The Journal of Neurophysiology Vol. 77 No. 1 January 1997, pp. 353-363
Copyright ©1997 The American Physiological Society

Synaptic and Synaptically Activated Intrinsic Conductances Underlie Inhibitory Potentials in Cat Lateral Amygdaloid Projection Neurons In Vivo

E. J. Lang and D. Paré

Département de Physiologie, Université Laval, Québec, Québec, G1K 7P4, Canada

Lang, E. J. and D. Paré. Synaptic and synaptically activated intrinsic conductances underlie inhibitory potentials in cat lateral amygdaloid projection neurons in vivo. J. Neurophysiol. 77: 353-363, 1997. The companion paper demonstrated that the responses of lateral amygdaloid (LAT) projection neurons to the stimulation of major input and output structures are dominated by monophasic hyperpolarizing potentials of large amplitude. To characterize the mechanisms underlying these inhibitory potentials, intracellular recordings of cortically evoked responses were obtained from morphologically and/or physiologically identified LAT projection neurons in barbiturate anesthetized cats. The reversal potential of the cortically evoked hyperpolarization was measured at its peak, and 115 ms later (tail), an interval corresponding to the peak latency of the -aminobuturic acid-B (GABA_B) response previously recorded in vitro. When recorded with K-acetate (KAc) pipettes, these reversal potentials were 86.9 ± 1.6 mV (peak; mean ± SE) and 90.7 ± 1.7 mV (tail), suggesting that both Cl and K⁺ conductances contribute throughout the cortically evoked hyperpolarization. The small, but consistent, difference between the two reversal potentials suggested that an additional slowly activating K⁺-mediated component contributed to the inhibitory postsynaptic potential (IPSP) tail. To determine whether Cl conductances contributed to the evoked hyperpolarization, recordings were performed with KCl; the peak (57.8 ± 2.2 mV) and tail (61.3 ± 2.1 mV) reversal potentials were ~15-20 mV more depolarized than those recorded with KAc pipettes. However, the difference between the peak and tail reversals remained. In an attempt to block the Cl conductance, recordings were obtained with pipettes filled with KAc or KCl and 4,4-diisothiocyanostilbene-2,2-disulphonic acid (DIDS), a Cl pump blocker that also was reported to block GABA_A responses. With KAc and DIDS, the initial depolarization was prolonged and the amplitude of the hyperpolarization decreased relative to that seen with KAc alone. However, with KCl and DIDS, the reversal potential was shifted to an even greater extent than with KCl pipettes with the evoked response consisting entirely of a large depolarization, which produced a spike burst. These results suggest that LAT neurons have a Cl pump that is blocked by DIDS, but that their Cl channels are not blocked by DIDS. To assess the contribution of K⁺ conductances to cortically evoked hyperpolarizing potentials, recordings were obtained with Cs-acetate pipettes. Under these conditions, the response reversed at more depolarized potentials (peak, 71.9 ± 1.0 mV; tail,72.0 ± 0.9 mV) compared with KAc recordings, with no difference between the peak and tail reversal potentials. These cells also had depolarized resting potentials (66.2 ± 1.8 mV) compared with those of cells recorded with KAc pipettes (73.6 ± 1.8 mV); however, this difference was too small to attribute the shift in reversals to a redistribution of Cl ions across the membrane. The action potentials generated by LAT neurons under Cs⁺ had a shoulder that prolonged their falling phase. The increased duration of the spikes was presumably due to a dendritic Ca²⁺ conductance because LAT amygdaloid neurons are known to possess such conductances and Cs⁺ blocks the delayed rectifier and some Ca²⁺-dependent K⁺ currents. The dramatic reduction of this shoulder by spontaneous and evoked IPSPs suggests that the activation of dendritic conductances by back-propagating somatic action potentials is regulated tightly by synaptic events. Intracellular injection of the Ca²⁺ chelating agent, 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid (100 mM) caused a depolarization of the peak (75.3 ± 1.3 mV) and tail (77.7 ± 1.7 mV) reversal potentials during a time course of 15-45 min. Concurrently, the amplitude of the excitatory postsynaptic potential increased whereas that of the hyperpolarization decreased, suggesting that a Ca²⁺-dependent K⁺ conductance contributes significantly to the evoked hyperpolarization. In conclusion, the large hyperpolarizing potentials that regulate the excitability of LAT projection neurons appear to be mediated primarily by a Cl, presumably GABA_A, IPSP, and a synaptically activated Ca²⁺-dependent K⁺ conductance. A relatively weak K⁺-mediated, possibly GABA_B, IPSP makes a small contribution to the later portions of the response.

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