Spontaneous discharge characteristic of neurons in the ventromedial nucleus of the rat hypothalamus in vivo

The ventromedial nucleus of the hypothalamus (VMN) is one of the main central regulators of two vital behaviours in rat: feeding behaviour and sexual behaviour. To better understand how these behaviours are regulated in the brain requires assessing how physiological stimuli are encoded by the electrical activity of populations of neurons, but there is still little known about the electrical activity of neurons in the VMN, in particular how it is regulated in vivo. Here, we recorded spontaneous firing activity from single VMN neurons in urethane-anaesthetized rats in vivo, and characterized their electrophysiological identities. For each of 271 cells, we constructed hazard functions from interspike interval histograms to show how the excitability of the cell changes with time after a spike. We completed the statistical characterization of each cell by analysis of its mean firing rate and coefficient of variation, and features of its interspike interval distribution, including kurtosis and skew (around the mean and around the mode). We thereby identified nine subpopulations of neurons in the VMN, which we named according to the main features of their firing pattern. One of the subpopulations fires very regularly, another almost randomly and another in intermittent clusters of two-three spikes, but perhaps the most interesting subpopulation are 'oscillatory cells' whose activity seems to be governed by an extrinsic 3-Hz rhythm. Whether these electrophysiologically distinct populations are also functionally and neurochemically distinct has now to be tested.