Karashima A, Katayama N, Nakao M. Enhancement of synchronization between hippocampal and amygdala theta waves associated with pontine wave density. J Neurophysiol 103: 2318 -2325, 2010. First published February 17, 2010 doi:10.1152/jn.00551.2009. Theta waves in the amygdala are known to be synchronized with theta waves in the hippocampus. Synchronization between amygdala and hippocampal theta waves is considered important for neuronal communication between these regions during the memory-retrieval process. These theta waves are also observed during rapid eye movement (REM) sleep. However, few studies have examined the mechanisms and functions of theta waves during REM sleep. This study examined correlations between the dynamics of hippocampal and amygdala theta waves and pontine (P) waves in the subcoeruleus region, which activates many brain areas including the hippocampus and amygdala, during REM sleep in rats. We confirmed that the frequency of hippocampal theta waves increased in association with P wave density, as shown in our previous study. The frequency of amygdala theta waves also increased with in associated with P wave density. In addition, we confirmed synchronization between hippocampal and amygdala theta waves during REM sleep in terms of the crosscorrelation function and found that this synchronization was enhanced in association with increased P wave density. We further studied theta wave synchronization associated with P wave density by lesioning the pontine subcoeruleus region. This lesion not only decreased hippocampal and amygdala theta frequency, but also degraded theta wave synchronization. These results indicate that P waves enhance synchronization between regional theta waves. Because hippocampal and amygdala theta waves and P waves are known to be involved in learning and memory processes, these results may help clarify these functions during REM sleep.
I N T R O D U C T I O NHippocampal theta waves are rhythmic field potentials recorded during particular behavioral activities and rapid eye movement (REM) sleep in cats and rats (Louie and Wilson 2001;Montgomery et al. 2008). The pontine reticular formation (PRF) nuclei, which have ascending projections to the septum/hippocampus via the diencephalon, are involved in the generation of hippocampal theta waves (Bland and Oddie 1998). During REM sleep, a group of PRF neurons begins to fire and/or increase their discharge rate coincident with the phasic field potential, which is called the ponto-geniculooccipital (PGO) wave in cats (Datta 1997;Koyama and Sakai 2000). In rats, a PGO-like phasic potential called the pontine (P) wave can be recorded in the subcoeruleus region (Datta et al. 1998;Heister et al. 2007), which has mutual connections with the other PRF (Datta et al. 1998(Datta et al. , 1999. Our previous studies on the relationships between PGO/P waves and theta waves during REM sleep have shown that an increase in the frequency of theta waves coincides with an increase in PGO wave density in cats and P wave density in rats (Karashima...