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upon spinal nociceptive processing (Almeida, et al., pearing after DRt activation seemed to be triggered
1999). Our results clearly show that DRt activation en- regardless of the type of primary afferents recruited
hanced/triggered and prolonged the post-discharge of since only the post-discharge but not fibre-mediated
nociceptive neurons, but did not affect the responses responses was affected. Accordingly, the activation of
mediated by primary afferents, whether they were in- only A$ fibres would also lead to an abnormal response
volved in pain transmission (A*- and C-fibres) or not of spinal neurons, turning then the light innocuous brush-
(A”$-fibres). ing of the skin into a painful experience. Other studies
Anatomical studies have described the existence of a are needed to clarify this issue.
descending projection from the DRt impinging upon The present study clearly demonstrates that the DRt
superficial dorsal horn neurons that project back to the exerts pronociceptive actions upon spinal nociceptive
DRt (Almeida, et al., 1993). At both levels, projecting neurons by potentiating their response to peripheral
neurons establish asymmetrical, presumably excitatory, stimulation. Through these actions, the DRt could play a
synapses forming therefore a reverberating loop proba- major role in chronic pain states, making it a reliable
bly aimed at amplifying the responses of superficial target for new approaches in pain treatment.
nociceptive dorsal horn neurons to noxious stimulation
(Almeida, et al., 1993, 2000). In the deep dorsal horn, References
neurons also receive descending projections from the
DRt and send axons back to it (Tavares and Lima, 1994; Almeida A, Tavares I, Lima D, Coimbra A. Descending projections from the
Almeida, et al., 2000), but the morphology of possible medullary dorsal reticular nucleus make synaptic contacts with spinal
synaptic contacts has not been established yet. Howev- cord lamina I cells projecting to that nucleus: an electron microscopic
tracer study in the rat. Neuroscience 1993; 55:1093-106.
er, in agreement with data showing that DRt lesion
Almeida A, Tjolsen A, Lima D, Coimbra A, Hole K. The medullary dorsal reticular
decreased the c-fos expression in the deep dorsal horn nucleus facilitates acute nociception in the rat. Brain Res Bul 1996;39:7-15.
induced by peripheral noxious stimulations (Almeida, et Almeida A, Storkson R, Lima D, Tjolsen A. The medullary dorsal reticular nucleus
al., 1999), our present study strongly suggests the facilitates pain behaviour induced by formalin in the rat. Eur J Neurosci
1999;11:110-22.
existence of a descending excitatory action, mediated
Almeida A, Tavares I, Lima D. Reciprocal connections between the medullary
by the DRt, upon deep dorsal horn nociceptive neurons. dorsal reticular nucleus and the spinal dorsal horn in the rat. Eur J Pain
From a clinical point of view, our results point out the 2000;4:373-87.
major role the DRt may play in chronic pain states. The Burgess PR, Perl ER. Cutaneous mechanoreceptors and nociceptors. In Iggo A
(ed). Handbook of Sensory Physiology. Vol 2. Somatosensory System.
dramatic changes in the excitability of spinal nocicep- Berlin: Springer-Verlag 1973:29-78.
tive neurons appearing after DRt activation are very Cervero F, Laird JMA. Mechanisms of touched-evoked pain (allodynia): a new
similar to one of the typical features of the sensitization model. Pain 1996;68:13-23.
Gasser HS, Erlanger J. The role played by the sizes of the constituent of a nerve
thought to sustain allodynia. In its major clinical form,
trunk in determining the form of its action potential wave. Am J Physiol
allodynia is a painful state produced by the activation of 1927;80:522-47.
low threshold A$ fibres that are normally devoted to the Lima D, Esteves F, Coimbra A. C-fos activation by noxious input of spinal neurons
transmission of mechanosensitive sensations (Woolf and projecting to the nucleus of the tractus solitarius in the rat. In: Gebhart GF,
Hammond DL, Jensen TS (eds). Proceedings of the Am. World Congress
Doubell, 1994). Since it is displayed by uninjured tissue,
on Pain Vol 2. Progress in Pain Research and Management. Seattle: IASP
allodynia is thought to involve central mechanisms ca- Press 1994.
pable of altering the spinal neurons’ properties (Cervero Tavares I, Lima D. Descending projections from the caudal medulla oblongata to
and Laird, 1996). Although the stimulation applied in our the superficial or deep dorsal horn of the rat spinal cord. Exp Brain Res
1994;99:455-63.
study was set to recruit both low- and high-threshold Woolf CJ, Doubell TP. The pathophysiology of chronic pain increases sensitivity to
fibres, the exaggerated response of spinal neurons ap- low threshold A__fibres. Curr Opin Neurobiol 1994;4: 525-34.

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