Postsynaptic calcium influx at single synaptic contacts between pyramidal neurons and bitufted interneurons in layer 2/3 of rat neocortex is enhanced by backpropagating action potentials.

Pyramidal neurons in layer 2/3 (L2/3) of the rat somatosensory cortex excite somatostatin-positive inhibitory bitufted interneurons located in the same cortical layer via glutamatergic synapses. A rise in volume-averaged dendritic [Ca2+]i evoked by backpropagating action potentials (APs) reduces glutamatergic excitation via a retrograde signal, presumably dendritic GABA. To measure the rise in local [Ca2+]i at synaptic contacts during suprathreshold excitation, we identified single synaptic contacts in the acute slice preparation in pairs of pyramidal and bitufted cells each loaded with a Ca2+ indicator dye. Repetitive APs (10-15 APs at 50 Hz) evoked in a L2/3 pyramidal neuron gave rise to facilitating unitary EPSPs in the bitufted cell. Subthreshold EPSPs evoked a transient rise in [Ca2+]i of 80-250 nM peak amplitude at the postsynaptic dendritic site. The local postsynaptic [Ca2+]i transient was restricted to 10 microm of dendritic length, lasted for 200 msec, and was mediated predominantly by NMDA receptor channels. When EPSPs were suprathreshold, the evoked AP backpropagated into the apical and basal dendritic arbor and increased the local [Ca2+]i transient at active contacts by approximately twofold, with a peak amplitude reaching 130-450 nM. This value is in the range of the half-maximal dendritic [Ca2+]i, evoking retrograde inhibition of glutamate release from boutons of pyramids. The localized enhancement of dendritic Ca2+ influx at synaptic contacts by synaptically evoked backpropagating APs could represent one mechanism by which a retrograde signal can limit the excitation of bitufted interneurons by L2/3 pyramids when these are repetitively active.