Changes in dendritic morphology such as dendritic pruning precede cell death in many neurodegenerative disorders, including HIV-1-associated neurocognitive disorders (HAND). Dendritic degeneration correlates with cognitive decline in HAND. A confocal imaging-based assay was developed to detect intact postsynaptic densities (PSDs) based on detection of clusters of the scaffolding protein PSD95 fused to green fluorescent protein (PSD95-GFP). In neuronal cultures, PSD95-GFP puncta were lost following exposure to factors released by HIV-1 infected cells including the HIV-1 proteins Tat and gp120. PSD loss induced by HIV-1 neurotoxins is reversible. This proposal will relate the loss and recovery of synapses to cognitive function in mouse models of neuroAIDS. In vivo multiphoton imaging will track PSD95-GFP puncta during expression of HIV-1 proteins and behavioral tests will monitor cognitive function. The first aim is to relate HIV-1 protein induced synapse loss and recovery to cognitive performance. Treatment with ifenprodil, an antagonist selective for NR2B-containing NMDA receptors, induced the recovery of synapses lost following in vitro exposure to the HIV-1 protein Tat. What remains unclear is whether synapses recover in vivo and if they do, whether cognitive function improves. Transgenic mice that express Tat under the control of an inducible promoter exhibit synaptic degeneration and cognitive decline following induction. The hypothesis that ifenprodil will evoke recovery of synapses lost in Tat-expressing animals and that synaptic recovery will correlate with improved cognitive function will be tested. Anticipated results may provide proof of the principle that drug-induced changes in synaptic number predict changes in cognitive function. The second aim is to determine the effects of cannabinoids, drugs given to AIDS patients clinically and widely used illicitly, on the synaptic and cognitive changes induced by HIV-1 proteins. Synapse loss and recovery is a dynamic process influenced by cannabinoids in vitro. The effects of cannabinoids on cognitive function in HIV-1 models are unknown. Agonists selective for cannabinoid receptor subtypes will be tested in transgenic mice expressing the HIV-1 proteins gp120 or Tat. The CB2 agonist JWH-133 is predicted to improve cognition in gp120- expressing mice. In contrast, the CB1 agonist arachidonyl-2'-chloroethylamide (ACEA) is predicted to impair ifenprodil-induced synapse recovery and limit cognitive improvement in Tat-expressing mice. If cannabinoids impair synaptic and cognitive recovery, this result would caution against recreational use of cannabinoids or their use as antiemetics and appetite stimulants in patients with HAND. In vivo multiphoton imaging has potentially broad applications for relating the effects of drugs on synaptic structure to their effects on behavior. This project will provide a foundation to guide th development of drugs to improve function in HAND patients and will identify sites where drugs of abuse might interact with the formation and loss of synapses.