Új ponto-thalamicus pálya sejtjeinek in vivo aktivitása

Abstract

A subset of thalamic nuclei, the so called higher order (i.e. nuclei which receive layer 5 cortical input) relays are targeted by a special type of inhibitory fibers arising from mesencephalic and diencephalic sources, other than the reticular nucleus, hence the name, extrareticular inhibition. We describe here a novel member of the extrareticular system, targeting seletively the intralaminar thalamic nucei (IL). The ascending activating system arising from the brainstem by its excitatory and depolarizing effect switch thalamocortical neurons from „burst” to tonic mode, the typical firing pattern of awake state. However, our recent analysis of glycine transporter type 2 (GlyT2) -EGFP transgenic mice revealed a novel inhibitory mixed glycinergic/GABAergic pathway from the midbrain reticular core (nucleus pontis oralis, PnO) that selectively innervated the intralaminar (IL) and midline thalamic nuclei. The glycinergic/GABAergic PnO axons in the IL formed large inhibitory terminals with multiple release sites on postsynaptic dendrites. In vitro photostimulation of the pathway revealed dual, GABAergicglycinergic IPSCs The response of the postsynaptic neuron was non-depressing even at high stimulation frequencies. To characterize the in vivo activity of PnO neurons we used juxtacellular single cell recording technique. In ketamine/xyliazine anesthesia thalamus projecting GlyT2-positive neurons in the PnO fired rhythmic clusters of action potentials at 50-90 Hz interspersed with high frequency doublets. The clusters were tightly coupled to cortical Up states recorded in the prefrontal (PF) cortex. Cortical inactivation abolished the rhythmic activity. Glycinergic neurons responded to cortical electrical stimulations in a frequency dependent manner. Two cells increased two decreased their firing activity to increased stimulation frequencies. These results indicate a cortex-brainstem-thalamus-cortex loop. In this loop high frequency inhibitory brainstem activity is modulated by cortical rhythmic inputs, which paces thalamic firing in the intralaminar nuclei. This activity reaches the cortex and the corticothalamic signal is fed back into the brainstem closing the loop. Thus, our data raise the possibility that pontine glycinergic-GABAergic neurons may have an important role in the regulation of cortical rhythm genesis via the thalamus.