Dopamine D1-like receptors, composed of D1 and D5 receptors, have been documented to modulate glutamate-mediated fast excitatory synaptic neurotransmission. Here, we report that dopamine D1 receptors modulate NMDA glutamate receptor-mediated functions through direct protein-protein interactions. Two regions in the D1 receptor carboxyl tail can directly and selectively couple to NMDA glutamate receptor subunits NR1-1a and NR2A. While one interaction is involved in the inhibition of NMDA receptor-gated currents, the other is implicated in the attenuation of NMDA receptor-mediated excitotoxicity through a PI-3 kinase-dependent pathway.

The spinal dorsal horn is the first level of the CNS in which nociceptive input from sensory afferents is integrated and transmitted. Although inhibitory control in this region has a crucial impact on pain transmission, the respective contribution of GABA and glycine to this inhibition remains elusive. We have previously documented co-release of GABA and glycine at the same inhibitory synapse in spinal laminas I-II of adult rats [older than postnatal day 30 (P30)]. However, despite this co-release, individual miniature inhibitory postsynaptic currents (mIPSCs) were mediated by either glycine receptors (GlyR) or GABA(A) receptors (GABA(A)R), yet never by the two together. In contrast, recent studies of ventral horn immature inhibitory synapses (

Colocalization of GABA and glycine in synaptic terminals of the superficial dorsal horn raises the question of their relative contribution to inhibition of different classes of neurons in this area. To address this issue, miniature IPSCs (mIPSCs) mediated via GABA A receptors (GABA A Rs) and glycine receptors (GlyRs) were recorded from identified laminae I-II neurons in adult rat spinal cord slices. GABA A R-mediated mIPSCs had similar amplitude and rise times, but significantly slower decay kinetics than GlyR-mediated mIPSCs. Lamina I neurons appeared to receive almost exclusively GlyR-mediated mIPSCs, even after application of hypertonic solutions. Yet, all neurons responded to exogenous applications of both GABA and glycine, indicating that they expressed both GABA A Rs and GlyRs. Given that virtually all glycinergic interneurons also contain GABA, the possibility was examined that GABA A Rs may be located extrasynaptically in lamina I neurons. A slow GABA A R-mediated component was revealed in large, but not minimally evoked monosynaptic IPSCs. Administration of the benzodiazepine flunitrazepam unmasked a GABA A R component to most mIPSCs, suggesting that both transmitters were released from the same vesicle. The isolated GABA A R component of these mIPSCs had rising kinetics 10 times slower than that of the GlyR component (or of GABA A R mIPSCs in lamina II). The slow GABA A R components were prolonged by GABA uptake blockers.It is concluded that, whereas GABA and glycine are likely released from the same vesicle of transmitter in lamina I, GABA A Rs appear to be located extrasynaptically. Thus, glycine mediates most of the tonic inhibition at these synapses. This differential distribution of GABA A Rs and GlyRs confers distinct functional properties to inhibition mediated by these two transmitters in lamina I. Key words: dorsal horn; substantia gelatinosa; nociception; miniature IPSCs; slice; inhibitionThe superficial laminae I and II of the dorsal horn play a pivotal role in the integration and relay of pain-related information (Perl, 1984;Willis, 1985;Light, 1992;Craig, 1996), and thus elucidating the nature of inhibitory control in this area is crucial for our understanding of nociceptive processing. Both GABA and glycine function as inhibitory neurotransmitters in the mammalian spinal cord (for review, see Todd and Spike, 1993), and blocking either of these control mechanisms produces a hypersensitivity characteristic of neuropathic pain syndromes (Yaksh, 1989;Sivilotti and Woolf, 1994;Sherman and L oomis, 1996;Sorkin and Puig, 1996). Previous studies report the coexistence of GABA and glycine as well as their respective receptors at many synapses in the superficial dorsal horn of the rat spinal cord (van den Pol

Excitotoxicity mediated by glutamate receptors plays crucial roles in ischemia and other neurodegenerative diseases. Whereas overactivation of ionotropic glutamate receptors is neurotoxic, the role of metabotropic glutamate receptors (mGluRs), and especially mGluR1, remains equivocal. Here we report that activation of NMDA receptors results in calpain-mediated truncation of the C-terminal domain of mGluR1alpha at Ser(936). The truncated mGluR1alpha maintains its ability to increase cytosolic calcium while it no longer activates the neuroprotective PI(3)K-Akt signaling pathways. Full-length and truncated forms of mGluR1alpha play distinct roles in excitotoxic neuronal degeneration in cultured neurons. A fusion peptide derived from the calpain cleavage site of mGluR1alpha efficiently blocks NMDA-induced truncation of mGluR1alpha in primary neuronal cultures and exhibits neuroprotection against excitotoxicity both in vitro and in vivo. These findings shed light on the relationship between NMDA and mGluR1alpha and indicate the existence of a positive feedback regulation in excitotoxicity involving calpain and mGluR1alpha.