Rationale: Recently, we reported that compared to haloperidol, chronic exposure to either the risperidone (RISP) or olanzapine (OLZ) resulted in superior effects on spatial learning performance as well as the cholinergic neurons, although the mechanism for these effects was not addressed.
Objectives: The objective of this study was to investigate one plausible mechanism whereby RISP and OLZ exert superior effects on cholinergic neurons, i.e. positive effects on nerve growth factor (NGF), which is known to regulate the brain cholinergic activity as well as cognitive function. Therefore, the effects of chronic exposure to HAL, RISP, or OLZ on the expression of NGF and choline acetyltransferase (ChAT) in the hippocampus (i.e. a brain area well known to be involved in cognitive function and known to receive major cholinergic projections from the medial septum) were compared.
Methods: Rats were treated with HAL (1 or 2 mg/kg per day), RISP (1.25 or 2.5 mg/kg per day), or OLZ (5 or 10 mg/kg per day) for 45 days in drinking water. NGF and ChAT were measured by immunohistochemistry and NGF protein was also measured by an enzyme-linked ImmunoSorbent assay.
Results: Compared to controls, HAL exposure resulted in a marked reduction in NGF immunoreactivity in the hippocampal dentate gyrus (DG), CA1 and CA3 areas. In contrast, RISP did not alter, while OLZ significantly increased levels of NGF. These changes in NGF levels corresponded well with changes in ChAT immunoreactivity in the hippocampus and the medial septum.
Conclusions: These preclinical data, combined with previously published behavioral results, support the premise that OLZ and RISP, in contrast to HAL, preserve cholinergic pathways and cognitive function via superior effects on NGF expression and are thus therapeutically superior for extended use.