V. Bohotin

Induction of long‐lasting changes of visual cortex excitability by five daily sessions of repetitive transcranial magnetic stimulation (rTMS) in healthy volunteers and migraine patients

We have shown that in healthy volunteers (HV) one session of 1 Hz repetitive transcranial magnetic stimulation (rTMS) over the visual cortex induces dishabituation of visual evoked potentials (VEPs) on average for 30 min, while in migraineurs one session of 10 Hz rTMS replaces the abnormal VEP potentiation by a normal habituation for 9 min. In the present study, we investigated whether repeated rTMS sessions (1 Hz in eight HV; 10 Hz in eight migraineurs) on 5 consecutive days can modify VEPs for longer periods. In all eight HV, the 1 Hz rTMS-induced dishabituation increased in duration over consecutive sessions and persisted between several hours (n = 4) and several weeks (n = 4) after the fifth session. In six out eight migraineurs, the normalization of VEP habituation by 10 Hz rTMS lasted longer after each daily stimulation but did not exceed several hours after the last session, except in two patients, where it persisted for 2 days and 1 week. Daily rTMS can thus induce long-lasting changes in cortical excitability and VEP habituation pattern. Whether this effect may be useful in preventative migraine therapy remains to be determined.

Excitability of visual V1-V2 and motor cortices to single transcranial magnetic stimuli in migraine: a reappraisal using a figure-of-eight coil

We used transcranial magnetic stimulation (TMS) with a figure-of-eight coil to excite motor and visual V1-V2 cortices in patients suffering from migraine without (MO) (n = 24) or with aura (MA) (n = 13) and in healthy volunteers (HV) (n = 33). Patients who had a migraine attack within 3 days before or after the recordings were excluded. All females were recorded at mid-cycle. Single TMS pulses over the occipital cortex elicited phosphenes in 64% of HV, 63% of MO and 69% of MA patients. Compared with HV, the phosphene threshold was significantly increased in MO (P = 0.001) and in MA (P = 0.007), but there was no difference between the two groups of migraineurs. The motor threshold tended to be higher in both migraine groups than in HV, but the differences were not significant. In conclusion, this study shows that two-thirds (64.86%) of patients affected by either migraine type present an increased phosphene threshold in the interictal period, which suggests that their visual cortex is hypoexcitable.

Vagus nerve stimulation in awake rats reduces formalin-induced nociceptive behaviour and fos-immunoreactivity in trigeminal nucleus caudalis

&NA; Besides its well‐established efficacy in epilepsy, vagus nerve stimulation (VNS) may be of potential interest in pain treatment. It has, however, not yet been assessed in animal pain models with the devices and stimulation protocols used in humans. We have therefore studied in awake rats the effects of left cervical VNS on trigeminal nociception using an implantable electrode and stimulator (NCP‐Cyberonics®). VNS was applied for 24 h at 2 mA intensity, 20 Hz frequency, 0.5 ms pulse width and a duty cycle of 20 s ON/18 s OFF. As a nociceptive stimulus, we injected formalin into the left mystacial vibrissae, assessed behaviour for 45 min and sacrificed the animals 45 min later. Fos‐immunoreactive (Fos‐Ir) neurons were counted in laminae I–II of trigeminal nucleus caudalis (TNC) on both sides. We used three groups of control animals: VNS without formalin, formalin without VNS and sham VNS (implanted without stimulation or formalin). Whereas sham VNS had no significant effect, VNS alone increased Fos expression in ipsilateral TNC in addition to the expected increase in nucleus tractus solitarius. It also significantly attenuated the increase of Fos‐Ir neurons observed in ipsilateral TNC laminae I–II after formalin injection. If the proper VNS effect on Fos‐expression was subtracted, the reduction of formalin‐induced nociceptor activation was 55%. VNS also reduced nociceptive behaviour on average by 96.1% during the early phase (0–6 min) and by 60.7% during the late phase (6–45 min) after the formalin injection. These results suggest that VNS applied with a device used in human therapy may have in awake rats a significant antinociceptive effect in a model of trigeminal pain.

Vagus nerve stimulation attenuates heat- and formalin-induced pain in rats

The analgesic effect of vagus nerve stimulation (VNS) has not yet been demonstrated in animals with the devices used in the clinic. We studied in awake rats the effects of two VNS protocols on the hind paw hot water test and compared the results with those previously obtained in the oro-facial formalin test. A stringent duty cycle (20 s on/18 s off) increased heat pain tolerance in both hind paws (average 188%) after 2 h of stimulation. VNS with parameters used in epilepsy (30 s on/5 min off) decreased heat tolerance after 2 h, but produced a significant antinociceptive effect after days of stimulation. VNS may thus be useful in pain disorders, even with the less stringent protocol.