The present study was undertaken to examine the involvement of descending pain modulatory systems from the brainstem rostral ventromedial medulla (RVM) in modulating visceral hyperalgesia produced by intracolonic instillation of zymosan. Three hours after intracolonic zymosan, the visceromotor response (VMR) to noxious colorectal distension (CRD, 80 mmHg, 20s) was increased significantly. This hyperalgesia was attenuated in a dose-dependent manner by the selective NMDA receptor antagonist APV (10-30 fmol, 1 microl) microinjected into the RVM. The hyperalgesia was also attenuated by intra-RVM administration of the nitric oxide synthase (NOS) inhibitor L-NAME. In support, there was a significant increase in the number of cells in the RVM labeled for NADPH diaphorase (NADPH-d) or neuronal NOS (nNOS) in zymosan-treated rats. In contrast to the effects of APV and L-NAME, administration of the non-NMDA receptor antagonist DNQX into the RVM further enhanced the already facilitated VMR to CRD in zymosan-treated rats. Taken together, these data suggest that zymosan-produced visceral hyperalgesia is influenced by two descending pain modulatory systems: a facilitatory system mediated by activation of NMDA receptors in the RVM and production of nitric oxide, and an inhibitory system mediated by activity at non-NMDA receptors in the RVM. The unmasking of one system by selective blockade of the other suggests simultaneous activation of both by colonic inflammation.

Over the last two decades, glutamate has been established as the main excitatory neurotransmitter in the mammalian brain. Glutamate released from synapses activates ion channel-forming receptors at postsynaptic cells and consequently mediates fast postsynaptic potentials. These receptors are termed ionotropic glutamate receptors (iGluRs). The subsequent discovery of metabotropic glutamate receptors (mGluRs) revealed that glutamate can also mediate slow synaptic potentials, modulate ion channels, and directly couple to GTP binding proteins. In contrast to the iGluRs, the mGluRs possess seven transmembrane domains and a large intracellular C-terminus that involves interactions with a variety of other intracellular signaling systems. Eight functionally distinct mGluR subtypes are known to be localized to specific neuron types at presynaptic and/or postsynaptic membranes. Their physiological functions involve the generation of slow excitatory and inhibitory synaptic potentials, modulation of synaptic transmission, synaptic integration, and plasticity. The classical role of glutamate as a fast excitatory synaptic transmitter was largely extended by mGluRs acting as a neuromodulator and even as an activator of inhibitory mechanisms at certain synapses.

We report an activity-induced green fluorescence signal observed when mouse cerebellar slices were illuminated with blue light and parallel fibre-Purkinje cell synapses were activated. The optical signal consisted of an initial increase in fluorescence that peaked within 1-2 s after the onset of stimulation, followed by a long lasting (40 s) transient decrease in fluorescence. Single or tetanic electrical stimuli applied to the molecular layer elicited 'beam-shaped' fluorescence changes along the trajectory of parallel fibres. These signals reported activation of Purkinje cells as they were depressed by antagonists of ionotropic and metabotropic glutamate receptors at Purkinje cells and correlated with Purkinje cell spiking activity. Optical responses induced by direct pharmacological activation of glutamate receptors were reduced by a calcium-free extracellular medium, consistent with the hypothesis that they reflect metabolic activity due to an increased intracellular calcium load associated with neuronal activation. We used these intrinsic fluorescence signals to address the question of whether granule cells excite Purkinje cells only locally via the ascending branches of their axons, or more widespread along the parallel fibre trajectory. White matter stimulation of the mossy fibres also elicited a beam-like fluorescence change along the trajectory of parallel fibres. Simultaneous imaging and extracellular recording demonstrated the association between the beam-like fluorescence signal and Purkinje cell spiking. This non-invasive imaging technique supports the notion that parallel fibre activity, evoked either locally or through the mossy fibre-granule cell pathway, can activate postsynaptic Purkinje cells along more than 3 mm of the parallel fibre trajectory.

l-Glutamate is the principal excitatory neurotransmitter in the vertebrate central nervous system, where it mediates many of its actions via G-protein-coupled metabotropic glutamate (mGlu) receptors. Since little is known about the dynamics of mGlu receptors at the plasma membrane, we have constructed a fusion protein comprising the mGlu receptor subtype 1alpha (mGlu1alpha) and green fluorescent protein (GFP). Using imaging of Ca2+ release from intracellular stores as a functional assay, the agonist pharmacology of this fluorescently tagged receptor was found to be similar to that of the wild-type receptor when expressed in HEK-293 cells. Receptor movement and function were measured simultaneously by combined imaging of Ca2+, using fura-red, and GFP fluorescence in single cells. Exposure to agonist induced a rapid loss of up to 30% of membrane-associated fluorescence, with a corresponding decrease in the functional response. Following removal of the agonist there was recovery of both the membrane fluorescence and the functional response. These data suggest that the surface expression of G-protein-coupled glutamate receptors might be rapidly regulated in response to agonist activation.

1 The presynaptic interactions between facilitatory b-adrenoreceptors and inhibitory 5-hydroxytryptamine (5-HT) receptors modulating glutamate release from cerebrocortical nerve terminals were examined. 2 4-Aminopyridine (4-AP, 1 mM)-evoked glutamate release was facilitated by the membrane permeant cyclic-3',5'-adenosine monophosphate (cAMP) analogue, 8-bromo-cAMP (8-Br-cAMP), used to directly activate cAMP-dependent protein kinase (PKA). 3 The b-adrenoreceptor agonist, isoprenaline (ISO), e ected a concentration-dependent potentiation of 4-AP-evoked glutamate release which was abolished by the b-adrenoreceptor antagonist, propranolol, and the PKA inhibitor, Rp-cyclic-3',5'-adenosine-monophosphothioate (Rp-cAMPS). 4 5-HT receptor activation by 100 mM 5-HT produced an inhibition of 4-AP-evoked glutamate release in nerve terminals. The inhibitory e ect of 5-HT could be mimicked by the selective 5-HT 1A receptor agonist, 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and antagonized by 1-(2-methoxyphenyl)-4-(4-phthalimidobutyl)piperazine . 5 When 5-HT (or 8-OH-DPAT) was used in conjunction with ISO or 8-Br-cAMP, the badrenoreceptor-and PKA-mediated potentiation of glutamate release was abrogated. 6 The inhibitory crosstalk of 5-HT 1A receptors to b-adrenoceptor-mediated facilitation of glutamate release was abolished in the presence of . 7 Examination of voltage-dependent Ca 2+ in¯ux revealed that, while ISO and 5-HT alone caused a respective potentiation and diminution of the 4-AP-evoked increase in [Ca 2+ ] c , the co-presence of 5-HT abolished the ISO mediated potentiation of Ca 2+ in¯ux. 8 Together, these results suggest that b-adrenoreceptors and 5-HT 1A receptors coexist on the cerebrocortical nerve terminals and that the cross-talk between the two receptor signalling pathways occurs at a locus downstream from cAMP production, possibly at the level of voltage-dependent Ca 2+ in¯ux.