Biomedical subjects

Sukumar Vijayaraghavan

Publications and source records attributed to Sukumar Vijayaraghavan.

8 recordsLinked to original sources

PubMed2006

ATP increases intracellular calcium in supraoptic neurons by activation of both P2X and P2Y purinergic receptors.

ATP increases intracellular calcium concentration ([Ca(2+)](i)) in supraoptic nucleus (SON) neurons in hypothalamo-neurohypophyseal system explants loaded with the Ca(2+)-sensitive dye, fura 2-AM. Involvement of P2X purinergic receptors (P2XR) in this response was anticipated, because ATP stimulation of vasopressin release from hypothalamo-neurohypophyseal system explants required activation of P2XRs, and activation of P2XRs induced an increase in [Ca(2+)](i) in dissociated SON neurons. However, the ATP-induced increase in [Ca(2+)](i) persisted after removal of Ca(2+) from the perifusate ([Ca(2+)](o)). This suggested involvement of P2Y purinergic receptors (P2YR), because P2YRs induce Ca(2+) release from intracellular stores, whereas P2XRs are Ca(2+)-permeable ion channels. Depletion of [Ca(2+)](i) stores with thapsigargin (TG) prevented the ATP-induced increase in [Ca(2+)](i) in zero, but not in 2 mM [Ca(2+)](o), indicating that both Ca(2+) influx and release of intracellular Ca(2+) contribute to the ATP response. Ca(2+) influx was partially blocked by cadmium, indicating a contribution of voltage-gated Ca(2+) channels. PPADS (pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid), and iso-PPADS, P2XR antagonists, attenuated, but did not abolish, the ATP-induced increase in [Ca(2+)](i). Combined treatment with PPADS or iso-PPADS and TG prevented the response. A cocktail of P2YR agonists consisting of UTP, UDP, and 2-methylthio-ADP increased [Ca(2+)](i) (with or without tetrodotoxin) that was markedly attenuated by TG. 2-Methylthio-ADP alone induced consistent and larger increases in [Ca(2+)](i) than UTP or UDP. MRS2179, a specific P2Y(1)R antagonist, eliminated the response to ATP in zero [Ca(2+)](o). Thus, both P2XR and P2YR participate in the ATP-induced increase in [Ca(2+)](i), and the P2Y(1)R subtype is more prominent than P2Y(2)R, P2Y(4)R, or P2Y(6)R in SON.

Adenosine Diphosphate↗

PubMed2006

Simultaneous exposure to ATP and phenylephrine induces a sustained elevation in the intracellular calcium concentration in supraoptic neurons.

Vasopressin (VP) release from the hypothalamo-neurohypophyseal system (HNS) is stimulated by ATP activation of P2X purinergic receptors and by activation of 1-adrenergic receptors by phenylephrine (PE). These responses are potentiated by simultaneous exposure to ATP+PE. Potentiation was blocked by depleting intracellular calcium stores with thapsigargin. To test the hypothesis that the synergistic response to ATP+PE reflects alterations in the intracellular calcium concentration ([Ca2+]i), [Ca2+]i was monitored in supraoptic neurons in HNS explants loaded with fura 2-AM. Both ATP and PE induced rapid, but transient, elevations in [Ca2+]i. In 0.3 mM Ca2+, the peak response to ATP was greater than to PE but did not differ from the peak response to ATP+PE. A sustained elevation in [Ca2+]i was induced by ATP+PE, that was greater than ATP or PE alone. In 2 mM Ca2+, the peak response to ATP+PE was significantly greater than to either ATP or PE alone, and the sustained response to ATP+PE was greater than to either agent alone. Responses were comparable in the presence of TTX. The sustained elevation in [Ca2+]i was also observed when ATP+PE was removed after 1 min, but it was eliminated by either thapsigargin or removing external calcium, indicating that both calcium influx and calcium release from internal stores are required. Some cells were vasopressinergic based on a VP-induced increase in [Ca2+]i. These observations support the hypothesis that simultaneous exposure to ATP+PE induces a different pattern of [Ca2+]i than either agent alone that may initiate events leading to synergistic stimulation of VP release.

Adenosine Triphosphate↗

PubMed2005

Store calcium mediates cholinergic effects on mIPSCs in the rat main olfactory bulb.

The significance of endoplasmic reticulum (ER) store calcium in modulating transmitter release is slowly gaining recognition. One transmitter system that might play an important role in store calcium modulation of transmitter release in the CNS is acetylcholine (ACh). The main olfactory bulb (OB) receives rich cholinergic innervation from the horizontal limb of the diagonal band of Broca and blocking cholinergic signaling in the bulb inhibits the ability of animals to discriminate between closely related odors. Here we show that exposing OB slices to carbamylcholine (CCh), a hydrolysis-resistant analog of Ach, increases gamma-aminobutyric acid (GABA) release at dendrodendritic synapses onto the mitral cells. This increase in transmitter release is mediated by the activation of the M1 class of muscarinic receptors and requires the mobilization of calcium from the ER. The site of action of CCh for this effect is developmentally regulated. In animals younger than postnatal day 10, the major action of CCh appears to be on mitral cells, enhancing GABA release by reciprocal signaling resulting from increased glutamate release from mitral cells. In animals older than postnatal day 10, CCh appears to modulate transmitter release from dendrites of the interneurons themselves. Our results point to modulation of inhibition as an important role for cholinergic signaling in the OB. Our data also strengthen the emerging idea of a role for store calcium in modulating transmitter release at CNS synapses.

Acetylcholine↗

PubMed2004

Reduction of all-trans retinal to all-trans retinol in the outer segments of frog and mouse rod photoreceptors.

The first step in the Visual Cycle, the series of reactions that regenerate the vertebrate visual pigment rhodopsin, is the reduction of all-trans retinal to all-trans retinol, a reaction that requires NADPH. We have used the fluorescence of all-trans retinol to study this reduction in living rod photoreceptors. After the bleaching of rhodopsin, fluorescence (excitation, 360 nm; emission, 457 or 540 nm) appears in frog and wild-type mouse rod outer segments reaching a maximum in 30-60 min at room temperature. With this excitation and emission, the mitochondrial-rich ellipsoid region of the cells shows strong fluorescence as well. Fluorescence measurements at different emission wavelengths establish that the outer segment and ellipsoid signals originate from all-trans retinol and reduced pyridine nucleotides, respectively. Using outer segment fluorescence as a measure of all-trans retinol formation, we find that in frog rod photoreceptors the NADPH necessary for the reduction of all-trans retinal can be supplied by both cytoplasmic and mitochondrial metabolic pathways. Inhibition of the reduction reaction, either by retinoic acid or through suppression of metabolic activity, reduced the formation of retinol. Finally, there are no significant fluorescence changes after bleaching in the rod outer segments of Rpe65(-/-) mice, which lack 11-cis retinal.

Animals↗

PubMed2003

Modulation of presynaptic store calcium induces release of glutamate and postsynaptic firing.

Action potential-independent transmitter release is random and produces small depolarizations in the postsynaptic neuron. This process is, therefore, not thought to play a significant role in impulse propagation across synapses. Here we show that calcium flux through presynaptic neuronal nicotinic receptors leads to mobilization of store calcium by calcium-induced calcium release. Recruitment of store calcium induces vesicular release of glutamate in a manner consistent with synchronization across multiple active zones in the CA3 region of the rat hippocampus. This modulation of action potential-independent release of glutamate is sufficient to drive the postsynaptic pyramidal cell above its firing threshold, thus providing a mechanism for impulse propagation.

Action Potentials↗

PubMed2002

Nicotinic receptors mediate changes in spinal motoneuron development and axonal pathfinding in embryonic zebrafish exposed to nicotine.

We show that transient exposure of embryonic zebrafish to nicotine delays the development of secondary spinal motoneurons. Furthermore, there is a long-lasting alteration in axonal pathfinding in secondary motoneurons that is not ameliorated by drug withdrawal. These effects of nicotine were reversed by mammalian nicotinic receptor antagonists. Coupled with these changes is a long-term alteration in swimming behavior. Our results show that transient embryonic exposure to nicotine leads to long-lasting effects on the vertebrate nervous system. These results also demonstrate that the zebrafish is a useful model to examine the effects of nicotine specifically, and drugs of abuse in general, on the development of the CNS in vertebrates.

Animals↗

PubMed2002

Calcium rise in cultured neurons from medial septum elicits calcium waves in surrounding glial cells.

One prerequisite for understanding the physiological relevance of intercellular calcium waves in glia is the examination of mechanisms that trigger these waves. Here, we show that stimulation of cultured septal neurons to produce a large and sustained calcium rise in the soma can initiate calcium waves in surrounding glial cells. The initiation of calcium waves is dependent on calcium influx through voltage-gated calcium channels on the neuron. The waves are not due to direct stimulation of the glial cells or to loss of neuronal membrane integrity. Mechanism of wave initiation is distinct from that involved in wave propagation and does not involve glutamate or acetylcholine release. Communication via gap junctions, or nitric oxide production, is not involved in the initial signaling between a stimulated neuron and the surrounding astrocytes. Suramin, a blocker of P2 receptors blocked the waves but failed to abolish the responses in glial cells immediately surrounding the stimulated neuron. Our results suggest that patterns of calcium rises on neurons, like those seen in glutamate cytotoxicity, can cause calcium waves in surrounding glial cells.

Acetylcholine↗

PubMed2002

Nicotinic receptor signaling in nonexcitable cells.

The finding that neuronal nicotinic acetylcholine receptors (nAChRs) are present in non-neuronal cells both within and outside the nervous system raises some interesting issues. The mechanisms underlying receptor signaling and its downstream consequences in these cells remain to be elucidated. Factors controlling the release of acetylcholine and the extent of its diffusion are likely to be different for these cells than for traditional neuronal synapses. Recent advances on the physiologic functions of some of these cell types have provided a better insight into possible functional roles for nAChRs in nonexcitable cells. The presence of nAChRs on these cells also implies a broader scope for the actions of nicotine that needs to be considered from a clinical viewpoint. Revealing the potential physiologic roles for nAChRs on nonexcitable cells is likely to provide a more complete understanding of cholinergic signaling.

Animals↗