USING THE SUBCORTICALLY LESIONED RAT CORTEX TO UNDERSTAND THE PHYSIOLOGICAL-ROLE OF AMYLOID PRECURSOR PROTEIN

Alzheimer's disease pathology is characterized by the presence of neuritic plaques and neurofibrillary tangles and specific neurotransmitter deficits in the cortex and hippocampus. Advances in the understanding of Alzheimer's disease have been hampered by the absence of appropriate animal model systems. Most in vivo rodent models have turned to aged animals, animals with experimentally induced lesions of various neurotransmitter systems, animals with pharmacologically induced neurotransmitter perturbations, and mice made transgenic for genes related to amyloid precursor protein. These models have been useful for the investigation of some discrete aspects of Alzheimer's disease, including deficits in forebrain cholinergic activity and the resulting cognitive deficits. However, none of these models have led to the development of the principal neuropathological hallmarks of Alzheimer's disease, neuritic plaques and neurofibrillary tangles. Furthermore, the relationship, if any, between the reduction of neurotransmitter activity and the formation of neuritic plaques and neurofibrillary tangles is unknown. The subcortically lesioned rat model system which we have used approximates the cortical neurotransmitter and the cognitive deficits of Alzheimer's disease. We have recently found that these same subcortical neurotransmitter system lesions alter the expression of amyloid precursor protein, the precursor of beta amyloid peptide, which is the principal component of neuritic plaques. Loss of functional subcortical innervation by either permanent lesions or transient inhibition of cortical neurotransmitter (acetylcholine) release resulted in the induction of amyloid precursor protein in the cortex. The induction was rapid and persistent with the permanent lesions or reversible with the transient inhibition. Lesions of cholinergic, serotonergic, and adrenergic neurotransmitter systems all resulted in the induction. The induction was accompanied by elevated secretion of amyloid precursor protein as evidenced by elevated levels of the secreted form of the protein in the cerebrospinal fluid of the lesioned animals. Although this alteration does not lead to formation of neuritic plaques in the brain, this animal represents a powerful model for investigating the normal physiological role of amyloid precursor protein in vivo and provides a means for integrating the relationship between neurotransmitter deficits, expression of amyloid precursor protein, and formation of neuritic plaques.