State-dependent and state-independent effects of thyrotropin-releasing hormone on medial septum neuronal activity in the brain slices of waking and hibernating ground squirrels

Abstract

Effects of thyrotropin-releasing hormone on spontaneous activity and responses to medial forebrain bundle stimulation were tested in the units of the medial septum-diagonal band complex in slices taken from the brain of hibernating and waking ground squirrels. Administration of thyrotropin-releasing hormone (0.1 microM) into the flow of incubating medium increased the frequency of spontaneous activity of all the medial septum-diagonal band complex neurons in hibernating ground squirrels and of the majority of neurons in the waking ground squirrels. However, in the septal slices of hibernating ground squirrels this increase was significantly more pronounced. In addition, the neuropeptide slightly increased the frequency of bursts in the majority of cells with rhythmic burst activity. The excitatory influence of thyrotropin-releasing hormone on the units was preserved in conditions of synaptic blockade. In neurons from other structures (lateral septum, medial preoptic area, hippocampus) in the brain slices of both hibernating and waking ground squirrels, thyrotropin-releasing hormone did not usually affect the level of spontaneous discharges. When studying the responses of the medial septum-diagonal band complex neurons to electrical stimulation of medial forebrain bundle it was found that application of thyrotropin-releasing hormone (0.1 microM) led to the disappearance of responses in 50 and 44% of units in the hibernating and waking ground squirrels, respectively; in the rest of the neurons a disturbance of stability and probability of responses was observed. The existence of a modulatory thyrotropin-releasing hormone system which participates post-, and, probably, presynaptically in the regulation of the medial septum-diagonal band complex neuronal activity is suggested. The role of thyrotropin-releasing hormone and of medial septum-diagonal band complex in the neural control of hibernation/euthermic waking cycle is discussed.