- Email: [email protected]
On establishing the genetic basis of mental disease
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,
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Onestablishingthegeneticbasisof mentaldisease Joseph S. Alper and Marvin R. Natowicz Many of the recent studies reporting genetic linkages for mental illnesses such as schizophrenia and manic depression have been retracted. The authors of this article argue that the fundamental reason for the difficulties in this research field tiesin the strongly held preconceived belief that the primary cause of these illnessesis in fact genetic. All scientists hold preconceived ideas. However, such ideas are more likely to result in erroneous conclusions in the study of human behavior than in other more 'objective' research areas. Moreover, it is especiallyimportant that researchersstudying human behavior be aware of their biases and learn to compensate for them because of the social consequencesof their work.
The remarkable advances in our understanding of the genetic basis of diseases such as Huntington's disease, cystic fibrosis and Duchenne muscular dystrophy have encouraged geneticists to investigate the basis of mental diseases using the tools of molecular genetics. During the past few years, genetic linkages have been reported for schizophrenia and manic depression I-3. However, it is now known that the evidence for essentially all the reported linkages is not nearly so convincing as it appeared in the original publications 4. Despite the recognition for at least four years of the difficulties of linkage analysis for psychiatric conditions 5, further weakened evidence for a linkage between manic depressive illness and X-chromosome markers was recently reported 6. In view of the seemingly inexorable progress in the genetic understanding of physical diseases, it is important to understand why investigations into the genetic basis of mental diseases continue to encounter such great difficulties. The widespread perception that there is a genetic basis for diseases like schizophrenia and manic depression is, in our opinion, the most important factor responsible for the difficulties. At the outset of this article, we emphasize that scientists should not be faulted for holding preconceived ideas and for having a bias towards the conclusions they are trying to establish. Scientific experiments are usually designed to show that a proposed hypothesis is true. Clearly, no scientist would devote months or years of work to a project if he or she did not care whether the proposed hypothesis were indeed true. Preconceived ideas and bias present few problems to physicists, chemists and even to researchers studying the basis of human physical diseases. What is special about the study of mental illnesses? The various pitfalls in this field have been clearly established. Three of the co-authors of the most recently published retraction of a linkage between manic depressive illness and X-chromosome markers have themselves warned of the difficulties in the field and suggested that researchers involved in linkage studies follow an extensive set of guidelines 7. Probably the most important source of difficulty noted by Baron et al. 7 is the fact that there is no single set of objective criteria for the diagnosis of a TINS, Vol. 16, No. 10, 1993
mental disease as there is in the diagnosis of physical diseases. Even the diagnostic criteria of guidelines such as the Diagnostic and Statistical Manual o f Mental Disorders (DSM-III-R) 8 are not uniformly applied. Thus, some researchers might use a broad definition, diagnosing the condition if only a few of the most important symptoms are present, while others might use a more narrow definition, requiring that most or all of the symptoms associated with the disease be present. In a study of the genetic basis of a mental disease, the researchers can choose which definition of the disease to use. If they believe that a genetic basis exists, they may tailor the definition of the illness so as to maximize the likelihood that the data will support the genetic hypothesis. The evidence for a genetic basis of a disease can also be enhanced by adjusting the degree of penetrance assumed for the genotype. Many traits are incompletely penetrant; not all individuals with a particular genotype will manifest the associated phenotype. The degree of penetrance for any of the major mental disorders (assuming that they indeed have a genetic basis) is unknown. Consequently, the degree of penetrance can be treated as a parameter in the model describing the linkage. Varying the degree of penetrance in the model has an effect equivalent to increasing diagnostic ambiguity. By varying the estimate of the penetrance, some subjects can, in effect, be classified as being affected because of their postulated genotype even though they are asymptomatic. This adjustment can be used to increase the predicted likelihood that the cause of the disease in the families studied is genetic. Both of these approaches - adjusting definitions and using adjustable parameters in a model - are accepted and useful scientific strategies. However, in non-human studies, the model can be tested on data sets other than the original data set that was used both to refine the definition of the 'property' (in this instance, the disease) under study and to optimize the values of the adjustable parameters. In studies involving human subjects, especially in linkage studies used to establish the genetic basis of a mental disease, the same data set is used both to establish the model and then to perform the statistical analysis. Lod (logarithm of the odds) scores are used to evaluate the likelihood that the correlation between the occurrence of the disease and the genetic markers in a family pedigree is due to a genotype associated with the disease, rather than simply occurring by chance. A Iod score of three means that the odds are a thousand to one that the correlation is the result of a gene linkage rather than chance. However, it has been pointed out that a Iod score of three is essentially equivalent to a 95% confidence interval7: if the study were repeated a hundred times, in five of those repetitions the linkage would not be found. Because a Iod score of three does not
© 1993, ElsevieSci r encePublishersLtd, (UK)
JosephS.Alperis at the Deptof Chemistry and Centerfor Geneticsand Public Policy, Universityof Massachusettsat Boston,Boston,MA 02125, USAand the Divisionof Medical Genetics,TheShriver Centerfor Mental Retardation, Waltham,MA 02254, USA,andMarvin R. Natowiczisat the Divisionof Medical Genetics, TheShriver Centerfor Mental Retardation, Waltham,/VIA02254, USAand the Deptsof Pediatrics and Pathology, HarvardMedical Schooland Massachusetts Genera/Hospital, Boston,MA 02115, USA.
387
provide overwhelming evidence of linkage, and in view of the fact that linkages with lod scores of three or better have been shown to be spurious, it has been suggested that a lod score significantly larger than three be required to establish significant evidence of linkage 7. However, imposing the need for a higher lod score will not solve all the problems arising in linkage analyses. It will mitigate the problem that arises in studies in which the correction of one or two misdiagnoses of individuals reduces the lod score from a highly significant one to one indicating no linkage. However, it will not solve the problems created by the fact that the lod score generated in a linkage analysis is derived from an analysis in which the same data set used to construct the model is then used to test it. Suppose, for example, that the genetic etiology found is peculiar to the families studied, or that in fact there is no genetic etiology at all but that a genetic linkage was found fortuitously as a result of adjustments in the model. In these cases, the same pattern of inheritance would not be expected to be found in other families, no matter how high a lod score was reported for the study of the original families. The methodological problems described above are elementary and are known to most researchers. We believe that the explanation for why these errors continue to be made lies in the strength of the preconceived view that there must be a genetic basis for mental diseases. As a consequence of this bias, researchers are willing to take the scientifically risky approach of adjusting their model using the same data set on which they will be performing the statistical analysis. As a result of the setbacks in the efforts to establish the genetic bases of mental diseases, scientists are beginning to acknowledge that environmental factors have been neglected. However, in remedying this neglect, the environmental factors cannot be simply 'added on' to the genetic factors. Instead, in the case of a mental disease, or any other trait, a phenotype must be acknowledged as being the result of a complex interaction between the environment and the genetic constitution of the individual. Despite the recognition of this interaction, the bias towards a genetic explanation of mental disease remains strong. Although a news article recently appeared in Science that outlined the current difficulties in this field, it nevertheless began by stating that '[t]here is good evidence that inherited susceptibilities underlie the development of psychiatric diseases '9. In addition, even though the editorial accompanying the most recent retraction on genetic linkage with a mental disorder discusses the probable importance of environmental factors, the author states that the evidence for the role of genetic factors in mental diseases is 'overwhelming '4. In view of the series of retractions that now leave few or no linkage studies unscathed, how can such statements be justified? The only remaining studies that support the idea of the genetic basis of mental illnesses are the 388
behavioral genetics studies involving identicai twins or adoption studies. There is much literature that discusses the weaknesses of such studies ~°, The difficulties include showing that separated twins are brought up in uncorrelated environments, that comonozygotic twins are not treated more similarly than are co-dizygotic twins or than other siblings, and analysing the covariance and interaction effects that arise in the calculation of heritability coefficients. In addition, the heritability estimates for mental diseases obtained by different researchers studying twins or adoption records have varied considerably 11 Moreover, unlike linkage studies, these studies cannot be re-analysed or repeated, partly because the raw data and subjects are usually no longer readily available. As a result, it would now be extremely difficult to prove that even a seriously flawed adoption or twin study were wrong. Nevertheless, a critical analysis of the assumptions of any adoption or twin study, coupled with the succession of retractions of the genetic linkage studies, indicates that the evidence for the genetic basis of mental illnesses is far from overwhelming. In view of the lack of scientific evidence for the hypothesis that there are genetic bases for mental diseases, we conclude that nonscientific beliefs play a major role in laying this hypothesis 12. Molecular geneticists are drawn to the field because of the possibility of applying their expertise to important and high-profile problems of great interest to society. In addition, some scientists believe that if a genetic basis can be found, then a treatment or cure based on genetics will follow. Others are drawn to the hypothesis for political and philosophical reasons. There is a tendency among the lay public to believe that genetic means unchangeable. This belief is false. For example, the invariably serious neurological effects of phenylketonuria (PKU), a genetic disease characterized by the inability to metabolize phenylalanine normally, can be largely prevented by providing the affected newborn with a phenylalanine-restricted dieL The belief that a genetic trait is unchangeable is often accompanied by the idea that neither the individuals with that trait nor society need feel responsible for their condition. In the late 1960s and during the 1970s, this biological determinist reasoning was used by some prominent academics to argue that the difference in performance on IQ tests between races was due to genetics, and that this difference could never be reduced 13. Recently, researchers studying the genetic basis of male homosexuality have argued that such a finding would reduce prejudice against homosexuals because, in their opinion, the public would realize that homosexuality is not the result of choice ~4. Today, many social commentators are concerned that the possible discovery of a genetic basis for violent criminal behavior might be used to excuse society from improving the living conditions of the poor in an attempt to ameliorate the problem of crime ~5. The study of the genetic basis of mental diseases is an extremely active area of research. Here, our TINS, Vol. 16, No. 10, 1993
intention is to emphasize that scientists must be aware of their biases, because these biases can have a dramatic effect on the outcome of their research. If technical progress is to be made in the understanding of the possible genetic bases of mental illnesses, then it will be essential that the biases be explicitly acknowledged and that extreme efforts be made to overcome their effects on scientific research.
Selected references 1 2 3 4 5
Sherrington,R. etal. (1988) Nature 336, 164-167 Baron, M. etaL (1987) Nature 326, 289-292 Egeland,J. A. etaL (1987) Nature 325,783-787 Pauls,D. L. (1993) Nature Genet. 3, 4-5 Barnes,D. M. (1989) Science 243,313-314
6 Baron, M. et al. (1993) Nature Genet. 3, 49-55 7 Baron,M., Endicott,J. and Ott, J. (1990) Br. J. Psychiatry 157, 645-655 8 Diagnostic and Statistical Manual of Mental Disorders, 3rd ed., Revised (1987), American Psychiatric Association, Washington DC, USA 9 Aldous, P. (1993) Science 259, 591-592 10 Billings, P. R., Beckwith, J. and Alper, J. S. (1992) Soc. Sci. Med. 35, 227-238 11 Kinney,D. K. (1990)in The Principles and Practice of Medical Genetics (2nd edn) (Emery, A. E. and Rimoin, D. L., eds), pp. 457-472, Churchill Livingstone 12 Alper, J. S. and Natowicz, M. R. (1992) Br. Med. J. 305, 666 13 Jensen,A. R. (1969) Harvard Educational Review 39, 1-123 14 Bailey, J. M. and Pillard, R. C. (1991) Arch. Gen. Psychiatry 48, 1089-1096 15 Goleman,D. (1992) New York Times, 15 September,C1, C7
Acknowledgements We wouldlike to thank membersof the GeneticScreening Study Group,Boston, MA for their valuable suggestions.This work wassupported in part by a grant from the Human GenomeInitiative through the US Departmentof Energy.
techniques Thedynamicdamp: artificial condudancesin biological ReuroR$ A n d r e w A. Sharp, Michael B. O'Neil, L. F. A b b o t t and Eve Ma rd e r The dynamic clamp is a novel method that usescomputer simulation to introduce conductances into biological neurons. This method can be used to study the role of various conductances in shaping the activity of single neurons, or neurons within networks. The dynamic clamp can also be used to form circuits from previously unconnected neurons. This approach makes computer simulation an interactive experimental tool, and will be useful in many applications where the role of synaptic strengths and intrinsic properties in neuronal and network dynamics is of interest.
A basic goal of neuroscience is to understand how membrane and synaptic conductances combine and interact to produce the behavior of neurons and neural circuits. The conventional method for altering the activity of single neurons or perturbing their activity in networks is to inject constant current using the current clamp. This allows the investigator to either depolarize or hyperpolarize a neuron, but does not correctly replicate the conductance changes produced by synaptic inputs or modified by neuromodulators. The primary tools used to determine the characteristics of individual neuronal conductances are the voltage clamp and the patch clamp. While essential for understanding the voltage- and time-dependence of a conductance, these methods are less useful for studying the interplay of conductances that determine how neurons act individually or in circuits. Most voltageand patch-clamp experiments halt the normal voltage excursions of the clamped neuron and they often employ pharmacological agents to isolate a single conductance. For these reasons, conventional voltage clamp methods do not allow the evaluation of the role of conductances during the normal dynamic evolution of the membrane potential. The classical solution to this problem is to simulate the electrical activity of a neuron using mathematical descriptions of its measured conductances 1. The limitation of this method is that it requires a detailed description of many, if not all, of the conductances in a neuron, and these data may be difficult or impossible to obtain. TINS, VoL 16, No. 10, 1993
The dynamic clamp is a new approach that allows an investigator to introduce artificial voltage- and time-dependent conductances into biological neurons (Refs 2 - 4 and Hutcheon, B. and Pull, E., unpublished observations). In a sense, the dynamic clamp uses biological neurons as simulators, allowing the investigator to evaluate the role of individual conductances in shaping the electrical activity of single neurons, as well as determining the consequences of synaptic strengths in networks. The dynamic clamp combines the control and flexibility of computer simulation with the accuracy and realism of electrophysiological recording, using computer modeling as an experimental tool.
AndrewA. Sharp, MichaelB. O'Neil, L. F.Abbott and Eve Marderare at the Deptof Biologyand Centerfor Complex Systems,Brandeis University, Waltham, MA 02254, USA.
The dynamic clamp produceschanges in conductance The basic set-up for the dynamic clamp is similar to a conventional voltage- or current-clamp rig. However, in the case of the dynamic clamp, the injected current is controlled by a computer program that duplicates the current that would flow through a real membrane or synaptic conductance (Box 1). Any conductance that can be modeled mathematically can be introduced into the neuron being studied. The capabilities and uses of the dynamic clamp are illustrated here, using examples from the stomatogastric ganglion (STG) of the crab Cancer borealis. The STG offers a small group of well-defined neurons, whose connections and circuit characteristics are well understood, including extrinsic inputs and neurotransmitters. Unlike current-clamp injection, the dynamic clamp duplicates both the voltage and the conductance changes caused, for example, by a neurotransmitter. In Fig. 1, the dynamic clamp mimics the response of an STG neuron to rapid bath application of the neurotransmitter ~,-aminobutyric acid (GABA). Hyperpolarizing constant current pulses were used to monitor the input impedance of the neuron. GABA increased the conductance of the neuron (seen as a decrease in the amplitude of the changes
© 1993.ElsevieSci r encePublishersLtd, (UK)
389
;~
;:
,
:~
~
........... ..........
Onestablishingthegeneticbasisof mentaldisease Joseph S. Alper and Marvin R. Natowicz Many of the recent studies reporting genetic linkages for mental illnesses such as schizophrenia and manic depression have been retracted. The authors of this article argue that the fundamental reason for the difficulties in this research field tiesin the strongly held preconceived belief that the primary cause of these illnessesis in fact genetic. All scientists hold preconceived ideas. However, such ideas are more likely to result in erroneous conclusions in the study of human behavior than in other more 'objective' research areas. Moreover, it is especiallyimportant that researchersstudying human behavior be aware of their biases and learn to compensate for them because of the social consequencesof their work.
The remarkable advances in our understanding of the genetic basis of diseases such as Huntington's disease, cystic fibrosis and Duchenne muscular dystrophy have encouraged geneticists to investigate the basis of mental diseases using the tools of molecular genetics. During the past few years, genetic linkages have been reported for schizophrenia and manic depression I-3. However, it is now known that the evidence for essentially all the reported linkages is not nearly so convincing as it appeared in the original publications 4. Despite the recognition for at least four years of the difficulties of linkage analysis for psychiatric conditions 5, further weakened evidence for a linkage between manic depressive illness and X-chromosome markers was recently reported 6. In view of the seemingly inexorable progress in the genetic understanding of physical diseases, it is important to understand why investigations into the genetic basis of mental diseases continue to encounter such great difficulties. The widespread perception that there is a genetic basis for diseases like schizophrenia and manic depression is, in our opinion, the most important factor responsible for the difficulties. At the outset of this article, we emphasize that scientists should not be faulted for holding preconceived ideas and for having a bias towards the conclusions they are trying to establish. Scientific experiments are usually designed to show that a proposed hypothesis is true. Clearly, no scientist would devote months or years of work to a project if he or she did not care whether the proposed hypothesis were indeed true. Preconceived ideas and bias present few problems to physicists, chemists and even to researchers studying the basis of human physical diseases. What is special about the study of mental illnesses? The various pitfalls in this field have been clearly established. Three of the co-authors of the most recently published retraction of a linkage between manic depressive illness and X-chromosome markers have themselves warned of the difficulties in the field and suggested that researchers involved in linkage studies follow an extensive set of guidelines 7. Probably the most important source of difficulty noted by Baron et al. 7 is the fact that there is no single set of objective criteria for the diagnosis of a TINS, Vol. 16, No. 10, 1993
mental disease as there is in the diagnosis of physical diseases. Even the diagnostic criteria of guidelines such as the Diagnostic and Statistical Manual o f Mental Disorders (DSM-III-R) 8 are not uniformly applied. Thus, some researchers might use a broad definition, diagnosing the condition if only a few of the most important symptoms are present, while others might use a more narrow definition, requiring that most or all of the symptoms associated with the disease be present. In a study of the genetic basis of a mental disease, the researchers can choose which definition of the disease to use. If they believe that a genetic basis exists, they may tailor the definition of the illness so as to maximize the likelihood that the data will support the genetic hypothesis. The evidence for a genetic basis of a disease can also be enhanced by adjusting the degree of penetrance assumed for the genotype. Many traits are incompletely penetrant; not all individuals with a particular genotype will manifest the associated phenotype. The degree of penetrance for any of the major mental disorders (assuming that they indeed have a genetic basis) is unknown. Consequently, the degree of penetrance can be treated as a parameter in the model describing the linkage. Varying the degree of penetrance in the model has an effect equivalent to increasing diagnostic ambiguity. By varying the estimate of the penetrance, some subjects can, in effect, be classified as being affected because of their postulated genotype even though they are asymptomatic. This adjustment can be used to increase the predicted likelihood that the cause of the disease in the families studied is genetic. Both of these approaches - adjusting definitions and using adjustable parameters in a model - are accepted and useful scientific strategies. However, in non-human studies, the model can be tested on data sets other than the original data set that was used both to refine the definition of the 'property' (in this instance, the disease) under study and to optimize the values of the adjustable parameters. In studies involving human subjects, especially in linkage studies used to establish the genetic basis of a mental disease, the same data set is used both to establish the model and then to perform the statistical analysis. Lod (logarithm of the odds) scores are used to evaluate the likelihood that the correlation between the occurrence of the disease and the genetic markers in a family pedigree is due to a genotype associated with the disease, rather than simply occurring by chance. A Iod score of three means that the odds are a thousand to one that the correlation is the result of a gene linkage rather than chance. However, it has been pointed out that a Iod score of three is essentially equivalent to a 95% confidence interval7: if the study were repeated a hundred times, in five of those repetitions the linkage would not be found. Because a Iod score of three does not
© 1993, ElsevieSci r encePublishersLtd, (UK)
JosephS.Alperis at the Deptof Chemistry and Centerfor Geneticsand Public Policy, Universityof Massachusettsat Boston,Boston,MA 02125, USAand the Divisionof Medical Genetics,TheShriver Centerfor Mental Retardation, Waltham,MA 02254, USA,andMarvin R. Natowiczisat the Divisionof Medical Genetics, TheShriver Centerfor Mental Retardation, Waltham,/VIA02254, USAand the Deptsof Pediatrics and Pathology, HarvardMedical Schooland Massachusetts Genera/Hospital, Boston,MA 02115, USA.
387
provide overwhelming evidence of linkage, and in view of the fact that linkages with lod scores of three or better have been shown to be spurious, it has been suggested that a lod score significantly larger than three be required to establish significant evidence of linkage 7. However, imposing the need for a higher lod score will not solve all the problems arising in linkage analyses. It will mitigate the problem that arises in studies in which the correction of one or two misdiagnoses of individuals reduces the lod score from a highly significant one to one indicating no linkage. However, it will not solve the problems created by the fact that the lod score generated in a linkage analysis is derived from an analysis in which the same data set used to construct the model is then used to test it. Suppose, for example, that the genetic etiology found is peculiar to the families studied, or that in fact there is no genetic etiology at all but that a genetic linkage was found fortuitously as a result of adjustments in the model. In these cases, the same pattern of inheritance would not be expected to be found in other families, no matter how high a lod score was reported for the study of the original families. The methodological problems described above are elementary and are known to most researchers. We believe that the explanation for why these errors continue to be made lies in the strength of the preconceived view that there must be a genetic basis for mental diseases. As a consequence of this bias, researchers are willing to take the scientifically risky approach of adjusting their model using the same data set on which they will be performing the statistical analysis. As a result of the setbacks in the efforts to establish the genetic bases of mental diseases, scientists are beginning to acknowledge that environmental factors have been neglected. However, in remedying this neglect, the environmental factors cannot be simply 'added on' to the genetic factors. Instead, in the case of a mental disease, or any other trait, a phenotype must be acknowledged as being the result of a complex interaction between the environment and the genetic constitution of the individual. Despite the recognition of this interaction, the bias towards a genetic explanation of mental disease remains strong. Although a news article recently appeared in Science that outlined the current difficulties in this field, it nevertheless began by stating that '[t]here is good evidence that inherited susceptibilities underlie the development of psychiatric diseases '9. In addition, even though the editorial accompanying the most recent retraction on genetic linkage with a mental disorder discusses the probable importance of environmental factors, the author states that the evidence for the role of genetic factors in mental diseases is 'overwhelming '4. In view of the series of retractions that now leave few or no linkage studies unscathed, how can such statements be justified? The only remaining studies that support the idea of the genetic basis of mental illnesses are the 388
behavioral genetics studies involving identicai twins or adoption studies. There is much literature that discusses the weaknesses of such studies ~°, The difficulties include showing that separated twins are brought up in uncorrelated environments, that comonozygotic twins are not treated more similarly than are co-dizygotic twins or than other siblings, and analysing the covariance and interaction effects that arise in the calculation of heritability coefficients. In addition, the heritability estimates for mental diseases obtained by different researchers studying twins or adoption records have varied considerably 11 Moreover, unlike linkage studies, these studies cannot be re-analysed or repeated, partly because the raw data and subjects are usually no longer readily available. As a result, it would now be extremely difficult to prove that even a seriously flawed adoption or twin study were wrong. Nevertheless, a critical analysis of the assumptions of any adoption or twin study, coupled with the succession of retractions of the genetic linkage studies, indicates that the evidence for the genetic basis of mental illnesses is far from overwhelming. In view of the lack of scientific evidence for the hypothesis that there are genetic bases for mental diseases, we conclude that nonscientific beliefs play a major role in laying this hypothesis 12. Molecular geneticists are drawn to the field because of the possibility of applying their expertise to important and high-profile problems of great interest to society. In addition, some scientists believe that if a genetic basis can be found, then a treatment or cure based on genetics will follow. Others are drawn to the hypothesis for political and philosophical reasons. There is a tendency among the lay public to believe that genetic means unchangeable. This belief is false. For example, the invariably serious neurological effects of phenylketonuria (PKU), a genetic disease characterized by the inability to metabolize phenylalanine normally, can be largely prevented by providing the affected newborn with a phenylalanine-restricted dieL The belief that a genetic trait is unchangeable is often accompanied by the idea that neither the individuals with that trait nor society need feel responsible for their condition. In the late 1960s and during the 1970s, this biological determinist reasoning was used by some prominent academics to argue that the difference in performance on IQ tests between races was due to genetics, and that this difference could never be reduced 13. Recently, researchers studying the genetic basis of male homosexuality have argued that such a finding would reduce prejudice against homosexuals because, in their opinion, the public would realize that homosexuality is not the result of choice ~4. Today, many social commentators are concerned that the possible discovery of a genetic basis for violent criminal behavior might be used to excuse society from improving the living conditions of the poor in an attempt to ameliorate the problem of crime ~5. The study of the genetic basis of mental diseases is an extremely active area of research. Here, our TINS, Vol. 16, No. 10, 1993
intention is to emphasize that scientists must be aware of their biases, because these biases can have a dramatic effect on the outcome of their research. If technical progress is to be made in the understanding of the possible genetic bases of mental illnesses, then it will be essential that the biases be explicitly acknowledged and that extreme efforts be made to overcome their effects on scientific research.
Selected references 1 2 3 4 5
Sherrington,R. etal. (1988) Nature 336, 164-167 Baron, M. etaL (1987) Nature 326, 289-292 Egeland,J. A. etaL (1987) Nature 325,783-787 Pauls,D. L. (1993) Nature Genet. 3, 4-5 Barnes,D. M. (1989) Science 243,313-314
6 Baron, M. et al. (1993) Nature Genet. 3, 49-55 7 Baron,M., Endicott,J. and Ott, J. (1990) Br. J. Psychiatry 157, 645-655 8 Diagnostic and Statistical Manual of Mental Disorders, 3rd ed., Revised (1987), American Psychiatric Association, Washington DC, USA 9 Aldous, P. (1993) Science 259, 591-592 10 Billings, P. R., Beckwith, J. and Alper, J. S. (1992) Soc. Sci. Med. 35, 227-238 11 Kinney,D. K. (1990)in The Principles and Practice of Medical Genetics (2nd edn) (Emery, A. E. and Rimoin, D. L., eds), pp. 457-472, Churchill Livingstone 12 Alper, J. S. and Natowicz, M. R. (1992) Br. Med. J. 305, 666 13 Jensen,A. R. (1969) Harvard Educational Review 39, 1-123 14 Bailey, J. M. and Pillard, R. C. (1991) Arch. Gen. Psychiatry 48, 1089-1096 15 Goleman,D. (1992) New York Times, 15 September,C1, C7
Acknowledgements We wouldlike to thank membersof the GeneticScreening Study Group,Boston, MA for their valuable suggestions.This work wassupported in part by a grant from the Human GenomeInitiative through the US Departmentof Energy.
techniques Thedynamicdamp: artificial condudancesin biological ReuroR$ A n d r e w A. Sharp, Michael B. O'Neil, L. F. A b b o t t and Eve Ma rd e r The dynamic clamp is a novel method that usescomputer simulation to introduce conductances into biological neurons. This method can be used to study the role of various conductances in shaping the activity of single neurons, or neurons within networks. The dynamic clamp can also be used to form circuits from previously unconnected neurons. This approach makes computer simulation an interactive experimental tool, and will be useful in many applications where the role of synaptic strengths and intrinsic properties in neuronal and network dynamics is of interest.
A basic goal of neuroscience is to understand how membrane and synaptic conductances combine and interact to produce the behavior of neurons and neural circuits. The conventional method for altering the activity of single neurons or perturbing their activity in networks is to inject constant current using the current clamp. This allows the investigator to either depolarize or hyperpolarize a neuron, but does not correctly replicate the conductance changes produced by synaptic inputs or modified by neuromodulators. The primary tools used to determine the characteristics of individual neuronal conductances are the voltage clamp and the patch clamp. While essential for understanding the voltage- and time-dependence of a conductance, these methods are less useful for studying the interplay of conductances that determine how neurons act individually or in circuits. Most voltageand patch-clamp experiments halt the normal voltage excursions of the clamped neuron and they often employ pharmacological agents to isolate a single conductance. For these reasons, conventional voltage clamp methods do not allow the evaluation of the role of conductances during the normal dynamic evolution of the membrane potential. The classical solution to this problem is to simulate the electrical activity of a neuron using mathematical descriptions of its measured conductances 1. The limitation of this method is that it requires a detailed description of many, if not all, of the conductances in a neuron, and these data may be difficult or impossible to obtain. TINS, VoL 16, No. 10, 1993
The dynamic clamp is a new approach that allows an investigator to introduce artificial voltage- and time-dependent conductances into biological neurons (Refs 2 - 4 and Hutcheon, B. and Pull, E., unpublished observations). In a sense, the dynamic clamp uses biological neurons as simulators, allowing the investigator to evaluate the role of individual conductances in shaping the electrical activity of single neurons, as well as determining the consequences of synaptic strengths in networks. The dynamic clamp combines the control and flexibility of computer simulation with the accuracy and realism of electrophysiological recording, using computer modeling as an experimental tool.
AndrewA. Sharp, MichaelB. O'Neil, L. F.Abbott and Eve Marderare at the Deptof Biologyand Centerfor Complex Systems,Brandeis University, Waltham, MA 02254, USA.
The dynamic clamp produceschanges in conductance The basic set-up for the dynamic clamp is similar to a conventional voltage- or current-clamp rig. However, in the case of the dynamic clamp, the injected current is controlled by a computer program that duplicates the current that would flow through a real membrane or synaptic conductance (Box 1). Any conductance that can be modeled mathematically can be introduced into the neuron being studied. The capabilities and uses of the dynamic clamp are illustrated here, using examples from the stomatogastric ganglion (STG) of the crab Cancer borealis. The STG offers a small group of well-defined neurons, whose connections and circuit characteristics are well understood, including extrinsic inputs and neurotransmitters. Unlike current-clamp injection, the dynamic clamp duplicates both the voltage and the conductance changes caused, for example, by a neurotransmitter. In Fig. 1, the dynamic clamp mimics the response of an STG neuron to rapid bath application of the neurotransmitter ~,-aminobutyric acid (GABA). Hyperpolarizing constant current pulses were used to monitor the input impedance of the neuron. GABA increased the conductance of the neuron (seen as a decrease in the amplitude of the changes
© 1993.ElsevieSci r encePublishersLtd, (UK)
389