential for their inhibitory action. The observation that high concentrations of extracellular ADP enhanced the outward component of the VRAC current in low Cl(-) hypotonic solution and shifted its reversal potential to negative potentials provides more direct evidence for the nucleotide permeability of VRAC. We conclude from these observations that VRAC is a nucleotide-permeable channel, which may serve as a pathway for HTS-induced ATP release in BAEC.

Enhancing effects of binary mixtures of acid with salt on the gustatory neural activity in the clawed toad, Xenopus laevis

Binary mixtures of acid with salt were observed to produce enhancing effects on oral chemoreceptor responses by recording the activity of the whole glossopharyngeal nerve of the clawed toad (Xenopus laevis). The mixtures of HCl with various inorganic salts elicited responses larger than the sum of the responses to individual component chemicals of each mixture. The results indicated that the mixture response was composed of a large HCI response and a negligibly small salt response. The concentration-response curve for HCl shifted toward lower concentrations due to the presence of NaCl without affecting the maximal response, and the Hill constant for the curve was unchanged, suggesting that the binding affinity of protons increases with an increase in the concentration of the mixed NaCl. The enhancing effects of the mixtures of HCl and various monovalent sodium salts having equal ionic strength showed distinct differences among the anion species of the salts. The responses to the mixtures of HCl with typical chloride salts having cations of various valencies depended on the Cl- ion concentration up to 0.1 M, whereas they depended on ionic strength as well, suggesting a possible role of salt cations in the enhancing effects of the mixtures. Thus, the enhancing effects of salts on the acid responses are interpreted mostly in terms of the interaction of the salt anions on the receptor membrane around the receptor sites for acid. The additive role of the cationic activity of the mixed salts, however, is also involved.

Theoretical analysis of fluorescence light shifts in optically pumped cesium beam frequency standards

Shirley's analysis (1985) is generalized to calculate the fluorescence light shifts in various optically pumped Cs beam standards, especially in the two-beam type frequency standard that enables a real-time correction of an end-to-end cavity phase shift. Using tables of light shift coefficients and mean fluorescence photon numbers per atom, fluorescence light shifts in one- and two-beam type optically pumped Cs beam frequency standards using noncycling detection and cycling detection, are evaluated. Results obtained are useful to correct the fluorescence light shifts in optically pumped Cs beam frequency standards.