Changes in Regional Brain Acetylcholine Content in Rats Following Unilateral and Bilateral Brainstem Lesions

Previous research (Adametz, 1959) has shown that two-step bilateral lesions of the reticular formation in cats produce minimal behavioral disruption compared to one-step bilateral lesions, which produce profound behavioral suppression. We systematically examined alterations in forebrain acetylcholine (ACh) content and neurologic tolerance to one-step and two-step bilateral and unilateral lesions of the pontomesencephalic reticular formation (PMRF) in rats. One-step and two-step bilateral lesions separated by 1 or 5 days produced irreversible bilateral motor dysfunction. Survival after lesioning was 10%, 20%, and 0%, respectively. Unilateral lesion or two-step bilateral lesions separated by 15 or 30 days produced transient (less than 3 days) contralateral motor dysfunction. Survival after lesioning was 90%, 90%, and 100%, respectively. Within 24 h after one-step bilateral lesions, ACh content was significantly decreased bilaterally in thalamus, frontal cortex, amygdala, hippocampus, and basal forebrain. Within 5 days after unilateral lesioning, ACh content was significantly decreased ipsilaterally in the thalamus, amygdala, and hippocampus and had returned to control values by day 10 in the thalamus and hippocampus. The increased neurologic tolerance and recovery of ACh content in two-step bilateral PMRF lesions demonstrate important functional and neurochemical plasticity to brain injury. Although not directly addressing mechanisms of neural plasticity, this research examined possible associations between neurologic tolerance to PMRF lesions and neurochemical markers of forebrain ACh activity.