Supramammillary neural discharge patterns and hippocampal EEG

Area of Science:

• Neuroscience
• Neurophysiology

Background:

• The supramammillary nucleus (SuM) plays a role in regulating hippocampal theta rhythms.
• The precise mechanisms governing SuM neuronal activity during different brain states are not fully understood.

Purpose of the Study:

• To investigate the origin and regulation of rhythmic and nonrhythmic neuronal activity in the SuM.
• To determine the relationship between SuM activity and hippocampal rhythms like theta and sharp-waves.

Main Methods:

• Electrophysiological recordings of multiunit activity in the SuM and EEG in the hippocampus.
• Pharmacological manipulation using carbachol infusion into the nucleus pontis oralis (RPO).
• Anatomical lesioning via bilateral transection anterior to the SuM.

Main Results:

• Rhythmic SuM activity, preceding dentate rhythmical slow-wave activity (RSA, theta), was observed after carbachol infusion into RPO.
• This rhythmic SuM activity persisted after transection anterior to SuM, indicating independence from septal or hippocampal descending influences.
• Increased carbachol led to hippocampal epileptiform sharp-waves and correlated nonrhythmic SuM discharge.
• Transection rostral to SuM abolished sharp-wave related SuM activity, suggesting dependence on descending hippocampal projections.

Conclusions:

• Rhythmic SuM discharge related to RSA is not dependent on descending inputs from the hippocampus or septum.
• Sharp-wave related SuM discharge appears to be dependent on descending projections from the hippocampus.