Response properties of hind limb single motor units in normal rats and after carrageenan-induced inflammation

The properties of single motor units from hind limb muscles and the changes in situations of hyperalgesia are not known in detail. We have therefore c haracterized the properties of single motor units in normal Wistar male rat s and in rats with carrageenan-induced inflammation, under ol-chloralose an aesthesia. Units were studied from three different muscles: peroneus longus , tibialis anterior and extensor digitorum longus. The properties of single motor units were not homogeneous in the three muscles studied in normal an imals, showing different sizes of cutaneous receptive fields, thresholds fo r natural and electrical stimulation, and encoding of responses at differen t: intensities of stimulation. Intraplantar injections of carrageenan induc ed a significant inflammation of the paw and a change in spontaneous behavi our observed in open field experiments. After inflammation, the responses t o cutaneous stimulation of the single motor units became more homogeneous. The threshold for mechanical stimulation was lower for peroneus longus and tibialis anterior but not for extensor digitorum longus units when compared to normal animals. The receptive fields were larger when mapped with a 500 mN von Frey hair but not when mapped using a threshold intensity hair. The threshold for thermal stimulation was lower after inflammation than in nor mal conditions in all cases, whereas the threshold for electrical stimulati on was lower in tibialis anterior and extensor digitorum longus units. An e nhancement of responses related to the increase of stimulus intensity was s een in normal animals in all muscles for mechanical and electrical stimuli (but not for thermal). After inflammation, a relationship between firing ra te and intensity of stimulation was seen in all cases studied. The firing of single motor units showed over 50% adaptation in the normal c ondition and over 75% after inflammation when stimulated for 10 s at mechan ical threshold intensity. After inflammation, the rate of adaptation was si gnificantly lower when suprathreshold intensity was used for mechanical sti mulation. No differences were seen in the adaptation of units to thermal st imulation. We conclude that, in situations of hyperalgesia due to inflammat ion, the threshold, encoding of stimulus intensity and adaptation of single motor units from different muscles changed, resulting in a narrower range of responses and a more homogeneous population of units. (C) 1999 IBRO. Pub lished by Elsevier Science Ltd.