Traumatic brain injury results in neuronal loss and associated neurological deficits. Although most research on the factors leading to trauma-induced damage focuses on synaptic or ionic mechanisms, the possible role of direct intercellular communication via gap junctions has remained unexplored. Gap junctions connect directly the cytoplasms of coupled cells; hence, they offer a way to propagate stress signals from cell to cell. We investigated the contribution of gap junctional communication (GJC) to cell death using an in vitro trauma model. The impact injury, induced by a weight dropped on the distal CA1 area of organotypic hippocampal slices, results in glutamate-dependent cell loss. The gap junctional blockers carbenoxolone and octanol decreased significantly post-traumatic cell death, measured by propidium iodide staining over a 72 hr period after the impact. Dye coupling in the pyramidal layers was enhanced immediately after the injury and decreased over the following 24 hr. To determine whether specific connexins were involved in the spread of trauma-induced cell death, we used organotypic slices from connexin43 (Cx43) knock-out mice, as well as acute knock-outs by incubation with antisense oligodeoxynucleotides. Simultaneous knockdown of two neuronal connexins resulted in significant neuroprotection. Slices from the null-mutant Cx43 mice, as well as the acute Cx43 knockdown, also showed decreased cell death after the impact. The gap junctional blockers alleviated the trauma-induced impairment of synaptic function as measured by electrophysiological field potential recordings. These results indicate that GJC enhances the cellular vulnerability to traumatic injury. Hence, specific gap junctions could be a novel target to reduce injury and secondary damage to the brain and maximize recovery from trauma.

The projections from the deep cerebellar nuclei and the sensorimotor cortex to the red nucleus were studied in the rat using anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (HRP-WGA). The anterogradely transported HRP-WGA was visualized ultrastructurally by using a modification of the tetramethylbenzidine (TMB) histochemical technique of Carson and Mesulam ('82). Following injection of HRP-WGA into the sensorimotor cortex, ultrastructural examination of anterograde labeling in the ipsilateral red nucleus revealed labeled synaptic terminals located on small-diameter dendrites of the parvocellular region. These terminals made asymmetrical contacts and contained round vesicles. HRP-WGA placement in the nucleus lateralis resulted in anterograde labeling of synaptic terminals which made asymmetrical contacts with small- to medium-sized dendrites of the parvocellular red nucleus. Similar placements in the nucleus interpositus gave rise to anterograde labeling of synaptic terminals which made asymmetrical contacts with somata and proximal dendrites of magnocellular neurons. In addition, retrograde labeling of magnocellular neurons was also observed following HRP-WGA placements in the nucleus interpositus. Anterogradely labeled interpositorubral synaptic terminals were located on retrogradely labeled rubrocerebellar neurons. The rat red nucleus thus receives topographically organized afferents which are characterized by their specificity in location at the cellular level.

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Following neonatal hemicerebellectomy, an aberrant ipsilateral cerebellorubral projection develops that maintains the topographic specificity of the normal contralateral projection. Similarly, neonatal lesions of the sensorimotor cortex lead to the appearance of an aberrant contralateral corticorubral projection that mirrors the topographic specificity of the normal ipsilateral input. The specificity of synaptic localization in these aberrant projections was studied by use of ultrastructural visualization of anterogradely transported HRP-WGA. Following neonatal ablations, adults received HRP-WGA injections in the unablated deep cerebellar nuclei or sensorimotor cortex. After 48 hours, animals were sacrificed and processed for ultrastructural localization of anterogradely transported HRP-WGA. In hemicerebellectomized animals, both the contralateral and ipsilateral interpositorubral projections terminated on the somatic and proximal dendritic membrane of magnocellular neurons. Some of these labeled synaptic terminals were located on somatic and dendritic spines. Following HRP-WGA injection in the unablated nucleus lateralis, anterogradely labeled synaptic terminals were located bilaterally on small- to medium-sized dendrites of parvicellular neurons. Injection of HRP-WGA in the remaining sensorimotor cortex of animals that had undergone neonatal unilateral ablation of the sensorimotor cortex resulted in labeled corticorubral synaptic terminals that contacted distal dendrites of ipsilateral and contralateral parvicellular neurons. These results demonstrate that, following neonatal deafferentation of the rat red nucleus, the topographic specificity of the aberrant rubral afferents is accompanied by a specificity of synaptic localization on discrete membrane areas of rubral neurons.