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Mental Disorders. Role of the Pallidum: Pathophysiological, Biochemical, and Therapeutic Aspects. Advances in Neurology, Vol.

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B.J., Schmechel, D., Sigmon, A . H . , Weinberg, T., Helms, M. J. and Swift, M. (1983) Ann. Neurol. 14, 507-515 Yates, C. M., Simpson, J., Gordon, A., Maloney, A. F.J., Allison, Y., Ritehie, I. M. and Urquhart, A. (1983) Brain. Res. 280, 119-126 Casanova, M. F., Walker, L.C., Whitehouse, P. J. and Price, D. L. (1985) Ann Neurol. 18, 310-313 Goudsmit, J.,Morrow, C. H.,Asher, D. M., Yangihara, R.T., Masters, C.L., Gibbs, C.J., Jr and Gajdusek, D.C. (1980) Neurology 30, 945-950 Prusiner, S. B., McKinley, M. P., Bowman, K.A., Bolton, D.C., Bendheim, P.E., Groth, D. F. and Gienner, G. G. (1983) Cell 35, 349-358 SaideI-Sulkowska, E. M. and Marotta, C. A. (1984) Science 225, 947-949 Calne, D. B., Duvoisin, R. C. and McGeer, E. (1984) in Purkinson-Speeific Motor and

Mental Disorders. Role of the PaUidum: Pathophysiological, Biochemical, and Therapeutic Aspects. Advances in Neurology, Vol. 40: (Hassler, R. G. and Christ, J. F., eds), pp. 353-360, Raven Press, New York 60 Langston, J. W., Ballard, P., Tetrud, J. W. and Irwin, I. (1983) Science 219, 979-980 61 Burns, R. S., Chiueh, C. C., Markey, S. P., Ebert, M. H., Jacobowitz, D. M. and Kopin, I. J. (1983) Proc. Nail Acad. Sci. USA 80, 4546--4550

Donald L. Price is in the Departments of Pathology, Neurology and Neuroscience. PeterJ. Whitehouse is in the Departments of Neurology and Neuroscienceand Robert G. Struble is in the Department of Pathology, Neuropathology Laboratory, 509 Pathology Building, The Johns Hopkins University School of Medicine, 600 North Wolfe Street, Baltimore, MD 21205, USA.

The role of NGF in sensory neurons/n v/vo Eugene M. Johnson, Jr, Keith M. Rich and Henry K. Yip Somatic sensory neurons have two target areas, one in the peripheral end organ and the other within the central nervous system. For many decades, it was thought that only the peripheral target was important f o r the survival o f developing and mature sensory neurons. Nerve growth factor (NGF), which is required f o r the survival o f embryonic sensory neurons in vitro, was thought to provide peripherally derived trophic support only during develapment and that mature sensory neurons were insensitive to NGF. In this article we review recent evidence which demonstrates a broader role f o r N G F and f o r the central target in the maintenance o f sensory neurons. We review the biology o f N G F in relationship to the life history o f sensory neurons, the potential use o f N G F as a pharmacological agent to ameliorate the effects o f injury, and new data establishing the central target tissue as a source o f neurotrophic support. The peripheral sensory neuron, a p s e u d o - u n i p o l a r cell, w h i c h d e v e l o p s f r o m n e u r a l crest o r e p i d e r m a l placodes, has two processes: one innervating a peripheral end organ and the o t h e r p r o j e c t i n g to t h e c e n t r a l n e r v o u s system (CNS). Like other neuronal types, t h e s e cells a r e critically d e p e n d e n t o n t h e i r t a r g e t o r g a n s for survival

d u r i n g d e v e l o p m e n t 1-3. T h e r e t r o g r a d e t r a n s f e r o f i n f o r m a t i o n is m e d i a t e d b y t r o p h i c f a c t o r s f r o m t h e t a r g e t tissue to its specific n e u r o n a l component. T h r o u g h this m e c h a n i s m t h e t a r g e t organ influences the innervating neurons. T h e i n t e r a c t i o n b e t w e e n t h e n e r v e cell b o d y a n d its field o f i n n e r v a t i o n is p a r t i c u l a r l y i m p o r t a n t

1986,ElsevierSciencePublishersB.V., Amsterdam 0378- 5912/86/$02.(]0

d u r i n g e a r l y d e v e l o p“‘I-NGF m e n t is w h e n t htransported e retrogradely n e u r o n s a r e s t i l l i n afrom c t i the v e periphery g r o wtot hthe cell a n body d in differentiation4,‘. Developing neurons adult animals in a specific receptorare able to make compensatory adjust- mediated manner very similar to that ments in accord with the volume of seen in sympathetic neurons. A second their targets by corresponding changes important observation was the demonin cell number and sizew. The depen- stration that DRG neurons contain a dence on the target for survival variety of putative neuromodulatory decreases gradually with increasing peptides15 (particularly substance P), age”. However, some neurons still made possible by the development of require the peripheral target for the immunohistochemical and radioimmaintenance of normal function and munoassay methods for their detection often for survival. and quantitation. Thus one can meaBy far the best evidence for the sure the effects of NGF or anti-NGF ‘trophic factor hypothesis’ of target- antibodies on what are considered neuron interaction derives from the differentiated functions of these neubiological characterization of nerve rons in the DRG. The effects of NGF growth factor (NGF). The discovery of and anti-NGF antibodies on these this factor by Levi-Montalcini and neuropeptides can be viewed as anaHamburger” stemmed from the obser- logous to their effects on noradrenaline vation that a secreted protein (NGF) and the levels of the associated from a tumor implanted in chick synthetic enzyme levels in sympathetic embryos could produce dramatic neurons. With this experimental aphypertrophy of sympathetic and sen- proach, substance P levels have been sory ganglia”‘~” The intensive study of shown to be increased in DRG and in the chemistry and biology of NGF over their targets in postnatal rats treated the succeeding two decades has pro- with NGF and to be decreased in vided considerable support for the animalsexposedtoNGFantibodies’6,’7. trophic factor hypothesis and some Evidence” has also been presented that understanding of the mode of action of substance P containing sensory neuNGF itself (see review of Thoenen and rons compete with sympathetic nerves Barde 1980)‘*. In contrast to the many studies of the in-viva effects of NGF A. deprivation (produced by antibodies to NGF) on sympathetic neurons, little central work was done on sensory neurons target throughout the 1960s and early 1970s. tissue Since in-vivo administration of NGF or anti-NGF antibodies does not produce dramatic changes in sensory neuronal morphology similar to those produced in sympathetic neurons, and because of observations on DRG explants and neurons in vitro, it was concluded that ‘these cells are receptive to the growthpromoting activity of the NGF only during a restricted period of their early embryoniclife’ (see Ref. 13). In fact, in contrast to the strong evidence for a physiological role of NGF in sympathetic neurons, provided by the deleterious effects of anti-NGF antibodies, the lack of a demonstrable effect of NGF antiserum on sensory neurons left in doubt the physiological role of NGF in sensory neurons. Role of NGF in perinatal period Several observations, beginning in the mid-1970s, furnished evidence for, and broadened our perspective on, the role of NGF in sensory neurons in vivo. Stoeckel et ~1.‘~ provided the first indication that there might be a trophic role for NGF in postpartum sensory nervous system by demonstrating that

ior NGF made by target tissues. Thus, at least this subpopulation of the postnatal DRG neurons is responsive to both NGF and NGF deprivation. Our understanding of the critical role of NGF throughout the life of sensory neurons was advanced by the introduction of the autoimmune approach to NGF deprivation’““. Several species, when immunized with mouse NGF, will produce antibodies that neutralize their own NGF and this produces chronic NGF deprivation. This approach provides a means of depriving the developing mammalian fetus of NGF via transplacentally transferred maternal antibodies. In the most dramatic instances, these offspring of anti-NGF producing mothers have a decrease of 85% in the number of neurons in DRG and trigeminal ganglia and a virtual absence of neurons in sympathetic ganglia. Exposure to maternal antibody via the milk during the postnatal period (using cross-foster paradigms in rats) results in loss of sympathetic neurons but not of sensory neurons*“. In striking contrast to the demonstrated effects of NGF deprivation on the developing neural crest-derived neurons, ganglia

Fig. 1. A schematic representation of the sources of trophic support for developing and mature sensory neurons. A. NGF(S) retrogradely transported to the cell body from both target tissues. The relative importance of the trophic support provided by either source changes as ofunction of age. B. Developing neurons require both sources (peripheral and central) of trophic support for survival. Loss of either source (e.g., axotomy) results in substantial cell death. In adult animals, moderate neuronal loss is observed only after lesion of theperipheralprocess. C. Exogenous NGFcan replace endogenous trophic factor(s) lost as o result of uxotomy of either or both processes

TINS-January 1986 (nodose, spiral) derived from placodes are unaffected22. Therefore, the vast majority, possibly all, of neural crestderived, but not placodally derived, sensory neurons go through a phase of dependence upon NGF for survival; however, this dependence is lost postnatally. The neurons are, as mentioned above, responsive in terms of peptide levels to NGF and anti-NGF in the postnatal period. That probably all populations of DRG neurons are NGF dependent during embryonic development is also supported by the observation of Hamburger et ai.23 who showed that the administration of NGF to chick embryos could retard or prevent the naturally occurring cell death of both the dorso-lateral and medio-ventral populations of sensory neurons seen in chick embryonic DRG. The role of NGF in mature DRG neurons - and in their response to

injury The inability of anti-NGF antibodies to produce cell death in mature sensory neurons (in contrast to the continued NGF dependence of adult sympathetic neurons) led to the conclusion that mature sensory neurons lose their dependence on NGF for survival. However, sensory neurons do retain a specific sensitivity to NGF. As previously mentioned, mature sensory neurons can transport radiolabelled NGF retrogradely via a receptor-specific mechanism from their peripheral terminals to their cell bodies in the DRG TM. In adult rats and guinea pigs chronically deprived of NGF by the autoimmune approach, substance P levels in DRG were reduced by twothirds24 and the DRG sensory neurons showed a modest atrophy25. Thus, even though adult sensory nerons are not apparently dependent on NGF for survival, they do require NGF for normal biochemical and morphologic homeostasis. The DRG neuron, like other types of neurons, undergoes a series of reactions after peripheral axotomy or injury. The individual neuronal reaction after injury varies from an anabolic response, with potential for regrowth of the injured axon to its peripheral target, to the dissolution of the perikaryon following neuronal death. The elements responsible for the variability of the response of individual neurons to injury are not known. Trophic factors or the loss of trophic support following axotomy are speculated to have a role in the response to injury26.

35 Response of DRG neurons to injury by sciatic nerve crush is not significantly altered in chronically NGFdeprived guinea pigs as determined by measurements of neuronal survival, size-frequency s~eetra, and regenerative capabilitiesz~. While loss of peripherally derived NGF support does not appear to alter significantly the response of the sensory neuron to injury, administration of exogenous NGF does ameliorate the adult sensory neuron's response to injury. The mature DRG neuron undergoes metabolic changes after peripheral axotomy including alterations in the levels of the polypeptide, substance P and in levels of the extra-lysosomal acid phosphatase enzyme27. Both these proteins are decreased in the DRG and also in the dorsal horn of the spinal cord following peripheral axotomy21'35. These biochemical changes in the dorsal horn of the spinal cord parallel morphological changes noted in the central terminals of the sensory neurons in the dorsal horn after axotomy. These include loss of synaptic contact and withdrawal of the afferent terminals in the dorsal horn after peripheral axotomy and are known as transganglionic degenerative atrophy. NGF partially prevents these changes seen in the dorsal horn after nerve transection. NGF delivered to the freshly transected sciatic nerve via a Gelita tampon obviates the marked decrease in substance P and extralysosomal acid phosphatase levels in the dorsal horn for up to a week 28. NGF supplied to the transection site via a silicone reservoir connected by a catheter to an indwellingosmotic pump impedes biochemical alterations in the dorsal horn for up to two weeks 29. In these animals treated with NGF at the injury site, electrophysiological changes, such as primary afferent depolarization, normally seen in the dorsal horn following peripheral section, are suppressed. Thus, NGF counteracts both biochemical and neurophysiological consequences in the dorsal horn of the spinal cord for up to two weeks after sciatic nerve section. Pharmacological administration of NGF prevents neuronal death in the DRG neurons of newborn rat pups following sciatic nerve injury3°. In adult rats, DRG neuronal death following sciatic nerve section can be completely forestalled for at least six weeks by supplying NGF locally at the transection site via silicone chambers sutured to the cut nerve (unpublished observations). Morphometric analysis shows an increase in neuronal cell size in the

groups treated with NGF when compared to control groups. The distinction between the physiological effects of NGF as defined by NGF deprivation and the pharmacological effects as described by exogenous NGF is a critical consideration 12. Similarly, the response of a mature, uninjured sensory neuron to NGF may differ significantly from the response of an injured sensory neuron whose axon has been severed from its target. DRG neurons which might not require NGF for survival or certain functions under normal conditions may respond to the pharmacological administration of NGF following injury. The pharmacological effects of NGF can influence survival, biochemical homeostasis, and neurophysiological properties of the injured mature sensory neuron and its central projections. The ability to prevent or to delay the effects of axonal injury suggests potential approaches wherein functional recovery could be enhanced by reducing the initial neuronal reaction to injury. This also provides a model for pharmacological intervention with trophic factors yet to be discovered which may be able to alter the neuronal response to injury. Role of the central process in maintenance of sensory neurons

The relationship of the target in the development of sensory neurons and the role played by NGF has previously centered on the peripheral fields of innervation. The sensory neuron has, of course, two targets: the peripheral innervated organs and the central nervous system. The role of the central target in the development and maintenance of sensory neurons has been regarded as virtually nonexistent. For example, in his extensive review, Lieberman states that 'the failure of sensory ganglion cells to undergo chromatolysis and to show increased protein synthesis after interruption of their central axons is one of the enigmas in neurobiology' (se.~ Ref. 26). The evidence for the co,clusion that the central process is unimportant in providing trophic support for sensory neurons, however, is neither comprehensive nor compelling. In particular, the effect of dorsal rhizotomy, which would separate the sensory ganglion from the central target, had never been carefully examined in immature animals. In addition, although attention had been given for several years to the retrograde transport of NGF from the periphery to the cell body, the

36

T I N S - J a n u a r y 1980

TABLE I. Effect of axotomy of central or/and peripheral D R G process on neuronal n u m b e r ~ in lumbar D R G of newborn rats treated with N G F and of adult guinea pigs i m m u n i z e d with N G F Lesion Group

Central

A B C D

-+ +

N e w b o r n (rats)

Peripheral

+ +

Adult (guinea pigs)

Untreated control

NGF treated

Untreated control

NGF immunized

1~) 50 b 50 b 31 ~

107 94 95 105

1(~) 97 79 b 71 b

100 75 h 81 h 59 b

a Expressed as percentage of untreated control. b Significantly different f r o m control (group A); p<0.02-0.05 D a t a were obtained from Refs 12 and 34.

possibility that 12SI-NGFmight also be transported from the CNS to the sensory neuron had not been examined. In late 1984, two papers appeared which independently demonstrated that t2SI-NGF can be retrogradely transported from dorsal spinal cord or nucleus gracilis to the lumbar DRG31.32. The properties of the transport are the same as those found in the periphery: a specific receptor-mediated mechanism with a rate of approximately 6-7 mm/hour. Thus, nerve terminals of the central, as well as the peripheral, axon bear NGF receptors. The fact that ~25I-NGF can be retrogradely transported from the CNS to the DRG does not mean that endogenous NGF /s transported, nor that this process is of any physiological significance. Further experiments, however, clearly demonstrate that trophic support of DRG neurons is provided by the CNS and that this is particularly important during early development. When newborn rats are subjected to dorsal rhizotomy, DRG neuronal numbers are decreased by 50% one week after surgery32 (see Table I), a decrease very similar to that observed after peripheral nerve lesion in animals of the same age 3°. This result indicates that, at that stage of development, the connection with the central target is as important for neuronal maintenance as the connection with the peripheral target. Whether the same 50% of cells die or if different neurons are differentially affected by the two lesions is not known. If NGF were the mediator of trophic support from the CNS, then supplying exogenous NGF could prevent the cell death. This is, indeed, the case. Administration of exogenous NGF prevents the death of DRG neurons after central, peripheral, or combined lesions32. Although this result is consistent with the idea that NGF is the trophic messenger supplied by the CNS, it does not prove the point. Some other molecule(s) might provide the

physiologically significant trophic signal; exogenous NGF might act pharmacologically to compensate for the loss of that signal. This possibility is suggested by recent work 33 showing that NGF can be detected in DRG and their peripheral axons but cannot be detected in the dorsal root or in the spinal cord of adult rats. Irrespective of whether NGF is the trophic signal, these results demonstrate that, at least during development, the central process is important in providing target-derived trophic support for the neuron. In retrospect, this result is entirely consistent with the hypothesis that naturally occurring neuronal death is mediated by competition for target-derived trophic factor(s) and that this competitive process serves as a mechanism to match the size of neuronal populations to the size of targets. It would seem just as essential for sensory neuron populations to be appropriately matched to their central targets as to their peripheral targets during the development of the mature nervous system. As described above, previous reports that heterologous antibodies against NGF or autoantibodies against NGF do not produce DRG neuronal loss in neonatal animals led to the conclusion that postnatal DRG neurons do not require NGF for survival. The results showing that, at least during development, the CNS provides trophic support (possibly NGF mediated), forces a re-evaluation of this conclusion. Anti-NGF antibodies would not be expected to gain access to the CNS and could not, therefore, deprive the DRG neuron of centrally derived NGF. Therefore, to determine whether deprivation of both peripherally and centrally derived NGF (or other factors) would produce DRG neuronal death in adults, we prepared NGF-autoimmunized guinea pigs (peripherally NGF-deprived) and then examined the long-term (100 days) effects of dorsal rhizotomy and/or a

crush of the peripheral nerve. Whereas neither autoimmunization alone nor dorsal rhizotomy alone produces any loss of DRG neurons, the combined insult results in a 29% neuronal loss in the DRG 34. That NGF is not the only trophic factor obtained from the periphery is suggested by the fact that the crushing of the peripheral nerve in a dorsal rhizotomized animal results in a greater (41%) cell loss when the animal had also been autoimmunized (Table I). These data suggest that there are at least three components to the trophic support of mature DRG neurons: a peripheral NGF component, a peripheral non-NGF component(s), and a central (NGF?) components(s). The acuteness of the dependency on, and the relative importance of, these various components clearly changes with age. Conclusions The vast majority, if not all, neural crest-derived sensory neurons require NGF for maintenance. The previously held view that NGF is a peripherally derived factor, essential for a subpopulation of these neurons during a limited period of development, underestimates the role of NGF in neural crestderived sensory neurons. Given the complexities of the sensory neurons by morphological, biochemical, and functional criteria, a great deal of work will be required to understand fully the role of NGF and other, as yet unknown, trophic factors derived from peripheral and central targets in the sensory neuron's economy. Fig. 1 shows a schematic drawing summarizing these points. In addition to the many open questions regarding the physiological role(s) of NGF, recent work indicates a pharmacological potential of NGF to ameliorate the effects of injury in sensory neurons. Thus, despite the quarter century of intensive study on the molecule, the rekindling of attention in the roles of NGF in sensory and, even more recently, in central neurons suggests that many more interesting and productive years lie ahead for the workers in this field. Acknowledgements The authors thank Patricia Osborne for her assistance in all phases of this work. Work cited in this review from the authors' laboratory was supported by NIH grant NS18071. Selected references 1 C r a g g , B. G. (1970) Brain Res. 23, 1-21 2 H a m b u r g e r , V. (1934) J. Exp. Zool. 68, 449473

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37 Nature (London) 284, 515-521 16 Kessler, J. T. and Black, I. B. (1980) Proc. Natl Acad. Sci. USA 77, 649-652 17 Otten, U., Goedert, M., Mayer, N. and Lembeck, F. (1980) Nature (London) 287, 158-159 18 Kessler, J. A., Bell, W. O. and Black, I. B. (1983) J. Neurosci. 3, 1301-1307 19 Gorin, P. D. and Johnson, E. M., Jr. (1979) Proc. Natl Acad. Sci. USA 76, 5382-5386 20 Johnson, E. M., Jr., Gorin, P. D., Brandeis, L. D. and Pearson, J. (1980) Science 210, 916-918 21 Johnson, E. M., Jr., Osborne, P. A., Rydel, R. E., Schmidt, R. E. and Pearson, J. (1983) Neurosci. 8, 631--642 22 Pearson, J., Johnson, E. M. and Brandeis, L. (1983) Dev. Biol. 96, 32-36 23 Hamburger, V., Brunso-Bechtold, J. K. and Yip, J. W. (1981) J. Neurosci. 1, 60-70 24 Schwartz, J. P., Pearson, J. and Johnson, E. M., Jr. (1982) Brain Res. 244, 378-381 25 Rich, K. M., Yip, H. K., Osborne, P. A., Schmidt, R.E. and Johnson, E.M., Jr. (1984) J. Comp. Nearol. 230, 110-118 26 Lieberman, A. R. (1971) Int. Rev. Neurobiol. 14, 49-124

27 Knyihar, E. and Csillik, B. (1976) Exp. Brain Res. 26, 73---87 28 Csillik, B. (1984) Z. Mikrosk-Anat. Forsch. 98, 11-16 29 Fitzgerald, M., Wall, P. D., Gesodert, M. and Erason, P. C. (1985) Brain Res. 332,131141 30 Yip, H. K., Rich, K. M., Lamp¢, P. A. and Johnson, E. M., Jr. (1984) J. Neurosci. 4, 2986-2992 31 Richardson, P. M. and Riopelle, R. J. (1984) J. Neurosci. 4, 1683-1689 32 Yip, H. K. and Johnson, E. M., Jr. (1984) Proc. Natl Acad. Sci. USA 81, 6245-6249 33 Korsching, S. and Thoenen, H. (1985) Neurosci. Lett. 54, 201-205 34 Johnson, E. M., Jr. and Yip, H. K. (1985) Nature (London) 314, 751-752 35 Jessell, T., Tsunoo, A., Kanazawa, I. and Otsuka, M. (1979) Brain Res. 168, 247-259

Eugene M. Johnson, Jr. and Henry K. Yip are at the Department of Pharmacology, Keith M. Rich is at the Department of Neurosurgery, Washington University, School of Medicine, St Louis, MO 63110, USA.

The role of the postsynaptic cytoskeleton in AChR organization Stanley C. Froehner ACh receptors (AChRs) occur at much higher concentrations immediately adjacent to the nerve terminal than elsewhere on the muscle membrane. The molecular mechanisms responsible for maintaining this distribution are largely unknown but are thought to involve other synaptic proteins. Of particular interest are those proteins that comprise the postsynaptic cytoskeleton. This article discusses the role that both general and synapse-specific cytoskeletal proteins may play in the postsynaptic organization of receptors. Proper alignment of transmitter release sites in the presynaptic terminal with areas of high receptor density in the postsynaptic membrane is a critical step in the formation of an efficient, rapidly transmitting chemical synapse. Nowhere is this better illustrated than at the neuromuscular junction. In this synapse, the postsynaptic membrane is extensively invaginated, giving a sixfold increase in membrane area. The AChRs, however, are concentrated at the crests of these folds (i.e. nearest the nerve terminal) where the density of the receptors is approximately 10000 molecules 0m -2 (Ref. 1). The concentration of receptors outside the synapse is at least 500-fold less. austering of AChRs underneath the nerve terminal occurs early during development of the neuromuscular junction 2. In embryonic muscles, AChRs are initially distributed diffusely over the muscle fiber surface but soon after innervation become concentrated under the nerve ter-

minal. At least some, and possibly most, of the receptors in clusters are recruited from pre-existing diffuse receptors by a process induced by the nerve 3-5 . Since the receptor, like most membrane proteins, is inherently capable of diffusion in the plane of the membrane 6, special mechanisms must exist to anchor receptors at sites designated by the nerve. Although the mechanisms responsible for anchoring are largely unknown, the cytoskeleton is likely to be a key component.

Postsynaptic cytoskeleton and receptor mobility Ultrastructural examination of vertebrate neuromuscular junctions has revealed an intracellular submembranous meshwork just beneath the postsynaptic membrane that appears to connect the membrane to underlying bundles of filaments7. The meshwork, composed of short, thin strands, is not found on membranes outside of the synapse and presumably corres-

ponds to the postsynaptic specialization seen in thin sections. Similar structures of 6 nm filaments have been observed at receptor clusters on aneural Xenopus cultured muscle cellss. Vesicles derived from Torpedo postsynaptic membranes negatively contrasted with tannic acid for thinsection EM display a bar of electrondense material on the cytoplasmic side of the membrane 9. The coextensive distribution of this material with structures identified as AChRs suggests that these two components may interact. Filamentous structures are frequently found associated with the subsynaptic density~0. These are precisely the types of structures that might anchor transmembrane proteins in place and are of particular interest since in muscle they are specific to the postsynaptic membrane. Brief treatment of Torpedo membranes with alkaline solution (pH 11) or with low concentrations of lithium di-iodosalicylate (a chaotropic agent) solubilizes the postsynaptic electrondense material9a 0, leaving the receptor associated with the membrane. Examination of the effects of alkaline extraction on the mobility of receptors in Torpedo membrane fragments suggests a role for the cytol~lasmic specialization in receptor anchoring.

1986,ElsevierSciencePublishersB.V.,Amsterdam 0378- 5912/86/$02.00