Hippocampal inputs mediate theta-related plasticity in anterior thalamus

Hippocampally-driven oscillatory activity at theta frequency is found in the diencephalon, but an understanding of the fundamental role of theta in the hippocampo-diencephalic circuit remains elusive. An important strategy in determining how activity modifies oscillatory properties of hippocampo-diencephalic circuitry comprises investigations of anterior thalamic responses to their main inputs: the descending dorsal fornix and the ascending mammillothalamic tract. Here, we show that the amplitude of thalamic theta spectral power selectively increases after plasticity-inducing stimulation of the dorsal fornix, but not of the mammillothalamic tract in urethane-anaesthetized young male rats. Furthermore, we show that low-frequency stimulation (LFS) significantly augments the fornix-driven theta ratio (theta over delta power, T-ratio), in parallel with depressing thalamic synaptic responses. However, the mammillothalamic synaptic response after LFS did not correlate with the slow band of theta oscillation (low T-ratio), but did correlate positively with the fast band of theta oscillation (high T-ratio). Our data demonstrate that the descending direct fornix projection is a pathway that modulates theta rhythm in the hippocampo-diencephalic circuit, resulting in dynamic augmentation of thalamic neuronal responsiveness. These findings suggest that hippocampal theta differentially affects synaptic integration in the different structures with which the hippocampus is reciprocally connected.