Modifications in the responsiveness of rat ventrobasal thalamic neurons at different stages of carrageenin-produced inflammation.

The present study was aimed at analyzing the responsiveness of the ventrobasal (VB) thalamic neurons in rats presenting with a hyperalgic carrageenin-produced inflammation. The following were studied: the responses of the same VB neuron, before and 15-145 min after the plantar injection of carrageenin in a part of its receptive field (RF) (acute phase); the responses of VB neurons located in the thalamus contralateral to the hyperalgesic inflamed paw, 24-96 h after the injection (subacute phase); and the effect of a local anesthetic injected in the inflamed paw, and that of an intravenous injection of Aspirin, on neuronal response modifications. Responses of VB neurons initially activated by light tactile stimuli (group 1; n = 4) and by moderate joint stimulation (group 3; n = 4) were not modified in the early period following the carrageenin injection. By contrast, in the first few minutes following the injection. VB neurons exclusively driven by noxious mechanical and thermal stimuli (group 2; n = 23), exhibited a clear enhancement of their responses, which persisted during the observation period. These modifications were also observed for responses obtained from part of the RF remote from the injection site; moreover there was an extension of the RF to areas distant from the injured paw. The local injection of an anesthetic (Xylocaine) in this paw, suppressed the modifications of responses of group 2 neurons, elicited not only from the injected paw, but also from the remote parts of the RF. At this time Aspirin was almost inefficient (even at the dose of 100 mg/kg) on responses of these group 2 neurons. In the subacute phase responses of 72 somatosensory neurons were analyzed. Twenty-five of 72 responded to rapid repetitive light tactile stimulation applied on a small contralateral RF (group 1); their responses were similar to those encountered in a normal situation. Thirty-three of 72 neurons responded to intense mechanical stimuli such as pinches (group 2). For half of them the response characteristics were similar to those described in the normal rat; for the other half responses appeared 'faded': short duration; absence of after-discharge; poor reproducibility. Fourteen of 72 neurons responded to moderate stimulation of the joints, deep tissues and/or surrounding cutaneous areas of the inflamed paw (group 3). Their RF was mostly unilateral, i.e. contralateral to the recording site; the responses were sustained during the stimulation but rarely exhibited after-discharge.(ABSTRACT TRUNCATED AT 400 WORDS)