Junctional versus Extrajunctional Glycine and GABA_AReceptor-Mediated IPSCs in Identified Lamina I Neurons of the Adult Rat Spinal Cord

Abstract

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

Glycine Receptors in CNS Neurons as a Target for Nonretrograde Action of Cannabinoids

“…Glycinergic synapses are found in many CNS regions, including the spinal cord, brainstem, and cerebellum (Jonas et al, 1998;Chery and de Koninck, 1999;O'Brien and Berger, 1999), where they play a well established role in the processing of motor and sensory information that controls movement, vision, and audition (Aprison, 1990;Moss and Smart, 2001). Our results suggest that this transmission can be directly modulated by endogenously released cannabinoids.…”

Integration Time in a Subset of Spinal Lamina I Neurons Is Lengthened by Sodium and Calcium Currents Acting Synergistically to Prolong Subthreshold Depolarization

“…Spontaneous synaptic events were detected and analyzed as described previously (Chéry and De Koninck, 1999). In all cells included for synaptic analysis, both EPSPs and EPSCs were recorded (in current and voltage clamp, respectively) to allow direct correlation between kinetics within the same cell.…”

“…Specifically, spontaneous EPSPs in tonic cells were significantly longer than EPSPs in other cells types, more than could be accounted for by differences in membrane or by differences in EPSP rise times; this latter point rules out kinetic differences arising from differences in the site of EPSP origin (i.e., distance from soma) and, in turn, argues against any differential capacity to space-clamp EPSCs depending on cell type. Furthermore, previous investigation specifically tested space-clamp issues in lamina I neurons and revealed these cells to be electrotonically compact (Chéry and De Koninck, 1999). Thus, we began the current study by investigating the basis for the unexplained differences in EPSP kinetics.…”

Inflammatory Pain Upregulates Spinal Inhibition via Endogenous Neurosteroid Production

“…The properties of mIPSCs in lamina II of lumbar spinal cord slices has recently been extensively characterized (Chéry and De Koninck, 1999;Keller et al, 2001Ahmadi et al, 2002). In the adult, lamina II neurons display only fast-decaying strychnine-sensitive glycine receptor (GlyR) mIPSCs and slow-decaying bicuculline-sensitive GABA A R mIPSCs.…”