The Astrocyte Surface NAAG Receptor and NAAG Peptidase Signaling Complex as a Therapeutic Target

There is evidence that schizophrenia and other neuropathies may involve malfunction of a unique N-acetylaspartylglutamate (NAAG) receptor and its associated NAAG peptidase, a receptor and enzyme found together on the astrocyte surface. NAAG is a peptide neurotransmitter released by stimulated neurons and specifically targeted to the group II metabotropic glutamate receptor 3 (mGlu(3)), activation of which initiates astrocyte Ca(2+) waves responsible for astrocyte-astrocyte and astrocyte-vascular system signaling and induction of vascular hyperemic responses that increase energy supplies to stimulated neurons. In this review, it is hypothesized that the receptor and enzyme exist as a cytostructural unit on the astrocyte surface, and the nature of this proposed mGlu(3)-NAAG peptidase complex is considered in terms of its physiological signaling role, and of the effect of drugs on this role. The mGlu(3) receptor has been the target of extrinsic antagonists and agonists that mimic NAAG structure and compete with natural NAAG for the receptor site. NAAG metabolism has also been the target of extrinsic NAAG-like substances that inhibit NAAG peptidase, competing with NAAG for the enzyme active site. Several drugs that affect the mGlu(3) receptor or NAAG peptidase have reached a stage of human testing. Two are agonists of the mGlu(3) receptor, and another is an NAAG peptidase inhibitor. These substances appear to have potential for treating schizophrenia and other cognitive neuropathies by interfering with a homeostatic NAAG activated neuron-astrocyte-vascular energy supply system.