Cell discharge correlates of posterior hypothalamic theta rhythm recorded in anesthetized rats and brain slices.

Kowalczyk et al. (Hippocampus 2014; 24:7-20) were probably the first to conduct a systemic study of posterior hypothalamic area (PHa) theta rhythm in anesthetized rats. They demonstrated that local PHa theta field potentials were tail-pinch resistant and could be generated in urethane-anesthetized rats independently of ongoing hippocampal formation theta rhythm. These in vivo data were also confirmed in PHa slice preparations perfused with cholinergic agonist, carbachol. In the current experiments we extend our earlier observations concerning PHa theta rhythm. Specifically, PHa field potentials were analyzed in relation to the ongoing local cell firing repertoire. Single-unit discharge patterns of cells localized in the posterior hypothalamic and supramammillary nuclei were characterized according to the criteria that was developed previously to classify theta-related cells in the hippocampal formation. The present study demonstrated that in addition to the earlier described theta-related cells (theta-on, theta-off and gating cells) the PHa also contains cells discharging in a very regular manner, which were labelled "timing cells". This type of neuron has not been previously documented. We suggest that "timing cells" form a part of the ascending brainstem synchronizing pathway, provideing a regular rhythmic signal which facilitates the transduction of tonic discharges of cells localized in the brain stem into theta-frequency rhythmic discharges. © 2016 Wiley Periodicals, Inc.