The intergeniculate leaflet (IGL) of the thalamus is an important structure of the circadian timing system. Its primary role is the integration of both photic and non-photic stimuli relevant for the regulation of the sleep-wake cycle. The consolidated information is then transmitted to the main generator of circadian rhythms – the suprachiasmatic nuclei (SCN). As the non-photic cues are delivered to the IGL via non-specific projections of the brainstem, we chose to investigate how one projec‑ tion, the norepinephrine (NE) system from the locus coe‑ ruleus (LC), influences IGL neuronal activity. Moreover, we divided recorded cells into two groups, anticipating that they reflect two different projections arising from the IGL. The first group, expressing T-type calcium cur‑ rent (T-type cells), putatively comprises the connection between the leaflets located in both hemispheres, while A-type potassium current expressing neurons (or A-type cells) most likely transmit information to the SCN. The influence of NE on IGL neurons was tested by patch clamp recordings in the current clamp mode on 250 μm brain slices from 2/3-week-old male Wistar rats. In each instance NE (20 μM) was applied twice, the second ap‑ plication in the presence of tetrodotoxin (TTX, 0.5 μM), to determine if the observed effect was postsynaptic or presynaptic. Both substances were applied by bath per‑ fusion. After the experiment, slices were immunostained against the neuropeptide Y to confirm that the recorded cell was within the borders of the IGL. For this purpose, ExtrAvidinCy3 (1:250) and anti-NPY antibodies (1:8000) were used. NE was shown to elicit a direct effect on a ma‑ jority of IGL neurons. The effects were diverse; however, all A-type cells, and most T-type cells, responded with depolarization. Some neurons within the T-type group showed no response and, interestingly, a few neurons were hyperpolarized. Our study shows for the first time the electrophysiological effects of NE on the neuronal activity of single IGL neurons. The variety of respons‑ es, probably through activation of different receptors, requires further study. We predict that the diverse re‑ sponses reflect the differential impact of NE on neurons foming different projections, but the variety of respons‑ es from the T-type cells indicate that this group may be more complex and consist of additional subpopulations.
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