Pruritic and nociceptive sensations and dysesthesias from a spicule of cowhage

Abstract

Although the trichomes (spicules) of a pod of cowhage (Mucuna pruriens) are known to evoke a histamine-independent itch that is mediated by a cysteine protease, little is known of the itch and accompanying nociceptive sensations evoked by a single spicule and the enhanced itch and pain that can occur in the surrounding skin. The tip of a single spicule applied to the forearm of 45 subjects typically evoked 1) itch accompanied by nociceptive sensations (NS) of pricking/stinging and, to a lesser extent, burning, and 2) one or more areas of cutaneous dysesthesia characterized by hyperknesis (enhanced itch to pricking) with or without alloknesis (itch to stroking) and/or hyperalgesia (enhanced pricking pain). Itch could occur in the absence of NS or one or more dysesthesias but very rarely the reverse. The peak magnitude of sensation was positively correlated for itch and NS and increased (exhibited spatial summation) as the number of spicules was increased within a spatial extent of 6 cm but not 1 cm. The areas of dysesthesia did not exhibit spatial summation. We conclude that itch evoked by a punctate chemical stimulus can co-exist with NS and cutaneous dysesthesias as may occur in clinical pruritus. However, cowhage itch was not always accompanied by NS or dysesthesia nor was a momentary change in itch necessarily accompanied by a similar change in NS or vice versa. Thus there may be separate neural coding mechanisms for itch, nociceptive sensations, and each type of dysesthesia.

FIG. 1.

The cowhage spicule and a method of applying multiple spicules to the skin. In some experiments, the tip of a single cowhage spicule was inserted ∼200 μm into the skin at an approximate angle of 30° by means of forceps. A: scanning electron micrograph (SEM) of the tip of a cowhage spicule. The total length of a spicule was typically 2–4 mm. B: SEM at a higher magnification of the end of the tip and of the protuberances formed on the outside of the spicule. C: Because a single spicule did not always evoke a sensation, in some experiments, multiple spicules were inserted into the skin by means of a “spicule inserter.” This consisted of 9 or 10 spicules fixed in a row along the cut end of a surgical sponge by means of nail polish. Application of the inserter typically resulted in the insertion of the tips of 7 spicules (±1). A sensation was always evoked.

FIG. 2.

The magnitude of itch, pricking/stinging, and burning evoked by a single spicule of cowhage. A: the mean rating of perceived intensity from 45 subjects obtained at successive intervals of 30 s after application (insertion) of the spicule. The SE is plotted every 5 min starting with the peak magnitude for the particular quality of sensation. Pricking/stinging is abbreviated as “prick” and burning as “burn.” The locations of 3 of the verbal descriptors are shown on the right vertical axis in correspondence with the numerical ratings of perceived intensity indicated on the left vertical axis. B: distributions of the categories chosen by the 45 subjects to indicate the peak magnitude of itch, pricking/stinging, and burning (from the same database used for calculating the means in A). The label corresponds to the upper limit of a numerical range defined as barely detectable (BD, ≤1), weak (WK, >1 to ≤6), moderate (MD, >6 to ≤17), strong (ST, >17 to ≤35), very strong (VS, >35 to ≤53), and greater than very strong (>VS, >53).

FIG. 3.

The sensory ratings of 3 different subjects each given 2 different tests with a single spicule. These findings indicate the diversity in sensory response profiles evoked by a single spicule.

FIG. 4.

The effects of increasing the number of spicules from 1 group of 7 to 4 groups applied at 2 cm spacing along the forearm. The increase in number of spicules from 7 to 28 resulted in the spatial summation in the magnitude of Itch, pricking/stinging and burning.

FIG. 5.

The perceived intensity of itch and pricking/stinging rated by a subject every 10 s in response to the application of 7 cowhage spicules. Also shown is the difference (change) in the magnitude rating of each sensory quality between the plotted rating and the rating obtained 10 s earlier. The ratings of itch and pricking/stinging are represented by solid and dashed lines, respectively. Solid bars, change in itch; open bars, change in pricking/stinging. Changes <10% are not shown.

FIG. 6.

The incidence of each type of dysesthesia evoked by a single cowhage spicule and the incidence in relation to the peak magnitude of itch. A: the incidence of hyperknesis (Hkn) and hyperalgesia (Halg) with or without alloknesis (Allo). B: the incidence of each type of dysesthesia in relation to the peak magnitude of itch. The peak magnitude ratings were categorized as less than weak (<WK), weak to moderate (WK–MD), moderate to strong (MD–ST), and greater than strong (>ST). In some instances, there was no dysesthesia even in the presence of a strong itch. However, there was rarely any alloknesis or hyperalgesia in the absence of at least a weak itch.

FIG. 7.

Areas of cutaneous dysesthesia evoked by cowhage. A: borders of alloknesis, hyperknesis, and hyperalgesia mapped on the volar forearm of a subject 20 min after the insertion of a single cowhage spicule. B: the mean log area of alloknesis, hyperknesis, and hyperalgesia that developed in response to a single spicule (open bars) or a group of 7 or 28 spicules (hashed and dotted bars, respectively).

FIG. 8.

Perceived intensity of itch and pricking/stinging in human compared with the mean discharge rates of mechano-heat sensitive nociceptors with C fibers in monkey each obtained in response to a group of cowhage spicules. The mean ratings of the perceived intensity of itch and pricking/stinging simultaneously obtained every 10 s and the mean discharge rates in impulses (imp)/s continuously recorded from 39 mechanosensitive nociceptive C-fibers in monkey were plotted at intervals of 10s after normalizing each to the peak mean perceived intensity of itch. The discharges in monkey were obtained by combining the responses of slowly and quickly adapting C-mechanoheat nociceptors in response to multiple spicules (Fig. 9A of Johanek et al. 2008).

FIG. 9.

Neuronal model of spatial summation and segmental inhibition of itch or nociceptive sensations evoked by cowhage. The model provides a candidate mechanism by which spatial summation of itch evoked by multiple spicules is absent within a local extent of 1 cm but present when spicules are spread over a greater extent, such as 6 cm. The same type of model could be used to explain the spatial summation of pricking/stinging or burning in pruriceptive spinothalamic tract (STT) neurons that mediate nociceptive sensations evoked by cowhage. As shown, 1 branch of a “mechano-chemo” nociceptive neuron (e.g., 1 with C-mechanoheat nociceptors), MCN2, responds sooner and more vigorously to a cowhage spicule than other branches of the same neuron or other MCNs (MCN1 and 3 whose branches are not shown). Action potentials generated by the most responsive peripheral branch of MCN2 is hypothesized as traveling not only orthodromically but also antidromically into neighboring branches of the same afferent, thereby resetting or fatiguing the generation of impulses from these fiber terminals (Peng et al. 2003). The central terminals of the same afferent are hypothesized as activating not only a 2nd-order pruriceptive neuron (mediating itch), P2, whose receptive field may be larger than shown, but also inhibitory interneurons (small filled circles). These inhibitory interneurons exert a weak inhibition (e.g., presynaptic) of weaker activity generated by neighboring MCNs whose inputs converge onto P2 (segmental afferent inhibition), accounting for the lack of spatial summation within approximately the size of the receptive field of a MCN. Not shown are the inhibitory interneurons activated by MCN1 and MCN3 similar to MCN2. Spatial summation of input from pruriceptive neurons is hypothesized to occur in 3rd-order neurons to account for the finding that there is spatial summation of itch when 4 groups of cowhage spicules are distributed along an extent of 6 cm on the arm. In this case, each group of spicules is hypothesized to activate some of the same but also different 2nd-order neurons (e.g., P4) whose primary afferent inputs are not subject to inhibitory effects from an adjacent group of spicules. The neuronal input and inhibitory mechanisms for P4 are identical to those of P2 but not shown for simplicity. The convergence of P2 and P4, each mediating itch, onto a spinothalamic tract neuron, S, provides the basis for the spatial summation of itch when the number of spicules is increased over a distance of 6 as opposed to 1 cm.

FIG. 10.

Schematic of how pruriceptive neurons might mediate cowhage evoked itch, pricking/stinging, and cutaneous dysesthesias. In this example, a mechanosensitive, nociceptive-specific dorsal root ganglion (DRG) neuron (MN), responsive to a nociceptive punctate stimulus such as a von Frey filament (a), terminates on a central neuron, e.g.,, an STT neuron (Sp), which when activated elicits a sensation of pricking (prick). Three mechano-chemo-sensitive (polymodal) nociceptive neurons (MCN 1, 2, and 3) are responsive to cowhage (spicule). MCN2 and MCN3 each terminate more superficially in the skin than MCN1 and are more responsive to fine, punctuate stimuli (e.g., 50 μm diam) (b) that can normally evoke itch. MCN2 and 3 have strong synaptic projections onto an STT neuron mediating itch (Si), whose cutaneous receptive field is shown on the left (large, dashed oval) and a weak projection (dashed line, shown for simplicity only for MCN2) to an STT neuron (Sp) mediating cowhage-evoked pricking/stinging. In contrast, MCN 1 projects weakly onto Si (dashed line) but strongly onto Sp. MCN2/3 and Si mediate cowhage-evoked itch. MCN 1 and Sp mediate cowhage-evoked pricking/stinging. A low-threshold mechanoreceptive neuron (LTM) terminates on neurons in the dorsal column nuclei (T) that mediate innocuous tactile sensations (touch), e.g., evoked by stroking the skin with a cotton swab (c), and sends collateral projections in the spinal dorsal horn that normally do not contribute to itch. Certain Si neurons, but not all, have the capacity of releasing a neurotransmitter from the terminals of collaterals (1 and 2) that produces a sustained sensitization of 2 types of interneurons (circles with crosshatch pattern). Collateral 1 projects to an interneuron that receives a weak (and normally ineffective) input from an LTM (via collateral 3) and/or an itch mediating, mechano-chemo nociceptive neuron (i.e., MCN2 via collateral 4), each with receptive fields adjacent to or remote from the cowhage application site. When the interneuron is sensitized, its responses to stroking and/or pricking conveyed to Si are enhanced, resulting in alloknesis and/or hyperknesis, respectively. Collateral 2 projects to an interneuron that receives input from MN alone (via collateral 5), and when sensitized, enhances the responses of Sp to pricking adjacent to or remote to the cowhage application site, thereby resulting in punctate hyperalgesia.