Traumatic or stroke-like injuries of the cerebral cortex result in the rapid retrograde degeneration of thalamic relay neurons that project to the damaged area. Although this phenomenon has been well documented, neither the basis for the relay neuron's extreme sensitivity to axotomy nor the mechanisms involved in the degenerative process have been clearly identified. Physiological and biochemical studies of the thalamic response to cortical ablation indicate that pathological overexcitation might contribute to the degenerative process. The responses of thalamic projection neurons, protoplasmic astrocytes, and inhibitory thalamic reticular neurons in adult mice were examined from one to 120 days following ablation of the somatosensory cortex as part of an investigation of the role of excitotoxicity in thalamic retrograde degeneration. The responses of thalamic neurons to cortical ablation were compared with those produced by intracortical injection of the convulsant excitotoxin kainic acid, since the degeneration of neurons in connected brain structures distant to the site of kainic acid injection is also thought to occur via an excitotoxic mechanism.
Within two days after either type of cortical injury, protoplasmic astrocytes in affected regions of the thalamic ventrobasal complex and the medial division of the posterior thalamic nuclei became reactive and expressed increased levels of immunohistochemically detectable glial fibrillary acidic protein. Within the affected regions of the ventrobasal complex an increased intensity of puncta positive for glutamate decar☐ylase immunoreactivity, presumably due to an increase in its content within the terminals of the reciprocally interconnected thalamic reticular neurons, was also evident. These immunohistochemically detectable alterations in the milieu of the damaged thalamic neurons preceded the disappearance of the affected relay neurons by at least two days following cortical ablation and by seven to 10 days following intracortical kainic acid injection. Regions of the thalamus containing reactive astrocytes corresponded very closely to the regions undergoing retrograde degeneration. Protoplasmic astrocytes in these areas remained intensely reactive up to 60 days after cortical injury. Levels of glutamate decar☐ylase were only transiently elevated in the degenerating regions of the ventrobasal complex following cortical ablation and returned to normal by 14 days. Increased glutamate decar☐ylase immunoreactivity was transiently seen through the entire ventrobasal complex following intracortical kainic acid injection but was markedly more intense in degenerating regions. These patterns of labeling did not return to normal until 50 days after intracortical kainic acid injection, well after the death of the relay neurons.
Cortical ablation and intracortical kainic acid injection produce similar alterations in thalamic neuronal and glial populations. The response of thalamic protoplasmic astrocytes to cortical injury may precipitate secondary pathological alterations that directly contribute to the overexcitation of affected relay neurons. Increased activity of inhibitory thalamic reticular neurons may contribute to the degenerative process by paradoxically increasing the excitability of relay neurons in degenerating thalamic regions.
These results are consistent with the hypothesis that the thalamic degeneration induced by cortical ablation and intracortical kainic acid injection may share a common excitotoxic mechanism.
