Encephalon 12: Medial Temporal Lobe and Hypocretin in Memory and Navigation
Memory is a complex cognitive process that involves encoding, storing, and recalling information. One of the critical regions in the brain associated with memory, particularly declarative or explicit memory, is the medial temporal lobe, which includes structures such as the hippocampus. The hippocampus is essential for the memory of people, objects, and places, and it plays a pivotal role in spatial memory. The importance of hippocampal structures in episodic memory was first highlighted in the study of patient H.M., who suffered from severe memory impairments following surgical removal of his hippocampus.
The medial entorhinal cortex (MEC) is another crucial component of the memory network. It forms part of the MEC-hippocampal circuit, which receives and sends inputs to the hippocampus. This circuit is vital for spatial memory and navigation. Cholinergic and monoaminergic wakefulness-promoting systems directly innervate the MEC and other brain regions related to spatial memory, indicating the importance of arousal and attention in memory processes.
Recent research (Y.Liao et al., 2024) has shed light on the role of hypocretin (Hcrt) neurons in memory. These neurons are activated during object-place exploration and exhibit higher activity during novel object-place exploration compared to novel context exploration. Hcrt neurons display dense innervation in the MEC, and their activity increases in the MEC during novel object-place exploration. This suggests that Hcrt neurons are engaged in novel object-place exploration and play a role in memory encoding.
The lateral hypothalamus (LH) Hcrt-MEC pathway is specifically involved in object-place memory encoding but not in recall. Hcrt stimulates presynaptic terminals of glutamatergic neurons to excite fast-spiking parvalbumin-positive (PV+) interneurons in the MEC. Gamma oscillations driven by PV+ neurons are associated with object-place memory encoding. The LH Hcrt-MEC pathway elevates gamma oscillations and signal-to-noise ratios (SNRs), supporting object-place memory encoding.
In conclusion, the medial temporal lobe, particularly the hippocampus and MEC, plays a crucial role in memory encoding and spatial navigation. The involvement of Hcrt neurons and the LH Hcrt-MEC pathway in modulating gamma oscillations and enhancing memory encoding highlights the intricate interplay between different brain regions and neurotransmitter systems in memory processes. Understanding these mechanisms provides valuable insights into the neural basis of memory and could inform the development of therapeutic strategies for memory disorders.
