Relationship between tonic inhibitory currents and phasic inhibitory activity in the spinal cord lamina II region of adult mice.

Phasic and tonic inhibitions are two types of inhibitory activities involved in inhibitory processing in the CNS. In the spinal cord dorsal horn, phasic inhibition is mediated by both GABAergic and glycinergic inhibitory postsynaptic currents. In contrast to phasic inhibitory currents, using patch-clamp recording technique on spinal cord slices prepared from adult mice we revealed that tonic inhibitory currents were mediated by GABAA receptors but not by glycine receptors in dorsal horn lamina II region. We found that there was a linear relationship (r = 0.85) between the amplitude of tonic inhibitory currents and the frequency of GABAergic inhibitory postsynaptic currents. Analysis of charge transfer showed that the charges carried by tonic inhibitory currents were about 6 times of charges carried by phasic inhibitory currents. The prominent charge transfer by tonic inhibitory currents and their synaptic activity dependency suggest a significant role of tonic inhibition in sensory processing.

Actions of midazolam on excitatory transmission in dorsal horn neurons of adult rat spinal cord.

BACKGROUND: Although intrathecal administration of midazolam, a water-soluble imidazobenzodiazepine derivative, has been found to produce analgesia, how it exerts this effect at the neuronal level in the spinal cord is not fully understood. METHODS: The effects of midazolam on electrically evoked and spontaneous excitatory transmission were examined in lamina II neurons of adult rat spinal cord slices using the whole cell patch clamp technique. RESULTS: Bath-applied midazolam (1 microm) diminished Adelta- and C-fiber evoked polysynaptic excitatory postsynaptic currents in both amplitude and integrated area. However, it affected neither Adelta- and C-fiber evoked monosynaptic excitatory postsynaptic currents in amplitude nor miniature excitatory postsynaptic currents in amplitude, frequency, and decay time constant. In the presence of a benzodiazepine receptor antagonist, flumazenil (5 microm), midazolam (1 microm) did not diminish Adelta-fiber evoked polysynaptic excitatory postsynaptic currents, suggesting that midazolam modulate the gamma-aminobutyric acid interneurons in the dorsal horn. CONCLUSIONS: Midazolam reduced excitatory synaptic transmission by acting on the gamma-aminobutyric acid type A/benzodiazepine receptor in interneurons, leading to a decrease in the excitability of spinal dorsal horn neurons. This may be a possible mechanism for the antinociception by midazolam in the spinal cord.

Substance P-driven feed-forward inhibitory activity in the mammalian spinal cord.

In mammals, somatosensory input activates feedback and feed-forward inhibitory circuits within the spinal cord dorsal horn to modulate sensory processing and thereby affecting sensory perception by the brain. Conventionally, feedback and feed-forward inhibitory activity evoked by somatosensory input to the dorsal horn is believed to be driven by glutamate, the principle excitatory neurotransmitter in primary afferent fibers. Substance P (SP), the prototypic neuropeptide released from primary afferent fibers to the dorsal horn, is regarded as a pain substance in the mammalian somatosensory system due to its action on nociceptive projection neurons. Here we report that endogenous SP drives a novel form of feed-forward inhibitory activity in the dorsal horn. The SP-driven feed-forward inhibitory activity is long-lasting and has a temporal phase distinct from glutamate-driven feed-forward inhibitory activity. Compromising SP-driven feed-forward inhibitory activity results in behavioral sensitization. Our findings reveal a fundamental role of SP in recruiting inhibitory activity for sensory processing, which may have important therapeutic implications in treating pathological pain conditions using SP receptors as targets.

Action of isoflurane on the substantia gelatinosa neurons of the adult rat spinal cord.

BACKGROUND: Although isoflurane, a volatile anesthetic, can block the motor response to noxious stimulation (immobility and analgesia) and suppress autonomic responsiveness, how it exerts these effects at the neuronal level in the spinal cord is not fully understood. METHODS: The effects of a clinically relevant concentration (1 rat minimum alveolar concentration [MAC]) of isoflurane on electrically evoked and spontaneous excitatory/inhibitory transmission and on the response to exogenous administration of the gamma-aminobutyric acid type A receptor agonist muscimol were examined in lamina II neurons of adult rat spinal cord slices using the whole cell patch clamp technique. The effect of isoflurane on the action potential-generating membrane property was also examined. RESULTS: Bath-applied isoflurane (1.5%, 1 rat MAC) diminished dorsal root-evoked polysynaptic but not monosynaptic excitatory postsynaptic currents. Glutamatergic miniature excitatory postsynaptic currents were also unaffected by isoflurane. In contrast, isoflurane prolonged the decay phase of evoked and miniature gamma-aminobutyric acid type A receptor-mediated inhibitory postsynaptic currents and increased the amplitude of the muscimol-induced current. Isoflurane had little effect on action potential discharge activity. CONCLUSIONS: Isoflurane augments gamma-aminobutyric acid-mediated inhibitory transmission, leading to a decrease in the excitability of spinal dorsal horn neurons. This may be a possible mechanism for the antinociceptive effect of isoflurane in the spinal cord.

Removal of GABAergic inhibition facilitates polysynaptic A fiber-mediated excitatory transmission to the superficial spinal dorsal horn.

Primary afferent A-fiber stimulation normally evokes fast mono- or polysynaptic EPSCs of short duration. However, in the presence of the GABA(A) receptor antagonist bicuculline, repetitive, long lasting, polysynaptic EPSCs can be observed following the initial, fast response. A-fiber-induced ERK activation is also facilitated in the presence of bicuculline. The frequency of miniature EPSCs and the amplitude of the monosynaptic A-fiber-evoked EPSCs are not affected by bicuculline or the GABA(A) receptor agonist muscimol, suggesting that GABA(A) receptors located on somatodendritic sites of excitatory interneurons are critical for this action. Bicuculline-enhanced polysynaptic EPSCs are completely eliminated by NMDA receptor antagonists APV and ketamine, as was the augmented ERK activation. This NMDA receptor-dependent phenomenon may contribute to bicuculline-induced allodynia or hyperalgesia, as well as the hypersensitivity observed in neuropathic pain patients.

Propofol enhances GABA(A) receptor-mediated presynaptic inhibition in human spinal cord.

Although the function of somatodendritic GABAA receptors is augmented by propofol, it is not known whether presynaptic GABAA receptor function is similarly affected. In the present study, we examined the action of propofol on the second positive wave (P2 component) of segmental spinal cord evoked potentials (seg SCEPs), which is believed to reflect GABAA receptor-mediated presynaptic inhibition of primary afferent terminals and can be recorded from spinal epidural space in man. In all seven patients tested while undergoing scoliosis surgery, a clinical dose of propofol (1 mg//kg, i.v.) significantly augmented the P2 component of seg SCEPs evoked by ulner nerve stimulation. We conclude that propofol enhances GABAA receptor-mediated presynaptic inhibition at primary afferent terminals in human spinal cord.

Baclofen inhibits more effectively C-afferent than Adelta-afferent glutamatergic transmission in substantia gelatinosa neurons of adult rat spinal cord slices.

Although intrathecal administration of baclofen, a selective GABA(B)-receptor agonist, is known to have an antinociceptive effect on various pain models, the role of presynaptic GABA(B) receptors in antinociception is not well characterized. In the present study, the action of baclofen on primary afferent-evoked glutamatergic excitatory transmission was examined in substantia gelatinosa (SG) neurons of an adult rat spinal cord slice with an attached dorsal root, prepared from the lumbar segment, by use of the blind whole-cell patch-clamp technique. Under the condition where a postsynaptic action of baclofen was inhibited, baclofen (1 microM) reduced the amplitudes of excitatory postsynaptic currents (EPSCs; V(H)=-70 mV) which were monosynaptically evoked by stimulating primary-afferent C- and/or Adelta-fibers and which were remarkably depressed by CNQX (10 microM). The identification of the C-fiber or Adelta-fiber EPSC was based on antidromic action potentials recorded from neurons of isolated dorsal root ganglia. The C-fiber EPSC was depressed in peak amplitude by baclofen (1 microM) to a larger extent than the Adelta-fiber EPSC (20 and 45% of control, respectively). Each of the baclofen actions was suppressed by a selective GABA(B)-receptor antagonist, CGP 35348 (50 microM). Baclofen (1 microM) did not affect a response of SG neurons to bath-applied AMPA (10 microM). These results indicate that baclofen inhibits the release of L-glutamate from Adelta and C primary-afferent terminals in the SG through the activation of GABA(B) receptor; this action is more effective to C-fiber than Adelta-fiber transmission. Considering that the SG is the main part of termination of Adelta- and C-fibers transmitting nociceptive information, the present finding would account for at least a part of the inhibitory action of baclofen on pain transmission.