Hippocampus

The hippocampus is a curved, seahorse-shaped structure buried within the medial temporal lobe, present as a paired region in each cerebral hemisphere. It is part of the limbic system and is essential to forming new memories and constructing the internal maps the brain uses to navigate physical space.

Without intact hippocampal function, new experiences cannot be encoded as lasting memories — a condition documented in patients whose hippocampal tissue was damaged or removed. Despite its modest size, it is one of the most thoroughly studied structures in neuroscience, sitting at the intersection of memory, emotion, and the brain's stress response.

How it works · its role

The hippocampus consolidates memories by strengthening the connections between neurons — a cellular process called long-term potentiation (LTP). NMDA receptors, which respond to the neurotransmitter glutamate, are central to this mechanism; they act as coincidence detectors, opening only when two connected neurons are active simultaneously, and this is thought to be the cellular basis of learning.

Specialised neurons called place cells fire when an individual occupies a specific location, building a cognitive map of the environment. This mapping function appears to extend beyond physical space: the hippocampus is thought to organise conceptual relationships between ideas in a similar way.

The hippocampus is also one of the few regions in the adult brain where new neurons continue to be generated, a process called neurogenesis. This ongoing renewal is thought to contribute to memory flexibility and, separately, to mood regulation.

Relevance to substances & effects

Cannabis exerts one of the most direct pharmacological effects on the hippocampus. THC binds to CB1 receptors, which are densely expressed here, suppressing the activity required to encode new information. This is the mechanism behind the short-term memory impairment commonly reported by cannabis users.

Dissociatives such as ketamine and PCP block NMDA receptors broadly, including in hippocampal circuits, disrupting memory consolidation and producing the spatial disorientation these substances are known for. At sub-anaesthetic doses, ketamine's effects on hippocampal plasticity are also thought to contribute to its rapid antidepressant action — possibly involving a burst of synaptic growth that standard antidepressants do not trigger.

Alcohol at high doses can overwhelm hippocampal function almost entirely, producing anterograde amnesia — the alcohol blackout — in which events are experienced but never recorded as memory.

Antidepressants, particularly SSRIs, are thought to promote hippocampal neurogenesis over weeks of use, and some researchers consider this one mechanism through which they gradually relieve depression. Classic psychedelics also alter activity in hippocampal and connected limbic networks, and preliminary evidence suggests some may stimulate neurogenesis as well, though what this means for human therapeutic effects remains an open question.