- Email: [email protected]
Goal-Oriented Behaviour: The Ventral Tegmental Area in Motivated Movements
Current Biology
Dispatches of the spliceosome and spliceosomal introns. REFERENCES 1. Roy, S.W., and Irimia, M. (2014). Diversity and evolution of spliceosomal systems. In Spliceosomal Pre-mRNA Splicing (Totowa: Humana Press), pp. 13–33. 2. Henriet, S., Sanmartı´, B.C., Sumic, S., and Chourrout, D. (2019). Evolution of the U2 spliceosome for processing numerous and highly diverse non- canonical introns in the chordate Fritillaria borealis. Curr. Biol. 29, 3193–3199. 3. Huff, J.T., Zilberman, D., and Roy, S.W. (2016). Mechanism for DNA transposons to generate introns on genomic scales. Nature 538, 533. , P., 4. Worden, A.Z., Lee, J.H., Mock, T., Rouze Simmons, M.P., Aerts, A.L., Allen, A.E., Cuvelier, M.L., Derelle, E., Everett, M.V., and Foulon, E. (2009). Green evolution and dynamic adaptations revealed by genomes of the marine picoeukaryotes Micromonas. Science 324, 268–272. 5. van der Burgt, A., Severing, E., de Wit, P.J., and Collemare, J. (2012). Birth of new spliceosomal introns in fungi by multiplication of introner-like elements. Curr. Biol. 22, 1260– 1265.
6. Rushforth, A.M., and Anderson, P. (1996). Splicing removes the Caenorhabditis elegans transposon Tc1 from most mutant premRNAs. Mol. Cell. Biol. 16, 422–429. 7. Purugganan, M., and Wessler, S. (1992). The splicing of transposable elements and its role in intron evolution. Genetica 86, 295–303. 8. Denoeud, F., Henriet, S., Mungpakdee, S., Aury, J.M., Da Silva, C., Brinkmann, H., Mikhaleva, J., Olsen, L.C., Jubin, C., Can˜estro, C., et al. (2010). Plasticity of animal genome architecture unmasked by rapid evolution of a pelagic tunicate. Science 330, 1381–1385. 9. Irimia, M., Penny, D., and Roy, S.W. (2007). Coevolution of genomic intron number and splice sites. Trends Genet. 23, 321–325. 10. Hudson, A.J., McWatters, D.C., Bowser, B.A., Moore, A.N., Larue, G.E., Roy, S.W., and Russell, A.G. (2019). Patterns of conservation of spliceosomal intron structures and spliceosome divergence in representatives of the diplomonad and parabasalid lineages. BMC Evol. Biol. 19, 1–18. 11. Zarnack, K., Ko¨nig, J., Tajnik, M., vant, I., Martincorena, I., Eustermann, S., Ste Zarnack, K., Ko¨nig, J., Tajnik, M., Martincorena, I., et al. (2013). Direct competition between hnRNP C and U2AF65 protects the transcriptome from the exonization of Alu elements. Cell 152, 453–466.
12. Jayasinghe, R.G., Cao, S., Gao, Q., Wendl, M.C., Vo, N.S., Reynolds, S.M., Zhao, Y., Climente-Gonza´lez, H., Chai, S., Wang, F., and Varghese, R. (2018). Systematic analysis of splice-site-creating mutations in cancer. Cell Rep. 23, 270–281. 13. Roca, X., and Krainer, A.R. (2009). Recognition of atypical 50 splice sites by shifted basepairing to U1 snRNA. Nat. Struct. Mol. Biol. 16, 176. 14. Stark, M.R., Dunn, E.A., Dunn, W.S., Grisdale, C.J., Daniele, A.R., Halstead, M.R., Fast, N.M., and Rader, S.D. (2015). Dramatically reduced spliceosome in Cyanidioschyzon merolae. Proc. Natl. Acad. Sci. USA 112, E1191–E1200. 15. Lesser, C.F., and Guthrie, C. (1993). Mutations in U6 snRNA that alter splice site specificity: implications for the active site. Science 262, 1982–1988. 16. Yang, F., Wang, X.Y., Zhang, Z.M., Pu, J., Fan, Y.J., Zhou, J., Query, C.C., and Xu, Y.Z. (2013). Splicing proofreading at 50 splice sites by ATPase Prp28p. Nucleic acids Res. 41, 4660– 4670. 17. Fischer, S.E., Butler, M.D., Pan, Q., and Ruvkun, G. (2008). Trans-splicing in C. elegans generates the negative RNAi regulator ERI-6/ 7. Nature 455, 491.
Goal-Oriented Behaviour: The Ventral Tegmental Area in Motivated Movements Laura Masullo and Marco Tripodi Neurobiology Division, MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge, CB2 0QH, UK Correspondence: [email protected] (L.M.), [email protected] (M.T.) https://doi.org/10.1016/j.cub.2019.08.041
The ventral tegmental area is a midbrain region known for the involvement of its dopaminergic neurons in encoding reward-related features, value and motivational states. New research suggests a role for inhibitory neurons of the ventral tegmental area in the orchestration of head movements, which might be instrumental in guiding animals towards spatial targets during motivated behaviour. The accurate performance of goaloriented behaviours requires the integration of information regarding reward and motivation with the execution of an appropriate motor plan. The ventral tegmental area (VTA) is a midbrain region traditionally implicated in reward-related behaviours [1]. A new study by Hughes et al. [2], reported in this issue of Current Biology, challenge the predominant view of the VTA as a brain area for encoding
reward-related features and provides new evidence for a direct role of the VTA in the control of goal-oriented head movements. Most of the conclusions regarding the functional relevance of the VTA in rewardrelated behaviours have been drawn from studies focusing on neurons releasing the neurotransmitter dopamine. These studies have stressed the role of the VTA in encoding reward [3], reward-prediction
R922 Current Biology 29, R918–R941, October 7, 2019 ª 2019 Elsevier Ltd.
[4] and reward history [5]. But a growing body of literature has begun to highlight the role of dopamine neurons, both in the VTA and in the substantia nigra (SN), in the control of movement kinematics [6–8], in line with the early body of work on nigrostriatal dopamine [9]. These studies question the view that VTA dopamine neurons solely encode reward-related signals and stress the importance of recognizing the impact of potential
Current Biology
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Upward pitch Vgat::stGtACR2 Current Biology
Figure 1. VTAVgat+ neurons direct three-dimensional head rotations. (A) Schematic representation of the behavioural set up used in the study by Hughes et al. [2] (left panel): The mouse is tasked with following a mobile port moving along a horizontal or a vertical slider in order to receive a food reward. Head position is tracked using an infrared camera recording markers (red dots) positioned on the head of the animal. Brain activity is recorded using either tetrodes or a microendoscope. During this task, the animal moves its head along the three axes of rotation: yaw, pitch and roll (right panel). (B) Recordings in vivo are targeted to Vgat+ neurons of the VTA, where three distinct subpopulations of motor-related neurons can be defined based on the dimension of head rotation they encode. (C) Each Vgat+ subpopulation representing one dimension of head rotation can be further subdivided according to their preferred direction of head movement (head angle, coloured lines; activity of ipsi-directional cells, solid lines; activity of contra-directional cells, dashed lines). (D) Optogenetic manipulation of VTAVgat+ neurons activity is sufficient to trigger head displacement: unilateral activation of these neurons with virally delivered ChR2 leads to ipsaversive yaw and roll rotations relative to the stimulation site, and downward pitch (top panel). Inhibition with virally delivered stGtACR2 causes contraversive yaw and roll rotations and upward pitch (bottom panel).
confounding kinematic variables in the interpretation of the role of VTA activity [6]. In this respect, it is interesting to note that, even within the canonical rewardrelated conceptual framework, dopamine neurons in both the SN and VTA show a preference for contralateral rewardpredicting cues [10,11]; yet, this feature appears difficult to reconcile exclusively with reward-related activity and would seem easier to interpret within a broader framework that also accounts for potential spatial-motor signals carried by dopamine neurons. Indeed, in line with a more spatial-motor perspective of VTA coding, tonic increase in dopamine levels in the striatum has also been shown to signal proximity to the reward [12]. Furthermore, a recent study [10] showed that both motor and reward-related signals can coexist within the VTA dopamine neuronal population. To further add to this complexity, the VTA contains not just dopamine neurons but also GABAergic and glutamatergic projection neurons, which are highly likely to play a significant role in orchestrating the VTA output but whose specific function
remains to be fully elucidated. In their new study, Hughes et al. [2] provide evidence for the existence of GABAergic neurons in the VTA deputed to the control of head movements along the three principal axes of rotation — yaw (left–right); roll, (clockwise–counterclockwise); pitch (up– down) — further contributing to the change in perspective about the function of the VTA. Hughes et al. [2] employed an extended version of a behavioural paradigm designed in an earlier study [13] in which mice are tasked with continuously moving their head to follow a mobile port located on a mechanic arm in front of them in order to receive a food reward (Figure 1A). Because the arm can move along both the horizontal and vertical axis, mice display head rotations about the three principal axes: yaw and roll are the predominant dimension of head displacements made to follow a target moving along the horizontal axis, while pitch movements are abundantly observed when mice follow the port as it moves vertically. These head displacements were monitored using markers placed on the
animal’s head, detected by infrared cameras and elaborated to extract information regarding instantaneous head angle, head position, rotation velocity and acceleration at any point of the recording session. Using tetrode recordings, Hughes et al. [2] were able to isolate units in the VTA and study the correlation of their spiking activity with these various variables associated with head motion. As the objective of the study was to elucidate the involvement of the GABAergic population of the VTA (VTAVgat+), the authors performed optrode recordings in mice expressing the light-gated ion channel channelrhodopsin-2 (ChR2) selectively in VTAVgat+ neurons, to allow the identification of inhibitory neurons among the recorded cells. Strikingly, they observed that the firing rates of a subgroup of VTAVgat+ neurons exhibited extremely high correlations with instantaneous head angles. The authors further determined that these head-anglerelated cells could be further classified according to the axis of motion with which their activity best correlates; the largest
Current Biology 29, R918–R941, October 7, 2019 R923
Current Biology
Dispatches proportion of neurons were tuned to roll displacements, other neurons fired preferentially in correlation with yaw events, and a smaller proportion of neurons was dedicated to the representation of pitch movements (Figure 1B). Each of these three distinct mono-dimensional subgroups could then be further refined by considering the direction for which cells became active. Indeed, for each axis of rotation, two opposite populations could be distinguished based on their direction preference — clockwise and anticlockwise for roll neurons, left and right for yaw neurons, and up or down for pitch neurons — leading the authors to conclude that there are six populations of VTAVgat+ neurons related to instantaneous head angles (Figure 1C). The authors further confirmed the existence of these functional subtypes, as well as their inhibitory nature, by performing chronic one-photon calcium imaging of VTAVgat+ neurons. Because the activity of these movement-related VTAVgat+ neurons was qualitatively observed to ramp up in the time window preceding movement execution, Hughes et al. [2] then went on to assess whether modulating the activity of these populations was sufficient to affect the pattern of head motion generated while following the moving reward port. Indeed, optogenetic activation of the VTAVgat+ neurons expressing channelrhodopsin was sufficient to promote the execution of ipsiversive head rotations (in yaw and roll; downwards in pitch; Figure 1D). In a complementary experiment, the authors demonstrated that the optogenetic inhibition of the VTAVgat+ population led, instead, to the occurrence of contralateral rotations of the head (in yaw and roll; upwards in pitch; Figure 1D). Furthermore, in line with their interpretation of the recordings data, the amplitude of the elicited head movements could be modulated by modifying either the duration or frequency of the pulse of stimulating light. The optogenetic manipulation of VTAVgat+ neurons resulted in a marked decrease in the ability of mice to track the reward port and, consequently, in a drop in reward consumption. These observations appear to be in line with the effects of the optogenetic stimulation on
the ongoing head motion. Indeed, both excitation and inhibition of VTAVgat+ neurons result in a swift head displacement being forced on the animal, leading to a head trajectory inappropriate for the requirements of the task and, hence, lack of reward delivery. However, lack of consumption following optogenetic stimulation of VTAVgat+ neurons [14], as well as the resulting place avoidance [15], in which animals avoid a location primed by the activation of VTAVgat+ neurons, has been previously linked to the manifestation of an aversive behavior rather than to the perturbation of movement kinematics. Indeed, while VTA dopamine neurons are activated by reward and reward–predicting cues, salient but aversive stimuli strongly inhibit dopaminergic neurons [16], an inhibition that is supposed to be mediated by the activity of the VTAVgat+ population [15] and to ultimately drive the behavioral response. This interpretation is also broadly supported by recent findings on the role of VTAVgat+ neurons in mediating innate defensive responses to looming stimuli [17], which are salient and inherently aversive. To test the potential intrinsic aversive nature of VTAVgat+ neuron signals, Hughes et al. [2] performed a real-time place preference assay that allowed the authors to assess whether optogenetic stimulation of VTAVgat+ neurons per se can be linked to the avoidance of an otherwise neutral environment. For this experiment, mice were left free to navigate in an openfield arena divided in two identical compartments; in baseline conditions, mice explored the entirety of the arena, spending a comparable amount of time in each of the two compartments. By applying optogenetic manipulation to VTAVgat+ neurons expressing ChR2, the authors were able to assess whether this balance could be perturbed by altering the activity state of this selected neuronal population. Stimulation of VTAVgat+ neurons had no effect on the place preference displayed by the mice, suggesting that there is no direct aversive signal being communicated downstream of the VTA by its GABAergic population. However, the use of distinct stimulation protocols as well as different behavioral contingencies makes it difficult to directly compare these results with earlier studies [14,15,17]. In particular, in the case of
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place aversion, this is mediated by direct inhibition of VTA dopamine neurons by the VTAVgat+ input [15] and it is unclear whether the light stimulation protocol used by Hughes et al. [2], albeit closely mirroring the physiological VTAVgat+ activity, is sufficient to drive such a sustained inhibition of VTA dopamine neurons. Overall, the new study by Hughes et al. [2] contributes to our understanding of the role of the VTA in two ways: firstly, by adopting a novel behavioural task that allows dissociating motor control and reward prediction error, the authors are able to extract the kinematic features of goal-directed behaviour controlled by VTAVgat+ neurons. Indeed, because the animal’s expectation of reward remains constant throughout the recording session, the authors make a convincing case for VTAVgat+ neurons being concerned with the execution of a particular motor plan, rather than with providing dopaminergic neurons locally with reward-related or motivational information. Secondly, the findings reported by Hughes et al. [2] suggest not only that VTAVgat+ neurons reliably represent head angles locally, but also that their activation is sufficient to generate desired head displacements. Therefore, the study abandons the valueand motivation-centered interpretation of the role of the VTA in aversive and appetitive processes and proposes a more general motor-centered role of the VTA in spatial orienting. One should also take note, however, that the role of VTAVgat+ neurons in directing head movements was only tested in the presence of reward and whether the VTA control of head kinematics is maintained outside of a rewarding contingency remains to be tested. In the future it will be interesting to assess whether other neuronal populations in the VTA, particularly dopamine neurons, also control head movement kinematics, as this might provide an even more direct link between value and action selection. Interestingly, other midbrain structures, most prominently the superior colliculus (SC), have been canonically regarded as responsible for the execution of spatially targeted head and eye movements in primates [18] as in mice [19,20]. Moving forward it will be interesting to understand
Current Biology
Dispatches how the VTA and SC coordinate the successful execution of goal-directed head movements and how target value, and the presence of reward in general, impact on head movement decisions and kinematics. The existence of a markedly motorrelated population within the VTA has interesting implications with respect to the organisation of brain circuits responsible for the execution of goaldirected behaviour. Given that such behaviour relies on the precise execution of a motor plan in three-dimensional space matched by an appropriate reward-driven motivational state, this and other recent work [10] support the idea that these two representations co-exist within the VTA and pave the way to future studies on the interaction of the neuronal populations responsible for their implementation. REFERENCES
specific signals for reward and punishment in the ventral tegmental area. Nature 482, 85–88. 4. Schultz, W., Dayan, P., and Montague, P.R. (1997). A neural substrate of prediction and reward. Science 275, 1593–1599. 5. Bayer, H.M., and Glimcher, P.W. (2005). Midbrain dopamine neurons encode a quantitative reward prediction error signal. Neuron 47, 129–141. 6. Barter, J.W., Li, S., Lu, D., Bartholomew, R.A., Rossi, M.A., Shoemaker, C.T., Salas-Meza, D., Gaidis, E., and Yin, H.H. (2015). Beyond reward prediction errors: the role of dopamine in movement kinematics. Front. Integr. Neurosci. 9, 39. 7. da Silva, J.A., Tecuapetla, F., Paixao, V., and Costa, R.M. (2018). Dopamine neuron activity before action initiation gates and invigorates future movements. Nature 554, 244–248. 8. Wang, D.V., and Tsien, J.Z. (2011). Conjunctive processing of locomotor signals by the ventral tegmental area neuronal population. PLoS One 6, e16528. 9. Campanella, G., Roy, M., and Barbeau, A. (1987). Drugs affecting movement disorders. Annu. Rev. Pharmacol. Toxicol. 27, 113–136.
1. Everitt, B.J., and Robbins, T.W. (2005). Neural systems of reinforcement for drug addiction: from actions to habits to compulsion. Nat. Neurosci. 8, 1481–1489.
10. Engelhard, B., Finkelstein, J., Cox, J., Fleming, W., Jang, H.J., Ornelas, S., Koay, S.A., Thiberge, S.Y., Daw, N.D., Tank, D.W., et al. (2019). Specialized coding of sensory, motor and cognitive variables in VTA dopamine neurons. Nature 570, 509–513.
2. Hughes, R.N., Watson, G.D.R., Petter, E.A., Kim, N., Bakhurin, K.I., and Yin, H.H. (2019). Precise coordination of three-dimensional rotational kinematics by ventral tegmental GABAergic neurons. Curr. Biol. 29, 3244– 3255.
11. Kim, H.F., Ghazizadeh, A., and Hikosaka, O. (2015). Dopamine neurons encoding long-term memory of object value for habitual behavior. Cell 163, 1165–1175.
3. Cohen, J.Y., Haesler, S., Vong, L., Lowell, B.B., and Uchida, N. (2012). Neuron-type-
12. Howe, M.W., Tierney, P.L., Sandberg, S.G., Phillips, P.E., and Graybiel, A.M. (2013). Prolonged dopamine signalling in striatum
signals proximity and value of distant rewards. Nature 500, 575–579. 13. Kim, N., Li, H.E., Hughes, R.N., Watson, G.D.R., Gallegos, D., West, A.E., Kim, I.H., and Yin, H.H. (2019). A striatal interneuron circuit for continuous target pursuit. Nat. Commun. 10, 2715. 14. van Zessen, R., Phillips, J.L., Budygin, E.A., and Stuber, G.D. (2012). Activation of VTA GABA neurons disrupts reward consumption. Neuron 73, 1184–1194. 15. Tan, K.R., Yvon, C., Turiault, M., Mirzabekov, J.J., Doehner, J., Labouebe, G., Deisseroth, K., Tye, K.M., and Luscher, C. (2012). GABA neurons of the VTA drive conditioned place aversion. Neuron 73, 1173–1183. 16. Ungless, M.A., Magill, P.J., and Bolam, J.P. (2004). Uniform inhibition of dopamine neurons in the ventral tegmental area by aversive stimuli. Science 303, 2040–2042. 17. Zhou, Z., Liu, X., Chen, S., Zhang, Z., Liu, Y., Montardy, Q., Tang, Y., Wei, P., Liu, N., Li, L., et al. (2019). A VTA GABAergic neural circuit mediates visually evoked innate defensive responses. Neuron 103, 473–488.e476. 18. Munoz, D.P., Guitton, D., and Pelisson, D. (1991). Control of orienting gaze shifts by the tectoreticulospinal system in the head-free cat. III. Spatiotemporal characteristics of phasic motor discharges. J. Neurophysiol. 66, 1642–1666. 19. Wilson, J.J., Alexandre, N., Trentin, C., and Tripodi, M. (2018). Three-dimensional representation of motor space in the mouse superior colliculus. Curr. Biol. 28, 1744– 1755.e1712. 20. Masullo, L., Mariotti, L., Alexandre, N., FreirePritchett, P., Boulanger, J., and Tripodi, M. (2019). Genetically defined functional modules for spatial orienting in the mouse superior colliculus. Curr. Biol. 29, 2892–2904.e8.
Water Balance: Abstaining from Obtaining While Retaining Sandra L. Martin Department of Cell and Developmental Biology, University of Colorado School of Medicine, Aurora, CO 80045, USA Correspondence: [email protected] https://doi.org/10.1016/j.cub.2019.08.038
Animals tightly regulate blood volume and solute concentrations. Water balance is usually achieved by a combination of managing intake and excretion but sometimes both drinking and urination are inconvenient. Hibernators have perfected internal mechanisms to maintain water balance without either. Thirst is a powerful drive — ask anyone who has exercised without drinking water in the heat of summer. This is because
even tiny changes in our blood osmolality trigger homeostatic mechanisms [1,2]. When water is depleted, the
hypothalamus region of the brain coordinates the incoming signals warning of decreased blood volume
Current Biology 29, R918–R941, October 7, 2019 ª 2019 Elsevier Ltd. R925
Dispatches of the spliceosome and spliceosomal introns. REFERENCES 1. Roy, S.W., and Irimia, M. (2014). Diversity and evolution of spliceosomal systems. In Spliceosomal Pre-mRNA Splicing (Totowa: Humana Press), pp. 13–33. 2. Henriet, S., Sanmartı´, B.C., Sumic, S., and Chourrout, D. (2019). Evolution of the U2 spliceosome for processing numerous and highly diverse non- canonical introns in the chordate Fritillaria borealis. Curr. Biol. 29, 3193–3199. 3. Huff, J.T., Zilberman, D., and Roy, S.W. (2016). Mechanism for DNA transposons to generate introns on genomic scales. Nature 538, 533. , P., 4. Worden, A.Z., Lee, J.H., Mock, T., Rouze Simmons, M.P., Aerts, A.L., Allen, A.E., Cuvelier, M.L., Derelle, E., Everett, M.V., and Foulon, E. (2009). Green evolution and dynamic adaptations revealed by genomes of the marine picoeukaryotes Micromonas. Science 324, 268–272. 5. van der Burgt, A., Severing, E., de Wit, P.J., and Collemare, J. (2012). Birth of new spliceosomal introns in fungi by multiplication of introner-like elements. Curr. Biol. 22, 1260– 1265.
6. Rushforth, A.M., and Anderson, P. (1996). Splicing removes the Caenorhabditis elegans transposon Tc1 from most mutant premRNAs. Mol. Cell. Biol. 16, 422–429. 7. Purugganan, M., and Wessler, S. (1992). The splicing of transposable elements and its role in intron evolution. Genetica 86, 295–303. 8. Denoeud, F., Henriet, S., Mungpakdee, S., Aury, J.M., Da Silva, C., Brinkmann, H., Mikhaleva, J., Olsen, L.C., Jubin, C., Can˜estro, C., et al. (2010). Plasticity of animal genome architecture unmasked by rapid evolution of a pelagic tunicate. Science 330, 1381–1385. 9. Irimia, M., Penny, D., and Roy, S.W. (2007). Coevolution of genomic intron number and splice sites. Trends Genet. 23, 321–325. 10. Hudson, A.J., McWatters, D.C., Bowser, B.A., Moore, A.N., Larue, G.E., Roy, S.W., and Russell, A.G. (2019). Patterns of conservation of spliceosomal intron structures and spliceosome divergence in representatives of the diplomonad and parabasalid lineages. BMC Evol. Biol. 19, 1–18. 11. Zarnack, K., Ko¨nig, J., Tajnik, M., vant, I., Martincorena, I., Eustermann, S., Ste Zarnack, K., Ko¨nig, J., Tajnik, M., Martincorena, I., et al. (2013). Direct competition between hnRNP C and U2AF65 protects the transcriptome from the exonization of Alu elements. Cell 152, 453–466.
12. Jayasinghe, R.G., Cao, S., Gao, Q., Wendl, M.C., Vo, N.S., Reynolds, S.M., Zhao, Y., Climente-Gonza´lez, H., Chai, S., Wang, F., and Varghese, R. (2018). Systematic analysis of splice-site-creating mutations in cancer. Cell Rep. 23, 270–281. 13. Roca, X., and Krainer, A.R. (2009). Recognition of atypical 50 splice sites by shifted basepairing to U1 snRNA. Nat. Struct. Mol. Biol. 16, 176. 14. Stark, M.R., Dunn, E.A., Dunn, W.S., Grisdale, C.J., Daniele, A.R., Halstead, M.R., Fast, N.M., and Rader, S.D. (2015). Dramatically reduced spliceosome in Cyanidioschyzon merolae. Proc. Natl. Acad. Sci. USA 112, E1191–E1200. 15. Lesser, C.F., and Guthrie, C. (1993). Mutations in U6 snRNA that alter splice site specificity: implications for the active site. Science 262, 1982–1988. 16. Yang, F., Wang, X.Y., Zhang, Z.M., Pu, J., Fan, Y.J., Zhou, J., Query, C.C., and Xu, Y.Z. (2013). Splicing proofreading at 50 splice sites by ATPase Prp28p. Nucleic acids Res. 41, 4660– 4670. 17. Fischer, S.E., Butler, M.D., Pan, Q., and Ruvkun, G. (2008). Trans-splicing in C. elegans generates the negative RNAi regulator ERI-6/ 7. Nature 455, 491.
Goal-Oriented Behaviour: The Ventral Tegmental Area in Motivated Movements Laura Masullo and Marco Tripodi Neurobiology Division, MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge, CB2 0QH, UK Correspondence: [email protected] (L.M.), [email protected] (M.T.) https://doi.org/10.1016/j.cub.2019.08.041
The ventral tegmental area is a midbrain region known for the involvement of its dopaminergic neurons in encoding reward-related features, value and motivational states. New research suggests a role for inhibitory neurons of the ventral tegmental area in the orchestration of head movements, which might be instrumental in guiding animals towards spatial targets during motivated behaviour. The accurate performance of goaloriented behaviours requires the integration of information regarding reward and motivation with the execution of an appropriate motor plan. The ventral tegmental area (VTA) is a midbrain region traditionally implicated in reward-related behaviours [1]. A new study by Hughes et al. [2], reported in this issue of Current Biology, challenge the predominant view of the VTA as a brain area for encoding
reward-related features and provides new evidence for a direct role of the VTA in the control of goal-oriented head movements. Most of the conclusions regarding the functional relevance of the VTA in rewardrelated behaviours have been drawn from studies focusing on neurons releasing the neurotransmitter dopamine. These studies have stressed the role of the VTA in encoding reward [3], reward-prediction
R922 Current Biology 29, R918–R941, October 7, 2019 ª 2019 Elsevier Ltd.
[4] and reward history [5]. But a growing body of literature has begun to highlight the role of dopamine neurons, both in the VTA and in the substantia nigra (SN), in the control of movement kinematics [6–8], in line with the early body of work on nigrostriatal dopamine [9]. These studies question the view that VTA dopamine neurons solely encode reward-related signals and stress the importance of recognizing the impact of potential
Current Biology
Dispatches Tetrodes and microendoscope recordings
Optogenetic manipulation C
A
D
Infrared camera
Head movement Ipsi-directional cell Contra-directional cell
z
Reward port
Horizontal slider
y Pitch
VTA
Vgat+ neuron
Yaw-related
Pitch-related
Roll-related
Time
Ipsaversive yaw and roll Downward pitch Vgat::ChR2
VTAVgat+ Contraversive yaw and roll
Cell firing
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Roll
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B
Pitch
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x Roll
Yaw
Head angle Head angle Head angle
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VTAVgat+
Upward pitch Vgat::stGtACR2 Current Biology
Figure 1. VTAVgat+ neurons direct three-dimensional head rotations. (A) Schematic representation of the behavioural set up used in the study by Hughes et al. [2] (left panel): The mouse is tasked with following a mobile port moving along a horizontal or a vertical slider in order to receive a food reward. Head position is tracked using an infrared camera recording markers (red dots) positioned on the head of the animal. Brain activity is recorded using either tetrodes or a microendoscope. During this task, the animal moves its head along the three axes of rotation: yaw, pitch and roll (right panel). (B) Recordings in vivo are targeted to Vgat+ neurons of the VTA, where three distinct subpopulations of motor-related neurons can be defined based on the dimension of head rotation they encode. (C) Each Vgat+ subpopulation representing one dimension of head rotation can be further subdivided according to their preferred direction of head movement (head angle, coloured lines; activity of ipsi-directional cells, solid lines; activity of contra-directional cells, dashed lines). (D) Optogenetic manipulation of VTAVgat+ neurons activity is sufficient to trigger head displacement: unilateral activation of these neurons with virally delivered ChR2 leads to ipsaversive yaw and roll rotations relative to the stimulation site, and downward pitch (top panel). Inhibition with virally delivered stGtACR2 causes contraversive yaw and roll rotations and upward pitch (bottom panel).
confounding kinematic variables in the interpretation of the role of VTA activity [6]. In this respect, it is interesting to note that, even within the canonical rewardrelated conceptual framework, dopamine neurons in both the SN and VTA show a preference for contralateral rewardpredicting cues [10,11]; yet, this feature appears difficult to reconcile exclusively with reward-related activity and would seem easier to interpret within a broader framework that also accounts for potential spatial-motor signals carried by dopamine neurons. Indeed, in line with a more spatial-motor perspective of VTA coding, tonic increase in dopamine levels in the striatum has also been shown to signal proximity to the reward [12]. Furthermore, a recent study [10] showed that both motor and reward-related signals can coexist within the VTA dopamine neuronal population. To further add to this complexity, the VTA contains not just dopamine neurons but also GABAergic and glutamatergic projection neurons, which are highly likely to play a significant role in orchestrating the VTA output but whose specific function
remains to be fully elucidated. In their new study, Hughes et al. [2] provide evidence for the existence of GABAergic neurons in the VTA deputed to the control of head movements along the three principal axes of rotation — yaw (left–right); roll, (clockwise–counterclockwise); pitch (up– down) — further contributing to the change in perspective about the function of the VTA. Hughes et al. [2] employed an extended version of a behavioural paradigm designed in an earlier study [13] in which mice are tasked with continuously moving their head to follow a mobile port located on a mechanic arm in front of them in order to receive a food reward (Figure 1A). Because the arm can move along both the horizontal and vertical axis, mice display head rotations about the three principal axes: yaw and roll are the predominant dimension of head displacements made to follow a target moving along the horizontal axis, while pitch movements are abundantly observed when mice follow the port as it moves vertically. These head displacements were monitored using markers placed on the
animal’s head, detected by infrared cameras and elaborated to extract information regarding instantaneous head angle, head position, rotation velocity and acceleration at any point of the recording session. Using tetrode recordings, Hughes et al. [2] were able to isolate units in the VTA and study the correlation of their spiking activity with these various variables associated with head motion. As the objective of the study was to elucidate the involvement of the GABAergic population of the VTA (VTAVgat+), the authors performed optrode recordings in mice expressing the light-gated ion channel channelrhodopsin-2 (ChR2) selectively in VTAVgat+ neurons, to allow the identification of inhibitory neurons among the recorded cells. Strikingly, they observed that the firing rates of a subgroup of VTAVgat+ neurons exhibited extremely high correlations with instantaneous head angles. The authors further determined that these head-anglerelated cells could be further classified according to the axis of motion with which their activity best correlates; the largest
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Dispatches proportion of neurons were tuned to roll displacements, other neurons fired preferentially in correlation with yaw events, and a smaller proportion of neurons was dedicated to the representation of pitch movements (Figure 1B). Each of these three distinct mono-dimensional subgroups could then be further refined by considering the direction for which cells became active. Indeed, for each axis of rotation, two opposite populations could be distinguished based on their direction preference — clockwise and anticlockwise for roll neurons, left and right for yaw neurons, and up or down for pitch neurons — leading the authors to conclude that there are six populations of VTAVgat+ neurons related to instantaneous head angles (Figure 1C). The authors further confirmed the existence of these functional subtypes, as well as their inhibitory nature, by performing chronic one-photon calcium imaging of VTAVgat+ neurons. Because the activity of these movement-related VTAVgat+ neurons was qualitatively observed to ramp up in the time window preceding movement execution, Hughes et al. [2] then went on to assess whether modulating the activity of these populations was sufficient to affect the pattern of head motion generated while following the moving reward port. Indeed, optogenetic activation of the VTAVgat+ neurons expressing channelrhodopsin was sufficient to promote the execution of ipsiversive head rotations (in yaw and roll; downwards in pitch; Figure 1D). In a complementary experiment, the authors demonstrated that the optogenetic inhibition of the VTAVgat+ population led, instead, to the occurrence of contralateral rotations of the head (in yaw and roll; upwards in pitch; Figure 1D). Furthermore, in line with their interpretation of the recordings data, the amplitude of the elicited head movements could be modulated by modifying either the duration or frequency of the pulse of stimulating light. The optogenetic manipulation of VTAVgat+ neurons resulted in a marked decrease in the ability of mice to track the reward port and, consequently, in a drop in reward consumption. These observations appear to be in line with the effects of the optogenetic stimulation on
the ongoing head motion. Indeed, both excitation and inhibition of VTAVgat+ neurons result in a swift head displacement being forced on the animal, leading to a head trajectory inappropriate for the requirements of the task and, hence, lack of reward delivery. However, lack of consumption following optogenetic stimulation of VTAVgat+ neurons [14], as well as the resulting place avoidance [15], in which animals avoid a location primed by the activation of VTAVgat+ neurons, has been previously linked to the manifestation of an aversive behavior rather than to the perturbation of movement kinematics. Indeed, while VTA dopamine neurons are activated by reward and reward–predicting cues, salient but aversive stimuli strongly inhibit dopaminergic neurons [16], an inhibition that is supposed to be mediated by the activity of the VTAVgat+ population [15] and to ultimately drive the behavioral response. This interpretation is also broadly supported by recent findings on the role of VTAVgat+ neurons in mediating innate defensive responses to looming stimuli [17], which are salient and inherently aversive. To test the potential intrinsic aversive nature of VTAVgat+ neuron signals, Hughes et al. [2] performed a real-time place preference assay that allowed the authors to assess whether optogenetic stimulation of VTAVgat+ neurons per se can be linked to the avoidance of an otherwise neutral environment. For this experiment, mice were left free to navigate in an openfield arena divided in two identical compartments; in baseline conditions, mice explored the entirety of the arena, spending a comparable amount of time in each of the two compartments. By applying optogenetic manipulation to VTAVgat+ neurons expressing ChR2, the authors were able to assess whether this balance could be perturbed by altering the activity state of this selected neuronal population. Stimulation of VTAVgat+ neurons had no effect on the place preference displayed by the mice, suggesting that there is no direct aversive signal being communicated downstream of the VTA by its GABAergic population. However, the use of distinct stimulation protocols as well as different behavioral contingencies makes it difficult to directly compare these results with earlier studies [14,15,17]. In particular, in the case of
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place aversion, this is mediated by direct inhibition of VTA dopamine neurons by the VTAVgat+ input [15] and it is unclear whether the light stimulation protocol used by Hughes et al. [2], albeit closely mirroring the physiological VTAVgat+ activity, is sufficient to drive such a sustained inhibition of VTA dopamine neurons. Overall, the new study by Hughes et al. [2] contributes to our understanding of the role of the VTA in two ways: firstly, by adopting a novel behavioural task that allows dissociating motor control and reward prediction error, the authors are able to extract the kinematic features of goal-directed behaviour controlled by VTAVgat+ neurons. Indeed, because the animal’s expectation of reward remains constant throughout the recording session, the authors make a convincing case for VTAVgat+ neurons being concerned with the execution of a particular motor plan, rather than with providing dopaminergic neurons locally with reward-related or motivational information. Secondly, the findings reported by Hughes et al. [2] suggest not only that VTAVgat+ neurons reliably represent head angles locally, but also that their activation is sufficient to generate desired head displacements. Therefore, the study abandons the valueand motivation-centered interpretation of the role of the VTA in aversive and appetitive processes and proposes a more general motor-centered role of the VTA in spatial orienting. One should also take note, however, that the role of VTAVgat+ neurons in directing head movements was only tested in the presence of reward and whether the VTA control of head kinematics is maintained outside of a rewarding contingency remains to be tested. In the future it will be interesting to assess whether other neuronal populations in the VTA, particularly dopamine neurons, also control head movement kinematics, as this might provide an even more direct link between value and action selection. Interestingly, other midbrain structures, most prominently the superior colliculus (SC), have been canonically regarded as responsible for the execution of spatially targeted head and eye movements in primates [18] as in mice [19,20]. Moving forward it will be interesting to understand
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Dispatches how the VTA and SC coordinate the successful execution of goal-directed head movements and how target value, and the presence of reward in general, impact on head movement decisions and kinematics. The existence of a markedly motorrelated population within the VTA has interesting implications with respect to the organisation of brain circuits responsible for the execution of goaldirected behaviour. Given that such behaviour relies on the precise execution of a motor plan in three-dimensional space matched by an appropriate reward-driven motivational state, this and other recent work [10] support the idea that these two representations co-exist within the VTA and pave the way to future studies on the interaction of the neuronal populations responsible for their implementation. REFERENCES
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Water Balance: Abstaining from Obtaining While Retaining Sandra L. Martin Department of Cell and Developmental Biology, University of Colorado School of Medicine, Aurora, CO 80045, USA Correspondence: [email protected] https://doi.org/10.1016/j.cub.2019.08.038
Animals tightly regulate blood volume and solute concentrations. Water balance is usually achieved by a combination of managing intake and excretion but sometimes both drinking and urination are inconvenient. Hibernators have perfected internal mechanisms to maintain water balance without either. Thirst is a powerful drive — ask anyone who has exercised without drinking water in the heat of summer. This is because
even tiny changes in our blood osmolality trigger homeostatic mechanisms [1,2]. When water is depleted, the
hypothalamus region of the brain coordinates the incoming signals warning of decreased blood volume
Current Biology 29, R918–R941, October 7, 2019 ª 2019 Elsevier Ltd. R925