The organization of efferent projections from the spinal cervical enlargement to the parabrachial (PB) area and the periaqueductal gray (PAG) was studied in the rat by using microinjections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into different laminae around the C7 level. The results demonstrated two areas of cervical enlargement which project in different ways to the PB area and PAG. First, the superficial laminae (I, II) showed a very dense projection, with a clear contralateral dominance at the coronal level where the inferior colliculus merges with the pons, to a restricted "superficial" portion of the PB area, namely the lateral crescent area, the dorsal lateral, the superior lateral (PBsl), and the outer portion of the external lateral PB subnuclei. Less dense projections were observed in the Kölliker-Fuse nucleus (KF) and in the ventrolateral/lateral quadrant of the caudal and mid PAG. By contrast, the labeling was weak or absent in the other PB subnuclei and the outer adjacent regions; in particular, no, or very little, labeling was found in the cuneiform nucleus. The PB area appeared to be the supraspinal target that received the densest projection from laminae I and II. Projections were less dense in the PAG and the thalamus and markedly less in other sites such as the ventrolateral medulla, the subnucleus reticularis dorsalis, and the nucleus of the solitary tract. Second, the reticular portion of lamina V, the medial portion of laminae IV-VI up to X and lamina VIII, showed bilateral projections with a weak ipsilateral dominance and a high to medium density on a very restricted portion of the PB area, namely the internal lateral PB subnucleus. A lesser projection was also observed in the adjacent portion of the PBsl, the KF, and the lateral quadrant of the PAG. These results suggest that signals carried by neurons from lamina I-II converge on a restricted superficial portion of the PB area and the ventral part of the lateral quadrant of the PAG. These results are discussed in the context of the role of the spino-PB and spino-PAG pathways in nociception.
In this study of the orofacial formalin test in rats, the effects of different formalin concentrations (0.2%, 0.5%, 1.5%, 2.5%, 5% and 10%) on the behavioural nociceptive response (face rubbing) was investigated. The histological responses of the skin were also evaluated. Increasing the concentration of formalin caused a parallel aggravation of histological signs of tissue inflammation and injury. All concentrations provoked an early phase of nociceptive response, but its intensity was not concentration-dependent. The 2nd phase of response to formalin only occurred for concentrations of 1.5% and higher. A positive relationship between the formalin concentration and the amplitude of the rubbing activity measured between 12 and 45 min after injection could be observed until 2.5% but with the highest concentrations (5 and 10%), the amplitude of the response decreased. Our findings indicate that the orofacial formalin test should be carried out using concentration between 0.5 and 2.5%. This is essential to assess increase as well as decrease in pain intensity. Moreover, this will have the effect of minimizing the suffering of the experimental animal.
Stimulation of small-diameter afferents supplying deep tissues has been shown to increase the excitability of spinal cord neurones responding to cutaneous afferent inputs. This facilitation has been implicated as integral central mechanisms of deep pain that may contribute to the tenderness and spread and/or referral of pain following injury of deep tissues. In view of the recent documentation of deep craniofacial afferent inputs, as well as cutaneous afferent inputs to the trigeminal (V) spinal tract nucleus, we wished to determine the effects of deep inputs excited by the small-fibre irritant mustard oil on trigeminal nociceptive neurones. The extracellular activity of single brain-stem neurones was recorded in subnuclei caudalis and oralis of the V spinal tract nucleus of anaesthetized rats. The neurones were classified as low-threshold mechanosensitive (LTM), wide dynamic range (WDR) and nociceptive specific (NS) on the basis of their cutaneous mechanoreceptive field properties and their responses evoked by electrical stimulation of their cutaneous afferent inputs. Injection of 5% mustard oil (2-5 microliters) into the deep masseter muscle produced a facilitatory effect in 12 of 27 nociceptive neurones tested in caudalis and in 5 of 12 nociceptive neurones in oralis. This effect was reflected in an expansion of the cutaneous mechanoreceptive field, an increase in spontaneous activity or an increase in responsivity to electrical stimulation of cutaneous afferent inputs to the neurones. The facilitation was reversible and typically became apparent within 3-5 min of the injection, reached its peak at 5-10 min, and lasted for 20-30 min.(ABSTRACT TRUNCATED AT 250 WORDS)
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