Abstract
The neurons of the locus coeruleus (LC) fire in response to novelty, and LC activation coupled with hippocampal afferent stimulation leads to long-term depression (LTD). The encoding of novel spatial information also involves activation of dopamine D1/D5 receptors. It is unclear if, or how, the noradrenergic and dopaminergic systems interact mechanistically in processing novelty. Novel spatial exploration when coupled with Schaffer collateral (SC) test-pulse stimulation results in short-term depression at SC-CA1 synapses, which is not observed in the absence of afferent stimulation. However, activation of D1/D5 receptors under these conditions without concomitant afferent stimulation enables slow-onset depression. LTD (>24 h) is facilitated when novel exploration occurs concurrently with low-frequency stimulation of CA1. Effects are not improved by a D1/D5 agonist. Facilitation of LTD (>4 h) by coupling LC stimulation with CA1 test-pulse stimulation was blocked by a D1/D5 antagonist, however, as was habituation to the holeboard environment. Novel spatial learning during LC stimulation did not enhance LTD further, whereas D1/D5 agonist treatment enabled LTD to persist for over 24 h. These data suggest that the regulation of hippocampal LTD by the LC is supported by D1/D5 receptors and that their contribution to information storage becomes important when the thresholds for persistent LTD have not been reached.
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