Priming of central and peripheral mechanisms with heat and cutaneous capsaicin facilitates secondary hyperalgesia to high-frequency electrical stimulation.

Heat/capsaicin sensitization and electrical high-frequency stimulation (HFS) are well-known models of secondary hyperalgesia, a phenomenon related to chronic pain conditions. This study investigated whether priming with heat/capsaicin would facilitate hyperalgesia to HFS in healthy subjects. Heat/capsaicin priming consisted of a 45°C heat stimulation for 5 min followed by a topical capsaicin patch (4 × 4 cm) for 30 min on the volar forearm of 20 subjects. HFS (100 Hz, 5 times 1 s, minimum 1.5 mA) was subsequently delivered through a transcutaneous pin electrode approximately 1.5 cm proximal to the heat/capsaicin application. Two sessions were applied in a crossover design; traditional HFS (HFS) and heat/capsaicin sensitization followed by HFS (HFS + HEAT/CAPS). Heat pain threshold (HPT), mechanical pain sensitivity (MPS), and superficial blood perfusion were assessed at baseline, after capsaicin removal, and up to 40 min after HFS. MPS was assessed with pin-prick stimulation (128 mN and 256 mN) in the area adjacent to both HFS and heat/capsaicin, distal but adjacent to heat/capsaicin and in a distal control area. HPT was assessed in the area of heat/capsaicin. Higher sensitivity to 128 mN pin-prick stimulation (difference from baseline and control area) was observed in the HFS + HEAT/CAPS session than in the HFS session 20 and 30 min after HFS. Furthermore, sensitivity was increased after HFS + HEAT/CAPS compared with after heat/capsaicin in the area adjacent to both paradigms, but not in the area distal to heat/capsaicin. Results indicate that heat/capsaicin causes priming of the central and peripheral nervous system, which facilitates secondary mechanical hyperalgesia to HFS.NEW & NOTEWORTHY High-frequency electrical stimulation (HFS) and heat/capsaicin sensitization are well-known models of secondary hyperalgesia. The results from the current study indicate that increased sensitivity to 128 mN pin-prick stimulation can be obtained when HFS is delivered following an already established heightened central hyperexcitability provoked by heat/capsaicin sensitization.

Multisine frequency modulation of intra-epidermal electric pulse sequences: A novel tool to study nociceptive processing.

A sustained sensory stimulus with a periodic variation of intensity creates an electrophysiological brain response at associated frequencies, referred to as the steady-state evoked potential (SSEP). The SSEPs elicited by the periodic stimulation of nociceptors in the skin may represent activity of a brain network that is primarily involved in nociceptive processing. Exploring the behavior of this network could lead to valuable insights regarding the pathway from nociceptive stimulus to pain perception. We present a method to directly modulate the pulse rate of nociceptive afferents in the skin with a multisine waveform through intra-epidermal electric stimulation. The technique was demonstrated in healthy volunteers. Each subject was stimulated using a pulse sequence modulated by a multisine waveform of 3, 7 and 13 Hz. The EEG was analyzed for the presence of the base frequencies and associated (sub)harmonics. Topographies showed significant central and contralateral SSEP responses at 3, 7 and 13 Hz in respectively 7, 4 and 3 out of the 9 participants included for analysis. As such, we found that intra-epidermal stimulation with a multisine frequency modulated pulse sequence can generate nociceptive SSEPs. The possibility to stimulate the nociceptive system using multisine frequency modulated pulses offers novel opportunities to study the temporal dynamics of nociceptive processing.