Long term changes in brain cholinergic markers and nerve growth factor levels after partial immunolesion

There are deficits in cholinergic basal forebrain neurons (CBFNs) in the aged brain and patients suffering Alzheimer's disease associated with a partial loss of the CBFNs. To mimic this partial loss and assess its long term effects on residual cholinergic activity and resultant target-derived nerve growth factor (NGF) levels, we produced a partial immunolesion to CBFNs with 192 IgG-saporin, an immunotoxin selectively taken up by p75NTR-bearing neurons. We measured two cholinergic markers, choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity, and NGF protein levels at 10 days, 1, 6 and 12 months postlesion. There were no significant changes in the cholinergic markers and the NGF protein levels in the sham-treated animal controls during the one year experiment. Ten days after 192 IgG-saporin treatment, ChAT activity decreased to 35-50% of controls in the olfactory bulb, hippocampus, and cortex. There was a minor but significant recovery of ChAT activity one year after the immunolesion in the hippocampus. Changes in AChE activity mirrored the ChAT changes but were less robust. There were transient increases in NGF protein levels in the hippocampus and cortex that returned to basal levels at 6 months and 12 months postlesion, respectively. In summary, partial immunolesions resulted in partial region-specific and time-dependent recoveries of cholinergic activity in the target areas of the basal forebrain after a partial elimination of CBFNs and a return to basal levels of NGF protein consistent with the hypothesis that the remaining CBFNs compensated for losses of ChAT and NGF due to changes in cholinergic innervation of basal forebrain target areas.