Inflammatory diseases and neuropathic insults are frequently accompanied by severe and debilitating pain, which can become chronic and often unresponsive to conventional analgesic treatment. A loss of synaptic inhibition in the spinal dorsal horn is considered to contribute significantly to this pain pathology. Facilitation of spinal gamma-aminobutyric acid (GABA)ergic neurotransmission through modulation of GABA(A) receptors should be able to compensate for this loss. With the use of GABA(A)-receptor point-mutated knock-in mice in which specific GABA(A) receptor subtypes have been selectively rendered insensitive to benzodiazepine-site ligands, we show here that pronounced analgesia can be achieved by specifically targeting spinal GABA(A) receptors containing the alpha2 and/or alpha3 subunits. We show that their selective activation by the non-sedative ('alpha1-sparing') benzodiazepine-site ligand L-838,417 (ref. 13) is highly effective against inflammatory and neuropathic pain yet devoid of unwanted sedation, motor impairment and tolerance development. L-838,417 not only diminished the nociceptive input to the brain but also reduced the activity of brain areas related to the associative-emotional components of pain, as shown by functional magnetic resonance imaging in rats. These results provide a rational basis for the development of subtype-selective GABAergic drugs for the treatment of chronic pain, which is often refractory to classical analgesics.

SUMMARY1. O2-,K+-and pH-sensitive microelectrodes were used to measure extracellular oxygen pressure (Po2), K' activity (aK.) and pH (pH.) in ventral regions of the medulla oblongata containing respiratory neurons in the in vitro brainstem-spinal cord preparation from 0 to 4-day-old rats.2. The location of respiratory neurons was mapped by extracellular recordings with conventional microelectrodes, or with the reference barrel of ion-sensitive microelectrodes. The major populations of respiratory neurons were distributed in the ventrolateral reticular formation near the nucleus ambiguus at depths of 300-600 ,um. In this area, aK. baseline increased from 3-2 to 3-8 mm whereas steadystate values of Po2 and pHo fell from 120 to 7 mmHg and from 6-9 to 6-7, respectively.3. During rhythmic inspiratory discharges recorded with suction electrodes from ventral roots of spinal (C3-C5) and cranial (IX, X, XII) nerves, aKo transiently increased by up to 100 /M, and Po2 fell maximally by 0 4 mmHg. During episodes of non-rhythmic neuronal discharge, aKo increased by as much as 0-4 mm and Po2 decreased by about 10 mmHg. In contrast, no variations in pHo could be detected during such activities.4. Activation of medullary neurons by tetanic electrical stimulation of axonal tracts in the ventrolateral column of the spinal cord at the level of the phrenic motoneuron pool produced aKo elevations of up to 5 mm, decreases of Po2 by up to 50 mmHg, and pHo increases by a maximum of 0 07 pH units. These aKo and Po2 transients were reduced by more than 80 % during blockade of synaptic transmission with 5 mm manganese (Mn2+) and completely blocked by 1 ,tm tetrodotoxin (TTX).5. The tissue Po2 gradient as well as activity-related decreases of P2 were completely abolished after block of oxidative cellular metabolism by addition of 2-10 mm cyanide (CN-) to the bathing solution.6. Inhibition of the Na+-K+ pump by addition of 3-50 /M ouabain (3-10 min)caused a reversible increase of aKo by 0-8-3 mm, a delayed recovery of stimulusinduced aKo elevations, and produced a disturbance of the respiratory rhythm.7. The sensitivity of the respiratory network to oxygen depletion was tested by superfusing the neuraxis with hypoxic solutions gassed with N2 instead of 02(5-20 min). The response of the respiratory network to such hypoxic exposure MS 9830 J. BROCKHA US AND OTHERS consisted of an initial increase in frequency of respiratory motor output, followed by a depression of respiratory activity that terminated in a reversible loss of the respiratory rhythm.8 10. It is concluded that the oxygen supply of the ventral respiratory network is sufficient to maintain aerobic neuronal metabolism and metabolically driven Na+-K+ pump activity which allows respiratory rhythmogenesis and network function in the in vitro neonatal rat brainstem-spinal cord preparation.

Interleukin-6 (IL-6) is a key mediator of inflammation. Inhibitors of IL-6 or of its signal transducing receptor gp130 constitute a novel class of anti-inflammatory drugs, which raise great hopes for improved treatments of painful inflammatory diseases such as rheumatoid arthritis. IL-6 and gp130 may enhance pain not only indirectly through their proinflammatory actions but also through a direct action on nociceptors (i.e., on neurons activated by painful stimuli). We found indeed that the IL-6/gp130 ligand-receptor complex induced heat hypersensitivity both in vitro and in vivo. This process was mediated by activation of PKC-␦ via Gab1/2/PI 3 K and subsequent regulation of TRPV1, a member of the transient receptor potential (TRP) family of ion channels. To assess the relevance of this direct pain promoting effect of IL-6, we generated conditional knock-out mice, which lack gp130 specifically in nociceptors, and tested them in models of inflammatory and tumor-induced pain. These mice showed significantly reduced levels of inflammatory and tumor-induced pain but no changes in immune reactions or tumor growth. Our results uncover the significance of gp130 expressed in peripheral pain sensing neurons in the pathophysiology of major clinical pain disorders and suggest their use as novel pain relieving agents in inflammatory and tumor pain.