The Three-Dimensional Morphology of Growing Dendrites.

The processes controlling the morphology of dendrites have been of great interest to a wide range of communities, since they are examples of an out-of-equilibrium pattern forming system, there is a clear connection with battery failure processes, and their morphology sets the properties of many metallic alloys. We determine the three-dimensional morphology of free growing metallic dendrites using a novel X-ray tomographic technique that improves the temporal resolution by more than an order of magnitude compared to conventional techniques. These measurements show that the growth morphology of metallic dendrites is surprisingly different from that seen in model systems, the morphology is not self-similar with distance back from the tip, and that this morphology can have an unexpectedly strong influence on solute segregation in castings. These experiments also provide benchmark data that can be used to validate simulations of free dendritic growth.

APOE genotype is a risk factor for neuropathy severity in diabetic patients.

This cross-sectional study tested the hypothesis that APOE genotype is a risk factor for diabetic neuropathy severity. A model with age, duration of diabetes, and APOE genotype was found to predict (p = 0.0083) severity on the Neuropathy Impairment Score in the Lower Limbs (NISLL). Considering genotype alone, patients with APOE 3/4 and 4/4 genotypes had 3 more NISLL points than patients with other genotypes. This impact on severity is equivalent to having 15 extra years of age or diabetes duration.

Internal carotid artery dissection in stroke from SCUBA diving: a case report.

Although diving with compressed air is generally safe, neurological problems resulting from infarction in SCUBA diving are well known, including arterial gas embolism and decompression sickness (caisson's disease, bends) involving the brain and spinal cord. While air gas embolism forms the overwhelming majority of causes for stroke in divers, internal carotid artery (ICA) dissection is another potential mechanism for central nervous system infarction in the setting of SCUBA diving. A 38 year-old female, who presented with complaints of headache, nausea, vomiting, and left sided hemiparesis after rapid ascent to the surface from a depth of 120 feet of seawater was initially treated for decompression illness in a hyperbaric chamber. Further neurological workup revealed a right ICA dissection. This case demonstrates the dangers of ICA dissection following rapid ascent to the surface from underwater and emphasizes an interesting presentation of stroke associated with SCUBA diving.

Cellular actions of topiramate: blockade of kainate-evoked inward currents in cultured hippocampal neurons.

PURPOSE: This study was undertaken to evaluate the effects of topiramate (TPM) on excitatory amino acid-evoked currents. METHODS: Kainate and N-methyl-D-aspartate (NMDA) were applied to cultured rat hippocampal neurons by using a concentration-clamp apparatus to selectively activate the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid)/kainate and NMDA receptor subtypes, respectively. The evoked membrane currents were recorded by using perforated-patch whole-cell voltage-clamp techniques. RESULTS: TPM partially blocked kainate-evoked currents with an early-onset reversible phase (phase I) and a late-onset phase (phase II) that occurred after a 10- to 20-min delay and did not reverse during a 2-h washout period. Application of dibutyryl cyclic adenosine monophosphate (cAMP; 2 mM) during washout after phase II block enhanced reversal, with the kainate current amplitude being restored by approximately 50%. Phase II but not phase I block was prevented by prior application of okadaic acid (1 microM), a broad-spectrum phosphatase inhibitor, suggesting that phase II block may be mediated through interactions with intracellular intermediaries that alter the phosphorylation state of kainate-activated channels. Topiramate at 100 microM blocked kainate-evoked currents by 90% during phase II, but had no effect on NMDA-evoked currents. The median inhibitory concentration (IC50) values for phase I and II block of kainate currents were 1.6 and 4.8 microM, respectively, which are within the range of free serum levels of TPM in patients. CONCLUSIONS: The specific blockade of the kainate-induced excitatory conductance is consistent with the ability of TPM to reduce neuronal excitability and could contribute to the anticonvulsant efficacy of this drug.

Regionally selective blockade of GABAergic inhibition by zinc in the thalamocortical system: functional significance.

The thalamocortical (TC) system is a tightly coupled synaptic circuit in which GABAergic inhibition originating from the nucleus reticularis thalami (NRT) serves to synchronize oscillatory TC rhythmic behavior. Zinc is colocalized within nerve terminals throughout the TC system with dense staining for zinc observed in NRT, neocortex, and thalamus. Whole cell voltage-clamp recordings of GABA-evoked responses were conducted in neurons isolated from ventrobasal thalamus, NRT, and somatosensory cortex to investigate modulation of the GABA-mediated chloride conductance by zinc. Zinc blocked GABA responses in a regionally specific, noncompetitive manner within the TC system. The regional levels of GABA blockade efficacy by zinc were: thalamus > NRT > cortex. The relationship between clonazepam and zinc sensitivity of GABA(A)-mediated responses was examined to investigate possible presence or absence of specific GABA(A) receptor (GABAR) subunits. These properties of GABARs have been hypothesized previously to be dependent on presence or absence of the gamma2 subunit and seem to display an inverse relationship. In cross-correlation plots, thalamic and NRT neurons did not show a statistically significant relationship between clonazepam and zinc sensitivity; however, a statistically significant correlation was observed in cortical neurons. Spontaneous epileptic TC oscillations can be induced in vitro by perfusion of TC slices with an extracellular medium containing no added Mg(2+). Multiple varieties of oscillations are generated, including simple TC burst complexes (sTBCs), which resemble spike-wave discharge activity. A second variant was termed a complex TC burst complex (cTBC), which resembled generalized tonic clonic seizure activity. sTBCs were exacerbated by zinc, whereas cTBCs were blocked completely by zinc. This supported the concept that zinc release may modulate TC rhythms in vivo. Zinc interacts with a variety of ionic conductances, including GABAR currents, N-methyl-D-aspartate (NMDA) receptor currents, and transient potassium (A) currents. D-2-amino-5-phosphonovaleric acid and 4-aminopyridine blocked both s- and cTBCs in TC slices. Therefore NMDA and A current-blocking effects of zinc are insufficient to explain differential zinc sensitivity of these rhythms. This supports a significant role of zinc-induced GABAR modulation in differential TC rhythm effects. Zinc is localized in high levels within the TC system and appears to be released during TC activity. Furthermore application of exogenous zinc modulates TC rhythms and differentially blocks GABARs within the TC system. These data are consistent with the hypothesis that endogenously released zinc may have important neuromodulatory actions impacting generation of TC rhythms, mediated at least in part by effects on GABARs.

GABA(A) receptor function in epileptic human dentate granule cells: comparison to epileptic and control rat.

Using patch clamp recording techniques in dentate granule cells (DGCs) isolated from patients undergoing temporal lobectomy for intractable epilepsy, we investigated basic properties of GABA(A) receptors (GABA(A)Rs) and pharmacological sensitivity of GABA-evoked currents to modulation by zinc and benzodiazepines (BZ). Properties of human DGC GABA(A)Rs were compared to DGC GABA(A)R properties in control and epileptic rats. Blockade of GABA evoked currents by zinc was significantly enhanced in epileptic human relative to control rat DGCs. Augmentation of the GABA(A)R current by the non-subunit selective BZ agonist, clonazepam (CNZ) and by the BZ1 specific agonist, zolpidem (ZOL), were not significantly different in human DGCs relative to control or epileptic rat. GABA potency was significantly higher in epileptic human DGCs than in control or epileptic rat DGCs. The significantly enhanced efficacy of zinc in blocking GABA currents in epileptic human DGCs mirrors that seen in epileptic rat DGCs, and was coupled with mossy fiber sprouting evident in both epileptic human and rat dentate gyrus. The aberrant mossy fibers provide a novel zinc delivery system within the epileptic dentate gyrus. The mossy fiber release of zinc onto DGCs coupled with the enhanced zinc sensitivity of GABA(A)Rs in epileptic DGCs, may lead to 'dynamic disinhibition' which could compromise inhibitory efficacy in the epileptic rat and human hippocampus.

Physiological analysis of Rasmussen's encephalitis: patch clamp recordings of altered inhibitory neurotransmitter function in resected frontal cortical tissue.

Rasmussen's encephalitis (RE) is a progressive, rare childhood disease characterized by severe epilepsy, hemiplegia, dementia, and inflammation of the brain. While one mechanism underlying the pathogenesis of RE has been hypothesized to be mediated by production of excitotoxic GluR3 autoantibodies to the AMPA receptor, other neuropathological etiologies have also been indicated. Whole-cell patch clamp recordings of GABA(A) receptor mediated responses were conducted in neurons acutely isolated from an RE patient, and compared to properties of non-focal human temporal cortical neurons. RE neurons appeared similar anatomically to control cortical neurons. Significant differences in GABAergic responses were evident between RE and control neurons. GABA was significantly more potent in RE than in control cortical neurons (EC50 of 13 microM vs 23 microM, respectively). In addition, the overall efficacy of GABA was significantly decreased in RE neurons, associated with a decrease in postsynaptic GABA current density in RE neurons (5.1 pA/microm2) in comparison to controls (9.2 pA/microm2). Augmentation of GABA responses by the benzodiazepine, clonazepam (CNZ), was significantly reduced in RE in comparison to control neurons (34% vs 99% augmentation at 100 nM). The RE-associated reduced functional efficacy and altered pharmacology of neuronal GABA(A) receptors is consistent with overall disinhibition in RE neurons, and could contribute to the generation of the severe epileptic activity evident in this disorder.

Differential epilepsy-associated alterations in postsynaptic GABA(A) receptor function in dentate granule and CA1 neurons.

Alterations in GABAergic function associated with the development of temporal lobe epilepsy (TLE) were examined with the use of patch-clamp recording techniques in dentate granule (DG) and CA1 neurons acutely isolated from control and spontaneously epileptic rats in which TLE was elicited by pilocarpine injection 3-17 wk before use. The maximal efficacy of gamma-aminobutyric acid (GABA) in activating whole cell GABA currents increased significantly in epileptic DG neurons relative to controls. This efficacy increase was due to a 78% enhancement in the functional capacitance-normalized GABA(A) receptor (GABA(A)R) current density in epileptic DG neurons. Increased DG GABA(A)R current density was not accompanied by alterations in GABA potency (EC50). However, the maximal sensitivity of DG GABA-evoked currents to blockade by zinc increased 190% in epileptic neurons. Augmentation of epileptic DG neuron GABA-evoked currents by the broad-spectrum anticonvulsant benzodiazepine clonazepam (100 nM) was enhanced 114% relative to controls, whereas augmentation by the benzodiazepine, (BZ1)-selective agonist zolpidem (100 nM) was decreased by 66%. In contrast to DG neurons, maximal efficacy of GABA in activating GABA currents decreased in epileptic CA1 neurons relative to controls, due to a 52% decrease in functional capacitance-normalized GABA(A)R current density. This altered efficacy of GABA was accompanied by an increased GABA potency (GABA EC50 was 35.8 and 24.5 microM in control and epileptic neurons, respectively). Sensitivity of GABA-evoked currents to blockade by zinc was unchanged in epileptic CA1 neurons, whereas clonazepam (100 nM) augmentation of CA1 GABA-evoked currents decreased 81% relative to controls. These regionally distinct epilepsy-associated modifications in hippocampal GABAergic function may be due to discrete structural alterations in postsynaptic GABA(A)Rs accompanying epileptogenesis, could be therapeutically important, and undoubtedly could contribute to the enhanced limbic excitability underlying TLE.

Physiological and pharmacological alterations in postsynaptic GABA(A) receptor function in a hippocampal culture model of chronic spontaneous seizures.

Cultured rat hippocampal neurons previously exposed to a media containing no added Mg2+ for 3 h begin to spontaneously trigger recurrent epileptiform discharges following return to normal medium, and this altered population epileptiform activity persisted for the life of the neurons in culture (> 2 wk). Neurons in "epileptic" cultures appeared similar in somatic and dendritic morphology and cellular density to control, untreated cultures. In patch-clamp recordings from hippocampal pyramidal cells from "epileptic," low Mg2+ pretreated hippocampal cultures, a rapid (within 2 h of treatment), permanent (lasting > or = 8 days) and statistically significant 50-65% reduction in the current density of functional gamma-aminobutyric acid-A (GABA(A)) receptors was evident when the GABA responses of these cells were compared with control neurons. Functional GABA receptor current density was calculated by determining the maximal response of a cell to GABA 1 mM application and normalizing this response to cellular capacitance. Despite the marked GABA efficacy differences noted above, the potency of GABA in activating chloride currents was not significantly different when the responses to control and "epileptic" pyramidal cells to multiple concentrations of GABA were compared. The EC50 for GABA was 4.5 +/- 0.2 (mean +/- SE) for control neurons and 3.5 +/- 0.4 microM, 5.2 +/- 0.5 microM, 3.7 +/- 0.3 microM, and 4.6 +/- 0.3 microM for epileptic neurons 2 h, 2 days, 3 days, and 8 days after low Mg2+ pretreatment, respectively. Modulation of GABA responses by the benzodiazepine, clonazepam, was significantly reduced in epileptic neurons compared with controls. The kinetically determined clonazepam 100 nM GABA augmentation efficacy decreased from 44.1% in control neurons to 9.3% augmentation in neurons recorded from cultures 10 days posttreatment. The kinetics of GABA current block by the noncompetitive antagonist picrotoxin were determined in hippocampal cultured neurons, and an IC50 of 14 microM determined. Bath application of picrotoxin at half of the IC50 concentration (7 microM) induced epileptiform activity in control cultures and this activity appeared very similar to the epileptiform activity induced by prior low Mg2+ treatment. This concentration of picrotoxin was determined experimentally to block 30% of the GABA(A)-mediated receptor responses in these cultures, and this level of block was sufficient to trigger spontaneous epileptiform activity. The 50% reduction of GABA responses induced as a permanent consequence of low Mg2+ treatment therefore was determined to be sufficient in and of itself to induce the spontaneous epileptiform activity, which was also a consequence of this treatment.

GABAA receptor function in developing rat thalamic reticular neurons: whole cell recordings of GABA-mediated currents and modulation by clonazepam.

1. Nucleus reticularis thalami (NRT) is a nucleus composed entirely of GABAergic neurons, which functions as a pacemaker to synchronize thalamocortical oscillations. To study the functional properties of GABAergic inhibition mediated through activation of gamma-aminobuturic acid-A (GABAA) receptors in these cells, neurons were isolated acutely from NRT at various postnatal developmental stages and recorded from using whole cell patch-clamp techniques. 2. Application of GABA to NRT neurons elicited a large, bicuculline sensitive current with an average reversal potential of -60.6 +/- 1.9 mV (mean +/- SD) in postnatal day (p) 19-21 neurons and -51.2 +/- 3.1 mV in p7 neurons, presumably mediated through activation of a GABAA-mediated chloride conductance. The potency of GABA in activating GABAA receptors decreased significantly with postnatal development in NRT neurons and was best fitted with EC50s of 24.9, 33.9, and 67.2 microM, in neurons isolated from p5-9, p18-25, and p58-74 rats, respectively. The density of GABAA receptors in the membranes of NRT neurons increased significantly with postnatal development. In addition, the GABA current decay rate slowed significantly in neurons isolated from older animals relative to early postnatal rat pups. 3. Both the potency and efficacy of benzodiazepine augmentation of GABAA responses in NRT neurons increased significantly with development. The EC50 of clonazepam decreased from 26 to 6 nM in p5-9 and p58-74 NRT neurons, respectively, whereas the efficacy increased from 126 to 163% augmentation over the same developmental range. 4. The elevated efficacy of clonazepam (CNZ) in NRT neurons relative to thalamic and cortical neurons, particularly in neurons isolated from adult (> p58) rats, is consistent with the anticonvulsant profile of this drug in controlling Generalized Absence epilepsy. Augmenting inhibition within NRT would enhance NRT/ NRT inhibitory connections and thereby decrease the amplitude of NRT-mediated synchronizing inhibitory postsynaptic potentials onto thalamus (where CNZ has low efficacy), in turn making it more difficult to elicit burst firing in thalamus due to deinactivation and subsequent activation of the low-threshold Ca2+ current. 5. The present developmental profile of GABAA responses in GABAergic NRT neurons provides data important in understanding the role of GABAergic inhibition within NRT in the modulation of normal and pathological thalamocortical rhythms in the brain and is also relevant in understanding potential differences in GABAA receptor physiology and pharmacology in GABAergic interneurons relative to glutamatergic neurons.

Characterization of GABAA receptor function in human temporal cortical neurons.

1. Surgically resected tissue from the tip of the human temporal lobe of seven patients undergoing temporal lobectomy was employed to study functional properties of GABAergic inhibition mediated through activation of GABAA receptors, using patch-clamp recording techniques in acutely isolated neurons and in slices of human temporal cortex. 2. Human temporal cortical pyramidal neurons from surgically resected tissue could be acutely isolated with the use of conventional methods. These neurons appeared normal in morphology, in their intrinsic membrane properties, and in their response to application of exogenous gamma-aminobutyric acid (GABA). 3. Application of GABA to acutely isolated human temporal cortical neurons elicited a large current with an average reversal potential of -65 mV, presumably mediated through a GABAA-activated chloride conductance. Application of varying concentrations of GABA generated a concentration/response relationship that could be well-fitted by a conventional sigmoidal curve, with an EC50 of 25.5 microM and a Hill coefficient of 1.0 4. Coapplication of the benzodiazepine clonazepam and 10 microM GABA augmented the amplitude of the GABA response. The concentration dependence of this benzodiazepine augmentation could be best-fitted by an equation assuming that the benzodiazepine interacted with two distinct binding sites, with differing potencies. The high-potency site had an EC50 of 0.06 nM and maximally contributed 38.5% augmentation to the total effect of clonazepam. The lower potency site had an EC50 of 16.4 nM, and contributed 66.1% maximal augmentation to the overall effect of clonazepam. These data derived from adult human temporal cortical neurons were very similar to our findings in adult rat sensory cortical neurons. 5. The effects of equimolar concentrations (100 nM) of clonazepam, a BZ1 and BZ2 agonist, and zolpidem, a selective BZ1 agonist, on acutely isolated human temporal cortical neurons were also investigated. Zolpidem and clonazepam were equally effective (71.5 vs. 65.0%, respectively) in potentiating GABA responses elicited by application of 10 microM GABA. This suggests that many of the functional benzodiazepine receptors in these neurons were of the BZ1 variety. 6. GABAergic synaptic inhibition was also studied with the use of patch-clamp recordings in slices of human temporal cortex. Extracellular stimulation at the white matter/gray matter border elicited compound synaptic events in layer II-V cortical neurons. These events usually consisted of an early excitatory postsynaptic potential (EPSP) and a late multiphasic inhibitory postsynaptic potential (IPSP). Application of either clonazepam or zolpidem (both at 100 nM) to the slice during extracellular stimulation reversibly augmented the late compound IPSP. 7. Spontaneous IPSPs were also recorded in approximately 50% of human temporal cortical neurons. These events did not have a preceding EPSP and were usually monopolar, with a single exponential rise and decay. This supported the idea that these events were triggered by spontaneous activity of GABAergic interneurons. Bath application of either clonazepam or zolpidem (both at 100nM) to the slice during ongoing spontaneous IPSP activity increased the amplitude and lengthened the time constant of decay of these events. 8. To our knowledge, this is one of the first detailed characterizations of the functional properties of GABAA-mediated inhibition in human cortical neurons using patch-clamp recordings in both isolated cells and slices of resected temporal cortex. Isolated pyramidal neurons exhibited GABAA-mediated currents that were comparable in many aspects with GABA currents recorded from adult rat cortical neurons, including similar GABA concentration/response curves, and similar two differing potency site effects for clonazepam augmentation of GABA currents. In addition, evoked and spontaneous IPSPs recorded in human cortical neurons appeared similar to IPSPs in rat cortical

Anticonvulsant drug effects on spontaneous thalamocortical rhythms in vitro: ethosuximide, trimethadione, and dimethadione.

Spontaneous generalized epileptiform discharges were elicited in rodent thalamocortical slices by perfusion with a medium containing no added Mg2+. In multiple-channel extracellular field potential recordings in thalamus and cortex, several distinct types of discharges were recorded, with two principal variants bearing marked similarity to spike-wave and generalized tonic-clonic seizure discharges recorded in patients with generalized seizure disorders. These discharges were termed sTBCs and cTBCs, respectively, for simple and complex thalamocortical burst complexes. The sensitivity of these discharges to the generalized absence anticonvulsants ethosuximide, trimethadione and dimethadione (the active metabolite of trimethadione) was studied. sTBCs were reduced or blocked by ethosuximide and dimethadione, when these drugs were applied in clinically relevant concentrations. The order of effectiveness of these agents was dimethadione > or = ethosuximide >> trimethadione. This paralleled the relative efficacy of these drugs in blocking T current in thalamic neurons. cTBCs were unaffected or exacerbated by these drugs. Structural control drugs including succinimide, the behaviorally inactive ring base of ethosuximide, and alpha, alpha-dimethyl-beta-methylsuccinimide, a convulsant succinimide, were inactive or exacerbated either sTBCs or cTBCs, respectively. These spontaneous generalized thalamocortical discharges in rodent thalamocortical slices may represent a potentially valuable in vitro model of generalized seizure discharges, with marked pharmacological and physiological similarities to various forms of clinical epileptic seizure activity.

Anticonvulsant drug effects on spontaneous thalamocortical rhythms in vitro: valproic acid, clonazepam, and alpha-methyl-alpha-phenylsuccinimide.

Spontaneous thalamocortical epileptiform activity was elicited in rodent thalamocortical slices by a medium containing no added Mg2+. Multiple varieties of activity were generated in these slices, including simple thalamocortical burst complex (sTBC) activity that resembled the spike-wave discharges of generalized absence epilepsy, and complex thalamocortical burst complex (cTBC) activity that resembled generalized tonic-clonic seizure discharges. In a further pharmacological characterization of this activity, the effects of the broad-spectrum anticonvulsants valproic acid, alpha-methyl-alpha-phenylsuccinimide (the active metabolite of methsuximide) and clonazepam were studied. All three drugs were found to be effective in controlling both sTBC and cTBC activity when applied in clinically relevant concentration ranges. The effectiveness of valproic acid against spontaneous rhythms in vitro was not due to augmentation of GABAergic inhibition. No effect of valproic acid on GABA-activated chloride currents was evident in patch-clamp recordings of acutely isolated thalamic or cortical neurons. The equivalent general clinical and experimental spectrum of action of broadly effective anticonvulsants provided an additional correlation between the clinical efficacy of anticonvulsant drugs and their effects against epileptiform discharges in rodent thalamocortical slices. This further validates spontaneous generalized low-Mg2+ thalamocortical activity as a potentially valuable in vitro model of the primary generalized epilepsies, in which the cellular mechanisms underlying generation and control of these seizure discharges can be studied.

Postnatal development of GABAA receptor function in somatosensory thalamus and cortex: whole-cell voltage-clamp recordings in acutely isolated rat neurons.

GABAergic inhibition synchronizes oscillatory activity in the thalamocortical system. To understand better the role of this neurotransmitter in generation of thalamocortical rhythmicity, the postnatal development of GABAergic function mediated through activation of GABAA receptors was studied in thalamus and cortex. GABA-evoked chloride currents were recorded in dissociated rat cortical and thalamic neurons during postnatal development. Kinetic fits of GABA concentration/response relationships revealed developmental and regional alterations in the potency of GABA. Early in postnatal development (p5-p8), both thalamic and cortical neurons exhibited reduced potency of GABA (27-31 microM KD). Potency increased by p18-p25 in thalamic and cortical neurons (19-22 microM KD), to a level maintained in adult thalamic neurons. Adult cortical neurons exhibited reduced potency of GABA (40 microM KD). Benzodiazepine modulation of GABAA currents was also studied. Kinetic analyses of benzodiazepine augmentation of GABAA currents were best fitted assuming two effective sites with different affinities for clonazepam. The high-affinity site (KD of 0.05-0.27 nM) showed little variation with development in cortical neurons, contributing about 16-23% potentiation at all postnatal ages. Developing thalamic neurons (p5-p25) showed similar potency and efficacy of the high-affinity benzodiazepine site to cortical neurons. High-affinity benzodiazepine effects disappeared in adult thalamic neurons. A lower-affinity benzodiazepine site (25-50 nM KD) was greater in efficacy in cortical neurons compared to thalamic neurons at all ages, with efficacy ranging from 50% to 110% in cortex and from 20% to 60% in thalamus. Knowledge of developmental and regional alterations in GABAA receptor function may aid in understanding mechanisms involved in generation and control of normal and pathological thalamocortical rhythms.

Use of the dexamethasone suppression test to detect depressive disorders of mentally retarded individuals.

The applicability of the Dexamethasone Suppression Test (DST) with 39 mentally retarded individuals was examined. Subjects were assigned to one of three groups based upon the presence of zero, one, or two (or more) primary behaviors: unprovoked aggressive/assaultive behavior, self-injurious behavior, and severely withdrawn behavior. Chi-square analysis revealed that individuals exhibiting two or more of these types of behavior were more likely to obtain a positive DST result than were individuals exhibiting only one behavior. Results support the hypothesis that retarded persons can become depressed (as defined by the DST) and may express the depression through these behaviors. Furthermore, these results suggest that the current diagnostic criteria for depression in the retarded population need to be revised.