Perforant path damage results in progressive neuronal death and somal atrophy in layer II of entorhinal cortex and functional impairment with increasing postdamage age

In vivo model systems that can evaluate neuronal death, survival, and regeneration are critical to revealing basic mechanisms of neuronal response and developing strategies for CNS repair. We propose a distinct experimental model of CNS degeneration following lesions to the perforant path connecting the hippocampus and the entorhinal cortex. Within 2 weeks of a unilateral aspirative perforant path lesion, 30% of the ipsilateral entorhinal cortex layer II (ECL II) projection neurons had died with no change in the contralateral ECL II population. Although there was no loss of ECL II neurons with normal aging, animals that survived for 15 months postlesion experienced an almost 50% loss of ipsilateral neurons compared to unlesioned controls. This progressive neuronal death was bilateral, with the contralateral ECL II experiencing a 30% decline in neuronal number relative to unlesioned controls. The use of unbiased stereology ensured that estimates of total number were not distorted by changes in the reference volume. The documented progressive neuronal death resulted in delayed behavioral impairment in spatial learning and performance (latency nearly 200% of controls). We propose, therefore, that the perforant path model is suitable for experimental investigation of neuronal survival and regeneration following CNS trauma.