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Zusammenfassung:
The presence of dopamine-(DA)-receptors-(DARs) and innervations in early sensory pathways has previously been demonstrated in monkeys and humans. Nonetheless, their possible role in the sensory processing is still far from being understood. Anatomical evidence has shown that DARs are expressed in early-visual-system. These studies indicated that D1Rs are found in primary-visual-cortex, while D2Rs are predominantly expressed in the lateral-geniculate-nucleus-(LGN). D1Rs have a facilitating effect on neuronal processing whereas D2Rs show a dampening effect. Given their differences in anatomical distribution and functionality the two kinds of DARs may have a differential effect on thalamocortical information transfer. Here, we set out to investigate DAergic impact on V1 by using combined fMRI, neurophysiology and neurochemistry measurements in anesthetized non-human-primates, during systemic-application of L-DOPA-Carbidopa (2.1/0.5mg/kg, respectively). Our results show that the stimulus-induced modulation of the BOLD-signal decreases by 40±5 for 10±3min (n=8,p < 0.05). This decrease is concomitant with an improvement in the signal-to-noise-ratio-(SNR) in multi-unit-activity-(MUA: 900-3200Hz) as well as in the CV (p< 0.05) of the theta (4-8Hz), low-gamma (20-60Hz) and gamma (65-120Hz) bands of LFP. In contrast, local application of DA in V1 did not induce any changes in neuronal activity indicating that the observed effects are most probably mediated by D2Rs of LGN. DAergic neuromodulation decreased the SNR of the neuronal recordings in V1 which reflects a sparse and dampened firing pattern. Neurochemical sampling in V1 has shown an increased glutamate/GABA-ratio which might reflect a change in the excitation/inhibition balance induced by DA. The additional measured lactate/pyruvate-ratio has also shown a change indicating a decreased metabolic demand. These findings suggest that the visual inputs are attenuated by the local DAergic-circuitry of LGN (D2Rs) generating sparse and precise neuronal firing in V1. At the same time, however, the reduced mass-activity appears to reduce the energy demands, and the stimulus-induced-modulation of the BOLD-signal.
