Changes in neuronal activities in the two ventral posterior medial thalamic nuclei in an experimental model of trigeminal pain in the rat by constriction of one infraorbital nerve
Bp. Vos et al., Changes in neuronal activities in the two ventral posterior medial thalamic nuclei in an experimental model of trigeminal pain in the rat by constriction of one infraorbital nerve, SOMAT MOT R, 17(2), 2000, pp. 109-122
The present study analyzes the activity of 120 neurons recorded in the two
ventro-postero-medial (VPM) nuclei of the thalamus in a rat model of trigem
inal neuropathic pain. Twenty-eight rats were tested 2 weeks after a chroni
c constriction injury (CCI) of the infraorbital nerve (IoN). These animals
exhibited violent pain-related reactions to extremely weak mechanical stimu
li applied to the lesioned and, to a lesser extent, unlesioned IoN territor
ies. The activities of neurons recorded in the VPMc (n = 80) and VPMi (n =
40), contralateral and ipsilateral to the injured nerve, respectively, were
compared with those of 62 neurons recorded in the VPM of ten normal rats (
VPMn). The neuronal background activity was higher in the VPM of CCI-IoN ra
ts than in normal rats (about 4 vs 1 spike/s). The proportion of neurons wh
ich were driven by mechanical stimulations applied contralaterally to their
recording site, was comparable in the three VPM (63-70%), but the effectiv
e stimulus modality differed significantly between the normal and the lesio
ned rats. In particular, the number of neurons driven by vibrissa or guard
hair movements dramatically decreased in the VPM of CCI-IoN rats, mainly in
the VPMc (67% of neurons with a receptive field (RF) in the VPMn vs 12% in
the VPMc). Inversely, a consistent number of neurons in both the VPMc and
VPMi were driven by other stimulus modalities applied to the IoN territory
(moderate pressure for VPMc neurons, pinch or pinprick for both VPMc and VP
Mi neurons). The responses so induced were especially intense and included
afterdischarges. In contrast to the VPMn neurons, the RFs of both the VPMc
and VPMi neurons included two vibrissae at least, and were occasionally dis
continuous and multimodal, including both vibrissae and cutaneous areas for
VPMc units. The bilateral changes in VPM responsiveness and in behavior su
ggest the involvement of central processing of sensory information, which a
re set off by the CCI-IoN. The putative mechanisms and functional implicati
on of the changes in the VPM neuronal activities are discussed.