Brain research
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As well as substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) have recently been found in the superficial dorsal horn of the spinal cord; NKA originating mainly in fine primary afferents. We have investigated the effects of these tachykinins and a range of analogues on somatosensory responses of single identified dorsal horn neurons, when applied ionophoretically to the region of the substantia gelatinosa. Behavioural reflex tests of thermal nociception were carried out in parallel. ⋯ Evidence from receptor-selective antagonists supports that obtained with agonists for the roles of particular NK receptors in somatosensory processing. NK-2, but not NK-1 or NK-3 antagonists attenuated endogenous thermal nociceptive responses, supporting the hypothesis that an NK-2 agonist (such as NKA) may normally participate in expression of thermal nociception in the superficial dorsal horn. Behavioural experiments showing increased response latencies with a putative NK-2 selective antagonist further supported the involvement of NK-2 receptors in thermal nociception.
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The effects of hyper- and hypocapnia on oxidative metabolism were evaluated by near-infrared (NIR) multiwavelength spectroscopy in intact brain and skeletal muscle tissues of the anesthetized cat. A 3-wavelength NIR algorithm was used to monitor cytochrome a,a3 oxidation state, regional blood volume, and tissue oxyhemoglobin and O2 stores simultaneously in brain and muscle in ventilated animals. Incremental hypercapnia was produced in 10 cats by raising arterial pCO2 from 27.0 +/- 1.3 to 95.1 +/- 1.9 mmHg with inspired CO2. ⋯ Hypocapnia produced by hyperventilation in 8 cats lowered paCO2 from 28.5 +/- 0.4 to 13.5 +/- 0.5 mmHg. Hypocapnia decreased cerebral HbO2, blood volume, and cytochrome a,a3 redox level (P less than 0.05), but NIR changes were not seen in skeletal muscle. These experiments demonstrate preferential distribution of oxygen to brain during hypercapnia and the ability of NIR spectroscopy to assess regional oxygenation in multiple tissues non-invasively.