Satoko Uemura

Actions of a novel water-soluble benzodiazepine-receptor agonist JM-1232(-) on synaptic transmission in adult rat spinal substantia gelatinosa neurons.

Although the intrathecal administration of JM-1232(-) reportedly produces antinociception, this action has not yet been examined at the cellular level. We examined the action of JM-1232(-) on synaptic transmission in spinal substantia gelatinosa (SG) neurons which play an important role in regulating nociceptive transmission from the periphery. The whole-cell patch-clamp technique was applied to the SG neurons of adult rat spinal cord slices. Bath-applied JM-1232(-) prolonged the decay phase of GABA(A)-receptor mediated spontaneous inhibitory postsynaptic current (sIPSC) and increased its frequency without a change in amplitude. The former but not latter action was sensitive to a benzodiazepine-receptor antagonist flumazenil. JM-1232(-) also increased glycinergic sIPSC frequency with no change in amplitude and decay phase. On the other hand, glutamatergic spontaneous excitatory transmission was unaffected by JM-1232(-). These results indicate that JM-1232(-) enhances inhibitory transmission by (1) prolonging the decay phase of GABAergic sIPSC through benzodiazepine-receptor activation and by (2) increasing the spontaneous release of GABA and glycine from nerve terminals without its activation. This enhancement could contribute to at least a part of the antinociceptive effect of intrathecally-administered JM-1232(-).

Oxytocin produces a membrane depolarization in adult rat spinal substantia gelatinosa neurons

The development of pain after peripheral nerve and tissue injury involves not only neuronal pathways but also glia. However, uncertainty still remains as to the relative contribution of different types of glial cells in the development of the pain-related enhanced response states. We examined the contribution of glial cells to the central sensitization in the rat spinal dorsal horn which is induced in neuropathic pain and inflammatory pain. In rats subjected to neuropathic pain, the immunoreactivity (IR) of microglial marker OX 42 was largely increased. In rats subjected to inflammatory pain, IR of astosytes marker GFAP was slightly increased. The optically-recorded neuronal excitation induced by single-pulse stimulation to the dorsal root was augmented in rats subjected to neuropathic pain and rats subjected to inflammatory pain by comparison to control rats. The bath application of a microglial inhibitor minocycline and a p38 mitogen-activated protein kinase inhibitor SB203580 inhibited the neuronal excitation in rats subjected to neuropathic pain, but not in control and rats subjected to inflammatory pain. PPADS slightly inhibited the neuronal excitation in all group. The additional perfusion of TNP-ATP in PPADS largely inhibited the neuronal excitation in rats subjected to neuropathic pain. In contrast, an astroglial toxin L-alpha-aminoadipate and a gap junction blocker carbenoxolone inhibited the neuronal excitation in rats subjected to inflammatory pain, but not in control and rats subjected to neuropathic pain. The larger number of cells in the spinal cord slice taken from rats subjected to neuropathic pain showed the Ca2+ signal by puff application of ATP to comparison with control and rats subjected to inflammatory pain. The Ca2+ signal was inhibited by minocycline and TNP-ATP. Research fund: KAKENHI22600005.

Opioids reduce the peak amplitude of action potential in adult rat dorsal root ganglion neurons

GABA and glycine are important inhibitory neurotransmitters in the central nervous system and are loaded into synaptic vesicles via the vesicular GABA transporter (VGAT). Due to the evidence linking alterations in GABAergic and/or glycinergic neurotransmission to various pain disorders, we investigated the possible influence of downregulation of VGAT on pain threshold in mice. The phenotypes of heterozygous VGAT knockout (VGAT+/−) mice were compared to wild-type (WT) mice using behavioral and electrophysiological assays. Western blot analysis showed significant reduction of VGAT protein levels in VGAT+/− mice brain (47.8% of WT mice) and spinal cord (61.0% of WT mice). However, HPLC revealed that glutamate, GABA, and glycine contents in the whole brain and spinal cord were no changed in VGAT+/− mice. Although behavioral analysis of VGAT+/− mice showed unchanged motor coordination, anxiety, memory performance and anesthetic sensitivity to propofol and ketamine, thermal nociception and inflammatory pain were enhanced. Patch-clamp recordings revealed that the frequency and amplitude of glycinergic mIPSCs in dorsal horn neurons were reduced in VGAT+/− mice. Genotype differences in glycinergic mIPSCs were more evident during sustained stimulation by high potassium solutions. It seemed that the estimated size of the readily releasable pool (RRP) of glycine-containing vesicles was reduced in VGAT+/− mice. Taken together, our results provide genetic, behavioral and electrophysiological evidence that reduction of VGAT-mediated inhibitory drive in the spinal cord alters very specific forms of pain processing. Research fund: KAKENHI (40447264).