- Email: [email protected]
Hot and cold in Drosophila larvae
Update
TRENDS in Neurosciences Vol.26 No.11 November 2003
6 Lendvai, B. et al. (2000) Experience-dependent plasticity of dendritic spines in the developing rat barrel cortex in vivo. Nature 404, 876– 881 7 Engert, F. and Bonhoeffer, T. (1999) Dendritic spine changes associated with hippocampal long-term synaptic plasticity. Nature 399, 66– 70 8 Maletic-Savatic, M. et al. (1999) Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity. Science 283, 1923 – 1927 9 Toni, N. et al. (1999) LTP promotes formation of multiple spine synapses between a single axon terminal and a dendrite. Nature 402, 421 – 425 10 Colicos, M.A. et al. (2002) Remodeling of synaptic actin induced by photoconductive stimulation. Cell 107, 605 – 616 11 Nikonenko, I. et al. Presynaptic remodeling contributes to activitydependent synaptogenesis. J. Neuroscience (in press) 12 Ahmari, S.E. et al. (2000) Assembly of presynaptic active zones from cytoplasmic transport packets. Nat. Neurosci. 3, 445 – 451
575
13 Friedman, H.V. et al. (2000) Assembly of new individual excitatory synapses: time course and temporal order of synaptic molecule recruitment. Neuron 27, 57 – 69 14 Marrs, G.S. et al. (2001) Rapid formation and remodeling of postsynaptic densities in developing dendrites. Nat. Neurosci. 4, 1006–1013 15 Chang, S. and De Camilli, P. (2001) Glutamate regulates actin-based motility in axonal filopodia. Nat. Neurosci. 4, 787– 793 16 Tashiro, A. et al. (2003) Bidirectional regulation of hippocampal mossy fiber filopodial motility by kainate receptors. A two-step model of synaptogenesis. Neuron 38, 773– 784 17 Ma, L. et al. (1999) Cyclic AMP induces functional presynaptic boutons in hippocampal CA3 – CA1 neuronal cultures. Nat. Neurosci. 2, 24 – 30 0166-2236/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tins.2003.08.010
Hot and cold in Drosophila larvae Troy Zars University of Missouri-Columbia, Division of Biological Sciences, 114 Lefevre Hall, Columbia, MO 65211, USA
Temperature is one of our most important physical cues, with extremes eliciting painful sensory warnings of tissue damage. Two recent publications present findings in Drosophila that implicate different neural substrates for low- and high-temperature responses, as well as indicating that a transient receptor potential (TRP) channel family member, painless, plays a role in these responses. We constantly monitor temperature, adapting to extremes of hot and cold with well-coordinated physiological and behavioral responses. That we expend so much effort making ourselves comfortable indicates the primary importance of body temperature control. Indeed, experience-independent subjective meanings of hot or cold for a given temperature implies selection of this knowledge through evolution. Pain, elicited by many stimuli including extreme temperatures, predicts tissue damage and ultimately infertility and death. It is perhaps not unexpected, therefore, that common mechanisms of temperature and pain sensation or perception will be found in widely different species. A novel approach first identified a new gene family within the transient receptor potential (TRP) ion channel super-family as crucial sensors of temperature [1]. The first channel to be isolated, the ‘vanilloid receptor’ TRPV1, responds to stimuli including the chemical that gives rise to the perception of spiciness in ‘hot’ peppers and temperature. Homology-based investigations now implicate many members of this gene family in responses to temperature, with different channels largely responding to different restricted temperature ranges [1– 8]. These studies have been extraordinarily important in providing insights into the molecular mechanisms of temperature Corresponding author: Troy Zars ([email protected]). http://tins.trends.com
sensation. However, this is obviously not the complete story. A single protein cannot alone transmit information to an organism about its environment. The cellular context of channel expression, the neuronal network supporting it, as well as the life and evolutionary history and motivational state of an organism, all influence a subjective interpretation of a temperature signal. The difficulty with addressing temperature perception (i.e. an analyzed representation [9] of temperature sensation measured in a behavioral response) in complex organisms is highlighted in the unexpectedly weak behavioral temperatureresponse phenotype of the TRPV1 knockout mouse [10,11]. Based on electrophysiological investigations, the TRPV1 knockout mouse could be expected to show severe decrements in perception of elevated temperatures. In actuality, these mutant mice turn out to have relatively subtle behavioral changes in this respect. This likely indicates that redundant systems (molecular or neural) can partially compensate for the loss of TRPV1. The complexity of the temperature perception problem can be reduced by examining simpler systems. Because of the relatively small number of CNS neurons, the advanced state of molecular genetic tools, and the broad behavioral repertoire of Drosophila [12,13] there is the exciting possibility of integrating the molecular mechanisms of temperature sensation into a neuro-behavioral model of temperature perception. This article focuses on recent investigations in Drosophila larval temperature perception [14,15] because recent reviews aptly cover this subject in other animal models [16 –18]. Larval temperature-related behaviors Drosophila larvae show temperature perception either by preferences on a temperature gradient or by escape behaviors when touched by a hot probe. In a slow (30 min) choice assay, larvae prefer temperatures of
576
Update
TRENDS in Neurosciences Vol.26 No.11 November 2003
, 188C compared with extremes of either 118C or 308C [15]. Interestingly, this response is not symmetrical. Larvae show a stronger avoidance of the high temperatures than of the low ones. This perhaps indicates the importance of the potentially more dangerous high temperatures as predictors of desiccation or tissue damage. The hot-poker approach involves touching a larva with a probe of defined temperature and measuring the latency in the initiation of rolling behavior, presumably as an indication of an escape response [14]. The average wild-type larval escape response to a high-temperature probe (e.g. 468C) is quick, , 0.4 s. Different temperature stimuli can also be used with this approach to test different theromsensors. Neuronal sites of larval thermosensation Approaching the problem from different directions, Tracey et al. [14] and Liu et al. [15] have now determined that synaptic transmission in different regions of the larval nervous system is necessary for sensing different temperature ranges. Low-temperature perception Neurons of the terminal organ in Drosophila larvae sense low temperatures. Using the transgenic cameleon Ca2þ sensor [19,20], the terminal organ of the larval head was found to increase and decrease Ca2þ influx with a decrease and increase in temperature, respectively [15]. These response characteristics are consistent with the presence of so-called cold cells [21]. However, other temperatureresponding cell types not detected with this technique might also be present in the terminal organ. No changes were detected in neurons innervating the dorsal organ (which is located adjacent to the terminal organ and receives primarily olfactory cues [22]) with changing temperatures. This indicates a specific effect of the temperature change on the neurons of the terminal organ. Extracellular recordings of neurons in the terminal organ revealed three types of responses to decreasing temperatures: increased firing rate, oscillatory activity induction, or a transient inhibition [15]. Tests for thresholds indicate that a change in temperature of as little as , 0.38C can be detected. All three cold-response types showed a decrease in firing rate with increases in temperature. The relationship between different responses and specific cell types awaits further experimentation. The neurons of the terminal organ are necessary for low-temperature perception. Expression of the tetanus toxin light chain (TeTxLC) in neurons of the terminal organ (and others cells) [23], blocking synaptobrevindependent synaptic transmission [24 – 26], reduced the preference for temperatures of 188C over 118C. Indeed, this preference was nearly abolished. There was no change in preference for 188C versus 308C in these larvae. These results, together with the physiological information, make it highly likely that the neurons innervating the terminal organ are responsible for sensing temperatures , 188C. http://tins.trends.com
High-temperature perception The multidendritic neurons of the larval abdominal segments are good candidates for sensing high temperatures, primarily because they are similar to nociceptors with ‘naked’ dendrites and have no other known function [14]. Again using the cameleon Ca2þ sensor, several types of multidendritic neurons were shown to change their Ca2þ levels with increasing or decreasing temperatures [15]. In well-isolated cells, the most prevalent response was a decrease in Ca2þ signal with decreasing temperature and no change in response to increasing temperatures. This temperature response profile has not been previously described in insects. A second cell type similarly showed a decreased Ca2þ signal following cooling but also showed an increase following warming. This fits descriptions of warm cells in other insects [21]. Additionally, clusters of multidendritic cells either responded in a similar way to the second cell type or displayed increased Ca2þ levels following both increases and decreases in temperature. Whether individual cells within these clusters respond in this fashion remains unclear. Electrophysiological recordings within abdominal segments indicate that at least two types of neurons respond to high temperature [14]. These neurons sharply increase activity in different temperature ranges (29 – 388C or 38 – 418C), consistent with the idea that these cells are tuned to specific temperature ranges [14]. The relationship between the cell types in which the Ca2þ imaging and electrophysiological recordings were performed is, unfortunately, not clear. Evidence that multidendritic neurons are necessary for sensing high temperatures comes from expressing TeTxLC in those cells and testing for the larval rolling response [14]. More than 95% of larvae expressing TeTxLC in their multidendritic neurons did not show escape responses to high temperature (468C) within the 10 s provided, in contrast to control larvae, which typically respond in , 1 s. This indicates that the multidendritic neurons are crucial for the perception of high temperatures.
Molecular mechanisms of temperature response The painless gene is the first cloned gene identified as being necessary for temperature perception in Drosophila [14]. A large proportion of painless mutant larvae have a delayed rolling response to a hightemperature probe of 468C. Importantly, the rolling response of painless larvae is normal with the highertemperature stimulus of 538C, indicating that the rolling motor program itself is normal. Molecular and genetic evidence indicates painless encodes a new member of the TRP ion channel super-family [17] and is distantly related to the vertebrate TRP family member TRPV1, which has been implicated in vertebrate high-temperature sensation [1,11]. Whether the Painless TRP channel responds directly to hightemperature, as shown in the case of TRPV1, awaits further study.
Update
TRENDS in Neurosciences Vol.26 No.11 November 2003
Does painless function within the multidendritic neurons? Electrophysiological measurements failed to detect a temperature-sensitive response in painless mutant larvae [14]. Assuming these measurements detected all possible cellular responses, painless has a role in the cells studied. Localization of the painless gene product labeled at least some of the multidendritic neurons in the embryo and this product appears to migrate to the dendritic terminals during development. It is tempting to speculate that painless acts within those cells to mediate a portion of the hightemperature response in Drosophila larvae. Clearly, expression of the painless transcript with a GAL4 driver in the multidendritic neurons in a mutant animal would address this possibility. Summary and implications In two studies [14,15], different regions of the Drosophila larva were implicated in thermal responses to different temperature ranges. The low-temperature response was detected in the neurons of the terminal organ and found to be necessary for normal coldtemperature avoidance. The multidendritic neurons of the abdominal segments respond to increases and decreases in temperature and are necessary for the initiation of an escape response to high-temperature. A key molecule in the moderately-high-temperature response is encoded by the painless gene and perhaps functions within the multidendritic neurons themselves. From a developmental point of view, it will be interesting to investigate the role of the structures implicated in the larval temperature response in the adult fly, where two thermosensors have been described [27,28]. The relationship between the thermosensory neural circuit and the neural systems underlying memory formation in paradigms using temperature as a reinforcer (e.g. the heat-box) should provide clues to the mechanisms of temperature perception and memory formation [29 – 31]. In broader terms, the molecular genetic tools and behavioral assays available in Drosophila should allow detailed understanding of the molecules, cells, and systems underlying temperature perception in a simple model, perhaps influencing our understanding of the mechanisms of pain. Acknowledgements I thank Douglas Guarnieri, Reginald B. Cocroft and the anonymous reviewers for helpful criticisms of the manuscript. Research in my laboratory is supported by the University of Missouri-Columbia and the University of Missouri Systems Research Board.
References 1 Caterina, M.J. et al. (1997) The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 389, 816– 824 2 Story, G.M. et al. (2003) ANKTM1, a TRP-like channel expressed in nociceptive neurons, is activated by cold temperatures. Cell 112, 819 – 829
http://tins.trends.com
577
3 Caterina, M.J. et al. (1999) A capsaicin-receptor homologue with a high threshold for noxious heat. Nature 398, 436 – 441 4 Peier, A.M. et al. (2002) A heat-sensitive TRP channel expressed in keratinocytes. Science 296, 2046– 2049 5 Xu, H. et al. (2002) TRPV3 is a calcium-permeable temperaturesensitive cation channel. Nature 418, 181 – 186 6 Smith, G.D. et al. (2002) TRPV3 is a temperature-sensitive vanilloid receptor-like protein. Nature 418, 186 – 190 7 Peier, A.M. et al. (2002) A TRP channel that senses cold stimuli and menthol. Cell 108, 705 – 715 8 McKemy, D.D. et al. (2002) Identification of a cold receptor reveals a general role for TRP channels in thermosensation. Nature 416, 52 – 58 9 Dudai, Y. (2002) Memory from A to Z, Oxford University Press 10 Davis, J.B. et al. (2000) Vanilloid receptor-1 is essential for inflammatory thermal hyperalgesia. Nature 405, 183 – 187 11 Caterina, M.J. et al. (2000) Impaired nociception and pain sensation in mice lacking the capsaicin receptor. Science 288, 306 – 313 12 Benzer, S. (1967) Behavioral mutants of Drosophila isolated by countercurrent distribution. Proc. Natl. Acad. Sci. U. S. A. 58, 1112– 1119 13 Heisenberg, M. (2003) Mushroom body memoir: from maps to models. Nat. Rev. Neurosci. 4, 266 – 275 14 Tracey, W.D. et al. (2003) painless, a Drosophila gene essential for nociception. Cell 113, 261 – 273 15 Liu, L. et al. (2003) Identification and function of thermosensory neurons in Drosophila larvae. Nat. Neurosci. 6, 267– 273 16 Patapoutian, A. et al. (2003) ThermoTRP channels and beyond: mechanisms of temperature sensation. Nat. Rev. Neurosci. 4, 529 – 539 17 Montell, C. et al. (2002) A unified nomenclature for the superfamily of TRP cation channels. Mol. Cell 9, 229– 231 18 Mori, I. (1999) Genetics of chemotaxis and thermotaxis in the nematode Caenorhabditis elegans. Annu. Rev. Genet. 33, 399 – 422 19 Miyawaki, A. et al. (1999) Dynamic and quantitative Ca2þ measurements using improved cameleons. Proc. Natl. Acad. Sci. U. S. A. 96, 2135– 2140 20 Fiala, A. et al. (2002) Genetically expressed cameleon in Drosophila melanogaster is used to visualize olfactory information in projection neurons. Curr. Biol. 12, 1877 – 1884 21 Altner, H. and Loftus, R. (1985) Ultrastructure and function of insect thermo- and hygroreceptors. Annu. Rev. Entomol. 30, 273 – 295 22 Stocker, R.F. (2001) Drosophila as a focus in olfactory research: mapping of olfactory sensilla by fine structure, odor specificity, odorant receptor expression, and central connectivity. Microsc. Res. Tech. 55, 284– 296 23 Heimbeck, G. et al. (1999) Smell and taste perception in Drosophila melanogaster larva: toxin expression studies in chemosensory neurons. J. Neurosci. 19, 6599 – 6609 24 Brand, A.H. and Perrimon, N. (1993) Targeted gene expression as a means of altering cell fates and generating dominant phenotypes. Development 118, 401– 415 25 Martin, J.R. et al. (2002) Targeted expression of tetanus toxin: a new tool to study the neurobiology of behavior. Adv. Genet. 47, 1 – 47 26 Sweeney, S.T. et al. (1995) Targeted expression of tetanus toxin light chain in Drosophila specifically eliminates synaptic transmission and causes behavioral defects. Neuron 14, 341 – 351 27 Zars, T. (2001) Two thermosensors in Drosophila have different behavioral functions. J. Comp. Physiol. [A], 187, 235 – 242 28 Sayeed, O. and Benzer, S. (1996) Behavioral genetics of thermosensation and hygrosensation in Drosophila. Proc. Natl. Acad. Sci. U. S. A. 93, 6079 – 6084 29 Putz, G. and Heisenberg, M. (2002) Memories in Drosophila heat-box learning. Learn. Mem. 9, 349 – 359 30 Zars, T. et al. (2000) Tissue-specific expression of a type I adenylyl cyclase rescues the rutabaga mutant memory defect: In search of the engram. Learn. Mem. 7, 18 – 31 31 Wustmann, G. et al. (1996) A new paradigm for operant conditioning of Drosophila melanogaster. J. Comp. Physiol. [A], 179, 429 – 436 0166-2236/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tins.2003.09.002
TRENDS in Neurosciences Vol.26 No.11 November 2003
6 Lendvai, B. et al. (2000) Experience-dependent plasticity of dendritic spines in the developing rat barrel cortex in vivo. Nature 404, 876– 881 7 Engert, F. and Bonhoeffer, T. (1999) Dendritic spine changes associated with hippocampal long-term synaptic plasticity. Nature 399, 66– 70 8 Maletic-Savatic, M. et al. (1999) Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity. Science 283, 1923 – 1927 9 Toni, N. et al. (1999) LTP promotes formation of multiple spine synapses between a single axon terminal and a dendrite. Nature 402, 421 – 425 10 Colicos, M.A. et al. (2002) Remodeling of synaptic actin induced by photoconductive stimulation. Cell 107, 605 – 616 11 Nikonenko, I. et al. Presynaptic remodeling contributes to activitydependent synaptogenesis. J. Neuroscience (in press) 12 Ahmari, S.E. et al. (2000) Assembly of presynaptic active zones from cytoplasmic transport packets. Nat. Neurosci. 3, 445 – 451
575
13 Friedman, H.V. et al. (2000) Assembly of new individual excitatory synapses: time course and temporal order of synaptic molecule recruitment. Neuron 27, 57 – 69 14 Marrs, G.S. et al. (2001) Rapid formation and remodeling of postsynaptic densities in developing dendrites. Nat. Neurosci. 4, 1006–1013 15 Chang, S. and De Camilli, P. (2001) Glutamate regulates actin-based motility in axonal filopodia. Nat. Neurosci. 4, 787– 793 16 Tashiro, A. et al. (2003) Bidirectional regulation of hippocampal mossy fiber filopodial motility by kainate receptors. A two-step model of synaptogenesis. Neuron 38, 773– 784 17 Ma, L. et al. (1999) Cyclic AMP induces functional presynaptic boutons in hippocampal CA3 – CA1 neuronal cultures. Nat. Neurosci. 2, 24 – 30 0166-2236/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tins.2003.08.010
Hot and cold in Drosophila larvae Troy Zars University of Missouri-Columbia, Division of Biological Sciences, 114 Lefevre Hall, Columbia, MO 65211, USA
Temperature is one of our most important physical cues, with extremes eliciting painful sensory warnings of tissue damage. Two recent publications present findings in Drosophila that implicate different neural substrates for low- and high-temperature responses, as well as indicating that a transient receptor potential (TRP) channel family member, painless, plays a role in these responses. We constantly monitor temperature, adapting to extremes of hot and cold with well-coordinated physiological and behavioral responses. That we expend so much effort making ourselves comfortable indicates the primary importance of body temperature control. Indeed, experience-independent subjective meanings of hot or cold for a given temperature implies selection of this knowledge through evolution. Pain, elicited by many stimuli including extreme temperatures, predicts tissue damage and ultimately infertility and death. It is perhaps not unexpected, therefore, that common mechanisms of temperature and pain sensation or perception will be found in widely different species. A novel approach first identified a new gene family within the transient receptor potential (TRP) ion channel super-family as crucial sensors of temperature [1]. The first channel to be isolated, the ‘vanilloid receptor’ TRPV1, responds to stimuli including the chemical that gives rise to the perception of spiciness in ‘hot’ peppers and temperature. Homology-based investigations now implicate many members of this gene family in responses to temperature, with different channels largely responding to different restricted temperature ranges [1– 8]. These studies have been extraordinarily important in providing insights into the molecular mechanisms of temperature Corresponding author: Troy Zars ([email protected]). http://tins.trends.com
sensation. However, this is obviously not the complete story. A single protein cannot alone transmit information to an organism about its environment. The cellular context of channel expression, the neuronal network supporting it, as well as the life and evolutionary history and motivational state of an organism, all influence a subjective interpretation of a temperature signal. The difficulty with addressing temperature perception (i.e. an analyzed representation [9] of temperature sensation measured in a behavioral response) in complex organisms is highlighted in the unexpectedly weak behavioral temperatureresponse phenotype of the TRPV1 knockout mouse [10,11]. Based on electrophysiological investigations, the TRPV1 knockout mouse could be expected to show severe decrements in perception of elevated temperatures. In actuality, these mutant mice turn out to have relatively subtle behavioral changes in this respect. This likely indicates that redundant systems (molecular or neural) can partially compensate for the loss of TRPV1. The complexity of the temperature perception problem can be reduced by examining simpler systems. Because of the relatively small number of CNS neurons, the advanced state of molecular genetic tools, and the broad behavioral repertoire of Drosophila [12,13] there is the exciting possibility of integrating the molecular mechanisms of temperature sensation into a neuro-behavioral model of temperature perception. This article focuses on recent investigations in Drosophila larval temperature perception [14,15] because recent reviews aptly cover this subject in other animal models [16 –18]. Larval temperature-related behaviors Drosophila larvae show temperature perception either by preferences on a temperature gradient or by escape behaviors when touched by a hot probe. In a slow (30 min) choice assay, larvae prefer temperatures of
576
Update
TRENDS in Neurosciences Vol.26 No.11 November 2003
, 188C compared with extremes of either 118C or 308C [15]. Interestingly, this response is not symmetrical. Larvae show a stronger avoidance of the high temperatures than of the low ones. This perhaps indicates the importance of the potentially more dangerous high temperatures as predictors of desiccation or tissue damage. The hot-poker approach involves touching a larva with a probe of defined temperature and measuring the latency in the initiation of rolling behavior, presumably as an indication of an escape response [14]. The average wild-type larval escape response to a high-temperature probe (e.g. 468C) is quick, , 0.4 s. Different temperature stimuli can also be used with this approach to test different theromsensors. Neuronal sites of larval thermosensation Approaching the problem from different directions, Tracey et al. [14] and Liu et al. [15] have now determined that synaptic transmission in different regions of the larval nervous system is necessary for sensing different temperature ranges. Low-temperature perception Neurons of the terminal organ in Drosophila larvae sense low temperatures. Using the transgenic cameleon Ca2þ sensor [19,20], the terminal organ of the larval head was found to increase and decrease Ca2þ influx with a decrease and increase in temperature, respectively [15]. These response characteristics are consistent with the presence of so-called cold cells [21]. However, other temperatureresponding cell types not detected with this technique might also be present in the terminal organ. No changes were detected in neurons innervating the dorsal organ (which is located adjacent to the terminal organ and receives primarily olfactory cues [22]) with changing temperatures. This indicates a specific effect of the temperature change on the neurons of the terminal organ. Extracellular recordings of neurons in the terminal organ revealed three types of responses to decreasing temperatures: increased firing rate, oscillatory activity induction, or a transient inhibition [15]. Tests for thresholds indicate that a change in temperature of as little as , 0.38C can be detected. All three cold-response types showed a decrease in firing rate with increases in temperature. The relationship between different responses and specific cell types awaits further experimentation. The neurons of the terminal organ are necessary for low-temperature perception. Expression of the tetanus toxin light chain (TeTxLC) in neurons of the terminal organ (and others cells) [23], blocking synaptobrevindependent synaptic transmission [24 – 26], reduced the preference for temperatures of 188C over 118C. Indeed, this preference was nearly abolished. There was no change in preference for 188C versus 308C in these larvae. These results, together with the physiological information, make it highly likely that the neurons innervating the terminal organ are responsible for sensing temperatures , 188C. http://tins.trends.com
High-temperature perception The multidendritic neurons of the larval abdominal segments are good candidates for sensing high temperatures, primarily because they are similar to nociceptors with ‘naked’ dendrites and have no other known function [14]. Again using the cameleon Ca2þ sensor, several types of multidendritic neurons were shown to change their Ca2þ levels with increasing or decreasing temperatures [15]. In well-isolated cells, the most prevalent response was a decrease in Ca2þ signal with decreasing temperature and no change in response to increasing temperatures. This temperature response profile has not been previously described in insects. A second cell type similarly showed a decreased Ca2þ signal following cooling but also showed an increase following warming. This fits descriptions of warm cells in other insects [21]. Additionally, clusters of multidendritic cells either responded in a similar way to the second cell type or displayed increased Ca2þ levels following both increases and decreases in temperature. Whether individual cells within these clusters respond in this fashion remains unclear. Electrophysiological recordings within abdominal segments indicate that at least two types of neurons respond to high temperature [14]. These neurons sharply increase activity in different temperature ranges (29 – 388C or 38 – 418C), consistent with the idea that these cells are tuned to specific temperature ranges [14]. The relationship between the cell types in which the Ca2þ imaging and electrophysiological recordings were performed is, unfortunately, not clear. Evidence that multidendritic neurons are necessary for sensing high temperatures comes from expressing TeTxLC in those cells and testing for the larval rolling response [14]. More than 95% of larvae expressing TeTxLC in their multidendritic neurons did not show escape responses to high temperature (468C) within the 10 s provided, in contrast to control larvae, which typically respond in , 1 s. This indicates that the multidendritic neurons are crucial for the perception of high temperatures.
Molecular mechanisms of temperature response The painless gene is the first cloned gene identified as being necessary for temperature perception in Drosophila [14]. A large proportion of painless mutant larvae have a delayed rolling response to a hightemperature probe of 468C. Importantly, the rolling response of painless larvae is normal with the highertemperature stimulus of 538C, indicating that the rolling motor program itself is normal. Molecular and genetic evidence indicates painless encodes a new member of the TRP ion channel super-family [17] and is distantly related to the vertebrate TRP family member TRPV1, which has been implicated in vertebrate high-temperature sensation [1,11]. Whether the Painless TRP channel responds directly to hightemperature, as shown in the case of TRPV1, awaits further study.
Update
TRENDS in Neurosciences Vol.26 No.11 November 2003
Does painless function within the multidendritic neurons? Electrophysiological measurements failed to detect a temperature-sensitive response in painless mutant larvae [14]. Assuming these measurements detected all possible cellular responses, painless has a role in the cells studied. Localization of the painless gene product labeled at least some of the multidendritic neurons in the embryo and this product appears to migrate to the dendritic terminals during development. It is tempting to speculate that painless acts within those cells to mediate a portion of the hightemperature response in Drosophila larvae. Clearly, expression of the painless transcript with a GAL4 driver in the multidendritic neurons in a mutant animal would address this possibility. Summary and implications In two studies [14,15], different regions of the Drosophila larva were implicated in thermal responses to different temperature ranges. The low-temperature response was detected in the neurons of the terminal organ and found to be necessary for normal coldtemperature avoidance. The multidendritic neurons of the abdominal segments respond to increases and decreases in temperature and are necessary for the initiation of an escape response to high-temperature. A key molecule in the moderately-high-temperature response is encoded by the painless gene and perhaps functions within the multidendritic neurons themselves. From a developmental point of view, it will be interesting to investigate the role of the structures implicated in the larval temperature response in the adult fly, where two thermosensors have been described [27,28]. The relationship between the thermosensory neural circuit and the neural systems underlying memory formation in paradigms using temperature as a reinforcer (e.g. the heat-box) should provide clues to the mechanisms of temperature perception and memory formation [29 – 31]. In broader terms, the molecular genetic tools and behavioral assays available in Drosophila should allow detailed understanding of the molecules, cells, and systems underlying temperature perception in a simple model, perhaps influencing our understanding of the mechanisms of pain. Acknowledgements I thank Douglas Guarnieri, Reginald B. Cocroft and the anonymous reviewers for helpful criticisms of the manuscript. Research in my laboratory is supported by the University of Missouri-Columbia and the University of Missouri Systems Research Board.
References 1 Caterina, M.J. et al. (1997) The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 389, 816– 824 2 Story, G.M. et al. (2003) ANKTM1, a TRP-like channel expressed in nociceptive neurons, is activated by cold temperatures. Cell 112, 819 – 829
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