- Email: [email protected]
Itching for an explanation
42 Stallone, D., Nicolaidis, S. and Gibbs, J. (1989) Am. J. Physiol. 256, R1138-R1141 43 Leibowitz, S. F. (1991) in Galanin: A New Multifunctional Peptide in the Neuro-Endocrine System (Hokfelt, T. and Bartfai, T., eds), pp. 393-406, Macmillan Press 44 Leibowitz, S. F. (1988)in Perspectives in Behavioral Medicine, Eating Regulation and Discontrol (Weiner, H. and Baum, A., eds), pp. 113-136, Lawrence Erlbaum Associates 45 Melander, T. et al. (1991) in Galanin: A New Multifunctional Peptide in the Neuro-Endocrine System (Hokfelt, T. and Bartfai, T., eds), pp. 107-116, Macmillan Press 46 Weisenfeld-Hallin, Z., Xu, X. J., Villar, M. J. and Hokfelt, T. (1990) Neurosci. Lett. 109, 217-221 47 Olson, G. A., Olson, R. D. and Kastin, A. J. (1991) Peptides 12, 1407-1432 48 Tempel, D. L., McEwen, B. S. and Leibowitz, S. F. Neuroendocrinology (in press) 49 Leibowitz, S. F. and Kim, T. Brain Res. (in press) 50 Leibowitz, S. F., Shor-Posner, G., Maclow, C. and Grinker, J. A. (1986) Brain Res. Bull. 17, 681--689 51 Vaccarino, F. J. (1990) Ann. NYAcad. Sci. 579, 227-232
52 Leibowitz, S. F., Lucas, D. J., Leibowitz, K. L. and Jhanwar, Y. S. (1991) Physiol. Behav. 50, 1167-1174 53 Drewnowski, A., Kurth, C., Holden-Wilste, J. and Saari, J. (1992) Appetite 18, 207-221 54 Sutton, S. W., Mitsugi, N., Plostsky, P. M. and Sarkar, D. K. (1988) Endocrinology 123, 2152-2154 55 Kant, G. J. et aL (1983) Psychoneuroendocrinology 8, 421-428 56 Dalvit-McPhillips, S. P. (1983) Physiol. Behav. 31, 209-212 57 Gabriel, S. M., Kaplan, L. M., Martin, J. B. and Koenig, J. I. (1989) Peptides 10, 369-374 58 Gabriel, S. M., Koenig, J. I. and Kaptan, L. M. (1990) Neuroendocrinology 51, 168-173 59 Frisch, R. E. (1991) Trends Endocrinol. Metab. 2, 191-197 60 Jhanwar-Uniyal, M., Chua, S. C., Jr and Leibowitz, S. F. (1991) Soc. Neurosci. Abstr. 17, 191 61 Mitchell, J. E. (1989) Ann. NYAcad. Sci. 575, 41-49 62 Kaye, W. H. (1992) in The Biology of Feast and Famine (Anderson, G. H., ed.), pp. 22-45, Academic Press 63 Williams, G. and Bloom, S. R. (1989) Diabetic Med. 6, 472-485
Acknowledgements Researchin the author'slaboratory describedin this articlewassupported by NIMHgrant MH43422.
Itching for an explanation Stephen B. McMahon and Martin Koltzenburg Itch is a distinct sensation arising from the superficial layers of skin and mucous membranes. It is elicited by histamine and probably other endogenous chemicals that excite subpopulations of unmyelinated primary afferents and spinal neurones projecting through the anterolateral quadrant to the brain. The two popular views, which propose either that itch is signalled by a labelled line system of peripheral and central itchspecific neurones or that itch is the subliminal form of pain, both fail to explain convincingly many known features. Alternative theories emphasize centra! ;~mcesses that extract the relevant information from afferents with broad sensitivity spectra for pruritogenic and noxious stimuli. Thus, itch presents an irritating challenge for the specificity theory of somatosensation. Itch is a well-appreciated but poorly understood sensation. Yet itch represents a major clinical problem affecting skin, mucous membranes and the upper respiratory tract, and its understanding is important for theories of somatosensation. This review will describe what is known about itch, and discuss the possible underlying neural mechanisms. To date there is no universally accepted single hypothesis that both explains all the observed features of itch and is clearly supported by all experimental evidence.
Itch, prickle, tickle and pain Itch is an unpleasant sensory experience associated with the desire to scratch. While the sensations of itch and pain share some features, the two can be distinguished on a number of grounds. Prickle and tickle are associated, yet distinct sensations. Prickle is typically caused by the movement of rough fabrics over the skin, and some evidence exists that prickly stimuli cause low levels of firing in peripheral nociceptors 1. Tickle is almost certainly mediated by lowthreshold mechanosensitive afferent neurones. While humans can quite easily recognize the distinct features of these sensations, many mammals respond with a similar stereotypical behaviour 2, and this has TINS, Vol. 15, No. 12, 1992
impeded the development of appropriate animal StephenB.McMahon isat the Deptof models of itch. Physiology,St Thomas'Hospital Starting from scratch - the historical MedicalSchool perspective (UMDS),Lambeth Itch was a sensation known in the ancient world. In PalaceRoad,London, 450 BC, Herodotus described the great effort of UKSE17EH,Martin Egyptians to avoid mosquito bites - sleeping on high Koltzenburgis at the towers and tightly wrapping themselves in fishing Neurologische nets. However, in early experimental studies on Universit~,ts-Klinik, cutar,eous sensations, itch was initially ignored. The Josef-Schneider-Str. theory of specific nerve energies of Johannes Mfiller~ 11, W-8700 did not recognize itch as a separate sensory experi- W~rzburg,FRG.
ence, and the description of 'skin spots' for different sensations did not originally include itch4. Later studies did recognize itch 'spots' (highly localized areas occurring about one per square millimetre) from which itch could be elicited by tactile stimuli. Initially these itch spots were thought to coincide with pain spots, and that itch resulted from weak stimuli and pain from stronger ones ~'6. Bishop 7 reported that electrical stimulation of these sites gave rise to itch, and Shelly and Arthur ~-1° made extensive studies of
BOX 1. Stimuli that can elicit or augment itch Physical Mechanical. Light touch, pressure, suction. Thermal. Warming. Electrical. Focal transcutaneous repetitive stimulation, transcutaneous constant current stimulation, intraneural microstimulation. Chemical Non-specific irritants. Acids, alkalis. Inflammatory mediators. Histamine, kallikrein, bradykinin, prostaglandins. Histamine-releasingsubstances. Compound 48/80, protamine, C3a. Peptidases. Mucunain, papain, trypsin, mast cell chymase. Neuropeptides. Substance P, vasoactive intestinal polypeptide, neurotensin, secretin. Opioids. Morphine, ~-endorphin, enkephalin analogues.
© 1992, ElsevierSciencePublishersLtd, (UK)
497
TABLE I. Comparison of the established features of itch and pain Itch
Pain
Skin, mucous membranes See Box I Occasionally AIIoknesis (itchy skin) Pronounced Scratching, pain, cooling
Most tissues Many stimuli Yes Hyperalgesia Present Tactile stimuli, cooling
C- and Ab-fibres Large ? Anterolateral funiculus ? Scratching, sneezing Yes ?
C- and A6-fibres Small NS, WDR Anterolateral funiculus Yes Flexion, guarding Yes Yes
Yes Probably not No
Chemogenic pain; yes Yes Yes
Psychophysiology Tissue Stimulus Intraneural microstimulation Secondary sensations Psychogenic modification Counterstimuli
Neurophysiology Primary afferent neurones Flare size Spinal cord neurones Spinal pathway Descending spinal control Protective reflexes Autonomic reflexes Central plasticity changes
Pharmacology Capsaicin sensitivity NSAID sensitivity Morphini~ sensitivity
Abbreviations: NS, nociceptive-specificneurones; WDR, wide dynamic range neurones; NSAID, non-steroidal anti-inflammatory drugs.
secondary sensations that are reminiscent of the features of secondary hyperalgesia evolving around a painful focus. A crucial observation is that itch and pain usually do not coexist in the same skin region and a mild noxious stimulus such as scratching is in fact the single most effective way to abolish itch. This abolition of itch can be prolonged producing an 'antiprufitic state 'is. Although mild scratch is often not painful, microneurographic recordings from humans have directly determined that such stimuli are among the most effective ways to excite cutaneous unmyelinated nociceptive afferents 21'22. A further important observation is that psychological factors more readily colour the conscious perception of itch compared to other sensations.
Neurophysiological aspects of itch There is a consensus that itch is itch spots in the 1950s using chemical stimuli. They conducted by unmyelinated and perhaps thin myeliningeniously used individual spicules of the pruritogenic ated primary afferents. Thus, humans can appreciate agent cowhage (Mucuna pruriens) to investigate the itch during a differential nerve block affecting myelinproperties of itch points. Their work clearly estab- ated axons 18'23, and intraneural electrical stimulation lished the importance of chemical mediators in the of unmyelinated fibres can occasionally evoke itch 24. generation of itch. They showed that cowhage Furthermore, the desensitization of fine chemospicules contained a proteinase that was itself prurito- sensitive afferents by topical application of capsaicin genic, and found that other compounds, including reportedly abohshes or reduces itch 23'25. histamine, trypsin and papain, were also active at itch Evidence that application of histamine can excite spots 8'1°. Histamine has now become the standard unmyelinated afferent fibres has also been directly pruritogenic stimulus in experimental studies n'lz documented 26-3°. In a recent human microneurobecause it produces an almost pure and graded graphic study, single unmyelinated primary neurones sensation of itch when delivered iontophorefically to were tested for their response to histamine iontothe skin. However, the relative ineffectiveness of phoresis (which induced itch) and topical application of antihistaminic drugs against many forms of exper- mustard oil (which evoked burning pain) 29. This study imental and clinical itch suggests that this compound is found that a subpopulation of unmyelinated mechanounlikely to be the only mediator in skin. Various sensitive afferents responded strongly to histamine, chemicals (Box 1) have been reported to elicit or but were also excited by application of mustard oil promote itch in humans m'13-17, although many of and other noxious stimuli. Another population of these may act indirectly by degranulafion of mast mechano-heat sensitive unmyelinated fibres (C-fibres) cells. responded only to mustard oil and not to histamine. Less is known about the CNS structures that What we know about itch mediate itch. Anterolateral cordotomy abolishes itch Any theory of the neural basis of itch needs to sensibility 31'32, consistent with the notion that the account for a number of observations. Table I lists the same spinal pathways transmit noxious and pruritic best established of these and compares them with the information. It is unknown which supraspinal strucproperties of pain, since many workers have been tures are involved in itch other than those that struck by the similarities of both sensations. process noxious information. Prufitogenic and noxious stimuli evoke essentially the same sympathetic reflex Psychophysical aspects of itch responses 33. Neurophysiological studies of central In contrast to the large variety of stimuli that are neurones have so far failed to obtain evidence for a capable of evoking pain in most tissues, itch can only specific central histamine-sensitive neurone 2°'z4. be felt in the superficial layers of the skin, mucous Rather, central neurones reflect essentially the same membranes and conjunctiva 1°' 13,18indicating that only properties seen in primary afferents. Neurones resubpopulations of primary afferents are capable of sponding to prufitogenic stimuli were also excited by encoding itch. Moreover, experimental focal itch noxious stimuli. Since neurophysiological studies stimuli are surrounded by a halo of seemingly using prufitogenic stimuli are still rare, and as most unaffected tissue where light tactile stimuli are have studied only mechanosensitive units, we cannot capable of eliciting itch-like sensations. The term itchy completely rule out the possibility that specific popuskin or alloknesis l°'n'lg'z° has been coined for these lations of histamine-sensitive neurones exist.
498
TINS, VoL 15, No. 12, 1992
Pharmacological aspects of itch Some drugs appear to have a differential response on itch and pain. It is said that opiates t4 and nonsteroidal anfi-inflam atory drugs 35 a r e ineffective in reducing itch. In fact, one disturbing side effect of opiates is the induction of itch - possibly as a consequence of mast cell degranulation. These differential effects suggest the existence of distinct central sensory processing pathways for itch and pain. However, this may not apply stringently because some pattern of excitation could be uniquely amenable to the drug's action.
A
Specificity ,1,-
~k~n g
? I
.iv ~ i
primary afferents
pain
centralneurons
Intensity m-
Q.
5
Theories of the neural basis of itch There is at present no single neuronal mechanism that is generally accepted as being able to explain all the features of itch (Table I). The evidence for and C Selectivity against different proposals is discussed below, and experimental approaches suggested that might yield definitive conclusions regarding the likelihood of these mechanisms. pain Specificity theory. Conceptually, the simplest explanation of itch is that a group of primary sensory neurones exist that respond to pruritogenic stimuli and no other (Fig. 1A), conforming to the specificity theory of somatosensation. Clearly such afferent Fig. 1. Schematic illustration of theories that have been neurones would have to be numbered among the proposed as the neural basis of itch. H, afferents respondsmall-diameter fibres and include some unmyelinated ing to pruritogenic stimufi notably histamine; M, afferents afferents, since itch sensibility is not lost until responding to noxious irritants such as mustard oil. See peripheral nerve blockade includes C-fibres. Un- text for details. fortunately, no such population of neurones has been observed in the rat, cat, monkey or humans. Many afferent fibres responding to pruritogenic chemicals response to prufitogenic stimuli could be the differhave been seen, but these have always also re- ential discharge patterns of unmyelinated afferents sponded to other, non-pruritogenic, noxious stimuli. producing itch or pain since there is not always a close It is possible that a special group of dedicated 'itch relation between the intensity of sensation and the fibres' (pruritoceptors) may have been overlooked, :magnitude of the flare response 4a. Finally a smaller and in this respect it is perhaps worth remembering - amount of cutaneous reflex vasoconstriction elicited that a group of somatic mechanically-insensitive by pruritogenic stimuli could underlie the large chemoreceptive afferents has only recently been histamine flare aa,44. The specificity theory also requires a 'labelled line' recognized36. However, from careful quantitative studies that have sought to sample unmyelinated within the CNS, dedicated to the processing of afferents in an unbiased fashion, it would appear that, pruritogenic stimuli. Positive experimental evidence numerically, this presumptive population of prurito- has not been sought, but in animal experiments nearly ceptors would have to be small, amounting to only a all spinal neurones identified by the presence of an few percent of cutaneous C-fibre afferents 37'38. ascending axon in the anterolateral quadrant have Nevertheless, as skin is innervated by several been found to respond to some form of nonhundred C-fibres per square millimetre 39'4°, a suf- pruritogenic stimulus 45. Thus, by exclusion, one can ficient density of units could still signal itch. These few conclude that only few spinal neurones can be fibres would have to be capable of producing the very dedicated to the processing of pruritogenic stimuli. The existence of a specific population of peripheral pronounced flare response seen, for instance, with histamine challenge in human skina4. afferents and central neurones could, of course, be Although the magnitude of histamine-induced itch is shown directly. It would be necessary to search often less than the intensity of chemically induced among the chemosensitive afferents (and methods pain, there is a perplexing difference in the sizes of now exist to identify these fibres37'38), and to use an the associated flare responses, which are typically appropriate animal species. The rat is probably unlarger after pruritogenic stirnl.di 11'23'41'42. This is suitable since histamine is not present in abundance intriguing, as excitation of a smaller number of in its skin, nor is there evidence that histamine sensory neurones should yield less vasodilatation. produces specifically itch-related (rather than painThis paradox is perhaps the only good evidence for related) behaviour in this species. While technically the existence of pruritoceptors that might possess demanding, the most revealing studies are likely to be especially powerful mechanisms for neurogenic vaso- in humans using microneurography. In summary, the weight of evidence does not dilatation. Alternatively, pnuritogenic stimuli may preferentially e~cite the most superficial sensory favour a model based on peripheral specificity. Intensity theory. Another way in which itch might be fibres and so produce vasodilatation of superficial blood vessels, which may be more readily visible. encoded is illustrated in Fig. lB. Here it is envisaged Another explanation for the unusually large flare that low levels of activity in nociceptive fibres signal TINS, Vol. 15, No. 12, 1992
499
A
Pattern coding
B
Population coding
Fig. 2. Schematic illustration of theories that could explain how different sensations of itch and pain could arise from the differential response of populations of nonspecific neurones. Open circles represent inactive neurones and filled circles the excited cells. The mechanism may either apply for primary afferents or central neurones and manifests as the temporal pattern of the firing or the pattern of activated units. See text for details.
itch, and higher levels of activity signal pain, suggesting that itch is a subliminal form of pain. Although this theory has been repeatedly proposed and is often stated in texts, there are many reasons to doubt its veracity. It is true that itch and pain share many features (Table I); however, direct testing with threshold forms of noxious stimuli (thermal, mechanical and chemical) do not consistently elicit itch. Moreover, many manoeuvres that inhibit pain do not inhibit itch, and vice versa. Thus, the differential sensitivity of the sensations to non-steroidal antiinflammatory drugs, opiates, noxious and innocuous counterstimuli all argue against this theory. Direct microneurographic recordings from human nociceptors have also determined that moderately itchy or painful stimuli yield similar tiring rates in nociceptors. In addition, when transcutaneous, intradermal or intraneural electrical stimulation techniques are used to elicit itch, increasing the frequency of the stimulus does not in general produce pain. In short, although this theory might be attractive in explaining the common features of itch and pain, the wealth of experimental evidence speaks against it. Selectivity theory. The principle of this theory is illustrated by one example in Fig. 1C. The theory recognizes the absence of specific populations of sensory neurones dedicated to the signalling of itch. It suggests instead that the subset of afferent nociceptors that respond to pruritogenic stimuli have different central connectivities, and activate separate central neurones. Noxious chemical stimuli, such as mustard oil, would activate the same subset of afferents and, additionally, other nociceptors. The theory requires that the widespread activation of nociceptive systems would mask the activity of ‘itchsignalling’ pathways, and this is consistent with the observation that itch and pain rarely coexist. The masking could arise from specific inhibitory mechanisms, as suggested in Fig. lC, or it could arise from inherent properties of the central signalling system, 500
where itch might arise from a form of parallel processing that requires activity in one population of neurones coupled with inactivity in another. The peripheral selectivity need not be confined to histamine-sensitive neurones. Other selective groups of nociceptive afferents could perform the same function, for instance those innervating superficial, rather than deep, layers of skin. This theory is consistent with known peripheral physiology, and could explain the absence of central neurones selectively responsive to pruritogenic stimuli. It can also explain the most basic feature of itch, namely the inhibition by noxious counterstimuli. Manipulations that interfere selectively with nociceptive transmission, such as opiate administration, would not be expected to inhibit itch, but perhaps enhance it. A direct test of this theory would require the detailed study of responsiveness of central neurones. If inhibition of central ‘itch-signalling’ neurones were a crucial mechanism, it should be directly demonstrable. Pattern theory. A final mechanistic model, which could apply to primary afferents and central neurones is that itch is encoded by the temporal or spatial firing of cells that are also capable of signalling other events (Fig. 2). Implicit in this theory is that individual neurones are ‘broadly tuned’ and respond to a variety of stimuli, which is true for the primary afferent neurones and (on meagre data) also for central neurones that respond to pruritogenic stimuli. The theory has the same explanatory power as the selectivity theory, accounting for the many similarities between itch and pain, and the inhibition of itch by pain. The major drawback is that the effective patterns are unspecified, and therefore untestable. One of the simplest patterns might be that pruritogenie stimuli activate one fraction of the available neuronal pool or temporal code, while noxious stimuli excite another (possibly overlapping) fraction or code (Fig. 2A). As detailed above, direct neurophysiological investigations endorse neither this view nor the possibility that the separate sensations are encoded by unique discharge pattems2g346. Perhaps the more plausible explanation is that itch is felt when one subpopulation is active, and if this proportion is exceeded the discharge might be interpreted as painfuP0 while the itch-signal becomes obscured, because of the lack of contrast between active neuronal pools (Fig. 2B). One implication of this theory is that activation of a single neuronal element should not elicit itch. However, itch can be elicited by focal intraneural or transcutaneous electrical stirnulation and this would imply that a suitable pattern could be generated from a relatively small number of afferents. Concluding remarks It is clear from the foregoing discussion that despite a century of experimental work our understanding of itch remains rudimentary. However, we have now reached a stage where the number of possible neuronal mechanisms is constrained, and techniques are available to test these. The outcome will be of interest both to clinicians, who face many patients who suffer from itching, and also to basic scientists, since several of the currently viable mechanistic models pose a direct challenge to the specificity theory of somatosensation. TINS, Vol. 15, No. 12, 1992
Selected references 1 Garnsworthy, R. K., Gully, R. L., Kenins, P., Mayfield, R. J. and Westerman, R. A. (1988) J. Neurophysiol. 59, 1083-1097 2 Woodward, D. F., Conway, J. L. and Wheeler, L. A. (1985) in Models in Dermatology (Maibach, H. I. and Lowe, N. J., eds), pp. 187-195, Karger 3 Miiller, J. (1838) Handbuch der Physiologie des Menschen (2nd edn), HOlscher 4 von Frey, M. (1896) Abh. Math. Phys. Kl. Saechs. Akad. Wiss. 23, 175-266 5 von Frey, M. (1922) Arch. Neerland. Physiol. 7, 142-145 6 Lewis, T., Grant, R. T. and Marvin, H. M. (1927) Heart 14, 139-160 7 Bishop, G. H. (1943) J. Neurophysiol. 6, 361-382 8 Arthur, R. P. and Shelly, W. B. (1955) J. Invest. Dermatol. 25, 341-346 9 Shelly, W. B. and Arthur, R. P. (1955) Arch. Dermatol. 72, 399-406 10 Shelly, W. B. and Arthur, R. P. (1957) Arch. DermatoL 76, 296-323 11 Simone, D. A. etal. (1987) Somatosens. Res. 5, 81-92 12 Magerl, W., Westerman, R. A., Mohner, B. and Handwerker, H. O. (1990) J. InvesL DermatoL 94, 347-352 13 Keele, C. A. and Armstrong, D. (1964) in Substances Producing Pain and Itch, pp:. 1-400, Arnold 14 Fjellner, B. and H&germark, O. (1982)Arch. Derm. Res. 274, 29-37 15 Fjellner, B. and H~germark, O. (1981) Acta Derm. Venerol. 61,245-250 16 H~germark, 6. (1974) Acta Derm. Venerol. 54, 397-400 17 H~germark, O., Strandberg, K. and Hamberg, M. (1977) J. Invest. DermatoL 69, 527-530 18 Bickford, R. G. (1938) Clin. Sci. 3, 337-386 19 Graham, D. T., Goodell, H. and Wolff, H. G. (1951) J. Clin. Invest. 30, 37-49 20 Simone, D. A., Alreja, M. and LaMotte, R. H. (1991) Somatosens. Mot. Res. 8, 271-279 21 TorebjOrk, E. (1985) Philos. Trans. R. Soc. London Ser. B 308, 227-234
22 Vallbo, A. B., Hagbarth, K. E., Torebj6rk, H. E. and Wallin, B. G. (1979) Physiol. Rev. 59, 919-957 23 Magerl, W. (1991) Allergologie 14, 395-405 24 Torebj6rk, H. E. and Ochoa, J. L. (1980) Acta PhysioL Scand. 110, 445-447 25 T6th-K&sa, I., Jancs6, G., Bognb.r, A., Musz, S. and Ob&l, F. (1986) Int. J. Clin. Pharm. Res. 6, 163-169 26 Torebj6rk, H. E. (1974) Acta PhysioL Scand. 92, 374-390 27 Fj~llbrant, N. and Iggo, A. (1961) J. Physiol. 156, 578-590 28 TuckeR, R. P. and Wei, J. Y. (1987) Brain Res. 413, 95-103 29 Handwerker, H. O., Forster, C. and Kirchoff, C. (1991) J. Neurophysiol. 66, 307-315 30 Handwerker, H. O. (1992) APSJ. 1, 135-138 31 White, J. C. and Sweet, W. H. (1969) Pain and the Neurosurgeon: A Forty Year Experience, C. C. Thomas 32 Nathan, P. W. (1990) J. Neurol. Neurosurg. Psychiat, 53, 935-939 33 Magerl, W., Koltzenburg, M. and Handwerker, H. O. (1991) Pfl6gers Arch. 418, R35 34 LaMotte, R. H. (1992) APSJ. 1, 115-126 35 H~germark, O. (1973) Acta Derm. Venerol. 53,363-368 36 McMahon, S. B. and Koltzenburg, M. (1990) Trends Neurosci. 13, 199-201 37 Meyer, R. A., Davis, K. D., Cohen, R. H., Treede, R-D. and Campbell, J. N. (1991) Brain Res. 561,252-261 38 Kress, M., Koltzenburg, M., Reeh, P. W. and Handwerker, H. O. (1992) J. Neurophysiol. 68, 581-593 39 Lynn, B. and Carpenter, S. E. (1982) Brain Res. 238, 29-43 40 Campbell, J. N., Raja, S. N., Cohen, R. H., Manning, A. A. K. and Meyer, R. A. (1989) in Textbook of Pain (2nd edn) (Wall, P. D. and Melzack, R,, eds), pp. 22-45, Churchill Livingstone 41 Simone, D. A., Baumann, T. K. and LaMotte, R. H. (1989) Pain 38, 99-107 42 Treede, R-D. (1992) Neurosci. Lett. 141, 169-172 43 Koltzenburg, M. and Handwerker, H. O. (1992) PflOgers Arch. 420, R47 44 Hornyak, M. E., Naver, H. K., Rydenhag, B. and Wallin, B. G. (1990) Acta Physiol. Scand. 139, 77-84 45 Willis, W. D. and Coggeshall, R. E. (1991) Sensory Mechanisms of the Spinal Cord (2nd edn), Plenum Press 46 TuckeR, R. P. (1982) J. Invest. Dermatol. 79, 368-373
Suppressionofprogrammedneuronaldeathbysustained elevationofcytoplasmiccalcium James L. Franklin and Eugene M. Johnson, Jr Chronic depolarization greatly increases the survival of many ~pes of neurons in culture. In at least some cases this enhancement of survival consists of the suppression of programmed cell death, a type of death occurring in developing neurons deprived of sufficient neurotrophic factor support. Available evidence suggests that the effect of depolarization on survival is mediated by a sustained rise of cytoplasmic free Ca e+, apparently caused by influx of Ca e+ through voltage-gated channels. This review discusses what is known about the mechanism by which prolonged depolarization and increased intracellular Ca e+ promote survival. Approximately half of all neurons produced in the developing vertebrate nervous system die before adulthood as a result of a process known as naturally occurring, or programmed, cell death (see Box 1). The primary role of this process seems to be to match innervation density appropriately with target size 1. Availability of specific neurotrophic substances, provided by the target or other tissues, is a major element determining which neurons survive this developmental stage. Those neurons obtaining sufTINS, Vol. 15, NO. 12, 1992
ficient quantities of relevant factors live, while those that do not, die. Removal of afferent input, pharmacological blockade of synaptic transmission or blockade of electrical activity can inhibit the growth and survival of some types of developing neurons, both in vivo 2-6 and in vitro7-1°, suggesting that electrical activity is also an important determinant of survival during the period of programmed death. While considerable work has been directed toward elucidating the mechanisms by which neurotrophic factors promote survival (or conversely, suppress programmed death), little effort has been aimed toward discerning the mechanism by which electrical activity supports survival. We describe here a long-estabfished cell culture technique that may serve as a paradigm for investigating this phenomenon.
JamesL. Franklinand EugeneM. Johnson, Jrare at the Dept of Molecular Biology and Pharmacology, Wash#Tgton UniversitySchoolof Medicine, St Louis, M063110, USA.
Chronic depolarization prevents the death of n e u r o n s in culture Scott and Fisher 11 showed more than two decades ago that the survival of embryonic chicken dorsal root ganglion neurons in culture is enhanced when they are maintained in a medium containing elevated
© 1992.ElsevierSciencePublishersLtd, (UK
501
52 Leibowitz, S. F., Lucas, D. J., Leibowitz, K. L. and Jhanwar, Y. S. (1991) Physiol. Behav. 50, 1167-1174 53 Drewnowski, A., Kurth, C., Holden-Wilste, J. and Saari, J. (1992) Appetite 18, 207-221 54 Sutton, S. W., Mitsugi, N., Plostsky, P. M. and Sarkar, D. K. (1988) Endocrinology 123, 2152-2154 55 Kant, G. J. et aL (1983) Psychoneuroendocrinology 8, 421-428 56 Dalvit-McPhillips, S. P. (1983) Physiol. Behav. 31, 209-212 57 Gabriel, S. M., Kaplan, L. M., Martin, J. B. and Koenig, J. I. (1989) Peptides 10, 369-374 58 Gabriel, S. M., Koenig, J. I. and Kaptan, L. M. (1990) Neuroendocrinology 51, 168-173 59 Frisch, R. E. (1991) Trends Endocrinol. Metab. 2, 191-197 60 Jhanwar-Uniyal, M., Chua, S. C., Jr and Leibowitz, S. F. (1991) Soc. Neurosci. Abstr. 17, 191 61 Mitchell, J. E. (1989) Ann. NYAcad. Sci. 575, 41-49 62 Kaye, W. H. (1992) in The Biology of Feast and Famine (Anderson, G. H., ed.), pp. 22-45, Academic Press 63 Williams, G. and Bloom, S. R. (1989) Diabetic Med. 6, 472-485
Acknowledgements Researchin the author'slaboratory describedin this articlewassupported by NIMHgrant MH43422.
Itching for an explanation Stephen B. McMahon and Martin Koltzenburg Itch is a distinct sensation arising from the superficial layers of skin and mucous membranes. It is elicited by histamine and probably other endogenous chemicals that excite subpopulations of unmyelinated primary afferents and spinal neurones projecting through the anterolateral quadrant to the brain. The two popular views, which propose either that itch is signalled by a labelled line system of peripheral and central itchspecific neurones or that itch is the subliminal form of pain, both fail to explain convincingly many known features. Alternative theories emphasize centra! ;~mcesses that extract the relevant information from afferents with broad sensitivity spectra for pruritogenic and noxious stimuli. Thus, itch presents an irritating challenge for the specificity theory of somatosensation. Itch is a well-appreciated but poorly understood sensation. Yet itch represents a major clinical problem affecting skin, mucous membranes and the upper respiratory tract, and its understanding is important for theories of somatosensation. This review will describe what is known about itch, and discuss the possible underlying neural mechanisms. To date there is no universally accepted single hypothesis that both explains all the observed features of itch and is clearly supported by all experimental evidence.
Itch, prickle, tickle and pain Itch is an unpleasant sensory experience associated with the desire to scratch. While the sensations of itch and pain share some features, the two can be distinguished on a number of grounds. Prickle and tickle are associated, yet distinct sensations. Prickle is typically caused by the movement of rough fabrics over the skin, and some evidence exists that prickly stimuli cause low levels of firing in peripheral nociceptors 1. Tickle is almost certainly mediated by lowthreshold mechanosensitive afferent neurones. While humans can quite easily recognize the distinct features of these sensations, many mammals respond with a similar stereotypical behaviour 2, and this has TINS, Vol. 15, No. 12, 1992
impeded the development of appropriate animal StephenB.McMahon isat the Deptof models of itch. Physiology,St Thomas'Hospital Starting from scratch - the historical MedicalSchool perspective (UMDS),Lambeth Itch was a sensation known in the ancient world. In PalaceRoad,London, 450 BC, Herodotus described the great effort of UKSE17EH,Martin Egyptians to avoid mosquito bites - sleeping on high Koltzenburgis at the towers and tightly wrapping themselves in fishing Neurologische nets. However, in early experimental studies on Universit~,ts-Klinik, cutar,eous sensations, itch was initially ignored. The Josef-Schneider-Str. theory of specific nerve energies of Johannes Mfiller~ 11, W-8700 did not recognize itch as a separate sensory experi- W~rzburg,FRG.
ence, and the description of 'skin spots' for different sensations did not originally include itch4. Later studies did recognize itch 'spots' (highly localized areas occurring about one per square millimetre) from which itch could be elicited by tactile stimuli. Initially these itch spots were thought to coincide with pain spots, and that itch resulted from weak stimuli and pain from stronger ones ~'6. Bishop 7 reported that electrical stimulation of these sites gave rise to itch, and Shelly and Arthur ~-1° made extensive studies of
BOX 1. Stimuli that can elicit or augment itch Physical Mechanical. Light touch, pressure, suction. Thermal. Warming. Electrical. Focal transcutaneous repetitive stimulation, transcutaneous constant current stimulation, intraneural microstimulation. Chemical Non-specific irritants. Acids, alkalis. Inflammatory mediators. Histamine, kallikrein, bradykinin, prostaglandins. Histamine-releasingsubstances. Compound 48/80, protamine, C3a. Peptidases. Mucunain, papain, trypsin, mast cell chymase. Neuropeptides. Substance P, vasoactive intestinal polypeptide, neurotensin, secretin. Opioids. Morphine, ~-endorphin, enkephalin analogues.
© 1992, ElsevierSciencePublishersLtd, (UK)
497
TABLE I. Comparison of the established features of itch and pain Itch
Pain
Skin, mucous membranes See Box I Occasionally AIIoknesis (itchy skin) Pronounced Scratching, pain, cooling
Most tissues Many stimuli Yes Hyperalgesia Present Tactile stimuli, cooling
C- and Ab-fibres Large ? Anterolateral funiculus ? Scratching, sneezing Yes ?
C- and A6-fibres Small NS, WDR Anterolateral funiculus Yes Flexion, guarding Yes Yes
Yes Probably not No
Chemogenic pain; yes Yes Yes
Psychophysiology Tissue Stimulus Intraneural microstimulation Secondary sensations Psychogenic modification Counterstimuli
Neurophysiology Primary afferent neurones Flare size Spinal cord neurones Spinal pathway Descending spinal control Protective reflexes Autonomic reflexes Central plasticity changes
Pharmacology Capsaicin sensitivity NSAID sensitivity Morphini~ sensitivity
Abbreviations: NS, nociceptive-specificneurones; WDR, wide dynamic range neurones; NSAID, non-steroidal anti-inflammatory drugs.
secondary sensations that are reminiscent of the features of secondary hyperalgesia evolving around a painful focus. A crucial observation is that itch and pain usually do not coexist in the same skin region and a mild noxious stimulus such as scratching is in fact the single most effective way to abolish itch. This abolition of itch can be prolonged producing an 'antiprufitic state 'is. Although mild scratch is often not painful, microneurographic recordings from humans have directly determined that such stimuli are among the most effective ways to excite cutaneous unmyelinated nociceptive afferents 21'22. A further important observation is that psychological factors more readily colour the conscious perception of itch compared to other sensations.
Neurophysiological aspects of itch There is a consensus that itch is itch spots in the 1950s using chemical stimuli. They conducted by unmyelinated and perhaps thin myeliningeniously used individual spicules of the pruritogenic ated primary afferents. Thus, humans can appreciate agent cowhage (Mucuna pruriens) to investigate the itch during a differential nerve block affecting myelinproperties of itch points. Their work clearly estab- ated axons 18'23, and intraneural electrical stimulation lished the importance of chemical mediators in the of unmyelinated fibres can occasionally evoke itch 24. generation of itch. They showed that cowhage Furthermore, the desensitization of fine chemospicules contained a proteinase that was itself prurito- sensitive afferents by topical application of capsaicin genic, and found that other compounds, including reportedly abohshes or reduces itch 23'25. histamine, trypsin and papain, were also active at itch Evidence that application of histamine can excite spots 8'1°. Histamine has now become the standard unmyelinated afferent fibres has also been directly pruritogenic stimulus in experimental studies n'lz documented 26-3°. In a recent human microneurobecause it produces an almost pure and graded graphic study, single unmyelinated primary neurones sensation of itch when delivered iontophorefically to were tested for their response to histamine iontothe skin. However, the relative ineffectiveness of phoresis (which induced itch) and topical application of antihistaminic drugs against many forms of exper- mustard oil (which evoked burning pain) 29. This study imental and clinical itch suggests that this compound is found that a subpopulation of unmyelinated mechanounlikely to be the only mediator in skin. Various sensitive afferents responded strongly to histamine, chemicals (Box 1) have been reported to elicit or but were also excited by application of mustard oil promote itch in humans m'13-17, although many of and other noxious stimuli. Another population of these may act indirectly by degranulafion of mast mechano-heat sensitive unmyelinated fibres (C-fibres) cells. responded only to mustard oil and not to histamine. Less is known about the CNS structures that What we know about itch mediate itch. Anterolateral cordotomy abolishes itch Any theory of the neural basis of itch needs to sensibility 31'32, consistent with the notion that the account for a number of observations. Table I lists the same spinal pathways transmit noxious and pruritic best established of these and compares them with the information. It is unknown which supraspinal strucproperties of pain, since many workers have been tures are involved in itch other than those that struck by the similarities of both sensations. process noxious information. Prufitogenic and noxious stimuli evoke essentially the same sympathetic reflex Psychophysical aspects of itch responses 33. Neurophysiological studies of central In contrast to the large variety of stimuli that are neurones have so far failed to obtain evidence for a capable of evoking pain in most tissues, itch can only specific central histamine-sensitive neurone 2°'z4. be felt in the superficial layers of the skin, mucous Rather, central neurones reflect essentially the same membranes and conjunctiva 1°' 13,18indicating that only properties seen in primary afferents. Neurones resubpopulations of primary afferents are capable of sponding to prufitogenic stimuli were also excited by encoding itch. Moreover, experimental focal itch noxious stimuli. Since neurophysiological studies stimuli are surrounded by a halo of seemingly using prufitogenic stimuli are still rare, and as most unaffected tissue where light tactile stimuli are have studied only mechanosensitive units, we cannot capable of eliciting itch-like sensations. The term itchy completely rule out the possibility that specific popuskin or alloknesis l°'n'lg'z° has been coined for these lations of histamine-sensitive neurones exist.
498
TINS, VoL 15, No. 12, 1992
Pharmacological aspects of itch Some drugs appear to have a differential response on itch and pain. It is said that opiates t4 and nonsteroidal anfi-inflam atory drugs 35 a r e ineffective in reducing itch. In fact, one disturbing side effect of opiates is the induction of itch - possibly as a consequence of mast cell degranulation. These differential effects suggest the existence of distinct central sensory processing pathways for itch and pain. However, this may not apply stringently because some pattern of excitation could be uniquely amenable to the drug's action.
A
Specificity ,1,-
~k~n g
? I
.iv ~ i
primary afferents
pain
centralneurons
Intensity m-
Q.
5
Theories of the neural basis of itch There is at present no single neuronal mechanism that is generally accepted as being able to explain all the features of itch (Table I). The evidence for and C Selectivity against different proposals is discussed below, and experimental approaches suggested that might yield definitive conclusions regarding the likelihood of these mechanisms. pain Specificity theory. Conceptually, the simplest explanation of itch is that a group of primary sensory neurones exist that respond to pruritogenic stimuli and no other (Fig. 1A), conforming to the specificity theory of somatosensation. Clearly such afferent Fig. 1. Schematic illustration of theories that have been neurones would have to be numbered among the proposed as the neural basis of itch. H, afferents respondsmall-diameter fibres and include some unmyelinated ing to pruritogenic stimufi notably histamine; M, afferents afferents, since itch sensibility is not lost until responding to noxious irritants such as mustard oil. See peripheral nerve blockade includes C-fibres. Un- text for details. fortunately, no such population of neurones has been observed in the rat, cat, monkey or humans. Many afferent fibres responding to pruritogenic chemicals response to prufitogenic stimuli could be the differhave been seen, but these have always also re- ential discharge patterns of unmyelinated afferents sponded to other, non-pruritogenic, noxious stimuli. producing itch or pain since there is not always a close It is possible that a special group of dedicated 'itch relation between the intensity of sensation and the fibres' (pruritoceptors) may have been overlooked, :magnitude of the flare response 4a. Finally a smaller and in this respect it is perhaps worth remembering - amount of cutaneous reflex vasoconstriction elicited that a group of somatic mechanically-insensitive by pruritogenic stimuli could underlie the large chemoreceptive afferents has only recently been histamine flare aa,44. The specificity theory also requires a 'labelled line' recognized36. However, from careful quantitative studies that have sought to sample unmyelinated within the CNS, dedicated to the processing of afferents in an unbiased fashion, it would appear that, pruritogenic stimuli. Positive experimental evidence numerically, this presumptive population of prurito- has not been sought, but in animal experiments nearly ceptors would have to be small, amounting to only a all spinal neurones identified by the presence of an few percent of cutaneous C-fibre afferents 37'38. ascending axon in the anterolateral quadrant have Nevertheless, as skin is innervated by several been found to respond to some form of nonhundred C-fibres per square millimetre 39'4°, a suf- pruritogenic stimulus 45. Thus, by exclusion, one can ficient density of units could still signal itch. These few conclude that only few spinal neurones can be fibres would have to be capable of producing the very dedicated to the processing of pruritogenic stimuli. The existence of a specific population of peripheral pronounced flare response seen, for instance, with histamine challenge in human skina4. afferents and central neurones could, of course, be Although the magnitude of histamine-induced itch is shown directly. It would be necessary to search often less than the intensity of chemically induced among the chemosensitive afferents (and methods pain, there is a perplexing difference in the sizes of now exist to identify these fibres37'38), and to use an the associated flare responses, which are typically appropriate animal species. The rat is probably unlarger after pruritogenic stirnl.di 11'23'41'42. This is suitable since histamine is not present in abundance intriguing, as excitation of a smaller number of in its skin, nor is there evidence that histamine sensory neurones should yield less vasodilatation. produces specifically itch-related (rather than painThis paradox is perhaps the only good evidence for related) behaviour in this species. While technically the existence of pruritoceptors that might possess demanding, the most revealing studies are likely to be especially powerful mechanisms for neurogenic vaso- in humans using microneurography. In summary, the weight of evidence does not dilatation. Alternatively, pnuritogenic stimuli may preferentially e~cite the most superficial sensory favour a model based on peripheral specificity. Intensity theory. Another way in which itch might be fibres and so produce vasodilatation of superficial blood vessels, which may be more readily visible. encoded is illustrated in Fig. lB. Here it is envisaged Another explanation for the unusually large flare that low levels of activity in nociceptive fibres signal TINS, Vol. 15, No. 12, 1992
499
A
Pattern coding
B
Population coding
Fig. 2. Schematic illustration of theories that could explain how different sensations of itch and pain could arise from the differential response of populations of nonspecific neurones. Open circles represent inactive neurones and filled circles the excited cells. The mechanism may either apply for primary afferents or central neurones and manifests as the temporal pattern of the firing or the pattern of activated units. See text for details.
itch, and higher levels of activity signal pain, suggesting that itch is a subliminal form of pain. Although this theory has been repeatedly proposed and is often stated in texts, there are many reasons to doubt its veracity. It is true that itch and pain share many features (Table I); however, direct testing with threshold forms of noxious stimuli (thermal, mechanical and chemical) do not consistently elicit itch. Moreover, many manoeuvres that inhibit pain do not inhibit itch, and vice versa. Thus, the differential sensitivity of the sensations to non-steroidal antiinflammatory drugs, opiates, noxious and innocuous counterstimuli all argue against this theory. Direct microneurographic recordings from human nociceptors have also determined that moderately itchy or painful stimuli yield similar tiring rates in nociceptors. In addition, when transcutaneous, intradermal or intraneural electrical stimulation techniques are used to elicit itch, increasing the frequency of the stimulus does not in general produce pain. In short, although this theory might be attractive in explaining the common features of itch and pain, the wealth of experimental evidence speaks against it. Selectivity theory. The principle of this theory is illustrated by one example in Fig. 1C. The theory recognizes the absence of specific populations of sensory neurones dedicated to the signalling of itch. It suggests instead that the subset of afferent nociceptors that respond to pruritogenic stimuli have different central connectivities, and activate separate central neurones. Noxious chemical stimuli, such as mustard oil, would activate the same subset of afferents and, additionally, other nociceptors. The theory requires that the widespread activation of nociceptive systems would mask the activity of ‘itchsignalling’ pathways, and this is consistent with the observation that itch and pain rarely coexist. The masking could arise from specific inhibitory mechanisms, as suggested in Fig. lC, or it could arise from inherent properties of the central signalling system, 500
where itch might arise from a form of parallel processing that requires activity in one population of neurones coupled with inactivity in another. The peripheral selectivity need not be confined to histamine-sensitive neurones. Other selective groups of nociceptive afferents could perform the same function, for instance those innervating superficial, rather than deep, layers of skin. This theory is consistent with known peripheral physiology, and could explain the absence of central neurones selectively responsive to pruritogenic stimuli. It can also explain the most basic feature of itch, namely the inhibition by noxious counterstimuli. Manipulations that interfere selectively with nociceptive transmission, such as opiate administration, would not be expected to inhibit itch, but perhaps enhance it. A direct test of this theory would require the detailed study of responsiveness of central neurones. If inhibition of central ‘itch-signalling’ neurones were a crucial mechanism, it should be directly demonstrable. Pattern theory. A final mechanistic model, which could apply to primary afferents and central neurones is that itch is encoded by the temporal or spatial firing of cells that are also capable of signalling other events (Fig. 2). Implicit in this theory is that individual neurones are ‘broadly tuned’ and respond to a variety of stimuli, which is true for the primary afferent neurones and (on meagre data) also for central neurones that respond to pruritogenic stimuli. The theory has the same explanatory power as the selectivity theory, accounting for the many similarities between itch and pain, and the inhibition of itch by pain. The major drawback is that the effective patterns are unspecified, and therefore untestable. One of the simplest patterns might be that pruritogenie stimuli activate one fraction of the available neuronal pool or temporal code, while noxious stimuli excite another (possibly overlapping) fraction or code (Fig. 2A). As detailed above, direct neurophysiological investigations endorse neither this view nor the possibility that the separate sensations are encoded by unique discharge pattems2g346. Perhaps the more plausible explanation is that itch is felt when one subpopulation is active, and if this proportion is exceeded the discharge might be interpreted as painfuP0 while the itch-signal becomes obscured, because of the lack of contrast between active neuronal pools (Fig. 2B). One implication of this theory is that activation of a single neuronal element should not elicit itch. However, itch can be elicited by focal intraneural or transcutaneous electrical stirnulation and this would imply that a suitable pattern could be generated from a relatively small number of afferents. Concluding remarks It is clear from the foregoing discussion that despite a century of experimental work our understanding of itch remains rudimentary. However, we have now reached a stage where the number of possible neuronal mechanisms is constrained, and techniques are available to test these. The outcome will be of interest both to clinicians, who face many patients who suffer from itching, and also to basic scientists, since several of the currently viable mechanistic models pose a direct challenge to the specificity theory of somatosensation. TINS, Vol. 15, No. 12, 1992
Selected references 1 Garnsworthy, R. K., Gully, R. L., Kenins, P., Mayfield, R. J. and Westerman, R. A. (1988) J. Neurophysiol. 59, 1083-1097 2 Woodward, D. F., Conway, J. L. and Wheeler, L. A. (1985) in Models in Dermatology (Maibach, H. I. and Lowe, N. J., eds), pp. 187-195, Karger 3 Miiller, J. (1838) Handbuch der Physiologie des Menschen (2nd edn), HOlscher 4 von Frey, M. (1896) Abh. Math. Phys. Kl. Saechs. Akad. Wiss. 23, 175-266 5 von Frey, M. (1922) Arch. Neerland. Physiol. 7, 142-145 6 Lewis, T., Grant, R. T. and Marvin, H. M. (1927) Heart 14, 139-160 7 Bishop, G. H. (1943) J. Neurophysiol. 6, 361-382 8 Arthur, R. P. and Shelly, W. B. (1955) J. Invest. Dermatol. 25, 341-346 9 Shelly, W. B. and Arthur, R. P. (1955) Arch. Dermatol. 72, 399-406 10 Shelly, W. B. and Arthur, R. P. (1957) Arch. DermatoL 76, 296-323 11 Simone, D. A. etal. (1987) Somatosens. Res. 5, 81-92 12 Magerl, W., Westerman, R. A., Mohner, B. and Handwerker, H. O. (1990) J. InvesL DermatoL 94, 347-352 13 Keele, C. A. and Armstrong, D. (1964) in Substances Producing Pain and Itch, pp:. 1-400, Arnold 14 Fjellner, B. and H&germark, O. (1982)Arch. Derm. Res. 274, 29-37 15 Fjellner, B. and H~germark, O. (1981) Acta Derm. Venerol. 61,245-250 16 H~germark, 6. (1974) Acta Derm. Venerol. 54, 397-400 17 H~germark, O., Strandberg, K. and Hamberg, M. (1977) J. Invest. DermatoL 69, 527-530 18 Bickford, R. G. (1938) Clin. Sci. 3, 337-386 19 Graham, D. T., Goodell, H. and Wolff, H. G. (1951) J. Clin. Invest. 30, 37-49 20 Simone, D. A., Alreja, M. and LaMotte, R. H. (1991) Somatosens. Mot. Res. 8, 271-279 21 TorebjOrk, E. (1985) Philos. Trans. R. Soc. London Ser. B 308, 227-234
22 Vallbo, A. B., Hagbarth, K. E., Torebj6rk, H. E. and Wallin, B. G. (1979) Physiol. Rev. 59, 919-957 23 Magerl, W. (1991) Allergologie 14, 395-405 24 Torebj6rk, H. E. and Ochoa, J. L. (1980) Acta PhysioL Scand. 110, 445-447 25 T6th-K&sa, I., Jancs6, G., Bognb.r, A., Musz, S. and Ob&l, F. (1986) Int. J. Clin. Pharm. Res. 6, 163-169 26 Torebj6rk, H. E. (1974) Acta PhysioL Scand. 92, 374-390 27 Fj~llbrant, N. and Iggo, A. (1961) J. Physiol. 156, 578-590 28 TuckeR, R. P. and Wei, J. Y. (1987) Brain Res. 413, 95-103 29 Handwerker, H. O., Forster, C. and Kirchoff, C. (1991) J. Neurophysiol. 66, 307-315 30 Handwerker, H. O. (1992) APSJ. 1, 135-138 31 White, J. C. and Sweet, W. H. (1969) Pain and the Neurosurgeon: A Forty Year Experience, C. C. Thomas 32 Nathan, P. W. (1990) J. Neurol. Neurosurg. Psychiat, 53, 935-939 33 Magerl, W., Koltzenburg, M. and Handwerker, H. O. (1991) Pfl6gers Arch. 418, R35 34 LaMotte, R. H. (1992) APSJ. 1, 115-126 35 H~germark, O. (1973) Acta Derm. Venerol. 53,363-368 36 McMahon, S. B. and Koltzenburg, M. (1990) Trends Neurosci. 13, 199-201 37 Meyer, R. A., Davis, K. D., Cohen, R. H., Treede, R-D. and Campbell, J. N. (1991) Brain Res. 561,252-261 38 Kress, M., Koltzenburg, M., Reeh, P. W. and Handwerker, H. O. (1992) J. Neurophysiol. 68, 581-593 39 Lynn, B. and Carpenter, S. E. (1982) Brain Res. 238, 29-43 40 Campbell, J. N., Raja, S. N., Cohen, R. H., Manning, A. A. K. and Meyer, R. A. (1989) in Textbook of Pain (2nd edn) (Wall, P. D. and Melzack, R,, eds), pp. 22-45, Churchill Livingstone 41 Simone, D. A., Baumann, T. K. and LaMotte, R. H. (1989) Pain 38, 99-107 42 Treede, R-D. (1992) Neurosci. Lett. 141, 169-172 43 Koltzenburg, M. and Handwerker, H. O. (1992) PflOgers Arch. 420, R47 44 Hornyak, M. E., Naver, H. K., Rydenhag, B. and Wallin, B. G. (1990) Acta Physiol. Scand. 139, 77-84 45 Willis, W. D. and Coggeshall, R. E. (1991) Sensory Mechanisms of the Spinal Cord (2nd edn), Plenum Press 46 TuckeR, R. P. (1982) J. Invest. Dermatol. 79, 368-373
Suppressionofprogrammedneuronaldeathbysustained elevationofcytoplasmiccalcium James L. Franklin and Eugene M. Johnson, Jr Chronic depolarization greatly increases the survival of many ~pes of neurons in culture. In at least some cases this enhancement of survival consists of the suppression of programmed cell death, a type of death occurring in developing neurons deprived of sufficient neurotrophic factor support. Available evidence suggests that the effect of depolarization on survival is mediated by a sustained rise of cytoplasmic free Ca e+, apparently caused by influx of Ca e+ through voltage-gated channels. This review discusses what is known about the mechanism by which prolonged depolarization and increased intracellular Ca e+ promote survival. Approximately half of all neurons produced in the developing vertebrate nervous system die before adulthood as a result of a process known as naturally occurring, or programmed, cell death (see Box 1). The primary role of this process seems to be to match innervation density appropriately with target size 1. Availability of specific neurotrophic substances, provided by the target or other tissues, is a major element determining which neurons survive this developmental stage. Those neurons obtaining sufTINS, Vol. 15, NO. 12, 1992
ficient quantities of relevant factors live, while those that do not, die. Removal of afferent input, pharmacological blockade of synaptic transmission or blockade of electrical activity can inhibit the growth and survival of some types of developing neurons, both in vivo 2-6 and in vitro7-1°, suggesting that electrical activity is also an important determinant of survival during the period of programmed death. While considerable work has been directed toward elucidating the mechanisms by which neurotrophic factors promote survival (or conversely, suppress programmed death), little effort has been aimed toward discerning the mechanism by which electrical activity supports survival. We describe here a long-estabfished cell culture technique that may serve as a paradigm for investigating this phenomenon.
JamesL. Franklinand EugeneM. Johnson, Jrare at the Dept of Molecular Biology and Pharmacology, Wash#Tgton UniversitySchoolof Medicine, St Louis, M063110, USA.
Chronic depolarization prevents the death of n e u r o n s in culture Scott and Fisher 11 showed more than two decades ago that the survival of embryonic chicken dorsal root ganglion neurons in culture is enhanced when they are maintained in a medium containing elevated
© 1992.ElsevierSciencePublishersLtd, (UK
501