The molecular mechanisms underlying the response to injury in the central nervous system are incompletely understood. Many cell activation systems may be involved. Tyrosine kinase receptors and their ligands play key roles in cell activation throughout life. The Eph family of tyrosine kinase receptors/ ligands are developmentally regulated and have been implicated in neural pathfinding. However, nothing is known about their role in the adult brain. We have used a model of central nervous system lesion in the rat, in which intraventricular injection of kainate was performed. This produced neuronal death in the CA3-CA4 fields and glial activation in the hippocampus. Highly degenerate primers, corresponding to the catalytic domain of the tyrosine kinase family, were used for reverse transcription-polymerase chain reaction of pooled RNA extracted from injured hippocampi. The amplified products were cloned and 100 clones (arbitrarily named TK1-TK100) were examined and inserts sequenced. We obtained four clones containing inserts which belong to the Eph receptor family. Two of these inserts (TK17 and TK63) were EphA4 and the other were EphB2 (TK25) and EphA5 (TK23). We performed in situ hybridization, and we found our clones to be present in all fields of the hippocampus, their expression being mainly neuronal. Three days after lesion, prominent expression appeared in CA1 as compared to the same field in the non-treated contralateral hippocampus. We performed northern blot analysis for quantification, and found that, three days after injury, the values decreased to 33 +/- 4%, 33 +/- 1% and 46 +/- 1% of control values for TK63 (EphA4), TK25 (EphB2) and TK23 (EphA5), respectively. Neuronal death in CA3-CA4 might account for this fact. Later, five days post-injury, the expression increased to 63 +/- 3%, 71 +/- 1% and 111 +/- 5% of control values, respectively. This increase was due to an up-regulation of these genes in the hippocampal neurons that survive after the injury, as indicated by in situ hybridization.