The receptors for mammalian sweet and umami taste

Sweet and umami (the taste of monosodium glutamate) are the main attractive taste modalities in humans. T1Rs are candidate mammalian taste receptors that combine to assemble two heteromeric G-protein-coupled receptor complexes: T1R1+3, an umami sensor, and T1R2+3, a sweet receptor. We now report the behavioral and physiological characterization of T1R1, T1R2, and T1R3 knockout mice. We demonstrate that sweet and umami taste are strictly dependent on T1R-receptors, and show that selective elimination of T1R-subunits differentially abolishes detection and perception of these two taste modalities. To examine the basis of sweet tastant recognition and coding, we engineered animals expressing either the human T1R2-receptor (hT1R2), or a modified opioid-receptor (RASSL) in sweet cells. Expression of hT1R2 in mice generates animals with humanized sweet taste preferences, while expression of RASSL drives strong attraction to a synthetic opiate, demonstrating that sweet cells trigger dedicated behavioral outputs, but their tastant selectivity is determined by the nature of the receptors.

The neuropeptide Y/agouti gene-related protein (AGRP) brain circuitry in normal, anorectic, and monosodium glutamate-treated mice

Neuropeptide Y (NPY) and the endogenous melanocortin receptor antagonist, agouti gene-related protein (AGRP), coexist in the arcuate nucleus, and both exert orexigenic effects. The present study aimed primarily at determining the brain distribution of AGRP. AGRP mRNA-expressing cells were limited to the arcuate nucleus, representing a major subpopulation (95%) of the NPY neurons, which also was confirmed with immunohistochemistry. AGRP-immunoreactive (-ir) terminals all contained NPY and were observed in many brain regions extending from the rostral telencephalon to the pons, including the parabrachial nucleus. NPY-positive, AGRP-negative terminals were observed in many areas. AGRP-ir terminals were reduced dramatically in all brain regions of mice treated neonatally with monosodium glutamate as well as of mice homozygous for the anorexia mutation. Terminals immunoreactive for the melanocortin peptide alpha-melanocyte-stimulating hormone formed a population separate from, but parallel to, the AGRP-ir terminals. Our results show that arcuate NPY neurons, identified by the presence of AGRP, project more extensively in the brain than previously known and indicate that the feeding regulatory actions of NPY may extend beyond the hypothalamus.

Descending modulation of pain

Although interest in descending modulation of spinal cord function dates back to the time of Sherrington, the modern era began in the late 1960s when it was shown that focal electrical stimulation in the midbrain of the rat produced analgesia sufficient to permit surgery. From this report evolved the concept of endogenous systems of pain modulation. Initial interest focused on descending inhibition of spinal nociceptive processing, but we now know that descending modulation of spinal nociceptive processing can be either inhibitory or facilitatory. As our understanding of descending facilitatory, or pro-nociceptive influences grows, so too has our appreciation of its potential importance. Accumulating evidence suggests that descending facilitatory influences may contribute to the development and maintenance of hyperalgesia and thus contribute to chronic pain states.

Taste-olfactory convergence, and the representation of the pleasantness of flavour, in the human brain

The functional architecture of the central taste and olfactory systems in primates provides evidence that the convergence of taste and smell information onto single neurons is realized in the caudal orbitofrontal cortex (and immediately adjacent agranular insula). These higher-order association cortical areas thus support flavour processing. Much less is known, however, about homologous regions in the human cortex, or how taste-odour interactions, and thus flavour perception, are implemented in the human brain. We performed an event-related fMRI study to investigate where in the human brain these interactions between taste and odour stimuli (administered retronasally) may be realized. The brain regions that were activated by both taste and smell included parts of the caudal orbitofrontal cortex, amygdala, insular cortex and adjoining areas, and anterior cingulate cortex. It was shown that a small part of the anterior (putatively agranular) insula responds to unimodal taste and to unimodal olfactory stimuli, and that a part of the anterior frontal operculum is a unimodal taste area (putatively primary taste cortex) not activated by olfactory stimuli. Activations to combined olfactory and taste stimuli where there was little or no activation to either alone (providing positive evidence for interactions between the olfactory and taste inputs) were found in a lateral anterior part of the orbitofrontal cortex. Correlations with consonance ratings for the smell and taste combinations, and for their pleasantness, were found in a medial anterior part of the orbitofrontal cortex. These results provide evidence on the neural substrate for the convergence of taste and olfactory stimuli to produce flavour in humans, and where the pleasantness of flavour is represented in the human brain.

Different functional roles of T1R subunits in the heteromeric taste receptors

The T1R receptors, a family of taste-specific class C G protein-coupled receptors, mediate mammalian sweet and umami tastes. The structure-function relationships of T1R receptors remain largely unknown. In this study, we demonstrate the different functional roles of T1R extracellular and transmembrane domains in ligand recognition and G protein coupling. Similar to other family C G protein-coupled receptors, the N-terminal Venus flytrap domain of T1R2 is required for recognizing sweeteners, such as aspartame and neotame. The G protein coupling requires the transmembrane domain of T1R2. Surprisingly, the C-terminal transmembrane domain of T1R3 is required for recognizing sweetener cyclamate and sweet taste inhibitor lactisole. Because T1R3 is the common subunit in the sweet taste receptor and the umami taste receptor, we tested the interaction of lactisole and cyclamate with the umami taste receptor. Lactisole inhibits the activity of the human T1R1/T1R3 receptor, and, as predicted, blocked the umami taste of l-glutamate in human taste tests. Cyclamate does not activate the T1R1/T1R3 receptor by itself, but potentiates the receptor's response to l-glutamate. Taken together, these findings demonstrate the different functional roles of T1R3 and T1R2 and the presence of multiple ligand binding sites on the sweet taste receptor.

Trpm5 null mice respond to bitter, sweet, and umami compounds

Trpm5 is a calcium-activated cation channel expressed selectively in taste receptor cells. A previous study reported that mice with an internal deletion of Trpm5, lacking exons 15-19 encoding transmembrane segments 1-5, showed no taste-mediated responses to bitter, sweet, and umami compounds. We independently generated knockout mice null for Trpm5 protein expression due to deletion of Trpm5's promoter region and exons 1-4 (including the translation start site). We examined the taste-mediated responses of Trpm5 null mice and wild-type (WT) mice using three procedures: gustatory nerve recording [chorda tympani (CT) and glossopharyngeal (NG) nerves], initial lick responses, and 24-h two-bottle preference tests. With bitter compounds, the Trpm5 null mice showed reduced, but not abolished, avoidance (as indicated by licking responses and preference ratios higher than those of WT), a normal CT response, and a greatly diminished NG response. With sweet compounds, Trpm5 null mice showed no licking response, a diminished preference ratio, and absent or greatly reduced nerve responses. With umami compounds, Trpm5 null mice showed no licking response, a diminished preference ratio, a normal NG response, and a greatly diminished CT response. Our results demonstrate that the consequences of eliminating Trmp5 expression vary depending upon the taste quality and the lingual taste field examined. Thus, while Trpm5 is an important factor in many taste responses, its absence does not eliminate all taste responses. We conclude that Trpm5-dependent and Trpm5-independent pathways underlie bitter, sweet, and umami tastes.

Signal transduction and information processing in mammalian taste buds

The molecular machinery for chemosensory transduction in taste buds has received considerable attention within the last decade. Consequently, we now know a great deal about sweet, bitter, and umami taste mechanisms and are gaining ground rapidly on salty and sour transduction. Sweet, bitter, and umami tastes are transduced by G-protein-coupled receptors. Salty taste may be transduced by epithelial Na channels similar to those found in renal tissues. Sour transduction appears to be initiated by intracellular acidification acting on acid-sensitive membrane proteins. Once a taste signal is generated in a taste cell, the subsequent steps involve secretion of neurotransmitters, including ATP and serotonin. It is now recognized that the cells responding to sweet, bitter, and umami taste stimuli do not possess synapses and instead secrete the neurotransmitter ATP via a novel mechanism not involving conventional vesicular exocytosis. ATP is believed to excite primary sensory afferent fibers that convey gustatory signals to the brain. In contrast, taste cells that do have synapses release serotonin in response to gustatory stimulation. The postsynaptic targets of serotonin have not yet been identified. Finally, ATP secreted from receptor cells also acts on neighboring taste cells to stimulate their release of serotonin. This suggests that there is important information processing and signal coding taking place in the mammalian taste bud after gustatory stimulation.

A second look at the barriers of the medial basal hypothalamus

The cell bodies of hypothalamic secretory neurons are localized in areas protected by the blood-brain barrier (BBB), whereas their axon terminals are localized in the median eminence, which lacks a BBB. This implies a complex barrier system, allowing neurons of the central nervous system to secrete into the blood stream without making the BBB leaky. In the present study, three experimental protocols were applied to clarify certain relevant aspects of the barriers operating in the medial basal hypothalamus of the rat. We established that the milieu of the arcuate nucleus is exposed to both the ventricular and the subarachnoidal cerebrospinal fluid (CSF). The median eminence milieu, the perivascular space of the portal vessels, and the subarachnoid space appear to be in open communication; also, beta2-tanycytes establish an efficient barrier between the median eminence milieu and the ventricular CSF. Similarly, beta1-tanycytes establish a lateral barrier, separating the intercellular space of the median eminence from that of the arcuate nucleus. We also found that the glucose transporter I (GLUT I), a BBB marker, is localized throughout the whole plasma membrane of beta1-tanycytes, but is missing from beta2-tanycytes. Expression of GLUT I by tanycytes progressively develops during the first postnatal weeks; while the degree of damage of the arcuate nucleus by administration of monosodium glutamate, at different postnatal intervals, parallels that of the GLUT I immunoreactivity of beta1-tanycytes. An explanation is offered for the selective destruction of the arcuate neurons by the parenteral administration of monosodium glutamate to infant rats.

Hypothalamic cocaine- and amphetamine-regulated transcript (CART) neurons: histochemical relationship to thyrotropin-releasing hormone, melanin-concentrating hormone, orexin/hypocretin and neuropeptide Y

Recent demonstrations of the feeding-inhibitory properties of putative peptides derived from cocaine- and amphetamine-regulated transcript (CART) raise the question of interactions between CART peptides and other messenger molecules implicated in the control of food intake. The present study investigated the histochemical relationship of CART to the neuropeptides thyrotropin-releasing hormone (TRH), melanin-concentrating hormone (MCH), orexin/hypocretin and neuropeptide Y (NPY) in the hypothalamus. Double-label in situ hybridization showed that CART to a great extent is co-expressed with TRH in hypothalamic paraventricular nucleus neurons. This technique was also used to demonstrate that MCH, but not orexin/hypocretin, mRNA colocalized with CART in neurons of the dorsomedial hypothalamic nucleus/lateral hypothalamic area. CART-peptide immunoreactive cell bodies in this region, as well as in the arcuate nucleus and the medial posterodorsal nucleus of the amygdala, were all seen to have close appositions formed by NPY-immunoreactive nerve terminals. Lastly, in a study of mice treated with the neurotoxin, monosodium glutamate, which targets the arcuate nucleus, a near-total ablation of CART peptide immunoreactive cell bodies in this nucleus was accompanied by decreased terminal staining for CART peptide in the paraventricular hypothalamic nucleus, the arcuate nucleus itself and in the dorsomedial hypothalamic nucleus. These findings further define the position of hypothalamic CART neurons within the hierarchy of brain circuitries regulating energy balance, demonstrating the presence of CART peptide in several cell populations that form putative down-stream targets of NPY terminals, including hypophysiotropic TRH neurons and lateral hypothalamic MCH neurons.

N-methyl-D-aspartate-activated channels of mouse central neurones in magnesium-free solutions

1. The whole-cell and outside-out configurations of the patch-clamp method were used to investigate the properties of the channels activated by N-methyl-D-aspartate (NMDA channels) in mouse central neurones in culture. Recording was made in Mg2+-free solutions. 2. In the whole-cell recording mode the currents induced by both NMDA and L-glutamate were accompanied by a large increase in noise. In both cases the noise power spectra were well fitted by single Lorentzian functions and the corresponding mean time constant, tau, was about 6 ms at room temperature. The single-channel conductance, gamma n, estimated from the ratio of the noise variance to the total current, varied between 22 and 40 pS. 3. Endogenous amino acids known to activate NMDA receptors (L-glutamate, L-aspartate, L-cysteine sulphinate and quinolinate) as well as exogenous NMDA agonists such as ibotenate and trans-2,3-piperidine dicarboxylate (trans-PDA) all produced similar responses. In particular, analysis of the current noise yielded tau values between 4 and 8 ms in all cases. 4. NMDA responses were antagonized by 2-amino-5-phosphonovalerate (APV) without any effect on gamma n or tau values measured by noise analysis; NMDA responses were also diminished by D-alpha-aminoadipate and cis-2,3-piperidine dicarboxylate. 5. In outside-out patches, it was observed that the single-channel current amplitude varies linearly as a function of membrane potential between -80 and +60 mV. The reversal potential is near 0 mV. NMDA channels are permeable to Na+, K+ and Cs+, but blocked by choline. The single-channel conductance, gamma e, varies between 40 and 50 pS at room temperature. 6. The NMDA channels open in bursts of short openings interrupted by brief closures. At -60 mV, the closures had a mean duration, tc, of 0.4 +/- 0.2 ms. The mean channel open time, to, was 5.9 +/- 1.0 ms for NMDA and 5.3 +/- 1.7 ms for L-glutamate. The mean burst duration, tb, was 10.5 +/- 0.7 ms for NMDA and 8.5 +/- 2.0 ms for L-glutamate. 7. When the temperature was increased between 14 and 24 degrees C, the NMDA channel conductance increased with a Q10 of 1.6 while the mean open time decreased with a Q10 close to 2. 8. The NMDA channel showed, in addition to the 'main' conductance state (40-50 pS), smaller conductance states of 15 and 35 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Substance P and spinal neurones

When applied by microiontophoresis, substance P (sP) had a strong, but slow and prolonged excitatory action on nearly half the neurones tested in the lumbar spinal cord of cats. Motoneuronal antidromic field potentials only occasionally showed a significant effect of sP. Cerebral cortical neurones in cats and rats were much less readily excited than spinal interneurones. Some unresponsive units showed evidence of a depressant effect of sP. Although sP may have a significant function in central afferent pathways, it is not likely to be a quickly-acting synaptic transmitter.

Neurotransmitters, neuropeptides and binding sites in the rat mediobasal hypothalamus: effects of monosodium glutamate (MSG) lesions

Indirect immunofluorescence histochemistry and receptor autoradiography were used to study the localization of transmitter-/peptide-containing neurons and peptide binding sites in the mediobasal hypothalamus in normal rats and in rats treated neonatally with repeated doses of the neurotoxin monosodium-glutamate (MSG). In the arcuate nucleus, the results showed a virtually complete loss of cell bodies containing immunoreactivity for growth hormone-releasing factor (GRF), galanin (GAL), dynorphin (DYN), enkephalin (ENK), corticotropin-like intermediate peptide (CLIP), neuropeptide Y (NPY), and neuropeptide K (NPK). Tyrosine hydroxylase(TH)-glutamic acid decarboxylase(GAD)-, neurotensin(NT)- and somatostatin(SOM)-immunoreactive (IR) cells were, however, always detected in the ventrally dislocated, dorsomedial division of the arcuate nucleus. In the median eminence, marked decreases in numbers of GAD-, NT-, GAL-, GRF-, DYN-, and ENK-IR fibers were observed. The numbers of TH-, SOM- and NPY-IR fibers were in contrast not or only affected to a very small extent, as revealed with the immunofluorescence technique. Biochemical analysis showed a tendency for MSG to reduce dopamine levels in the median eminence of female rats, whereas no effect was observed in male rats. Autoradiographic studies showed high to moderate NT binding sites, including strong binding over presumably dorsomedial dopamine cells. In MSG-treated rats, there was a marked reduction in GAL binding in the ventromedial nucleus. The findings implicate that most neurons in the ventrolateral and ventromedial arcuate nucleus are sensitive to the toxic effects of MSG, whereas a subpopulation of cells in the dorsomedial division of the arcuate nucleus, including dopamine neurons, are not susceptible to MSG-neurotoxicity. The results indicate, moreover that the very dense TH-IR fiber network in the median eminence predominantly arises from the dorsomedial TH-IR arcuate cells, whereas the GAD-, NT-, GAL-, GRF- and DYN-IR fibers in the median eminence to a large extent arise from the ventrolateral arcuate nucleus. Some ENK- and NPK-positive cells in the arcuate nucleus seem to project to the lateral palisade zone of the median eminence, but most of the ENK-IR fibers in the median eminence, located in the medial palisade zone, seem to primarily originate from an area(s) located outside the arcuate nucleus, presumably the paraventricular nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurotoxin induced nerve cell degeneration: possible involvement of calcium

Neurotoxin induced nerve cell degeneration has been studied in sensory ganglia of newborn and in the area postrema of adult rats following the administration of the selective sensory neurotoxin, capsaicin and the amino acid excitotoxin, glutamic acid, respectively. Light microscopic histochemical, autoradiographic, electroncytochemical and X-ray microanalytical studies revealed that degeneration of certain small-sized, type B primary sensory neurons, induced by capsaicin, was associated with a marked accumulation of calcium predominantly in mitochondria of the damaged ganglion cells. Similarly, monosodium glutamate treatment resulted in the appearance of calcium-containing electron-dense granules in mitochondria of degenerating area postrema neurons. In addition, after a combined administration of 45Ca2+ and capsaicin or monosodium glutamate, significantly higher levels of radioactivity have been detected by liquid scintillation spectroscopy in the Gasserian ganglia and the area postrema, respectively. It is concluded that an enhancement in intracellular calcium level may be intimately involved in the process of neuronal cell death and may represent a common basic mechanism responsible for the development of cellular events leading ultimately to the degeneration of nerve cells.

Amyotrophic lateral sclerosis-linked glutamate transporter mutant has impaired glutamate clearance capacity

We have investigated the functional impact of a naturally occurring mutation of the human glutamate transporter GLT1 (EAAT2), which had been detected in a patient with sporadic amyotrophic lateral sclerosis. The mutation involves a substitution of the putative N-linked glycosylation site asparagine 206 by a serine residue (N206S) and results in reduced glycosylation of the transporter and decreased uptake activity. Electrophysiological analysis of N206S revealed a pronounced reduction in transport rate compared with wild-type, but there was no alteration in the apparent affinities for glutamate and sodium. In addition, no change in the sensitivity for the specific transport inhibitor dihydrokainate was observed. However, the decreased rate of transport was associated with a reduction of the N206S transporter in the plasma membrane. Under ionic conditions, which favor the reverse operation mode of the transporter, N206S exhibited an increased reverse transport capacity. Furthermore, if coexpressed in the same cell, N206S manifested a dominant negative effect on the wild-type GLT1 activity, whereas it did not affect wild-type EAAC1. These findings provide evidence for a role of the N-linked glycosylation in both cellular trafficking and transport function. The resulting alteration in glutamate clearance capacity likely contributes to excitotoxicity that participates in motor neuron degeneration in amyotrophic lateral sclerosis.

The response of individual sympathetic preganglionic neurones to microelectrophoretically applied endogenous monoamines

In anaesthetized cats the effect on antidromically identified single sympathetic preganglionic neurones (SPN) in the third thoracic segment of microelectrophoretically applied monoamines, amino acids and acetyl choline was examined. 5-Hydroxytryptamine (5-HT) creatinine sulphate and bimaleate excited a majority of SPN. A few cells were inhibited by 5-HT creatinine sulphate. These effects were observed on spontaneously active SPN (cardiac and non-cardiac type) and on silent SPN. Noradrenaline, adrenaline and dopamine inhibited all 'types' of SPN, including spontaneously active neurons, silent neurones activated by glutamate or DL-homocysteic acid and neurones synaptically activated by electrically stimulating a brain stem excitatory region. Acetyl choline had no effect on different types of SPN.

Membrane ionic mechanisms activated by noradrenaline in cells isolated from the rabbit portal vein

1. Membrane currents were recorded in cells freshly dispersed from the rabbit portal vein with patch pipette techniques in the whole-cell configuration of recording. In potassium-containing solutions at a holding potential of -50 mV noradrenaline usually evoked an inward current and enhanced greatly the outward current evoked by depolarizing voltage steps. 2. The ionic mechanism of the inward current was investigated in potassium-free solutions in which the inward current elicited by noradrenaline was produced by an increase in membrane conductance. 3. In the first series of experiments NaCl was the main salt in the patch pipette solution (representing the intracellular milieu) and the ionic composition of the bathing solution was altered. In these conditions the reversal potential of the noradrenaline-induced current was close to the chloride equilibrium potential (ECl). 4. When sodium glutamate was the major salt in the pipette solution the reversal potential of the noradrenaline-evoked current was influenced by the cation rather than the anion gradient. 5. With 89 mM-BaCl2 in the external solution (and sodium glutamate in the pipette) noradrenaline produced a membrane current with a reversal potential which was much more positive than ECl or ENa. These results indicate that noradrenaline opens a chloride-selective channel and a cation channel which is permeable to monovalent and divalent cations. 6. Bath application of 10 mM-caffeine evoked a membrane current with a reversal potential close to ECl with either NaCl or sodium glutamate in the pipette. This is interpreted to mean that the increase in membrane chloride conductance can occur as a consequence of a rise in intracellular calcium concentration. It is less evident that the cation channel is calcium activated.

Inhibition of spinal nociceptive transmission from the midbrain, pons and medulla in the rat: activation of descending inhibition by morphine, glutamate and electrical stimulation

It is generally believed that morphine activates a descending system(s) of inhibition, an effect contributing significantly to the analgesia produced. There has arisen, however, considerable controversy on this point. To address whether morphine inhibits spinal nociceptive transmission when given into the brainstem, the effects of focal electrical stimulation and monosodium S-glutamate (Glu) given in the periaqueductal gray (PAG), the locus coeruleus/subcoeruleus (LC/SC) and/or the nucleus raphe magnus (NRM) on spinal unit responses to noxious heating (50 degrees C) of the skin were examined and compared with effects produced by morphine (Mor). Focal electrical stimulation in 46 sites in the midbrain, dorsolateral pons and ventromedial medulla reliably inhibited unit responses to noxious heating of the skin (mean 34% of control). Microinjections of Glu (50 nmol, 0.5 microliter) were made into 17 sites in the midbrain, 10 sites in the LC/SC and 11 sites in the NRM, inhibiting unit responses to a mean 57% at 22 of the 38 sites of microinjection. Mor (10-20 micrograms, 0.5 microliter) was microinjected into 15 sites in the midbrain, 13 sites in the LC/SC and 11 sites in the NRM, inhibiting unit responses to heat to 63% of control at 24 sites of microinjection. The effects of morphine were shown to be receptor specific by antagonism with naloxone administered either intravenously or into the brainstem at the same site of microinjection as morphine. In 31 sites in the midbrain, dorsolateral pons and ventromedial medulla, microinjections of both Mor and Glu into the same sites attenuated unit responses to heating of the skin to a mean 77% and 71% of control, respectively. The results support the hypothesis that Mor acts supraspinally to modulate spinal nociceptive transmission by activating an endogenous descending inhibitory system(s). Focal electrical stimulation, glutamate and morphine modulated spinal nociceptive transmission by activation of descending inhibitory systems whose cell bodies of origin are in the PAG, the LC/SC or the NRM.

Production of gamma-aminobutyric acid by Streptococcus salivarius subsp. thermophilus Y2 under submerged fermentation

Gamma-aminobutyric acid (GABA), a major inhibitory neurotransmitter in the central nervous system, has several well-known physiological functions and has been applied to the production of many drugs and functional foods. The technology of GABA production via submerged fermentation by Streptococcus salivarius subsp. thermophilus Y2 was investigated in this paper. It indicated that the GABA production was related to the biochemical characteristics of glutamate decarboxylase (GAD) of S. salivarius subsp. thermophilus Y2. After 24 h of fermentation at 37 degrees C, which is the suitable culture conditions for GAD-production, then the culture condition were adjusted to the optimal temperature (40 degrees C) and pH (4.5) for the GAD reaction activity in biotransformation of cells and pyridoxal 5'-phosphate (0.02 mmol/l) were added to the broth at the 48 h, the GABA production was increased up to 1.76-fold, reaching 7984.75 +/- 293.33 mg/l. The strain shows great potential use as a starter for GABA-containing yoghurt, cheese and other functional fermented food productions.

Quantitative characterization of ceruleospinal inhibition of nociceptive transmission in the rat

A total of 51 dorsal horn units responsive to heat were isolated and their receptive fields characterized (i.e., response properties and adequate stimuli determined) in pentobarbital-anesthetized, paralyzed rats. In 39 of the 51 units, the descending inhibition of heat-evoked activity produced by focal electrical stimulation in the locus ceruleus/subceruleus (LC/SC) was examined. All units studied responded to mechanical stimulation, to electrical stimulation of the ipsilateral tibial nerve at intensities supramaximal to activate A-alpha, delta- and C-fibers, and to noxious heating (50 degrees C) of the footpad. The cutaneous receptive fields of all units were confined to the glabrous skin of the toes and footpad. All neurons examined were located in the dorsal horn of the spinal cord in laminae I-VI. Tracking experiments established that inhibition of heat-evoked dorsal horn unit activity could be reliably produced by focal electrical stimulation in both the contralateral and ipsilateral LC/SC. The inhibition produced by electrical stimulation in the LC/SC was intensity-, pulse duration-, and frequency-dependent. In six experiments, the efficacy of LC/SC stimulation-produced inhibition of heat-evoked activity was compared using two pulse durations (100 and 400 microseconds); greater inhibition of heat-evoked activity was produced at lower intensities of stimulation at the 400-microseconds pulse duration. In 10 experiments, the frequency of stimulation was varied (25-200 Hz); stimulation at a frequency of 100 Hz resulted in maximal inhibition of heat-evoked activity for stimulation sites both inside (n = 7) and outside (n = 3) the LC/SC. Inhibition of heat-evoked dorsal horn unit activity could be reliably produced by focal electrical stimulation in sites inside the LC/SC (n = 18). Significant descending inhibition of noxious heat-evoked spinal neuronal activity could also be produced by stimulation in pontine sites located outside the LC/SC, however, not as reliably. Systematic electrode tracks were made through the pons, using a searching stimulus of 100 microA, to locate sites medial, lateral, and ventral to the LC/SC from which significant descending inhibition could be produced. Stimulation in 156 sites outside the LC/SC at 100 microA produced inhibition of heat-evoked spinal unit activity to 50% of control or less in only 37 sites. Descending inhibition was characterized quantitatively from 14 of these 37 sites; the mean intensities of stimulation to inhibit heat-evoked activity to 50% of control were experimentally determined, and the mean thresholds of stimulation for inhibition and the mean recruitment indices were calculated.(ABSTRACT TRUNCATED AT 400 WORDS)

Spinal serotonin receptors mediate descending facilitation of a nociceptive reflex from the nuclei reticularis gigantocellularis and gigantocellularis pars alpha in the rat

Electrical stimulation in the nucleus reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) produces facilitation and/or inhibition of spinal nociceptive transmission in behavioral and electrophysiological studies. The present study examined spinal neurotransmitter receptors mediating descending facilitation from the NGC/NGC alpha. As previously demonstrated, electrical stimulation in the NGC/NGC alpha at low intensities (approximately equal to 10 microA) produced facilitation and at greater intensities (approximately equal to 38 microA) inhibition of the tail-flick (TF) reflex. Intrathecal pretreatment with the non-selective serotonin (5-HT) receptor antagonist methysergide attenuated or completely abolished facilitation of the TF reflex produced by electrical stimulation in the NGC/NGC alpha; intrathecal pretreatment with atropine, phentolamine, naloxone or mecamylamine was without effect on stimulation-produced facilitation. Descending inhibition from the NGC/NGC alpha produced by electrical stimulation was attenuated or completely abolished by bilateral transection of the dorsolateral funiculi (DLF) of the cervical spinal cord. Descending facilitation produced by electrical stimulation, however, was unaffected or enhanced following DLF transections. Glutamate microinjections (1.7 nmol/0.17 microliters) into the NGC/NGC alpha produced a rapid, repeatable and short-duration facilitation of the TF reflex in rats with bilateral DLF transections and such facilitation was attenuated by intrathecal pretreatment with methysergide, but not atropine, xylamidine (5-HT2 selective receptor antagonist) or MDL-72222 (5-HT3 selective receptor antagonist). These findings suggest that facilitation of the TF reflex from the activation of the cell bodies in the NGC/NGC alpha is mediated by a descending serotonergic pathway traveling in the ventrolateral funiculi and by spinal 5-HT1 receptors.

Sympathetic nervous system activation by glutamate injections into the paraventricular nucleus

The purpose of the present study was to determine the overall cardiovascular and sympathetic nervous system responses to stimulation of neuronal cell bodies in the paraventricular nucleus (PVN) of the hypothalamus. Bilateral microinjections (50 nl) of monosodium glutamate or sodium acetate were made into the PVN of conscious unrestrained rats. Blood pressure, heart rate and plasma concentrations of norepinephrine and epinephrine were measured. The injection of sodium acetate as an osmotic control was without effect on any of the recorded variables. In contrast, the injections of glutamate were associated with a rapid increase in both blood pressure and heart rate. At doses of 15, 25, and 50 nmol blood pressure increased by 13 +/- 2, 14 +/- 3 and 16 +/- 1 mmHg while heart rate increased by 64 +/- 15, 73 +/- 8 and 50 +/- 8 bpm. These responses were associated with increases in plasma norepinephrine concentrations of 51 +/- 8, 100 +/- 16 and 62 +/- 13 pg/ml while epinephrine concentrations rose by 42 +/- 17, 58 +/- 18 and 38 +/- 17 pg/ml. The responses of glutamate (25 nmol) were not affected by blockade of vascular vasopressin receptors with d(CH2)5Tyr(Me)AVP (10 micrograms/kg) (blood pressure: pre 15 +/- 3 vs post 13 +/- 3 mmHg, heart rate: pre 77 +/- 9 bpm vs post 91 +/- 7 bpm, plasma norepinephrine: pre 106 +/- 22 vs post 121 +/- 28 pg/ml and plasma epinephrine: pre 61 +/- 25 vs post 34 +/- 30 pg/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

A new specific ageusia: some humans cannot taste L-glutamate

A new specific ageusia was found in human subjects for monosodium L-glutamate (MSG). Four tests were successively applied to discriminate non-tasters and hypotasters from tasters. (i) NaCl and MSG thresholds, and (ii) suprathreshold sensitivity were evaluated using the up-and-down procedure. Only 73% of 109 subjects common to both tests demonstrated a sensitivity for MSG significantly higher than their sensitivity to NaCl, and hence a specific sensitivity to L-glutamate. The remaining 27% who showed no significant difference in sensitivity to MSG and NaCl solutions were considered as putative hypotasters. (iii) Perception profiles (time-intensity) for MSG and NaCl were tested in 58 subjects and appeared significantly different in 47 tasters (81%). This technique helped in identifying among putative hypotasters of tests 1 and 2 a few tasters who perceived equal intensity for isoconcentration of NaCl and MSG but who could discriminate isomolar solutions on other cues. Thus, 19% of subjects, for whom no significant differences were found between MSG and NaCl time-intensity profiles, remained in the hypotaster group. (iv) A discrimination task including 24 triangular presentations per subject of NaCl and MSG 29 mM applied to the eight most severe hypotasters showed that two subjects at least (two of 58; 3.5%) could not discriminate between both stimuli. Moreover, these subjects probably perceived identical sensations for MSG and NaCl solutions. The six other hypotasters (10.3%) could discriminate both stimuli at the limit of significance. None of these eight subjects were able to identify the typical umami taste in 29 mM MSG.