Glutamate May Be an Efferent Transmitter That Elicits Inhibition in Mouse Taste Buds

Recent studies suggest that l-glutamate may be an efferent transmitter released from axons innervating taste buds. In this report, we determined the types of ionotropic synaptic glutamate receptors present on taste cells and that underlie this postulated efferent transmission. We also studied what effect glutamate exerts on taste bud function. We isolated mouse taste buds and taste cells, conducted functional imaging using Fura 2, and used cellular biosensors to monitor taste-evoked transmitter release. The findings show that a large fraction of Presynaptic (Type III) taste bud cells (∼50%) respond to 100 µM glutamate, NMDA, or kainic acid (KA) with an increase in intracellular Ca2+. In contrast, Receptor (Type II) taste cells rarely (4%) responded to 100 µM glutamate. At this concentration and with these compounds, these agonists activate glutamatergic synaptic receptors, not glutamate taste (umami) receptors. Moreover, applying glutamate, NMDA, or KA caused taste buds to secrete 5-HT, a Presynaptic taste cell transmitter, but not ATP, a Receptor cell transmitter. Indeed, glutamate-evoked 5-HT release inhibited taste-evoked ATP secretion. The findings are consistent with a role for glutamate in taste buds as an inhibitory efferent transmitter that acts via ionotropic synaptic glutamate receptors

P2 receptor-mediated modulation of neurotransmitter release—an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions

P2 receptors are involved in the mediation of motivation-related behavior

The importance of purinergic signaling in the intact mesolimbic–mesocortical circuit of the brain of freely moving rats is reviewed. In the rat, an endogenous ADP/ATPergic tone reinforces the release of dopamine from the axon terminals in the nucleus accumbens as well as from the somatodendritic region of these neurons in the ventral tegmental area, as well as the release of glutamate, probably via P2Y1 receptor stimulation. Similar mechanisms may regulate the release of glutamate in both areas of the brain. Dopamine and glutamate determine in concert the activity of the accumbal GABAergic, medium-size spiny neurons thought to act as an interface between the limbic cortex and the extrapyramidal motor system. These neurons project to the pallidal and mesencephalic areas, thereby mediating the behavioral reaction of the animal in response to a motivation-related stimulus. There is evidence that extracellular ADP/ATP promotes goal-directed behavior, e.g., intention and feeding, via dopamine, probably via P2Y1 receptor stimulation. Accumbal P2 receptor-mediated glutamatergic mechanisms seem to counteract the dopaminergic effects on behavior. Furthermore, adaptive changes of motivation-related behavior, e.g., by chronic succession of starvation and feeding or by repeated amphetamine administration, are accompanied by changes in the expression of the P2Y1 receptor, thought to modulate the sensitivity of the animal to respond to certain stimuli

A Neuron-Glial Perspective for Computational Neuroscience

International audienceThere is growing excitement around glial cells, as compelling evidence point to new, previously unimaginable roles for these cells in information processing of the brain, with the potential to affect behavior and higher cognitive functions. Among their many possible functions, glial cells could be involved in practically every aspect of the brain physiology in health and disease. As a result, many investigators in the field welcome the notion of a Neuron-Glial paradigm of brain function, as opposed to Ramon y Cayal's more classical neuronal doctrine which identifies neurons as the prominent, if not the only, cells capable of a signaling role in the brain. The demonstration of a brain-wide Neuron-Glial paradigm however remains elusive and so does the notion of what neuron-glial interactions could be functionally relevant for the brain computational tasks. In this perspective, we present a selection of arguments inspired by available experimental and modeling studies with the aim to provide a biophysical and conceptual platform to computational neuroscience no longer as a mere prerogative of neuronal signaling but rather as the outcome of a complex interaction between neurons and glial cells

<it>Echinacea</it>-induced cytosolic Ca²⁺elevation in HEK293

<p>Abstract</p> <p>Background</p> <p>With a traditional medical use for treatment of various ailments, herbal preparations of <it>Echinacea </it>are now popularly used to improve immune responses. One likely mode of action is that alkamides from <it>Echinacea </it>bind to cannabinoid type 2 (CB2) receptors and induce a transient increase in intracellular Ca²⁺. Here, we show that unidentified compounds from <it>Echinacea purpurea </it>induce cytosolic Ca²⁺elevation in non-immune-related cells, which lack CB2 receptors and that the Ca²⁺elevation is not influenced by alkamides.</p> <p>Methods</p> <p>A non-immune human cell line, HEK293, was chosen to evaluate <it>E. purpurea </it>root extracts and constituents as potential regulators of intracellular Ca²⁺levels. Changes in cytosolic Ca²⁺levels were monitored and visualized by intracellular calcium imaging. U73122, a phospholipase C inhibitor, and 2-aminoethoxydiphenyl borate (2-APB), an antagonist of inositol-1,4,5-trisphosphate (IP₃) receptor, were tested to determine the mechanism of this Ca²⁺signaling pathway. <it>E. purpurea </it>root ethanol extracts were fractionated by preparative HPLC, screened for bioactivity on HEK293 cells and by GC-MS for potential constituent(s) responsible for this bioactivity.</p> <p>Results</p> <p>A rapid transient increase in cytosolic Ca²⁺levels occurs when <it>E. purpurea </it>extracts are applied to HEK293 cells. These stimulatory effects are phospholipase C and IP₃receptor dependent. <it>Echinacea</it>-evoked responses could not be blocked by SR 144528, a specific CB2 receptor antagonist, indicating that CB2 is not involved. Ca²⁺elevation is sustained after the <it>Echinacea</it>-induced Ca²⁺release from intracellular Ca²⁺stores; this longer-term effect is abolished by 2-APB, indicating a possible store operated calcium entry involvement. Of 28 HPLC fractions from <it>E. purpurea </it>root extracts, six induce cytosolic Ca²⁺increase. Interestingly, GC-MS analysis of these fractions, as well as treatment of HEK293 cells with known individual and combined chemicals, indicates the components thought to be responsible for the major immunomodulatory bioactivity of <it>Echinacea do not </it>explain the observed Ca²⁺response. Rather, lipophilic constituents of unknown structures are associated with this bioactivity.</p> <p>Conclusions</p> <p>Our data indicate that as yet unidentified constituents from <it>Echinacea </it>stimulate an IP₃receptor and phospholipase C mediation of cytosolic Ca²⁺levels in non-immune mammalian cells. This pathway is distinct from that induced in immune associated cells via the CB2 receptor.</p