Hippocampal release of dopamine and norepinephrine encodes novel contextual information.

The detection and processing of novel information encountered in our environment is crucial for proper adaptive behavior and learning. Hippocampus is a prime structure for novelty detection that receives high-level inputs including context information. It is of our interest to understand the mechanisms by which the hippocampus processes contextual information. For this, we performed in vivo microdyalisis in order to monitor extracellular changes in neurotransmitter levels during Object Location Memory (OLM), a behavioral protocol developed to evaluate contextual information processing in recognition memory. Neurotransmitter release was evaluated in the dorsal hippocampus and insular cortex during OLM in 3-month-old B6129SF2/J mice. We found a simultaneous release of dopamine and norepinephrine in hippocampus during OLM, while neurochemical activity remained unaltered in the cortex. Additionally, we administered 6-hydroxy-dopamine (6-OHDA), a neurotoxic compound selective to dopaminergic and noradrenergic neurons, in the dorsal hippocampus in a different group of mice. Depletion of catecholaminergic terminals in the hippocampus by 6-OHDA impaired OLM but did not affect novel object recognition. Our results support the relevance of hippocampal catecholaminergic neurotransmission in recognition memory. The significance of catecholaminergic function may be extended to the clinical field as it has been reported that innervation of hippocampus by the noradrenergic and dopaminergic system is reduced and atrophied in aging and Alzheimer's disease brain.