Midline thalamic region: widespread excitatory input to the entorhinal cortex and amygdala.

The midline thalamus has a role in memory formation and has well described projections to multiple limbic sites including the hippocampus, amygdala, and entorhinal cortex. Stimulation of this region evokes excitatory responses in the CA1 region of the hippocampus, but nothing is known about the nature of thalamic influence on other limbic sites such as the entorhinal cortex and the amygdala. In this study we electrically stimulated the midline thalamus in anesthetized rats to determine whether responses could be evoked in the amygdala or entorhinal cortex. In addition we examined the distribution of the responses within the target regions as well as the effect of short interval paired or high-frequency tetanizing stimulation. We found reproducible responses in the entorhinal cortex and the amygdala with a distribution of responses that matched the described synaptic input from the thalamus. In addition, high-frequency stimulation induced a consistent long-term potentiation in the two sites. Paired stimulation resulted in depression of the test response in the amygdala, but a facilitation in the entorhinal cortex. These findings indicate that the midline has a significant monosynaptic excitatory influence in the amygdala and the entorhinal cortex. Combined with the previous work in the hippocampus, this study suggests that the midline thalamus plays a significant role in limbic physiology and may serve to synchronize activity in this system.