DIFFERENCES IN INHIBITORY SYNAPTIC INPUT BETWEEN LAYER-II-III AND LAYER-V NEURONS OF THE CAT NEOCORTEX

1. The goal of this study was to compare the relative effectiveness of intrinsic inhibitory synaptic inputs in different layers of the cat m otor cortex. Postsynaptic potentials (PSPs) were evoked in neurons loc ated in the superficial (layer II-III) or deep layers (layer V) by loc al extracellular stimulation in vitro. Electrophysiological properties and intracellular filling indicated that the recorded neurons were py ramidal cells. 2. The shape and time course of the evoked PSPs differe d. Layer II-III cells showed stereotyped triphasic PSPs consisting of a fast excitatory PSP (fEPSP) and a fast and slow inhibitory PSP (fIPS P and sIPSP, respectively). PSPs in layer V cells, in contrast, were m uch more variable, mainly depolarizing at resting membrane potential, and lacked a hyperpolarizing IPSP in 84% of neurons tested at rest. 3. Blockade of glutaminergic neurotransmission with 6-cyano-7-nitroquino xaline-2,3-dione (CNQX) and D-2-amino-5-phosphonovaleric acid (AP5) re vealed that fIPSPs could be evoked in all layer V cells by local stimu lation of the superficial or deep layers, even in those that showed sm all or no IPSPs in control perfusate. Small (<1 mV) isolated sIPSPs we re evoked in only one-fifth of layer V cells when the deep layers were stimulated, and in about one-half of the layer V cells when the super ficial layers were stimulated. In layer II-III cells, stimulation of t he superficial layers always resulted in fIPSP-sIPSP combinations. No IPSPs could be evoked in layer II-III neurons by stimulating the deep layers after glutaminergic blockade. Selective blockade of gamma-amino butyric acid-A (GABA(A)) or GABA(B) receptor-mediated neurotransmissio n showed that in both cell types fIPSPs were due to GABA(A) receptor s timulation, whereas sIPSPs were mediated by GABA(B) receptors. 4. Isol ated fIPSPs were recorded in perfusate containing CNQX, AP5, and the G ABA(B) antagonist CGP 35348. The rise and decay times of the fIPSPs in layer II-III cells were significantly longer than those in layer V ce lls. Rise and decay times normalized for differences in membrane time constant were not significantly different, however, suggesting that th e intrinsic membrane properties of the postsynaptic membrane account f or the difference in time course of the fIPSPs in these two cell types . 5. Selective blockade of the inward rectifier current I-h with extra cellular Cs+ showed that this conductance functions to shorten and att enuate fIPSPs in layer V cells. In contrast, I-h is absent or small in layer II-III cells, and, consequently, Cs+ had little or no effect on the fIPSPs evoked in these cells. In layer V cells that had sIPSPs, e xtracellular Cs+ enhanced sIPSP amplitude, but these sIPSPs were still small compared with those in layer II-III cells. These results indica te that I-h contributes to the relatively weak and short-lived IPSPs r ecorded in layer V cells. In contrast, the presence of sIPSPs depends more strongly on differences in inhibitory circuitry or properties of GABAB synapses between deep and superficial layers of the cortex. 6. W hen trains of IPSPs were evoked by repetitive extracellular stimulatio n after blockade of glutaminergic neurotransmission, our experiments s howed that the amount of temporal summation and ''accumulation'' of hy perpolarizing inhibition depended both on the presence of GABA(B)-medi ated sIPSPs and the duration of the fIPSP. The small or absent sIPSP a nd the short fIPSP in layer V cells makes temporal summation of inhibi tory synaptic input less effective. 7. When GABA release was enhanced by the convulsant drug 4-aminopyridine (with excitation blocked), spon taneous and evoked biphasic IPSPs resembling fast and slow IPSPs were observed in all layer V cells tested. This indicates that GABA(B)-medi ated IPSPs require a larger release of GABA to be visible in layer V c ompared with layer II-III neurons. 8. Evoked single fIPSPs were able t o veto action potentials generated in response to intracellular stimul ation, whereas single sIPSPs were more suited to modulate firing rate but were able to do so over longer time periods. 9. In conclusion, our studies show that pyramidal neurons in the superficial layers of the cat motor cortex are controlled by a powerful local inhibitory circuit that operates through both GABA(A)- and GABA(B)-mediated IPSPs. This dual-component inhibitory input tends to hyperpolarize these cells awa y from firing threshold during bursts of synaptic input. In comparison , intrinsic membrane properties and weak or absent sIPSPs in layer V p yramidal cells appear to be designed to rapidly bring these neurons to firing threshold during repeated synaptic input. These results sugges t that there are laminar-specific inhibitory circuits operating in the cat motor cortex.