SINGLE AXON IPSPS ELICITED IN PYRAMIDAL CELLS BY 3 CLASSES OF INTERNEURONS IN SLICES OF RAT NEOCORTEX

1. Using dual intracellular recordings in slices of adult rat neocorte x, twenty-four IPSPs activated by single presynaptic interneurones wer e studied in simultaneously recorded pyramidal cells. Fast spiking int erneurones inhibited one in four or fire of their close pyramidal neig hbours. No reciprocal connections were observed. After recordings neur ones were filled with biocytin. 2. Interneurones that elicited IPSPs w ere classified as classical fast spiking (n = 10), as nonclassical fas t spiking (n = 3, including one buret-firing interneurone), as unclass ified, or slow interneurones (n = 8), or as regular spiking interneuro nes (n = 3), i.e. interneurones whose electrophysiological characteris tics were indistinguishable from those of pyramidal cells. 3. All of t he seven classical fast spiking cells anatomically fully recovered had aspiny, beaded dendrites. Their partially myelinated axons ramified e xtensively, varying widely in shape and extent, but randomly selected labelled axon terminals typically innervated somata and large calibre dendrites on electron microscopic examination. One 'autapse' was demon strated. One presumptive regular spiking interneurone axon made four s omatic and five dendritic connections with unlabelled targets. 4. Full anatomical reconstructions of labelled classical fast spiking interne urones and their postsynaptic pyramids (n = 5) demonstrated one to fiv e boutons per connection. The two recorded IPSPs that were fully recon structed morphologically (3 and 5 terminals) were, hen-ever, amongst t he smallest recorded (< 0.4 mV). Some connections may therefore involv e larger numbers of contacts. 5. Single axon IPSPs were between 0.2 an d 3.5 mV in average amplitude at -55 to -60 mV. Extrapolated reversal potentials were between -70 and -82 mV. IPSP time course correlated wi th tile type of presynaptic interneurone, but not with IPSP latency, a mplitude, reversal potential, or sensitivity to current injected at th e soma. 6. Classical fast spiking interneurones elicited the fastest I PSPs (width at half-amplitude 14.72 +/- 3.83 ms, n = 10) and unclassif ied, or slow interneurones the slowest (56.29 +/- 23.44 ms, n = 8). Re gular spiking interneurone IPSPs had intermediate half-widths (27.3 +/ - 3.68 ms, n = 3). 7. Increasingly brief presynaptic interspike interv als increased the peak amplitude of but not the area under, the summed IPSP. Only at interspike intervals between 10 and 20 ms did IPSP inte grals exhibit paired pulse facilitation. Paired pulse depression was a pparent at (10 and 20-60 ms. During longer spike trains, summing IPSPs decayed to a plateau potential that was relatively independent of fir ing rate (100-250 Hz). Thereafter, the voltage response could increase again. 8. Summed IPSPs elicited by two to fifteen presynaptic spike t rains decayed as, or more rapidly than, single-spike IPSPs. Summed IPS Ps elicited by > 20 spikes (> 150 Hz), however, resulted in an additio nal, more slowly decaying component (latency > 50 ms, duration > 200 m s). The possible involvement of GABA(B) receptors in this component is discussed. 9. It is suggested that three broad classes of interneuron es may activate GABA(A) receptors on relatively proximal portions of n eocortical pyramidal neurones. The different time courses of the IPSPs elicited by the three classes may reflect different types of postsyna ptic receptor rather than dendritic location. An additional class, bur st firing, spiny interneurones appear to activate GABA(A) receptors on more distal sites.