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TRANSLATIONAL PHYSIOLOGY

Excitability of human motor and visual cortex before, during, and after hyperventilation

¹Institute of Neuroscience and Biophysics, Department of Medicine, Research Centre Juelich, Juelich; and ²Interdisciplinary Centre for Clinical Research BIOMAT and ³Department of Neurology, Rheinisch-Westfälische Technische Hochschule, Aachen University, Aachen, Germany

Submitted 12 July 2006 ; accepted in final form 20 September 2006

ABSTRACT

In humans, hyperventilation (HV) has various effects on systemic physiology and, in particular, on neuronal excitability and synaptic transmission. However, it is far from clear how the effects of HV are mediated at the cortical level. In this study we investigated the effects of HV-induced hypocapnia on primary motor (M1) and visual cortex (V1) excitability. We used 1) motor threshold (MT) and phosphene threshold (PT) and 2) stimulus-response (S-R) curves (i.e., recruitment curves) as measures of excitability. In the motor cortex, we additionally investigated 3) the intrinsic inhibitory and facilitatory neuronal circuits using a short-interval paired-pulse paradigm. Measurements were performed before, during, and after 10 min of HV (resulting in a minimum end-tidal PCO₂ of 15 Torr). HV significantly increased motor-evoked potential (MEP) amplitudes, particularly at lower transcranial magnetic stimulation (TMS) intensities. Paired-pulse stimulation indicated that HV decreases intracortical inhibition (ICI) without changing intracortical facilitation. The results suggestthat low PCO₂ levels modulate, in particular, the intrinsic neuronal circuits of ICI, which are largely mediated by neurons containing -aminobutyric acid. Modulation of MT probably resulted from alterations of Na⁺ channel conductances. A significant decrease of PT, together with higher intensity of phosphenes at low stimulus intensities, furthermore suggested that HV acts on the excitability of M1 and V1 in a comparable fashion. This finding implies that HV also affects other brain structures besides the corticospinal motor system. The further exploration of these physiological mechanisms may contribute to the understanding of the various HV-related clinical phenomenona.

partial pressure of carbon dioxide; phosphenes; threshold; paired-pulse transcranial magnetic stimulation; intracortical facilitation; intracortical inhibition