Changes in excitability of the theta activity generating substrate by ACTH 4-10 in the rat

The present experiments tested the sensitivity of theta activity to ACTH 4-10 a peptide known to facilitate the maintenance of conditioned behaviors. Hippocampal theta activity was induced by electrical stimulation of the reticular formation in freely moving rats. The administration of ACTH 4-10 produced a typical 0.5 c/s shift indominant frequency and an increase in 7.5-9.0 c/s components of induced theta activity. The effects were maximal between 60-120 min after subcutaneous injection and lasted several hours. The fact, that a similar effect could be obtained by an increase in stimulus intensity suggests an elevated excitability in the theta generating system in the presence of the peptide. The possible significance of this finding for the maintenance of learned behavior is discussed.