aggregates that may be toxic to neurons (9-11). Neurotransmitters can regulate APP processing to favor the secretion of APPS (12). In human embryonic kidney (HEK) 293 cells stably expressing the human muscarinic receptor subtypes ml or m3, stimulation with the muscarinic agonist carbachol increased APP, secretion. The muscarinic receptors mediating this effect (ml and m3 but not m2 or m4) are coupled to intracellular signaling pathways via second messengers, diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (InsP3), which are generated by phosphatidylinositol 4,5-bisphosphate (PtdInsP2) hydrolysis. In other established cell lines, accelerated APP metabolism produced by direct stimulation of protein kinase C (PKC) or inhibition of phosphatase activity also increased APP, secretion (13-16). In HEK 293 cells overexpressing ml receptors, there was a reciprocal relationship between APP, and AP3 secretion after receptor activation with carbachol (17); thus, receptor activation can enhance APP, secretion and also suppress AP3 formation.Abundant levels of both APP message and protein are present in neurons. The detection of APP, in plasma and cerebrospinal fluid (18,19) suggests that proteolysis of APP into soluble derivatives occurs in the central nervous system. Because receptor activation can regulate APP processing, impaired neurotransmission could conceivably exacerbate amyloid formation in AD, particularly in cortex and hippocampus. In addition to the loss of cholinergic basal forebrain neurons (20), glutamatergic corticocortical connections and major projections of the hippocampus are especially vulnerable to damage in AD (21). These pathways atrophy during the early stages of the disease, and in postmortem brain glutamate concentrations are decreased by as much as 80% (22). Because glutamate activates PKC in nervous tissue (23, 24), we tested the hypothesis that glutamate might divert APP processing from amyloidogenic pathways and, instead, favor increased APPs secretion.Metabotropic glutamate receptors (mGluR) are coupled to the formation of multiple second messengers via activation of phospholipase enzymes (23-25). Nonselective glutamate agonists [e.g., L-glutamate, quisqualate (QA)] can interact with both mGluR and ionotropic glutamate receptors (iGluR), but the selective mGluR agonist, trans-(1S,3R)-l-amino-1,3-cyclopentane dicarboxylic acid (ACPD) initiates signal transduction without affecting the iGluR. mGluR exist as seven subtypes and are categorized into three major groups on the

It has previously been shown that stimulation of muscarinic m1 or m3 receptors can, by generating diacylglycerol (DAG) and activating protein kinase C (PKC), accelerate the breakdown of the amyloid precursor protein (APP) to form soluble, non-amyloidogenic peptides (APPs). This relationship has been demonstrated in human glioma and neuroblastoma cells as well as in transfected human embryonic kidney (HEK) cells and PC12 cells. We now provide evidence that stimulation of metabotropic glutamate receptors (mGluRs), which also are coupled to DAG and PKC, similarly accelerates processing of APP into non-amyloidogenic APPs in hippocampal neurons and cortical astrocytes derived from normal fetal rats. The mGluR antagonist, L(+)-2-amino-3-phosphonopropionic acid (L-AP3), and GF 109203X, an inhibitor of PKC, both blocked the release of APPs from hippocampal neurons and astrocytes evoked by glutamate receptor stimulation. Inasmuch as glutamatergic neurons in cortex and hippocampus are known to be damaged in Alzheimer's disease, our findings suggest that amyloid formation may be enhanced by the resulting glutamate deficiency and that selective mGluR agonists may be useful in facilitating synaptic efficacy and treating the disease.