Itch and motivation to scratch: an investigation of the central and peripheral correlates of allergen- and histamine-induced itch in humans.

Intense itch and urge to scratch are the major symptoms of many chronic skin ailments, which are increasingly common. Vicious itch-scratch cycles are readily established and may diminish quality of life for those afflicted. We investigated peripheral and central processing of two types of itch sensation elicited by skin-prick tests of histamine and allergen solutions. Itch-related skin blood flow changes were measured by laser Doppler in 14 subjects responsive to type I allergens and 14 nonatopic subjects. In addition, this study examined central processing of both types of itch using functional magnetic resonance imaging (fMRI). Itch perception and blood flow changes were significantly greater when itch was induced by allergens compared with histamine. Both types of itch correlated significantly with activity in the genual anterior cingulate, striatum, and thalamus. Moreover, itch elicited by allergens activated orbitofrontal, supplementary motor, and posterior parietal areas. Histamine-induced itch also significantly correlated with activation in the insula bilaterally. The identification of limbic and ventral prefrontal activation in two types of itch processing likely reflects the subjects' desire to relieve the itch sensation by scratching, and these regions have been repeatedly associated with motivation processing. A dysfunction of the striato-thalamo-orbitofrontal circuit is believed to underlie the failure to regulate motivational drive in disorders associated with strong urges, e.g., addiction and obsessive compulsive disorder. The patterns of itch-induced activation reported here may help explain why chronic itch sufferers frequently self-harm through uncontrollable itch-scratch cycles.

Combining fMRI with a pharmacokinetic model to determine which brain areas activated by painful stimulation are specifically modulated by remifentanil.

We present a method for investigating the dynamic pharmacological modulation of pain-related brain activity, measured by BOLD-contrast fMRI. Noxious thermal stimulation was combined with a single infusion and washout of remifentanil, a short-acting opioid analgesic agent. The temporal profile of the effect site concentration of remifentanil, estimated from a pharmacokinetic model, was incorporated into a linear model of the fMRI data. The methodology was tested in nine healthy male subjects. During each imaging session the subjects received noxious thermal stimulation to the back of the left hand, prior to infusion, during infusion to a remifentanil effect site concentration of 1.0 ng/ml, and during washout of the remifentanil. Infusions were repeated with saline. Remifentanil-induced analgesia was confirmed from subjective pain intensity scores. Pain-related brain activity was identified in a matrix of regions using a linear model of the transient BOLD responses to noxious stimulation. Of those regions, there was a significant fractional reduction in the amplitude of the pain-related BOLD response in the insular cortex contralateral to the stimulus, the ipsilateral insular cortex, and the anterior cingulate cortex. Statistical parametric mapping of the component of pain-related BOLD responses that was linearly scaled by remifentanil concentration confirmed the contralateral insular cortex as the pain-processing region most significantly modulated by remifentanil compared to saline. The mapping of specific modulation of pain-related brain activity is directly relevant for understanding pharmacological analgesia. The method of examining time-dependent pharmacological modulation of specific brain activity may be generalized to other drugs that modulate brain activity other than that associated with pain.

Imaging how attention modulates pain in humans using functional MRI.

Current clinical and experimental literature strongly supports the phenomenon of reduced pain perception whilst attention is distracted away from noxious stimuli. This study used functional MRI to elucidate the underlying neural systems and mechanisms involved. An analogue of the Stroop task, the counting Stroop, was used as a cognitive distraction task whilst subjects received intermittent painful thermal stimuli. Pain intensity scores were significantly reduced when subjects took part in the more cognitively demanding interference task of the counting Stroop than in the less demanding neutral task. When subjects were distracted during painful stimulation, brain areas associated with the affective division of the anterior cingulate cortex (ACC) and orbitofrontal regions showed increased activation. In contrast, many areas of the pain matrix (i.e. thalamus, insula, cognitive division of the ACC) displayed reduced activation, supporting the behavioural results of reduced pain perception.