Locus coeruleus potentiation of dentate gyrus responses: evidence for two systems

Glutamate activation of the locus coeruleus (LC) and norepinephrine (NE) have both been shown to potentiate the perforant path (PP)-evoked population spike. This potentiation may be short-lasting, the population spike returning to baseline levels within minutes after NE-application or LC activation, or can be long-lasting, persisting 20 minutes or more after termination of the NE or glutamate manipulation. In the present study LC electrical stimulation (333 Hz, 15 msec) initiated 40 msec prior to a PP stimulus reliably caused short-lasting potentiation of the dentate gyrus population spike amplitude (mean maximal = 161%, N = 22). With 50 LC-PP pairings a long-lasting potentiation (greater than 30 min after offset of LC stimulation) was seen in 10/22 experiments. Propranolol (20-30 mg/kg IP) did not block the potentiating effect of LC electrical simulation but completely suppressed the potentiating effect of glutamate activation of the LC in the same animals (N = 5). The beta receptor dependence of short-and long-lasting hippocampal NE potentiation has been previously demonstrated. The inability of a beta receptor antagonist to attenuate the potentiation induced by LC electrical stimulation suggests there are two distinct systems. Both the beta-NE-dependent and the beta-NE-independent system are capable of inducing long-lasting potentiation of the PP-evoked potential.