A.R. Castro’s research while affiliated with University of Porto and other places

Secondary hyperalgesia in the monoarthritic rat is accompanied by a decrease in nociceptive activation of spinal neurons expressing GABA(B) receptors and by the opposite effect in the cells expressing neurokinin 1 (NK1)-receptors. In order to ascertain the relative role of each receptor, the effects of intrathecal administration of SP-saporin (SP-SAP), baclofen or both were evaluated, using a model of secondary hyperalgesia that consists of mechanical stimulation of the hindlimb skin close to an inflamed joint. Four days after the induction of monoarthritis by intraarticular injection of Complete Freund's Adjuvant (CFA), a cannula was implanted at T(13)-L(1) level and 10 microl of saline or SP-SAP (10(-6) M) were intrathecally (i.t.) injected. Fourteen days after CFA-injection, half of the animals from each group received i.t. injections of 10 microl saline and the remainder were injected with the same volume of baclofen (1 microg). Ten minutes later, the animals were behaviorally evaluated by the von Frey test or submitted to noxious mechanical stimulation to analyze c-fos expression. The von Frey thresholds increased after the treatments, but more pronouncedly after baclofen or SP-SAP plus baclofen. In segments L(2)-L(3), the spinal area that receives input from the stimulated skin close to the inflamed joint, the numbers of Fos-immunoreactive neurons were reduced after the three treatments both in the superficial and deep dorsal horn. In segments T(13)-L(1), the numbers of Fos-immunoreactive neurons were significantly reduced after treatment with SP-SAP plus baclofen in both dorsal horn regions, and in the deep dorsal horn after baclofen treatment. We conclude that both GABA(B) and NK1 receptors of spinal dorsal horn neurons participate in secondary hyperalgesia in the monoarthritic rat, although the decrease in GABA inhibition appears to play a more important role than the increase in SP-mediated effects.

The balance between excitatory and inhibitory input exerted upon spinal cord neurones that belong to spinofugal pathways determines the ultimate type of information transmitted to the brain. We compared the relative expression of NK1 and GABAB receptors in two spinomedullary pathways targeting an antinociceptive area and a pronociceptive centre, respectively, the lateral part of the caudal ventrolateral medulla (VLMlat) and the dorsal reticular nucleus (DRt). Spinal cord sections of rats injected in the VLMlat or DRt with the retrograde tracer cholera toxin subunit B were triple-immunoreacted for the tracer, NK1 receptors and GABAB receptors. The dorsal horn neurones labelled from the VLMlat mainly co-localized the two receptors while those labelled from the DRt mainly expressed GABAB receptors, which was particularly evident in neurones of laminae IV-V. The morphological classification of lamina I neurones projecting to the VLMlat showed that fusiform, flattened and pyramidal cells mainly co-localized NK1 and GABAB receptors. As to lamina I neurones projecting to the DRt, multipolar neurones mainly expressed GABAB receptors while the majority of flattened and pyramidal neurones co-localized NK1 and GABAB receptors. The present results suggest that the expression of NK1 and GABAB receptors varies in neurones participating to different spinofugal pathways. The importance of the present findings in the knowledge of the endogenous supraspinal pain control system is discussed.

The neurochemical changes that operate in nociceptive spinal cord circuits during secondary hyperalgesia are largely unknown, in particular with respect to the balance between excitatory and inhibitory neurotransmission. In this study we evaluated the expression of NK1 and GABA(B) receptors in nociceptive spinal neurons in a model of secondary hyperalgesia consisting of noxious mechanical stimulation of the hindlimb skin close to a joint chronically inflamed by complete Freund's adjuvant. In spinal segments receiving input from that skin area, Fos-immunodetection was combined with immunocytochemistry for NK1 receptors, GABA(B) receptors or both receptors. In control and monoarthritic animals, neurons double-labeled for Fos and each receptor occurred mainly in laminae I and IV-V. In lamina I, the percentage of NK1 neurons expressing Fos was higher in monoarthritics while lower percentages of GABA(B) neurons expressed Fos. The percentage of Fos-positive cells expressing NK1 immunoreaction did not change in monoarthritics but that of Fos cells with GABA(B) immunoreaction was lower in these animals. In laminae IV-V, a large increase in Fos expression was detected in monoarthritic rats but the relative proportions of Fos-positive neurons expressing each receptor were similar in the two groups. Co-localization of NK1 and GABA(B) receptors occurred only in lamina I neurons in both experimental groups with no differences between control and monoarthritic animals in the percentages of Fos-positive neurons that expressed the receptors. Considering the participation of lamina I neurons bearing NK1 and GABA(B) receptors in several spinofugal systems, it is possible that the imbalance between excitatory and inhibitory actions exerted, respectively, by substance P and GABA may subserve secondary hyperalgesia by increasing ascending transmission of nociceptive input.