Responsiveness of rat substantia gelatinosa neurones to mechanical but not thermal stimuli revealed by in vivo patch-clamp recording

Autor: Eiichi Kumamoto, Megumu Yoshimura, Hidemasa Furue, Keita Narikawa
Rok vydání: 1999
Předmět:
Zdroj: The Journal of physiology.
ISSN: 0022-3751
Popis: Synaptic responses of 46 substantia gelatinosa (SG) neurones in the spinal dorsal horn to cutaneous mechanical and/or thermal stimuli were investigated in an in vivo rat preparation with whole-cell patch-clamp recordings. The clamped neurones were identified as being in the SG based on either their morphological features by intrasomatic injection of biocytin or the depth of the neurones from the surface of the spinal cord. In all SG neurones examined where spontaneous EPSCs occurred, pinch (noxious) and air (innocuous) stimuli applied to the ipsilateral hindlimb elicited a barrage of EPSCs (some of which initiated an action potential under current-clamp conditions), which subsided just after cessation of the stimuli without any residual slow current (or after-discharge). The spontaneous and evoked EPSCs were reversibly abolished by a non-N-methyl-D-aspartate (non-NMDA) receptor antagonist, CNQX (20 μm). Noxious (≥ 45 °C) or innocuous (≤ 40 °C) thermal stimuli did not elicit any synaptic responses in all 18 SG neurones tested which were sensitive to mechanical stimuli. Noxious cold stimulation (≤ 10 °C) also failed to produce any responses (n = 6). It is concluded that both noxious and innocuous mechanical information to SG neurones are transmitted primarily by activation of non-NMDA receptors, probably without any involvement of slow synaptic transmission, and that thermal information is conveyed to areas of the dorsal horn other than SG. Substantia gelatinosa (SG) neurones in the dorsal horn (lamina II of Rexed; Rexed, 1952) of the spinal cord are of great interest because of their possible role in the modulation of nociceptive transmission from the periphery to the CNS (Cervero & Iggo, 1980; Brown, 1982). Primary afferent inputs to the SG are thought to derive both from high-threshold mechanoreceptive afferents conducting in the Aδ-fibre range and from polymodal nociceptors conducting in the C-fibre range (Light & Perl, 1979; Sugiura et al. 1986); these central terminals contain various neurotransmitters, e.g. L-glutamate (Greenamyre et al. 1984) and substance P (Hokfelt et al. 1977; see Willis & Coggeshall, 1991, for review). Microelectrode and patch-clamp techniques have been adapted to SG neurones in a spinal cord slice with an attached dorsal root to investigate synaptic responses to peripheral nerve stimulation (Schneider & Perl, 1988; Yoshimura & Jessell, 1989; Randic et al. 1993; Yoshimura & Nishi, 1993; Baba et al. 1994; Jeftinija & Urban, 1994; Yajiri et al. 1997; Yang et al. 1999; Nakatsuka et al. 1999). These studies revealed that SG neurones exhibit a variety of excitatory and inhibitory synaptic responses which range in duration from milliseconds to minutes (see Yoshimura, 1996, for review). It remains, however, to be settled what kinds of stimulation applied to the skin elicit these responses. In vivo intracellular recordings from SG neurones have been used to measure synaptic responses evoked by cutaneous stimuli (Bennett et al. 1980; Woolf & Fitzgerald, 1983; Light & Kavookjian, 1988). However, attempts to record synaptic activities from them have rarely been sufficiently stable for prolonged investigation because of their small size (∼10 μm in diameter; Woolf & Fitzgerald, 1983) and also because of spinal cord pulsations caused by respiration and heart beat. Furthermore, a limitation of microelectrode methods is the trade-off between the small electrode-tip size needed to impale neurones and the low resistance needed to pass currents through the microelectrode. This compromise is largely avoided by the whole-cell patch-clamp technique (Sakmann & Neher, 1983). In the present study, we have developed a method for making whole-cell recordings from SG neurones in vivo in order to analyse functional synaptic responses by cutaneous mechanical and thermal stimuli.
Databáze: OpenAIRE
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