Figure 1.

Circuit Models of Theta-Gamma Oscillations in the Dentate Gyrus

(A) Classical model, in which theta rhythm originates in the medial septum and gamma rhythm is generated in interneuron loops. Adapted from Stewart and Fox (1990). For cholinergic innervation of the dentate gyrus, see Frotscher and Léránth (1986).

(B) Alternative model in which both theta and gamma rhythms are relayed from the entorhinal cortex via glutamatergic synapses. Such a model is motivated by the finding that bilateral chronic lesions of the entorhinal cortex markedly reduce the power of both theta and gamma oscillations (Bragin et al., 1995).

(C) Alternative model derived from the present experimental results. Theta rhythmic activity is primarily relayed from the entorhinal cortex via glutamatergic synapses, while gamma activity is generated intrinsically by local oscillators involving interneurons. Note that the schemes for simplicity only illustrate direct inputs to the dentate gyrus GC-interneuron network; polysynaptic pathways (e.g., septal and entorhinal input to entorhinal cortex neurons or hilar mossy cells) are omitted. MS, medial septum; DG, dentate gyrus; EC, entorhinal cortex; gc, granule cell; st, stellate cell; int, interneuron.