SINGLE AXON FAST INHIBITORY POSTSYNAPTIC POTENTIALS ELICITED BY A SPARSELY SPINY INTERNEURON IN RAT NEOCORTEX

Many of the different morphological types of interneurons in mammalian neocortex are presumed to be inhibitory, but to date, conclusive func tional data have been lacking. Using paired intracellular recordings i n slices of adult rat somatosensory cortex, we present a sparsely spin y, burst firing interneuron that elicits in a simultaneously recorded pyramid a fast inhibitory postsynaptic potential, reversing at -78 mV. Neither inhibitory postsynaptic potential time course, nor paired pul se depression (inter-spike interval 5-120 ms), was affected by additio n of the GABA(B) antagonist/partial agonist 2-OH-Saclofen (250 mu M), but increasing extracellular [Ca2+] enhanced inhibitory postsynaptic p otential amplitude at low firing rates and increased paired pulse depr ession at higher rates. Light microscopic examination of the biocytin- filled neurons revealed the presynaptic cell to be a sparsely spiny in terneuron and the postsynaptic to be a small pyramidal neuron, both in layer II. Ultrastructural examination of 16 terminals of the presynap tic interneuron revealed that they formed symmetric contacts with unla belled neurons, four with neuronal somata, 10 with dendritic shafts an d two with spine shafts. This, therefore, is the first report of the p roperties of a single axon inhibitory postsynaptic potential in neocor tex resulting from action potentials in an electrophysiologically and morphologically identified interneuron. We propose that at least some of the sparsely spiny, burst firing interneurons inhibit pyramidal neu rons via GABA(A) receptors.