2DandE). == Localization of Synaptic Proteins and Vesicles in Receptor Axons. be inhibited by bafilomycin A1 and botulinum toxin A. Ca2+transients in OECs evoked by ATP, axonal stimulation, and laser photolysis of NP-EGTA resulted in constriction of adjacent blood vessels. Our results indicate that ATP and glutamate Rhosin hydrochloride are released ectopically by vesicles along axons and mediate neurovascular coupling via glial Ca2+signaling. Keywords:neuronglia interactions, olfactory bulb, olfactory ensheathing cells, purinergic signaling In the conventional view, a neuron is divided into morphologically and functionally distinct compartments, comprising dendrites receiving synaptic input, axons conducting information via action potentials, and synaptic terminals transmitting information by neurotransmitter release. Recent evidence, however, indicates that neurotransmitters also can be released along axons (e.g., in optic nerve, olfactory nerve and corpus callosum) (14). Axonal neurotransmitter release may include the cotransmitter ATP, which is coreleased with glutamate from axons in optic and olfactory nerves (1,2). However, the mechanisms of this ectopic neurotransmitter release, and in particular the mechanism of neuronal ATP release, are controversial. Early studies suggested vesicular release of ATP at Rhosin hydrochloride varicosities of peripheral nerves such as the vagus and enteric nerves (5). In few instances, vesicular ATP release could Rabbit Polyclonal to FZD4 be demonstrated at central Rhosin hydrochloride synapses (6,7), and a vesicular ATP transporter has been described recently (8). However, other studies question the hypothesis of vesicular ATP release. Release of ATP from peripheral nerves, for example, is affected differently by presynaptic modulation than is the release of vesicular noradrenaline (9), and suppressing vesicular release with botulinum toxin and tetanus toxin does not inhibit ATP release from cholinergic synaptosomes (10,11). Alternative modes of ATP release include pores formed by gap junctional hemichannels or P2X7 purinoceptors (1214). Recent data demonstrate axonal ATP release from cultured neurons through volume-regulated anion channels (VRAC) upon electrical stimulation Rhosin hydrochloride (15). To date, it is unknown which mechanisms mediate ectopic ATP release in axon tracts such as optic and olfactory nerves. The main targets of axonal neurotransmitter release are glial cell receptors, and functional neuronglia interactions often are mediated by these neurotransmitters. In the corpus callosum and optic nerve, for example, glutamate released by axons evokes AMPA receptor-mediated currents in NG2-positive glial precursors (3,4), whereas axonal glutamate and ATP release leads to Ca2+signals in glial cells in optic and olfactory nerves (1,2). However, the function of these Ca2+signals is still obscure. In synaptic regions, transmitter-induced Ca2+signaling in astrocytes links neuronal activity to changes in blood vessel diameter (neurovascular coupling) (16), but whether neurovascular coupling occurs via glial cells other than astrocytes and in brain regions devoid of synapses, such as axon tracts, is not known. Therefore, we used olfactory ensheathing glial cells (OECs), which are closely associated with axons in the nerve layer of the olfactory bulb (17), as a monitor for glutamate and ATP to study (i) the mechanism of ectopic axonal transmitter release and (ii) the possibility of neurovascular coupling in the olfactory nerve via Ca2+signaling in OECs. Our results show that ATP and glutamate release from axons is Ca2+-dependent and is sensitive to impairment of vesicle fusion, suggesting ectopic transmitter release via vesicles. The Ca2+increase in OECs evoked by these neurotransmitters triggered vasoconstriction in the nerve layer, demonstrating that OECs are able to mediate neurovascular coupling. == Results == == Release of ATP and Glutamate from Receptor Axons Stimulates Ca2+Signaling in OECs. == In the superficial layers of the mouse olfactory bulb, Fluo-4 acetoxymethyl ester (Fluo-4 AM) preferentially labels olfactory ensheathing cells (OECs), which can be distinguished readily from other cells by their position in the nerve layer, their size, and their shape (2). Electrical stimulation of receptor axons in the nerve layer (individual pulse length: 1 ms; 20 Hz, 30 V) for 3 s evoked increases in the cytosolic Ca2+concentration in OECs, reflected by an increase in fluorescence of 42.5 3.1% F (n= 49) (Fig. 1A). A single stimulation pulse evoked a Ca2+rise of 30.8 8.0% F in 27% of the cells that responded to a 3-s train (n= 26), indicating that some OECs are able to detect the amount of neurotransmitter that is released during a single compound action potential in the nerve layer (Fig. S1). Stimulation for 3 s elicited inward currents in mitral cells of 806.7 257.1 pA (n= 7); the stimulation-evoked responses in mitral cells and OECs were suppressed in the presence of tetrodotoxin (Fig. S1). After type 1 metabotropic glutamate receptors (mGluR1s) were blocked with 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (CPCCOEt) (100 M) or P2Y1 receptors were blocked with N(6)-methyl-2-deoxyadenosine-3,5-bisphosphate (MRS2179) (60 M) (Fig. S2), the stimulation-induced Ca2+increase in OECs was reduced by 53.3 4.6% (n= 49;P< 0.005) and 57.1 4.1% (n= 33;P< 0.005), respectively (Fig. 1A). When both types of receptors were blocked simultaneously, the stimulation-induced Ca2+transients in OECs were suppressed entirely (n= 64), indicating that glial responses.