Effects of locus coeruleus activation and inactivation on hippocampal formation theta rhythm in anesthetized rats

Previously obtained data suggests that noradrenaline (NE) released from the efferent locus coeruleus (LC) endings in hippocampal formation (HPC) may serve as an important modulating signal involved in the pharmacological mechanisms responsible for the production of type 2 theta rhythm in rats. Hence, two distinct hypotheses were tested in the present study: 1/ if the decrease in HPC level of NE is correlated with the desynchronization of HPC field potential, then the inhibition of LC would be expected to abolish HPC type 2 theta rhythm; 2/ if the increase in HPC NE level is correlated with synchronization of HPC field potential, then the stimulation of LC would be expected to produce type 2 theta. The experiments were performed using an experimental model of HPC type 2 theta rhythm recorded in urethanized rats. It was demonstrated that electrical stimulation of LC produced type 2 theta rhythm whereas procaine injection into LC, in contrast, reversibly abolished type 2 theta. The possible relation of type 2 theta rhythm with some disturbances of Alzheimer disease are addressed.