Figure 8. Summary and schematic model.

The data have shown that there are two distinct groups of interneuronal responses that we propose originate from functionally distinct interneuron classes, types 1 and 2. Type 1 interneurons are stimulated by CCh, and IPSPs originating from these neurons are inhibited by endocannabinoids, conotoxin and activation of GABA_B receptors. Type 2 interneurons are activated by CCK, and IPSPs originating from these neurons are inhibited by agatoxin, but are insensitive to endocannabinoids or conotoxin. Our data also suggest that CCh causes the liberation of CCK, perhaps from type 1 cells. Activation of type 2 cells by CCK feeds back GABA, which by activation of GABA_B receptors, partially suppresses the type 1 interneurons. Because their pharmacological profiles are consistent with the identification of type 1 as CCK-expressing interneurons and type 2 with PV interneurons, we tentatively propose this conclusion. Predictions of the model remain to be tested.