Conditioned pain modulation affects the N2/P2 complex but not the N1 wave: A pilot study with laser-evoked potentials

Area of Science:

• Neuroscience
• Pain Research
• Neurophysiology

Background:

• The 'pain-inhibits-pain' phenomenon is mediated by diffuse noxious inhibitory controls (DNIC) or conditioned pain modulation (CPM).
• Laser-evoked potentials (LEPs) components, N1 wave and N2/P2 complex, reflect lateral and medial pain pathways, respectively.
• The N1 wave originates from the lateral thalamic nuclei (LT) projecting to the somatosensory cortex, while the N2/P2 complex originates from medial thalamic nuclei (MT) projecting to limbic cortices.

Purpose of the Study:

• To investigate the effect of CPM on the medial and lateral pain pathways.
• To determine if DNIC selectively modulates specific pain transmission routes.
• To assess the utility of N1 wave and N2/P2 complex as neurophysiological markers for pain modulation.

Main Methods:

• A conditioned pain modulation (CPM) protocol was applied using a cold pressor test as the conditioning stimulus and laser stimulation for the test stimulus.
• Laser-evoked potentials (LEPs) were recorded from 15 healthy subjects.
• LEPs were measured during baseline, heterotopic noxious conditioning stimulation (HNCS), and post-HNCS conditions.

Main Results:

• A significant reduction in the N2/P2 complex amplitude was observed during HNCS, returning to baseline post-HNCS.
• The N1 wave amplitude remained unchanged during and after HNCS.
• These findings indicate that CPM selectively affects the medial pain system.

Conclusions:

• CPM primarily modulates pain through the medial pain system, specifically impacting pain transmission via the spinoreticulothalamic (SRT) pathway.
• The lateral pain pathway, mediated by the spinothalamic tract (STT), is not affected by CPM.
• The N1 wave is a reliable neurophysiological parameter for assessing STT function, as it is not influenced by CPM.