Synaptic strength as a function of post- versus presynaptic expression of the neural cell adhesion molecule NCAM

To evaluate the contributions of the pre- versus postsynaptic expression of NCAM in regulation of synaptic efficacy, we cultured dissociated hippocamp al cells from NCAM-deficient and wild-type mice in homo- and heterogenotypi c combinations. Double recordings from synaptically coupled neurons maintai ned in heterogenotypic cocultures showed that synaptic strength of excitato ry but not inhibitory synapses depended on expression of NCAM post- but not presynaptically. This correlated with higher levels of potentiation and sy naptic coverage of NCAM-expressing neurons compared to NCAM-deficient neuro ns in heterogenotypic cocultures. Synaptic density was the same in homogeno typic cultures of NCAM-deficient and wild-type neurons as well as in hetero genotypic cocultures in which glutamate receptors were blocked. These obser vations indicate that the relative levels of postsynaptic NCAM expression c ontrol synaptic strength in an activity-dependent manner by regulating the number of synapses.