autoimmune channelopathies are becoming one of the exciting areas of neurological diseases in clinical practice because , though relatively uncommon ( collectively perhaps 20 per million per year ) , diagnosis of these conditions usually indicates a significant clinical improvement following immunotherapies that reduce autoantibody levels . the field stems from three decades of research into myasthenia gravis and the lambert - eaton myasthenic syndrome ; in these conditions , autoantibodies to muscle nicotinic acetylcholine receptors ( achrs ) or voltage - gated calcium channels ( p / q - type ) , respectively , are the main pathogenic agents and cause destruction and/or downregulation of their targets , leading to neuromuscular junction transmission failure ( table 1 ) which can be demonstrated in animal models . newer disorders of peripheral neurotransmission include ( a ) peripheral nerve hyperexcitability syndromes with antibodies binding to i - dendrotoxin - labelled shaker - type ( kv1 ) voltage - gated potassium channels ( vgkcs ) extracted from mammalian cortex   and ( b ) autonomic neuropathies with antibodies to i - epibatidine - labelled ganglionic nicotinic achrs . achr , acetylcholine receptor ; vgcc , voltage - gated calcium channel ; vgkc , voltage - gated potassium channel . over the last decade or so , a new family of antibody - associated diseases has emerged that is beginning to overturn previous concepts that regarded the brain as immune - privileged and protected by an impermeable blood - brain barrier . first , glutamate receptor ( glur3 ) antibodies were present in children with the very rare but devastating form of epilepsy called rasmussen encephalitis , but these findings were not always confirmed in other cohorts of patients , and the main pathology is now thought to be cellular rather than antibody - driven . the paradigm shift really began with the finding of very high vgkc antibody levels in patients with limbic encephalitis - which includes seizures , psychological disturbance , memory loss and high signal on magnetic resonance imaging ( mri ) in the medial temporal lobes - who responded convincingly to immunotherapies such as plasma exchange ( which removes circulating plasma components such as antibodies and replaces them with substitute plasma proteins ; see figure 1 ) [ 8 - 10 ] . until then , limbic encephalitis was almost always recognised as paraneoplastic ( that is , associated with a t cell - mediated immune response to a tumour ) and with a poor response to treatments . the vgkc antibody - associated central nervous system ( cns ) phenotypes are now recognised widely , are usually nonparaneoplastic and include patients with some form of epilepsy   or morvan syndrome . however , despite colocalisation with antibodies to different kv1 subtypes on brain tissue and in transfected hela cells expressing different kv1 subtypes , the specificity of the antibodies is not altogether clear ; a new finding is that many of the antibodies bind not to the kv1s themselves but to other juxtaparanodal proteins , such as contactin - associated protein 2 ( caspr2 ) , that form part of the vgkc complex after extraction from brain tissue . antibodies to caspr2 are particularly frequent in patients with morvan syndrome ( s irani , s alexander , a vincent , unpublished data ) . ( a ) above , magnetic resonance imaging evidence of hippocampal inflammation and swelling ( arrows ) in a patient with voltage - gated potassium channel ( vgkc ) antibodies . below , graph of dramatic decrease in the antibodies after successful treatment in another patient . ( b ) in the images above , antibodies ( red anti - human igg ) bind to human embryonic kidney cells expressing n - methyl - d - aspartate receptors ( nmdars ) ( coexpressed with enhanced green fluorescent protein [ egfp ] ; some cells express egfp without detectable nmdars ) in a young girl who developed an encephalopathy with mutism and catatonia and made a complete recovery after immunotherapies ( see   and the video available online ) . in the image below , nmdar antibodies ( green ) bind to unpermeabilised hippocampal neurons in culture , which were then fixed , permeabilised and stained for synaptophysin ( blue ) and nr1 ( red ) ( the binding of antibodies to the live cell surface does not colocalise well with the intracellular nr1 which appears throughout the neuron ) . ( c ) in the images above , antibodies to aquaporin-4 ( aqp4 ) in a patient with neuromyelitis optica ( nmo ) bind to cells transfected with aqp4-egfp . in this case , the directly tagged aqp4-egfp can also be solubilised from the cells and , as seen in the graph , the antibodies can be measured quantitatively by counting the fluorescence in the immunoprecipitates . in these assays , images courtesy of s irani , l zuliani , mi leite , p waters and b lang . some patients presenting with symptoms of cognitive problems , psychiatric disturbance or epilepsy were found to have antibodies that bound to the proximal dendrites of the hippocampal neuropil , distinct from the binding of vgkc antibodies more distally ; many of these antibodies were subsequently shown to be directed against n - methyl - d - aspartate receptors ( nmdars ) ( nr1/nr2b ) , with nr1 as the main target . most of these patients progressed to a more complex phenotype with movement disorders or catatonia , mutism , sleep disturbance and autonomic dysfunction . at first , the syndrome was associated with ovarian teratomas in young women , but in these cases , unlike the traditional paraneoplastic disorders , the conditions improved when the tumour was removed and immunotherapies given . now many nonparaneoplastic cases are being identified and the phenotype is widening to include both male and female adults , teenagers , and even young children   ( s irani , a vincent , unpublished data ) . these nmdar antibodies may be different from those measured by binding to linear peptide sequences of nr2a / nr2b seen in neuropsychiatric patients   and have the potential to be pathogenic since they target extracellular domains on nr1/nr2b transfected human embryonic kidney cells and substantially reduce the expression of these subunits in primary cultures of hippocampal neurons . the nmdar antibodies were most easily detected in the cerebrospinal fluid ( csf ) ( at 1:10 ) compared with serum ( at 1:400 ) , and there is substantial intrathecal synthesis of the specific antibody   ( table 2 ) ; nevertheless , in absolute terms , serum levels are higher than csf levels . recently , antibodies to ampar ( -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor ) glur1/glur2 were identified in another form of limbic encephalitis that was mostly cancer - related . this is calculated as ( cerebrospinal fluid [ csf ] titre of specific antibody / concentration of csf igg ) / ( serum titre of specific antibody / concentration of serum igg ) . values above 1.5 are usually considered indicative of synthesis within the csf compartment rather than the result of passive leakage . it needs to be appreciated that the normal serum igg concentration is about 400 times higher than the normal csf igg concentration . therefore , even in the presence of substantial intrathecal synthesis , the serum concentration of specific antibody will be higher than the csf concentration . ab , antibody ; ampar , -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor ; aqp4 , aquaporin-4 ; cns , central nervous system ; eaat1 , excitatory amino acid transporter 2 ; glur , glutamate receptor ; glyr , glycine receptor ; nmdar , n - methyl - d - aspartate receptor ; vgkc , voltage - gated potassium channel . meanwhile , a completely different condition was found to be associated with antibodies to a water channel . neuromyelitis optica ( nmo , or devic disease ) has usually been considered to be part of the spectrum of inflammatory demyelinating disorders , of which multiple sclerosis is the best known . however , nmo is a distinct inflammatory condition of the optic nerves which involves severe visual failure and inflammation of the spinal cord causing longitudinally extensive transverse myelitis ( at least three spinal cord segments with high signal on mri ) , that leads to para- or tetraparesis , sensory deficits and bladder disturbances . patients show variable recovery with immunomodulatory treatments but accumulate disability over time , and mortality is high if the disease is not appropriately treated . in 2004 , antibodies binding around small vessels , under the pia and in virchow - robin spaces were defined by immunofluoresence , and the target was subsequently identified as aquaporin-4 ( aqp4 ) , the only water channel expressed strongly in the brain ( and also in kidney and stomach ) . antibodies to aqp4 bind to the astrocyte endfeet that abut cns blood vessels and are thought to be important contributors to the integrity of the blood - brain barrier . the antibodies lead to substantial loss of surface aqp4 by internalisation and activate complement with formation of the membrane attack complex , leading to cellular damage . they also reduce astrocyte expression of excitatory amino acid transporter 2 ( eaat2 ) with reduced reuptake of glutamate   and hence potential excitotoxic damage . whether these changes alone lead to the substantial inflammatory infiltrates , areas of demyelination , loss of aqp4 and sometimes necrosis that are found in lesions   is not yet clear , but increases in antibody levels are associated with clinical relapses , and aqp4 antibodies decrease in parallel with clinical improvement after immunosuppression ; overall , there seems little doubt that the antibodies contribute to the pathology . finally , another receptor target is emerging in patients with rare spinal and brain stem syndromes . in one adult male who presented with excessive startle and progressive encephalomyelitis with rigidity and myoclonus ( perm ) , a form of stiff person syndrome , antibodies to glycine receptor alpha 1 pentamers ( glyr1s ) were identified . these antibodies are now being found in other patients with related disorders ( a vincent , i leite , h - m meinck , unpublished data ) . once defined , the antibodies are best identified by binding to native proteins extracted from mammalian tissue in mild detergents ( vgkcs ) , or better still to the native protein expressed in an appropriate human cell line ( nmdars , ampars , aqp4 , glyr ) , rather than to short peptides that do not represent the native conformation of the target antigen . the protein must be expressed on the cell surface and the cells should be unpermeabilised so that only cell surface - binding antibodies are detected ( this ensures that they are potentially pathogenic , in contrast to those antibodies to intracellular components found in paraneoplastic disorders ) . clustering of the antigen by use of intracellular scaffolding proteins can increase sensitivity and specificity as recently demonstrated for achr antibodies . in addition , the antibodies should be shown to bind to the extracellular surface of neurons or astrocytes cultured from mammalian tissues and to induce relevant biological changes in such cultures . in the future , one hopes that these studies will extend to examining the effects of these recently discovered antibodies on neuronal activity in brain slices in vitro and in animal models in vivo . considering the diversity of ion channels and receptors in the nervous system , it would be strange if there were no other autoimmune channelopathies to be discovered , diagnosed and treated . until now , most of the target channels have been identified by a candidate approach , but if the target for binding to the cultured cells is sufficiently abundant , as appears to be the case for ampars , it is possible to immunoprecipitate the target using the relatively pure csf igg from the patients ; this technique has potential for identifying new targets in the future . even the total patient plasma igg can be used to identify antigens by this approach when a suitable cell preparation or cell line is identified . in each of these diseases , csf antibodies are found , and there is often evidence of high concentrations of csf - specific antibody relative to csf igg concentration when compared with similar measurements in serum (  intrathecal synthesis , table 2 ) , but the absolute concentration of antibody is still higher in serum than in csf . a major question , therefore , is whether the antibodies that are pathogenic come directly from the blood into the cns parenchyma via a  leaky or damaged blood - brain barrier or whether the disorders require the presence of specific antibodies in the csf . the latter could be the result of passive diffusion across the choroid plexus and/or intrathecal synthesis by b cells that have gained entry to the cns and synthesise the antibodies in the intrathecal compartment . these considerations are not purely academic . does intrathecal synthesis decrease with current systemic treatments and increase if the patient relapses ? do immune responses ever begin in the cns and remain undetectable in the serum ? and importantly , should drugs and therapies be specifically targeted to the csf compartment rather than to the systemic immune system ? these are just some of the questions that arise from the identification of these new autoimmune disorders , and the answers will likely come from both focused human studies and animal models .