Pro- and anti-apoptotic evidence for cholinergic denervation and hippocampal sympathetic ingrowth in rat dorsal hippocampus

In rat, injection of the specific cholinotoxin, 192 IgG-saporin, into the medial septum results not only in a selective cholinergic denervation of hippocampus, but in an ingrowth of peripheral sympathetic fibers, originating from the superior cervical ganglion, into the hippocampus. A similar process, in which peripheral noradrenergic axons invade hippocampus, may also occur in Alzheimer's disease. Since apoptotic cell death has been demonstrated in the selective neuronal loss found in Alzheimer's disease, the aim of this study was to measure apoptotic protein expression and DNA fragmentation in hippocampal sympathetic ingrowth and cholinergic denervation. Western blot, TdT-mediated dUTP nick end labeling, and oligo ligation techniques were used. Choline acetyltransferase activity and norepinephrine concentrations were also measured. As seen in our previous results, an increase in apoptotic markers was induced by cholinergic denervation alone (medial septum lesion + ganglionectomy), while hippocampal sympathetic ingrowth (medial septum + sham ganglionectomy) reduced or normalized apoptotic effects to control group levels. A decrease in choline acetyltransferase activity was also found in the dorsal hippocampus of hippocampal sympathetic ingrowth and cholinergic denervation groups. An increase in norepinephrine concentration was found in hippocampal sympathetic ingrowth but not in cholinergic denervation group. Results of this study suggest that cholinergic denervation is responsible for most of the proapoptotic responses, while hippocampal sympathetic ingrowth produces a protective effect in the process of programmed cell death in rat dorsal hippocampus. This effect may be a secondary to an altered relationship between norepinephrine–acetylcholine.