Laminar and temporal heterogeneity of NMDA/metabotropic glutamate receptorbinding in posterior cingulate cortex

Both N-methyl-D-aspartate (NMDA) and quisqualate/AMPA-insensitive metabotro pic glutamate (mGlu) receptors mediate plasticity induction in neocortex, b ut their interlaminar distribution in cortical microcircuits is largely unk nown. We used (+)H-3-MK801 and H-3-glutamate binding plus saturating concen trations of NMDA, AMPA, and quisqualate to autoradiographically map NMDA an d mGlu receptor sites by lamina in posterior cingulate cortex in adult male rats. Specific binding at NMDA receptor sites in laminae II/III and VI was significantly reduced in comparison to other laminae. Brains prepared from rats killed during dark phase of a 12h/12h light/dark cycle showed a mean 129% increase in overall (+)H-3-MK801 binding versus light phase brains but retained reduced binding densities in laminae II/III and VI. In contrast t o NMDA findings, specific binding at mGlu sites was consistently elevated d uring light phase in both laminae II/III and VI. Specific H-3-glutamate bin ding in dark-phase brains showed an overall 147% increase versus light phas e binding but did not retain significant interlaminar heterogeneity. Interp reted in accordance with our physiologically derived models of hippocampo-c ortical microcircuitry, these results suggest that spatial and temporal var iations in glutamate receptor distribution may play an important role in in tracingulate neural processing of afferent input from hippocampus.