Figure 2. GABA release from PV⁺ basket cells is not required for the CCK-induced suppression of GABA release from CCK⁺ basket cells.

A, left, camera lucida drawing of a representative CCK⁺ basket cell recorded in this study; note that the axons were located within or adjacent to the CA1 pyramidal cell layer. The insets show photomicrographs indicating the immunopositivity of the cell for CCK. Right, upper panel, example traces of current-clamp recordings of the CCK⁺ BC in response to current steps (−100 and 160 pA, from −60 mV); note the spike frequency adaptation. Right, lower panel, averaged traces of presynaptic action potentials (top) elicited in the CCK⁺ BC, and the respective postsynaptic responses (bottom; voltage clamp configuration) in the pyramidal cell. Note the fast and large unitary IPSCs; 100 postsynaptic responses are shown in grey, together with their averages (black). Abbreviations: Str. L.-M.: stratum lacunosum-moleculare; Str. Rad.: stratum radiatum; Str. Pyr.: stratum pyramidale; Str. Oriens: stratum oriens. B, CCK (500 nm) significantly inhibited the amplitudes of the effective unitary IPSCs (euIPSC, including both successful events and failures; see Methods) in the pyramidal cell (lower traces) evoked by the presynaptic CCK⁺ BC (upper traces). The individual IPSCs are grey and the averages are black. Summary data are shown on the right (open circles: individual pairs; filled circles: means). C, agatoxin TK (250 nm) produced a complete suppression of the unitary IPSCs in pyramidal cells evoked by presynaptic PV⁺ BCs. Note the irreversible suppression. D, in slices preincubated (>1 h) in agatoxin TK (250 nm), 500 nm CCK still attenuated the amplitude of unitary IPSCs in pyramidal cells evoked by presynaptic CCK⁺ BCs. Note that the degree of CCK-induced suppression of unitary IPSCs is similar between control (B) and agatoxin TK preincubated slices (D). Error bars represent s.e.m., and the asterisks denote P < 0.05.