Differential contributions of adenosine to hypoxia-evoked depressions of three neuronal pathways in isolated spinal cord of neonatal rats

Background and purpose: Hypoxic effects on neuronal functions drastically vary with experimental conditions, but its mechanism is unclear. Adenosine has been reported to play a key role in depression of neuronal activities in the CNS during acute hypoxia. In this study, we examined the effect of acute hypoxia on different spinal reflex potentials and the contribution of adenosine to them. Experimental approach: Spinal reflex potentials, monosynaptic reflex potential (MSR), slow ventral root potential (sVRP) and dorsal root potential (DRP), were measured in the isolated spinal cord of the neonatal rat. Adenosine release was measured by using enzymatic biosensors. Key results: In the spinal cord preparation isolated from postnatal day 5-8 (P5-8) rats at 27℃, acute hypoxia released adenosine and depressed three reflex potentials. However, in postnatal day 0-3 (P0-3) rats at 27℃, the hypoxic adenosine release and depression of MSR were negligible, while the depression of sVRP and DRP were perceptible responses. In P0-3 rats at 33℃, hypoxia evoked adenosine release and depression of MSR. An adenosine A1 receptor selective antagonist and a high [Ca2+]o, which suppressed adenosine release, abolished the hypoxic depression of MSR but not those of sVRP and DRP. Conclusions and implications: These results indicate that the hypoxic depression of MSR depends on adenosine release, which is highly susceptible to age, temperature and [Ca2+]o. On the other hand, a large part of the depressions of DRP and sVRP were mediated via adenosine-independent mechanisms. This differential contribution of adenosine to depression is suggested to be an important factor for the varying influence of hypoxia on neuronal functions

Zinc modulates primary afferent fiber-evoked responses of ventral roots in neonatal rat spinal cord in vitro

Zinc ions (Zn2+) are known to modulate the functions of a variety of channels, receptors and transporters. We examined the effects of Zn2+ on the reflex potentials evoked by electrical stimulation and responses to depolarizing agents in the isolated spinal cord of the neonatal rat in vitro. Zn2+ at low concentrations (0.5–2μM) inhibited, but at high concentrations (5 and 10μM) augmented, a slow depolarizing component (slow ventral root potential). Zn2+ had no effect on fast components (monosynaptic reflex potential; fast polysynaptic reflex potential). Unlike Zn2+, strychnine (5μM), a glycine receptor antagonist, and (S),9(R)-(−)-bicuculline methobromide (10μM), a GABAA receptor antagonist, potentiated both fast polysynaptic reflex potential and slow ventral root potential. Zn2+ (5μM) did not affect depolarizing responses to glutamate and N-methyl-d-aspartate. Zn2+ enhanced the substance P-evoked depolarization in the absence of tetrodotoxin (0.3μM) but not in its presence. The dorsal root potential was inhibited by (S),9(R)-(−)-bicuculline methobromide (10μM) but not by Zn2+ (5μM). The Zn2+-potentiated slow ventral root potential was inhibited by the N-methyl-d-aspartate receptor antagonists, ketamine (10μM) and dl-2-amino-5-phosphaonovaleric acid (50μM) but not by P2X receptor antagonists, pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid (30μM) and 2′,3′-O-(2,4,6-trinitrophenyl)ATP (10μM). Ketamine (10μM) and dl-2-amino-5-phosphaonovaleric acid (50μM) almost abolished spontaneous activities increased by Zn2+. It is concluded that Zn2+ potentiated slow ventral root potential induced by primary afferent stimulation, which was mediated by the activation of N-methyl-d-aspartate receptors but not by activation of P2X receptors or blockade of glycinergic and GABAergic inhibition. Zn2+ does not seem to directly affect N-methyl-d-aspartate receptors. The release of glutamate from interneurons may play an important role in Zn2+-induced potentiation of slow ventral root potential in the spinal cord of the neonatal rat

Inhibitory effects of dopamine on spinal synaptic transmission via dopamine D1-like receptors in neonatal rats

BACKGROUND AND PURPOSE: Dopamine released from the endings of descending dopaminergic fibre in the spinal cord is suggested to be involved in modulating functions such as locomotion and nociception. Here, we examined the effects of dopamine on spinal synaptic transmissions in rats. EXPERIMENTAL APPROACH: Spinal reflex potentials, monosynaptic reflex potential (MSR) and slow ventral root potential (sVRP), were measured in the isolated spinal cord of the neonatal rat. Dopamine release was measured by using HPLC. KEY RESULTS: Dopamine at lower concentrations ( 1 μM), in addition to a potent sVRP depression, dopamine depolarized baseline potential and slightly depressed MSR. Depression of sVRP by dopamine was partially reversed by dopamine D1-like but not by D2-like receptor antagonists. SKF83959 and SKF81297, D1-like receptor agonists, and methamphetamine, an endogenous dopamine releaser, also caused the inhibition of sVRP. Methamphetamine also depressed MSR, which was inhibited by ketanserin, a 5-HT_[2A/2C] receptor antagonist. Methamphetamine induced the release of dopamine and 5-HT from spinal cords, indicating that the release of endogenous dopamine and 5-HT depresses sVRP and MSR respectively. CONCLUSION AND IMPLICATIONS: These results suggest that dopamine at lower concentrations preferentially inhibits sVRP, which is mediated via D1-like and unidentified receptors. The dopamine-evoked depression is involved in modulating the spinal functions by the descending dopaminergic pathways

Polymodal TRPC signaling: Emerging role in phenotype switching and tissue remodeling

TRPC proteins have been implicated in a large array of Ca2+ signaling processes and are considered as pore-forming subunits of unique polymodal channel sensors. The mechanisms of TRPC activation are so far incompletely understood but appear to involve a concert of signals that are generated typically downstream of receptor-mediated activation of phospholipase C. Specifically for the TRPC1/4/5 subfamily the activating scenario is ill-defined and appears enigmatic due to the observation of multiple modes of activation. TRPC4 was initially described as a store-operated cation channel and was repeatedly proposed as a pivotal element of the store-operated signaling pathways of various tissues. However, classical reconstitution of TRPC4 complexes in expression systems as well as recent knock-down strategies provided evidence against store-dependent regulation of this channel and raised considerable doubt in its proposed prominent role agonist-induced Ca2+ signaling. Recent analysis of the function of TRPC4 in vascular endothelial cells of divergent phenotype revealed a novel aspect of TRPC signaling, extending the current concept of TRPC regulation by a phenotype-dependent switch between Ca2+ transport and a potential intracellular scaffold function of the TRPC protein