Spontaneous and evoked activities of anterior thalamic neurons during waking and sleep states

Summary In contrast with most dorsal thalamic nuclei, the anterior thalamic (AT) complex is devoid of input from the reticular (RE) thalamic nucleus. Instead, it is interposed in a limbic circuit linking the mammillary bodies (MB) to various allo- and periallocortical fields. To investigate the electrophysiological consequences of this peculiar pattern of connectivity, a sample of 92 AT cells, physiologically identified by their short-latency response to MB stimulation, was recorded in chronically-implanted animals and compared to a group of rostral intralaminar centralis-paracentralis (CL-PC) thalamic neurons receiving a dense innervation from the RE nucleus. Numerous similarities were found between the state-related fluctuations in spontaneous and evoked activities of AT and CL-PC neurons. For instance, both types of cells displayed stereotyped, high-frequency (250–300 Hz) bursts of 2–4 spikes in S and a more tonic discharge pattern in W and D. However, the high-frequency bursts of the S state occurred randomly in AT cells, whereas in CL-PC neurons they appeared rhythmically in clusters closely related to spindle waves. In addition, we found that a period of decreased responsiveness consistently followed the ortho- or antidromic activation of AT an CL-PC cells. This inhibitory period lasted less than 80 ms in AT cells and contrasted with the much longer (120ms) period of decreased excitability observed in CL-PC neurons during S. We suggest that these differences reflect the lack of RE input to the AT group. Since the AT complex is, through the MB, intimately linked to the hippocampal formation, it was expected that such prominent hippocampal activities as the theta rhythm would be channeled to the AT nuclei. Surprisingly, no theta-related modulation of neuronal activity was observed in identified AT cells during D epochs. This finding suggests that dramatic transformations are made on the signals emitted by the hippocampal formation somewhere along the pathway linking the hippocampal formation to the AT complex.