The goal of this study was to determine how ascending sensory information underlying histaminergic and nonhistaminergic forms of itch is carried in primate STT neurons. Only two studies have examined the responses of STT neurons to applications of histamine (Andrew and Craig, 2001
; Simone et al., 2004
), and none have examined the responses of STT neurons to nonhistaminergic forms of itch. We used antidromic activation to identify primate spinothalamic tract neurons and then tested each neuron for sensitivity to histamine and cowhage. We found that 21% of examined STT neurons responded to histamine and 12% were activated by cowhage. Surprisingly, STT neurons responded either to histamine or to cowhage, but never to both, indicating that there are at least two, separate populations of STT neurons that convey pruritic information to the brain.
Most STT neurons activated by histamine responded between 5 and 25 min. This time course reflects the expected time course of itch in humans injected with the same amount of histamine (Simone et al., 1987
). Each histamine-responsive STT neuron was also activated by noxious mechanical stimuli and was classified as either HT or WDR. Sixty-seven percent of histamine-responsive neurons also responded to a noxious heat (50°C) stimulus. Histamine-responsive STT neurons were located in both the marginal zone as well as in the nucleus proprius.
STT neurons responded to cowhage usually after a short latency period of 10–20 s and with a time course of several minutes. The latency and duration of the responses to cowhage parallel the sensation of itch reported in psychophysical studies (Graham et al., 1951
; Shelley and Arthur, 1957
; Tuckett, 1982
; Johanek et al., 2007
). All cowhage-responsive STT neurons were classified as HT or WDR and 67% responded to a 50°C heat ramp. STT neurons responsive to cowhage were located within the marginal zone and the lateral reticulated area of the dorsal horn. Cowhage could be applied repeatedly without a decrement of the response, suggesting that the pathway is not subject to tachyphylaxis. Furthermore, in about half of the pruritogen responsive STT neurons, the second pruritogen applied (cowhage or histamine) produced the response indicating that cross-tachyphylaxis between the two types of pruritogens does not occur.
Histamine-responsive neurons with unidentified axonal projections have been recorded in the rat superficial and deep dorsal horn (Carstens, 1997
; Jinks and Carstens, 1998
). These neurons in rats all responded to other noxious mechanical or chemical stimuli, consistent with our results of histamine (and cowhage)-responsive neurons in primate dorsal horn. However, rats do not scratch to intracutaneous injections of histamine (Jinks and Carstens, 2002
); therefore, it could be concluded that the responses to histamine in rats are nociceptive rather than pruriceptive. In contrast, both histamine and cowhage elicit scratching directed to the application site in monkeys (R. H. LaMotte, personal communication). Additionally, like humans, monkeys have been shown to scratch in response to intrathecal morphine, and this scratching is not attenuated by antihistamines (Ko and Naughton, 2000
; Ko et al., 2004
). These data support the use of monkeys as an appropriate animal model to investigate the central neural mechanisms of both histaminergic and nonhistaminergic itch.
Our data predict that STT neurons receive input from populations of primary afferent fibers that respond to histamine or cowhage, but not both. However, the current understanding of pruritic mechanisms in the periphery is incomplete. Several lines of evidence indicate that separate peripheral mechanisms exist for coding histamine- or cowhage-produced itch. As noted, cowhage produced an itch without flare and the itch was not blocked by antihistamine. In addition, desensitization of the skin with topical capsaicin prevented cowhage-induced, but not histamine-induced itch (Johanek et al., 2007
). Furthermore, cowhage applied to the hairy skin of cats excited a majority of polymodal C-fibers tested (Tuckett and Wei, 1987
). In contrast, histamine applied iontophoretically weakly excited only a subset of polymodal C-fibers in human (Handwerker et al., 1991
; Schmelz et al., 1997
). These data indicate that polymodal C-fibers encode the itch produced by cowhage. However, a previous study in monkeys demonstrated that single polymodal C-fibers can be excited by both an application of cowhage as well as an intradermal injection of histamine (Johanek et al., 2005
). Thus, the role of polymodal C-fibers in encoding histamine-induced itch remains enigmatic. Additional studies are needed to determine the sources of input to the two types of pruritogen-responsive STT neurons in monkeys.
Important studies of itch have advocated the specificity theory (Schmelz et al., 1997
; Andrew and Craig, 2001
), which states that itch is generated by a subpopulation of neurons in the periphery and spinal cord that respond exclusively to itch-producing stimuli. The seminal observation in support for this idea came from microneurography experiments that identified a population of slowly conducting C-fibers that were mechanically insensitive and responded to histamine for a duration that matched the duration of itch sensation in humans (Schmelz et al., 1997
). Because these histamine-sensitive fibers failed to respond to mechanical stimuli and also to the chemical algogen mustard oil, they were initially labeled “itch specific.” However, additional examination with capsaicin and bradykinin demonstrated that these histamine-sensitive fibers also responded to painful chemicals (Schmelz et al., 2003
). Moreover, the ability of many dorsal root ganglion neurons to respond to histamine has been shown previously to be contingent on the expression of the capsaicin-responsive receptor [transient receptor potential vanilloid 1 (TRPV1)], and TRPV1-null mice exhibit diminished scratching in response to histamine compared with wild-type (Shim et al., 2007
) (but see Nicolson et al., 2002
). Thus, even the strongest candidate primary afferent fibers for the transduction of histamine-produced itch also respond to painful chemicals. Similarly, primary afferent fibers that respond to cowhage are also all activated by noxious mechanical and heat stimuli (Tuckett and Wei, 1987
; Johanek et al., 2005
). These findings suggest that itch and pain are inexorably linked in the peripheral nervous system. Our results demonstrate that STT neurons responsive to histamine or cowhage can also be activated by capsaicin, noxious mechanical, and heat stimuli, indicating that the link between noxious and pruritic information is preserved in the responses of primate STT neurons.
Responses mirroring those of the afferent fibers thought to be itch specific were recorded from STT neurons in the marginal zone of cat (Andrew and Craig, 2001
). In that study, neurons from the deep dorsal horn were not examined for responses to histamine, nor were marginal zone neurons that responded to mechanical stimuli. Our present findings indicate that mechanically sensitive STT neurons in the deep and superficial dorsal horn of primates are capable of contributing to the itch sensation. Importantly, in the study by Andrew and Craig (2001)
, STT neurons that responded to histamine were not tested for sensitivity to capsaicin. In our study, all primate STT neurons that responded to histamine or cowhage also responded to capsaicin. This is consistent with previous observations in primates (Simone et al., 2004
). Because each pruritogen-responsive neuron responded to mechanical stimulation and to the painful chemical capsaicin, our data further complicate support for a specific itch pathway. Although a population of truly itch-specific peripheral fibers and spinal cord neurons may yet be found in primates, alternatives to the specificity theory have been considered in the past (Handwerker, 1992
; McMahon and Koltzenburg, 1992
; LaMotte, 1996
) and deserve re-evaluation in light of the lack of evidence for a labeled-line pathway for itch. Our data are consistent with a theory of itch that recognizes that there are no primary afferent fibers that respond exclusively to pruritogens. Instead, itch is generated when a subset of central neurons is activated by inputs from primary afferent fibers responsive to both noxious and pruritic stimuli. However, pain is perceived when nociceptive-specific (i.e., not pruritic) primary afferent fibers excite additional nonpruritic spinal neurons. In primates, we found that roughly two-thirds of STT neurons respond to noxious, but not to pruritic, stimuli and that about one-third respond to both pruritic and noxious stimuli. Noxious mechanical and chemical stimuli would likely activate both classes of STT neurons and produce pain. Sole activation of the STT neurons that are activated by pruritic and noxious stimuli could signal itch.
Our results indicate that separate, parallel populations of STT neurons carry ascending sensory information underlying the itch produced by histamine or cowhage. It will be valuable in future studies to determine whether other types of nonhistaminergic itch stimuli, possibly including those that more closely mirror clinical forms of itch, activate cowhage-responsive or a separate population of STT neurons. In addition, it will be interesting to determine whether processing of histaminergic and nonhistaminergic forms of itch is also performed by separate populations of neurons within higher levels of the CNS.