Effects of reversible inactivation of locus coeruleus on long-term potentiation in perforant path-DG synapses in rats.

The locus coeruleus (LC) is the main source of noradrenergic innervations to the forebrain and the hippocampal formation but does not receive noradrenergic projections itself. Previous studies have suggested that hippocampal neural response is modulated by the noradrenergic pathway and that the experimental activation of the LC can potentiate hippocampal responses. Most studies have suggested that the noradrenergic system has controversial effects on long-term potentiation (LTP) in hippocampal neurons, because its influence on synaptic plasticity in perforant path-DG synapses is ambiguous. The aim of this article was to study the LC's role in baseline activity and LTP in perforant path-DG cells of hippocampus by in vivo LC inactivation. Rats were anesthetized with urethane, and LC was temporarily suppressed by intra-LC injection of lidocaine. Population spike (PS) amplitude and excitatory postsynaptic potential (EPSP) slope in DG were recorded 10 min before and 5, 10, 20, 40, 60, and 120 min after tetanization (400 Hz). Saline or lidocaine was injected during the baseline recording (experiment 1), 5 min before tetanus (experiment 2), and 5 min after tetanus (experiment 3). The results from this study indicated that the LC inactivation has no effect on baseline activity of granular cells or maintenance of LTP after tetanization. Moreover, LC inactivation before tetanus had no effect on LTP induction but decreased PS-LTP amplitude 60 and 120 min after tetanization. Taken together, the LC noradrenergic system likely influences LTP induction in later time periods while it has no effect on LTP in earlier time periods.