Association of the paternally transmitted copy of common Valine allele of the Val66Met polymorphism of the brain-derived neurotrophic factor (BDNF) gene with susceptibility to ADHD

Attention deficit hyperactivity disorder (ADHD) is a common, highly heritable, neurodevelopmental disorder with onset in early childhood. Genes involved in neuronal development and growth are, thus, important etiological candidates and brain-derived neurotrophic factor (BDNF), has been hypothesized to play a role in the pathogenesis of ADHD. BDNF is a member of the neurotrophin family and is involved in the survival and differentiation of dopaminergic neurons in the developing brain (of relevance because drugs that block the dopamine transporter can be effective therapeutically). The common Val66Met functional polymorphism in the human BDNF gene (rs 6265) was genotyped in a collaborative family-based sample of 341 white UK or Irish ADHD probands and their parents. We found evidence for preferential transmission of the valine (G) allele of BDNF (odds ratio, OR=1.6, P=0.02) with a strong paternal effect (paternal transmissions: OR=3.2, P=0.0005; maternal transmissions: OR=1.00; P=1.00). Our findings support the hypothesis that BDNF is involved in the pathogenesis of ADHD. The transmission difference between parents raises the possibility that an epigenetic process may be involved.

Attention deficit hyperactivity disorder with reading disabilities: preliminary genetic findings on the involvement of the ADRA2A gene

BACKGROUND

Attention deficit/hyperactivity disorder (ADHD) and reading disability (RD) tend to co-occur and quantitative genetic studies have shown this to arise primarily through shared genetic influences. However, molecular genetic studies have shown different genes to be associated with each of these conditions. Neurobiological studies have implicated noradrenergic function in the aetiology of ADHD that is comorbid with RD. This paper examines the neurobiological evidence and presents preliminary testing of the hypothesis that the ADRA2A receptor gene is contributing to ADHD and comorbid RD.

METHODS

One hundred and fifty-two children (140 boys, 12 girls) of British Caucasian origin, aged between 6 and 13 years and with a diagnosis of ADHD, were recruited. The children's reading ability was tested. Children were identified as having ADHD or ADHD plus RD (n=82). DNA was available for 110 parent child trios and 42 parent child duos. Genotyping was undertaken for an ADRA2A polymorphism.

RESULTS

For those with ADHD plus RD there was evidence of association with the alpha 2A adrenergic receptor (ADRA2A) polymorphism with the G allele being preferentially transmitted.

CONCLUSIONS

The preliminary evidence together with other neurobiological research findings suggests that the ADRA2A gene may contribute to comorbid ADHD and RD and needs to be properly examined.

A family based study of catechol-O-methyltransferase (COMT) and attention deficit hyperactivity disorder (ADHD)

Neurobiological studies have suggested that altered dopaminergic function may contribute to the etiology of attention deficit hyperactivity disorder (ADHD). The gene encoding catechol-O-methyltransferase (COMT) is an attractive candidate for ADHD susceptibility as it plays a major role in the degradation of dopamine. Moreover, a functional Val158Met polymorphism in COMT that alters the activity of the encoded protein has been strongly implicated in frontal lobe function, with the high activity Valine allele being associated with poorer performance, and ADHD is thought to involve fronto-striatal pathways. We have examined this functional variant for association with ADHD in a family based association sample comprising 279 probands and their parents. We have also examined two other markers in the COMT gene (rs737865, rs165599) which, together with the Val/Met variant, have recently been shown to be associated with altered COMT expression rather than enzyme activity. No evidence for association was observed with any single marker or haplotype in a sample of 279 affected children and their parents.

Direct analysis of the genes encoding G proteins G alpha T2, G alpha o, G alpha Z in ADHD

We have followed up the extensive replicated evidence that the dopamine DRD4 receptor is involved in the aetiology of ADHD by undertaking direct analysis of genes encoding other proteins in this effector system. We prioritised the genes encoding G protein alpha subunits G alpha(T2), G alpha(o), G alpha(Z) as these have been shown to transduce the effects of ligand binding at DRD4. We screened the exons of all three genes for sequence variation in 28 unrelated subjects with ADHD and identified 13 novel polymorphisms. All were tested for possible association with ADHD using a combination of pooled and individual genotyping. The results of our study do not suggest that polymorphisms in these genes contribute to susceptibility to ADHD.

Follow-up of genetic linkage findings on chromosome 16p13: evidence of association of N-methyl-D aspartate glutamate receptor 2A gene polymorphism with ADHD

Attention deficit hyperactivity disorder (ADHD) is a childhood onset disorder, for which there is good evidence that genetic factors contribute to the aetiology. Recently reported linkage findings suggested evidence of a susceptibility locus on chromosome 16p13 (maximum LOD score of 4.2, P=5 x 10(-6)). The GRIN2A (glutamate receptor, ionotropic, N-methyl D-aspartate 2A) gene that encodes the N-methyl D-aspartate receptor subunit 2A (NMDA2A) maps to this region of linkage. As this is also a good functional candidate gene for ADHD, we undertook family-based association analysis in a sample of 238 families. We found significant evidence of association with a GRIN2A exon 5 polymorphism (chi(2)=5.7, P=0.01). Our data suggest that genetic variation in GRIN2A may confer increased risk for ADHD and that this, at least in part, might be responsible for the linkage result on 16p reported by Smalley et al. We conclude that replication is required and that further work examining for association of GRIN2A polymorphisms with ADHD is warranted.

Evidence to suggest biased phenotypes in children with Attention Deficit Hyperactivity Disorder from completely ascertained trios

The transmission disequilibrium test (TDT) is widely used as a robust statistical method to test for genetic association due to linkage based upon analysis of parent-proband trios. The TDT and other family-based tests (eg haplotype relative risk method) are commonly used in association studies including those of ADHD because of concerns that the case-control design has a strong tendency for false positives due to poor matching between cases and controls. Unfortunately, it is not always possible to obtain DNA from both parents in studies of this design, even where the onset of disorder is in childhood, and usually the missing parent is the father. Despite the fact that methods exist for analysis where one parent is missing, many family-based studies are based on the collection or analysis of complete trios only. However this selection process might potentially introduce bias, particularly for studies of behavioural phenotypes like ADHD because the phenotype of proband or parents might influence family stability and therefore complete parental ascertainment. We set out to examine whether children with ADHD and for whom DNA samples from fathers were missing ('duos') differed phenotypically from children for whom genotype information was available from both parents ('trios'). Children from duos showed a significantly higher frequency of DMS-IV ADHD-combined type, significantly more co-morbid conduct disorder and conduct disorder symptoms, and a trend for higher total ADHD symptom scores. Excluding duos from sample collection and analysis may result in systematic bias. If comorbid conduct disorder and ADHD-combined type index increased genetic liability, exclusion of duos could further reduce the power of the TDT (and similar tests) to detect susceptibility genes for ADHD, or replicate effects detected by case-control analysis.

Serotonergic system and attention deficit hyperactivity disorder (ADHD): a potential susceptibility locus at the 5-HT(1B) receptor gene in 273 nuclear families from a multi-centre sample

Attention deficit hyperactivity disorder (ADHD) is a highly heritable and heterogeneous disorder, which usually becomes apparent during the first few years of childhood. Imbalance in dopamine neurotransmission has been suggested as a factor predisposing to ADHD. However, evidence has suggested an interaction between dopamine and serotonin systems in the pathophysiology of the disorder. Studies using selective agonists of the different 5-HT receptors microinjected into selected brain structures have shown a positive modulating effect on the functional activities of the mesotelencephalic dopaminergic system. This suggests that some of the genetic predisposition to ADHD might be due to DNA variation at serotonin system genes. In this study, we investigated polymorphisms in HTR(1B) and HTR(2A) (which encode the serotonin receptors 5-HT(1B) and 5-HT(2A) respectively) in a European ADHD sample. Using haplotype based haplotype relative risk (HHRR) and transmission disequilibrium test (TDT) analyses, we observed significant preferential transmission of the allele 861G of the HTR(1B) in the total sample (for HHRR; chi(2) = 7.4, P = 0.0065 and TDT; (chi(2) = 6.4, P = 0.014). Analysis of HTR(2A) failed to reveal evidence of association or linkage between the His452Tyr polymorphism and ADHD in the total sample. However, a significantly increased transmission of the allele 452His was observed in the Irish sample alone (chi(2) = 4.9, P = 0.026). These preliminary data suggest an important role for the serotonin system in the development of ADHD. Further studies, preferentially including different ethnic groups are required to substantiate these findings.

Geraniol, a component of plant essential oils, sensitizes human colonic cancer cells to 5-Fluorouracil treatment

Differentiation of human colonic cancer cells at confluency has been correlated to their increased resistance to chemotherapeutic agents. The aim of this study was to determine whether blocking Caco-2 cell differentiation could sensitize the cells to 5-fluorouracil (5-FU) treatment. We show that in cells at confluency, geraniol (400 microM) prevented the formation of brush-border membranes and inhibited the expression of intestinal hydrolases (sucrase, lactase, alkaline phosphatase). The antiproliferative effect of geraniol (400 microM) together with 5-FU (5 microM) was twice that of 5-FU alone. The cytotoxicity induced by 5-FU was enhanced in the presence of geraniol, as shown by a 50% increase of lactate dehydrogenase release in the culture medium. These effects are related to enhanced intracellular accumulation of 5-FU in the presence of geraniol as shown by a 2-fold increase in intracellular 5-[6-(3)H]FU (1.5 microCi/ml). It is concluded that geraniol sensitizes colonic cancer cells to 5-FU treatment, by increasing the cytotoxicity of the drug, and that this results from the facilitated transport of 5-FU and the blockade of the morphological and functional differentiation of the cancer cells.

The hypophysis controls expression of SNAP-25 and other SNAREs in the adrenal gland

SNAP-25 (Synaptosomal Associated Protein of 25 kDa), in association with two other SNARE (soluble NSF attachment protein receptor) proteins, syntaxin and Vesicle Associated Membrane Protein, VAMP, is implicated in regulated and constitutive exocytosis in neurones and neuroendocrine cells. Our previous studies have shown that it is expressed more by noradrenergic than adrenergic chromaffin cells in the rat adrenal gland. Since certain hormones under hypophyseal control play an essential role in determining chromaffin cell phenotype, the present study examined the effect of hypophysectomy on SNAP-25 expression. Hypophysectomy was found by immunoblotting and RT-PCR analysis to increase adrenal gland SNAP-25, syntaxin-1 and VAMP-2 levels, without modifying the relative expression of SNAP-25 isoforms: immunocytochemistry showed a dramatic increase in SNAP-25 expression in former adrenergic chromaffin cells. Since adrenal glucocorticoids are considerably reduced by hypophysectomy, the effect of corticosterone replacement therapy was investigated. This did not change levels of SNAP-25, syntaxin-1 or VAMP-2. SNARE expression was also unmodified in pheochromocytoma cells treated with a synthetic glucocorticoid. In contrast, subcutaneous injection of hypophysectomized rats with thyroid hormone decreased adrenal SNAP-25, demonstrating the potential importance of the pituitary-thyroid axis. The current data thus demonstrate that the hypophysis exerts an inhibitory control on adrenal gland SNARE proteins. They suggest that glucocorticoids are unlikely to be directly responsible for this but provide evidence that thyroid hormones are implicated in this phenomenon. The putative role of hormonal regulation on SNARE function is discussed.

Regularization in a neural model of motion perception

Neurons in sensory systems encode and transmit information about attributes of the environment. Much of the information transmitted by spiking neurons appears to be encoded in the rate at which they fire. This rate necessarily has a positive value. In this paper, the implication of this constraint for models of motion detection is examined. The detection of image motion is represented mathematically as a quadratic programming problem in which variables used to represent image speed are restricted to positive values. This novel representation requires that additional constraints are introduced to stabilize motion computations because quadratic programming problems require a surplus of unknowns to code for image speed. Two further constraints are introduced into the model to take into account possible cases of image degeneracy. They are based upon (i) an a priori preference for small image speeds, and (ii) the assumption that image motion parallel to contours of constant intensity for a one-dimensional signal is zero. The latter assumption is shown to account for perceived biases in speed reported for type I plaid patterns [Castet, E. & Morgan, M. (1996). Apparent speed of type-I symmetrical plaids. Vision Research 36, 223-32]. The model suggests that the visual system uses separate constraints to stabilize motion computations. One set of constraints arises from the nature of the motion detection process itself, while another two constraints take into account possible cases of degeneracy where image contrast is low or near zero and where the image function is one-dimensional and the aperture problem prevails.

SNAREs during development

Soluble N-ethylmaleimide-sensitive factor attached protein receptor (SNARE) molecules are implicated in many fundamental cellular processes that require membrane fusion, and the interactions of the SNARE proteins, SNAP-25, syntaxin and VAMP/synaptobrevin, have been extensively studied. This review documents recent data on their role at different stages of development. SNARE proteins are expressed very early and play important roles in fertilization and in cell division during early embryogenesis. In the developing nervous system, they are important for neurite outgrowth and transformation of the growth cone into the mature synapse. In the neuroendocrine system, in addition to neurosecretion, they are involved in processes related to morphological plasticity. Although few data exist on regulation of SNARE proteins during development, growth factors, intracellular messengers and depolarization are known to modify their cellular expression. The putative importance of these factors during development is discussed.

Adrenal gland SNAP-25 expression is altered in thyroid hormone receptor knock-out mice

SNAP-25 is a protein in neurons and neuroendocrine cells, which is involved, together with syntaxin and VAMP, in neurotransmitter release and neurite outgrowth. Since the thyroid hormone receptors TR alpha and TR beta are essential for nervous system development, their possible role in regulating the expression of these vesicle trafficking proteins was examined by analysing SNAP-25 levels in TR alpha and TR beta knock-out mice. Immunoblotting and RT-PCR showed that SNAP-25 levels are increased in the adrenal gland, but not in cerebellum, in knock-out mice, while syntaxin-1 and VAMP-2 are unaffected in either tissue. Treatment of the pheochromocytoma-derived cell line PC12 with the thyroid hormone L-3,5,3'-triiodothyronine (T3) decreased SNAP-25 expression. Together, these data suggest that thyroid hormones exert a negative regulatory effect on SNAP-25 in adrenal medullary neuroendocrine cells.

NGF enhances depolarization effects on SNAP-25 expression: induction of SNAP-25b isoform

The 25 kDa synaptosomal associated protein (SNAP-25), which is implicated in neuronal plasticity and neurosecretion, exists as two isoforms generated by alternative splicing of exons 5a and 5b. The aim of the present study was to characterize factors influencing isoform expression. We report that chronic depolarization of PC12 cells alone or in the presence of NGF induces the expression of isoform-b, in addition to a 1.8- to 3-fold increase in SNAP-25 mRNA and protein as determined by immunoblotting and combined RT-PCR and Southern blot analysis. When cerebellar granule neurons were cultured in elevated K+, the predominant isoform switched from SNAP-25a to SNAP-25b. Taken together these results suggested that chronic depolarization regulates the transcription and processing of SNAP-25 mRNA.

Phospholipid composition in late infantile neuronal ceroid lipofuscinosis

BACKGROUND

Neuronal ceroid lipofuscinosis (NCL) is a relatively common group of inherited neurodegenerative disorders characterised by the accumulation of autofluorescent lipopigments (ceroid) similar to lipofuscin. Because of this property, studies have concentrated on fatty acid metabolism and lipid peroxidation.

METHODS

In the present study, the fatty acid composition of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) and the molecular species compositions of diacylglycerophosphocholine (diacyl GPC), diacylglycerophosphoethanolamine (diacyl GPE) and alkenylacyl GPE (plasmalogens) were investigated in cultured skin fibroblasts from three patients with a confirmed diagnosis of the late infantile form of the disease (LINCL, CLN2) and three healthy age-matched controls.

RESULTS

Relatively minor differences in the fatty acid compositions of PC and PE were observed between patients and controls. However, dimethyl acetals of plasmalogens were found to be 40% higher in the patients compared to in the controls. Control and LINCL fibroblasts displayed only slight differences in the molecular compositions of diacyl GPE and diacyl GPC. In contrast, compared with normal cells, LINCL fibroblasts had higher levels of alkenylacyl GPE species containing both 18 : 1 and polyunsaturated fatty acids, but lower levels of species with 16 : 0 or 18 : 0 in the sn-1 position.

CONCLUSION

The molecular composition of PC and PE subclasses in skin fibroblasts of healthy subjects and patients suffering from LINCL is here described for the first time. While few differences are noticeable in the fatty acid composition of PC and PE and the molecular species distribution of diacylGPC and diacylGPE, the alkenylacyl GPE (or ethanolamine plasmalogens) were found to differ significantly between patients and healthy controls.

A model of motion adaptation and motion after-effects based upon principal component regression

A computational model to help explain effects of adaptation to moving signals is compared with established energy (linear regression) models of motion detection. The proposed model assumes that processed image signals are subject to error in both dimensions of space and time. This assumption constrains models of motion perception to be based upon principal component regression rather than linear regression. It is shown that response suppression of model complex cell neurons that input into the model may account for (1) increases in perceived speed after adaptation to static patterns and testing with slowly moving patterns, (2) significant increases in perceived speed after adaptation to patterns moving at a medium speed and testing at high speed, and (3) decreases in perceived speed in the opponent direction to a quickly moving adapting signal. Neither of predictions (2) or (3) are general features of established accounts of motion detection by visual processes based upon linear regression. Comparisons of the proposed model's speed transfer function with existing psychophysical data suggests that the visual system processes motion signals with the tacit assumption that image measurements are subject to error in both space and time.

7beta-hydroxysterol is cytotoxic to neonatal rat astrocytes in primary culture when cAMP levels are increased

We have shown previously that 7beta-hydroxycholesterol (7betaOHCH) and 7beta-hydroxycholesteryl-3-oleate (7betaOHCH- 3-OL) are potent inhibitors of lesion-induced astrogliosis in the rat cortex or spinal cord; these substances reduce reactive astrocyte proliferation and hypertrophy. In this study, we employed cultured newborn rat astrocytes with increased cAMP levels as an in vitro model of reactive astrocytes. Treatment with either dibutyryl-cAMP (dbcAMP) or isoproterenol resulted in morphologic differentiation of astrocytes which became fibrous. Concomitant incubation with 30 microM 7betaOHCH and dbcAMP (or isoproterenol) provoked the cells to retract and was cytotoxic. When the beta-adrenergic receptor-mediated cAMP increase was abolished by propranolol, the 7betaOHCH cytotoxicity was inhibited. Immunocytochemical labelling for glial fibrillary acidic protein (GFAP) and beta-tubulin and electron microscopy suggested that intermediate filament and microtubular organizations were modified by 7betaOHCH. Analysis of the activity of cAMP-dependent protein kinase (PKA) in astrocytes treated with dbcAMP and 7betaOHCH showed a rapid and marked inhibition of the phosphotransferase activity which lasted for 24 hr. We suggest that this culture system provides an experimental system to study the molecular mechanisms involved in the effect of oxysterols on astrocytic hypertrophy. The cytotoxicity of 7betaOHCH seems to be mediated by inhibition of PKA, which phosphorylates intermediate filaments and the transcription factor cyclic AMP responsive element binding.

SNAP-25 regulation during adrenal gland development: comparison with differentiation markers and other SNAREs

Synaptosomal-associated protein of 25 kDa (SNAP-25) is one of a limited number of soluble N-ethylmaleimide-sensitive fusion attachment protein receptors (SNAREs) that play a major role in membrane docking of synaptic vesicles and secretory granules during regulated exocytosis. We have previously shown that SNAP-25 levels differ between noradrenergic and adrenergic chromaffin cell populations of the adult adrenal gland. We examine SNAP-25 expression by immunofluoresence in cells of the sympathoadrenal lineage in the rat during late embryonic and postnatal development. In parallel, tyrosine hydroxylase was used to identify sympathoadrenal cells, phenylethanolamine N-methyltransferase to distinguish adrenergic from noradrenergic chromaffin cells, and chromogranin A to define the presence of secretory granules. In addition, SNAP-25 protein and mRNA levels were followed in adrenal gland extracts by immunoblotting and reverse transcription-polymerase chain reaction (RT-PCR). Protein levels were compared with those of other molecules also implicated in organelle trafficking, including syntaxin 1 and vesicle-associated membrane protein (VAMP-2) and the nonneuronal analogues SNAP-23 and cellubrevin. This study provides evidence that SNAP-25 is expressed early during development in sympathoadrenal neurons and migrating cells. It is detected in intra-adrenal chromoblasts as soon as they enter the adrenal primordium. Its differential expression between catecholamine chromaffin cell phenotypes is already evident from the 17th embryonic day, future noradrenergic cells appearing to express higher levels than adrenergic cells. The granule maturation marker chromogranin A is expressed in chromaffin cells later than SNAP-25. Both SNAP-25 protein and mRNA increased rapidly in the adrenal gland in the perinatal period to peak during the first postnatal week, after which levels dropped dramatically to adult values. In contrast, levels of both syntaxin and SNAP-23 appeared to remain fairly constant throughout adrenal gland development. VAMP-2 expression increased gradually around birth to reach maximal levels during the first two postnatal weeks, and then decreased slightly. Cellubrevin levels also appeared to increase gradually until adult values were attained by the end of the second postnatal week. The threefold increase of SNAP-25 mRNA shortly after birth compared to the low adult levels suggests that during this period SNAP-25 is implicated in additional functions than regulated secretion, possibly associated with cellular growth or maturation.

Recursive implementations of temporal filters for image motion computation

Efficient algorithms for image motion computation are important for computer vision applications and the modelling of biological vision systems. Intensity-based image motion computation proceeds in two stages: the convolution of linear spatiotemporal filter kernels with the image sequence, followed by the non-linear combination of the filter outputs. If the spatiotemporal extent of the filter kernels is large, then the convolution stage can be very intensive computationally. One effective means of reducing the storage required and computation involved in implementing the temporal convolutions is the introduction of recursive filtering. Non-recursive methods require the number of frames of the image sequence stored at any given time to be equal to the temporal extent of the slowest temporal filter. In contrast, recursive methods encode recent stimulus history implicitly in the values of a small number of variables updated through a series of feedback equations. Recursive filtering reduces the number of values stored in memory during convolution and the number of mathematical operations involved in computing the filters' outputs. This paper extends previous recursive implementations of gradient- and correlation-based motion analysis algorithms [Fleet DJ, Langley K (1995) IEEE PAMI 17: 61-67; Clifford CWG, Ibbotson MR, Langley K (1997) Vis Neurosci 14: 741-749], describing a recursive implementation of causal band-pass temporal filters suitable for use in energy- and phase-based algorithms for image motion computation. It is shown that the filters' temporal frequency tuning curves fit psychophysical estimates of the temporal properties of human visual filters.

Molecular markers of sympathoadrenal cells

Cells constituting the sympathoadrenal (SA) cell lineage originate from the neural crest and acquire a catecholaminergic fate following migration to the dorsal aorta. Subsequently, SA cells migrate to sites widely dispersed throughout the body. In addition to endocrine chromaffin and "small intensely fluorescent" cells in adrenal glands and in extra-adrenal tissues such as the paraganglia, this lineage also includes neurones located in sympathetic ganglia and in the adrenal gland. It is widely assumed that these cells are all derived from the same precursors, which then differentiate along divergent pathways in response to different external stimuli. During embryonic differentiation, SA cells lose some of their early traits and acquire other distinguishing features. To help understand how the lineage diverges in terms of phenotype and function, this article examines the cellular expression of a variety of "marker" proteins that characterize the individuals of the lineage. In particular, differences between adrenal medullary adrenergic and noradrenergic chromaffin cells in the expression of proteins, such as the neural adhesion molecule L1, the growth-associated protein GAP-43 and molecules involved in the secretory process, are emphasized. Factors that might differentially regulate such molecular markers in these cells are discussed.

Physiological aspects of exocytosis in chromaffin cells of the adrenal medulla

The adrenal medulla is composed principally of groups of adrenergic and noradrenergic chromaffin cells, with minor populations of small intensely fluorescent cells and ganglionic neurones. Different molecular stimuli evoke distinct secretory events in the gland, involving the release of either adrenaline or noradrenaline together with various neuroactive peptides. The nature of the secretory response can be controlled at a central level or regulated locally within the gland. Specific innervation patterns to the different types of chromaffin cell have been implicated in central regulatory mechanisms, while several explanations for regulating secretion locally have been proposed. The differential distribution of various types of receptors between cell phenotypes, such as muscarinic or nicotinic acetylcholine receptors, histamine receptors, angiotensin receptors and different classes of opiate receptors between the two principal chromaffin cell populations could be involved in local control. In addition exocytosis parameters could be modulated differently in adrenergic and noradrenergic cells by phenotype-specific mechanisms, possibly involving molecules like Growth Associated Protein-43, Synaptosomal Associated Protein-25 isoforms or the p11 annexin subunit. The distribution of the various types of calcium channels is also known to vary between chromaffin cell subtypes. This short review examines possible ways in which specific innervation patterns in the adrenal gland could be programmed and discusses exocytosis mechanisms that could differ between chromaffin cell phenotypes. Data reviewed here suggest that the adrenal medulla should no longer be viewed as a homogeneous entity but as consisting of an ensemble of individual cell subpopulations each with a distinct secretory response that could in part reflect its local history.

Cultured glial cells express the SNAP-25 analogue SNAP-23

Astrocytes release glutamate and aspartate in response to elevated intracellular calcium levels, and it has been proposed that this occurs by a vesicular release mechanism, in which SNARE proteins are implicated. Although syntaxin, synaptobrevin, and cellubrevin have been shown to be expressed by cultured astrocytes, SNAP-25 has not been detected. By using immunocytochemical, immunoblotting, and polymerase chain reaction techniques, the present study demonstrates that SNAP-23, an analogue of SNAP-25, is expressed by astrocytes both in culture and in rat cerebellum. These findings provide additional evidence that astrocytes release excitatory amino acids by a vesicular mechanism involving SNARE proteins. SNAP-23 and also syntaxin 1 and cellubrevin were found to be expressed in glial precursor cells, oligodendrocytes, and microglia. These data suggest that the t-SNAREs SNAP-23 and syntaxin 1 and the v-SNARE cellubrevin participate in general membrane insertion mechanisms involved in diverse glial cell functions such as secretion, phagocytosis, and myelinogenesis.

Stereopsis from contrast envelopes

We report two experiments concerning the site of the principal nonlinearity in second-order stereopsis. The first exploits the asymmetry in perceiving transparency with second-order stimuli found by Langley et al. (1998) (Proceedings of the Royal Society of London B, 265, 1837-1845) i.e. the product of a positive-valued contrast envelope and a mean-zero carrier grating can be seen transparently only when the disparities are consistent with the envelope appearing in front of the carrier. We measured the energy at the envelope frequencies that must be added in order to negate this asymmetry. We report that this amplitude can be predicted from the envelope sidebands and not from the magnitude of compressive pre-cortical nonlinearities measured by other researchers. In the second experiment, contrast threshold elevations were measured for the discrimination of envelope disparities following adaptation to sinusoidal gratings. It is reported that perception of the envelope's depth was affected most when the adapting grating was similar (in orientation and frequency) to the carrier, rather than to the contrast envelope. These results suggest that the principal nonlinearity in second-order stereopsis is cortical, occurring after orientation- and frequency-selective linear filtering.

Clustering of activating mutations in c-KIT's juxtamembrane coding region in canine mast cell neoplasms

The proto-oncogene c-KIT encodes a growth factor receptor, KIT, with ligand-dependent tyrosine kinase activity that is expressed by several cell types including mast cells. c-KIT juxtamembrane coding region mutations causing constitutive activation of KIT are capable of transforming cell lines and have been identified in a human mast cell line and in situ in human gastrointestinal stromal tumors, but have not been demonstrated in situ in neoplastic mast cells from any species. To determine whether c-KIT juxtamembrane mutations occur in the development of mast cell neoplasms, we examined canine mastocytomas, which are among the most common tumors of dogs and which often behave in a malignant fashion, unlike human solitary mastocytomas. Sequencing of c-KIT cDNA generated from tumor tissues removed from seven dogs revealed that three of the tumors contained a total of four mutations in an intracellular juxtamembrane coding region that is completely conserved among vertebrates. In addition, two mutations were found in three mast cell lines derived from two additional dogs. One mutation from one line matched that found in situ in one of the tumors. The second was found in two lines derived from one dog at different times, indicating that the mutation was present in situ in the animal. All five mutations cause high spontaneous tyrosine phosphorylation of KIT. Our study provides in situ evidence that activating c-KIT juxtamembrane mutations are present in, and may therefore contribute to, the pathogenesis of mast cell neoplasia. Our data also suggest an inhibitory role for the KIT juxtamembrane region in controlling the receptor kinase activity.