Changes In Dendritic Spine Density Research Articles

Developmental Stage-dependent Persistent Impact of Propofol Anesthesia on Dendritic Spines in the Rat Medial Prefrontal Cortex

Recent observations demonstrate that anesthetics rapidly impair synaptogenesis during neuronal circuitry development. Whether these effects are lasting and depend on the developmental stage at which these drugs are administered remains, however, to be explored. Wistar rats received propofol anesthesia at defined developmental stages during early postnatal life. The acute and long-term effects of these treatments on neuronal cytoarchitecture were evaluated by Neurolucida and confocal microscopy analysis after iontophoretic injections of Lucifer Yellow into layer 5 pyramidal neurons in the medial prefrontal cortex. Quantitative electron microscopy was applied to investigate synapse density. Layer 5 pyramidal neurons of the medial prefrontal cortex displayed intense dendritic growth and spinogenesis during the first postnatal month. Exposure of rat pups to propofol at postnatal days 5 and 10 significantly decreased dendritic spine density, whereas this drug induced a significant increase in spine density when administered at postnatal days 15, 20, or 30. Quantitative electron microscopy revealed that the propofol-induced increase in spine density was accompanied by a significant increase in the number of synapses. Importantly, the propofol-induced modifications in dendritic spine densities persisted up to postnatal day 90. These new results demonstrate that propofol anesthesia can rapidly induce significant changes in dendritic spine density and that these effects are developmental stage-dependent, persist into adulthood, and are accompanied by alterations in synapse number. These data suggest that anesthesia in the early postnatal period might permanently impair circuit assembly in the developing brain.

Changes in dendritic spine density on layer 2/3 pyramidal cells within the cingulate cortex of late pregnant and postpartum rats

A rapid upregulation of astrocytic protein expression within area 2 of the cingulate cortex (Cg2) of the maternal rat occurs within 3 h postpartum and persists throughout lactation. Previous studies have shown that similar changes in astrocytic proteins can signal changes in local synapses and dendritic spines. Thus, here we used the Golgi–Cox impregnation technique to compare spine density in layer 2 and 3 pyramidal cells of Cg2, the CA1 region of the hippocampus and the parietal cortex (ParCx) among metestrus, late pregnant (LP), 3-hour postpartum (3H PP) and 16-day postpartum rats (D16 PP). Rats in the 3H PP group had higher numbers of dendritic spines/10 μm on the apical dendrites of pyramidal neurons in both Cg2 and CA1 than the other groups, which did not differ. A similar pattern was observed in basilar dendrites but this failed to reach significance. In Cg2, Sholl analysis revealed that rats in the D16 PP group had a significantly greater extent of dendritic arborization in the basilar region than any other group. These data suggest that the changes in astrocytic proteins that occur in Cg2 in the postpartum period are associated with neuronal plasticity in pyramidal layers 2 and 3.

“TOR” rents of Excitement over Rapamycin's Antiepileptogenic Potential

Response of a Neuronal Model of Tuberous Sclerosis to Mammalian Target of Rapamycin (mTOR) Inhibitors: Effects on mTORC1 and Akt Signaling Lead to Improved Survival and Function. Meikle L, Pollizzi K, Egnor A, Kramvis I, Lane H, Sahin M, Kwiatkowski DJ. J Neurosci 2008;28(21):5422–5432. Tuberous sclerosis (TSC) is a hamartoma syndrome attributable to mutations in either TSC1 or TSC2 in which brain involvement causes epilepsy, mental retardation, and autism. We have reported recently (Meikle et al., 2007) a mouse neuronal model of TSC in which Tsc1 is ablated in most neurons during cortical development. We have tested rapamycin and RAD001 [40- O-(2-hydroxyethyl)-rapamycin], both mammalian target of rapamycin mTORC1 inhibitors, as potential therapeutic agents in this model. Median survival is improved from 33 d to more than 100 d; behavior, phenotype, and weight gain are all also markedly improved. There is brain penetration of both drugs, with accumulation over time with repetitive treatment, and effective reduction of levels of phospho-S6, a downstream target of mTORC1. In addition, there is restoration of phospho-Akt and phospho-glycogen synthase kinase 3 levels in the treated mice, consistent with restoration of Akt function. Neurofilament abnormalities, myelination, and cell enlargement are all improved by the treatment. However, dysplastic neuronal features persist, and there are only modest changes in dendritic spine density and length. Strikingly, mice treated with rapamycin or RAD001 for 23 d only (postnatal days 7–30) displayed a persistent improvement in phenotype, with median survival of 78 d. In summary, rapamycin/RAD001 are highly effective therapies for this neuronal model of TSC, with benefit apparently attributable to effects on mTORC1 and Akt signaling and, consequently, cell size and myelination. Although caution is appropriate, the results suggest the possibility that rapamycin/RAD001 may have benefit in the treatment of TSC brain disease, including infantile spasms. Rapamycin Prevents Epilepsy in a Mouse Model of Tuberous Sclerosis Complex. Zeng LH, Xu L, Gutmann DH, Wong M. Ann Neurol 2008;63(4):444–453. OBJECTIVE: Tuberous sclerosis complex (TSC) represents one of the most common genetic causes of epilepsy. TSC gene inactivation leads to hyperactivation of the mammalian target of rapamycin signaling pathway, raising the intriguing possibility that mammalian target of rapamycin inhibitors might be effective in preventing or treating epilepsy in patients with TSC. Mice with conditional inactivation of the Tsc1 gene primarily in glia ( Tsc1GFAPCKO mice) develop glial proliferation, progressive epilepsy, and premature death. Here, we tested whether rapamycin could prevent or reverse epilepsy, as well as other cellular and molecular brain abnormalities in Tsc1GFAPCKO mice. METHODS: Tsc1GFAPCKO mice and littermate control animals were treated with rapamycin or vehicle starting at postnatal day 14 (early treatment) or 6 weeks of age (late treatment), corresponding to times before and after onset of neurological abnormalities in Tsc1GFAPCKO mice. Mice were monitored for seizures by serial video-electroencephalogram and for long-term survival. Brains were examined histologically for astrogliosis and neuronal organization. Expression of phospho-S6 and other molecular markers correlating with epileptogenesis was measured by Western blotting. RESULTS: Early treatment with rapamycin prevented the development of epilepsy and premature death observed in vehicle-treated Tsc1GFAPCKO mice. Late treatment with rapamycin suppressed seizures and prolonged survival in Tsc1GFAPCKO mice that had already developed epilepsy. Correspondingly, rapamycin inhibited the abnormal activation of the mammalian target of rapamycin pathway, astrogliosis, and neuronal disorganization, and increased brain size in Tsc1GFAPCKO mice. INTERPRETATION: Rapamycin has strong efficacy for preventing seizures and prolonging survival in Tsc1GFAPCKO mice.

Cocaine Regulates MEF2 to Control Synaptic and Behavioral Plasticity

Repeated exposure to cocaine causes sensitized behavioral responses and increased dendritic spines on medium spiny neurons of the nucleus accumbens (NAc). We find that cocaine regulates myocyte enhancer factor 2 (MEF2) transcription factors to control these two processes in vivo. Cocaine suppresses striatal MEF2 activity in part through a mechanism involving cAMP, the regulator of calmodulin signaling (RCS), and calcineurin. We show that reducing MEF2 activity in the NAc in vivo is required for the cocaine-induced increases in dendritic spine density. Surprisingly, we find that increasing MEF2 activity in the NAc, which blocks the cocaine-induced increase in dendritic spine density, enhances sensitized behavioral responses to cocaine. Together, our findings implicate MEF2 as a key regulator of structural synapse plasticity and sensitized responses to cocaine and suggest that reducing MEF2 activity (and increasing spine density) in NAc may be a compensatory mechanism to limit long-lasting maladaptive behavioral responses to cocaine.

Automated quantification of dendritic spine density and spine head diameter in medium spiny neurons of the nucleus accumbens

Dendritic spines are postsynaptic specializations thought to regulate the strength of synaptic transmission and play a critical role in neuronal plasticity. While changes in dendritic spine density can be pharmacologically- or environmentally-induced, the widespread utility of this important measure of synaptic plasticity in vivo has been hampered by the labor-intensive nature, and potential for bias and inconsistency inherent in manual spine counting. Here we report a method for obtaining high-resolution, three-dimensional confocal images of accumbens spiny neurons labeled with a diolistically delivered lipophilic fluorescence dye (DiI) that permits automated analysis of spine density and spine head diameter. The automated quantification was verified by manual counts of spine density and electron microscopic measures of spine head diameter. The density of spines was relatively constant over 2nd to 4th order dendrites within a neuron, and spine density was normally distributed. The mean spine density (2.68 spines/microm; N = 45 neurons) was higher than previous reports, due in part to analysis in three rather than two dimensions and the capacity of lipophilic dyes to fill very thin spines. The distribution of spine head diameters was continuous and skewed to the right (mean = 0.43 microm; N = 8,891), and approximately 25% of all spines were thin and filopodia-like (< or = 0.20 microm diameter). The density of spines was not correlated with average spine head diameter or with the number of filopodia-like spines. The capacity to rapidly assess spine density and spine head diameter will facilitate quantifying spine plasticity induced by pharmacological and environmental manipulations.

Response of a Neuronal Model of Tuberous Sclerosis to Mammalian Target of Rapamycin (mTOR) Inhibitors: Effects on mTORC1 and Akt Signaling Lead to Improved Survival and Function

Tuberous sclerosis (TSC) is a hamartoma syndrome attributable to mutations in either TSC1 or TSC2 in which brain involvement causes epilepsy, mental retardation, and autism. We have reported recently (Meikle et al., 2007) a mouse neuronal model of TSC in which Tsc1 is ablated in most neurons during cortical development. We have tested rapamycin and RAD001 [40-O-(2-hydroxyethyl)-rapamycin], both mammalian target of rapamycin mTORC1 inhibitors, as potential therapeutic agents in this model. Median survival is improved from 33 d to more than 100 d; behavior, phenotype, and weight gain are all also markedly improved. There is brain penetration of both drugs, with accumulation over time with repetitive treatment, and effective reduction of levels of phospho-S6, a downstream target of mTORC1. In addition, there is restoration of phospho-Akt and phospho-glycogen synthase kinase 3 levels in the treated mice, consistent with restoration of Akt function. Neurofilament abnormalities, myelination, and cell enlargement are all improved by the treatment. However, dysplastic neuronal features persist, and there are only modest changes in dendritic spine density and length. Strikingly, mice treated with rapamycin or RAD001 for 23 d only (postnatal days 7-30) displayed a persistent improvement in phenotype, with median survival of 78 d. In summary, rapamycin/RAD001 are highly effective therapies for this neuronal model of TSC, with benefit apparently attributable to effects on mTORC1 and Akt signaling and, consequently, cell size and myelination. Although caution is appropriate, the results suggest the possibility that rapamycin/RAD001 may have benefit in the treatment of TSC brain disease, including infantile spasms.

Repeated social defeat stress-induced sensitization to the locomotor activating effects of d-amphetamine: role of individual differences

In this study, we sought to examine individual differences in stress-induced behavioral sensitization to d-amphetamine after repeated social defeat stress. In an effort to understand what mechanisms underlie stress-induced sensitization to d-amphetamine, we examined striatal gene expression of the dopamine receptor D(2). Additionally, we investigated if repeated social defeat was associated with changes in dendritic spine density in the hippocampus, prefrontal cortex, and nucleus accumbens of rats that exhibit stress-induced sensitization. Male rats were classified into high responders (HR) and low responders (LR) based on their locomotor response to a novel environment. Then, rats were either handled as a control or defeated on four occasions by aggressive rats. Two weeks after the last defeat, animals were challenged with one of three doses of d-amphetamine and their locomotor activity was recorded. Non-defeated HR rats exhibited higher locomotor activity in response to d-amphetamine when compared to LR non-defeated rats. Fourteen days from the last repeated social defeat, LR rats and HR rats were behaviorally identical in response to acute injections of amphetamine. Furthermore, HR non-defeated rats had less D(2) mRNA expression in the nucleus accumbens core and dorsal striatum than do LR non-defeated rats. However, after repeated social defeat, HR and LR rats had identical D(2) mRNA expression in both the core and dorsal striatum. Finally, there were no changes in dendritic spine density in any of the brain areas examined in LR rats. Repeated social defeat abolishes individual differences in behavioral responses to d-amphetamine which may be due to a down-regulation of striatal dopamine D(2) receptors in LR rats.

Running induces widespread structural alterations in the hippocampus and entorhinal cortex

Physical activity enhances hippocampal function but its effects on neuronal structure remain relatively unexplored outside of the dentate gyrus. Using Golgi impregnation and the lipophilic tracer DiI, we show that long-term voluntary running increases the density of dendritic spines in the entorhinal cortex and hippocampus of adult rats. Exercise was associated with increased dendritic spine density not only in granule neurons of the dentate gyrus, but also in CA1 pyramidal neurons, and in layer III pyramidal neurons of the entorhinal cortex. In the CA1 region, changes in dendritic spine density are accompanied by changes in dendritic arborization and alterations in the morphology of individual spines. These findings suggest that physical activity exerts pervasive effects on neuronal morphology in the hippocampus and one of its afferent populations. These structural changes may contribute to running-induced changes in cognitive function.

A model of activity-dependent changes in dendritic spine density and spine structure

Recent evidence indicates that the morphology and density of dendritic spines are regulated during synaptic plasticity. See for instance a review by [1]. High-frequency stimuli that induce long-term potentiation (LTP) have been associated with increases in the number and size of spines. In contrast, low-frequency stimuli that induce long-term depression (LTD) are associated with decreases in the number and size of spines. Decreases in spine density also occur due to excitotoxicity associated with very high levels of activity such as during seizures. In this work, we extend previous modeling studies [2] by combining a model for activity-dependent spine density with one for calcium-mediated spine stem restructuring. The model is based on the standard dimensionless cable equation for the changes in membrane potential in a passive dendrite. An additional equation characterizes the activity-dependent changes in spine density along the dendrite. For this continuum model, a typical Hodgkin-Huxley type current balance equation represents the change in membrane potential in an isopotential compartment representing a spine head. Both the cable equation and the current balance equation rely on the spine stem current to represent current flow between the spines and the dendrite. The model also includes equations for activity-dependent changes in the calcium concentration in spines as well as changes in spine stem resistance that depend on the level of calcium in an individual spine. The calcium-mediated changes in spine density and spine stem resistance are based on a conceptual model proposed by Segal et al. [1] where low calcium concentrations lead to spine shrinkage and pruning, an increase in calcium concentration leads to spine elongation and formation of new spines, and significantly higher values cause spine shrinkage and pruning. We use computational studies to investigate the changes in spine density and structure for differing synaptic inputs and demonstrate the effects of these changes on the input-output properties of the dendritic branch. Moderate amounts of high-frequency synaptic activation to dendritic spines cause an increase in spine stem resistance, which is correlated with spine stem elongation. In addition, the spine density increases both inside and outside the input region. The model is formulated so that this LTP-inducing stimulus eventually leads to structural stability. In contrast, a prolonged low-frequency stimulation paradigm that would typically induce LTD results in a decrease in stem resistance (correlated with spine shortening) and an eventual decrease in spine density.

A model of activity-dependent changes in dendritic spine density and spine structure

Recent evidence indicates that the morphology and density of dendritic spines are regulated during synaptic plasticity. See, for instance, a review by Hayashi and Majewska [9]. In this work, we extend previous modeling studies [27] by combining a model for activity-dependent spine density with one for calcium-mediated spine stem restructuring. The model is based on the standard dimensionless cable equation, which represents the change in the membrane potential in a passive dendrite. Additional equations characterize the change in spine density along the dendrite, the current balance equation for an individual spine head, the change in calcium concentration in the spine head, and the dynamics of spine stem resistance. We use computational studies to investigate the changes in spine density and structure for differing synaptic inputs and demonstrate the effects of these changes on the input-output properties of the dendritic branch. Moderate amounts of high-frequency synaptic activation to dendritic spines result in an increase in spine stem resistance that is correlated with spine stem elongation. In addition, the spine density increases both inside and outside the input region. The model is formulated so that this long-term potentiation-inducing stimulus eventually leads to structural stability. In contrast, a prolonged low-frequency stimulation paradigm that would typically induce long-term depression results in a decrease in stem resistance (correlated with stem shortening) and an eventual decrease in spine density.

Dendritic spine loss in the hippocampus of young PDAPP and Tg2576 mice and its prevention by the ApoE2 genotype

Postmortem AD brains exhibit dendritic spine loss in the hippocampus. To determine whether this pathology may be associated with amyloid burden, the present study used the Golgi stain technique to assess age- and genotype-dependent changes in dendritic spine density in CA1 hippocampus of two transgenic mouse lines that produce high levels of Aβ. Tg2576 and PDAPP mice, as well as a group of Tg2576 mice crossed with human apoE2-expressing transgenic mice, were compared to respective transgene-negative controls. Since the time course of amyloid plaque deposition in the PDAPP and Tg2576 mice is well characterized, we examined changes in spine density at ages that corresponded to different levels of amyloid plaque load. The data show age- and genotype-dependent reductions in spine density in both Tg2576 and PDAPP mice, albeit at somewhat different time courses. The spine loss occurred prior to plaque deposition and was ameliorated by the overexpression of human apoE2. These results suggest that a soluble Aβ species may affect hippocampal synapses and thereby contribute to functional deficits evident in these animals.

Synchronous development of pyramidal neuron dendritic spines and parvalbumin-immunoreactive chandelier neuron axon terminals in layer III of monkey prefrontal cortex

Postnatal development of the primate cerebral cortex involves an initial proliferation and the subsequent attrition of cortical synapses. Although these maturational changes in synaptic density have been observed across the cortical mantle, little is known about the precise time course of developmental refinements in synaptic inputs to specific populations of cortical neurons. We examined the postnatal development of two markers of excitatory and inhibitory inputs to a subpopulation of layer III pyramidal neurons in area 9 and 46 of rhesus monkey prefrontal cortex. These neurons are of particular interest because they play a major role in the flow of information both within and between cortical regions. Quantitative reconstructions of Golgi-impregnated mid-layer III pyramidal neurons revealed substantial developmental changes in the relative density of dendritic spines, the major site of excitatory inputs to these neurons. Relative spine density on both the apical and basilar dendritic trees increased by 50% during the first two postnatal months, remained at a plateau through 1.5 years of age, and then decreased over the peripubertal age range until stable adult levels were achieved. As a measure of the postnatal changes in inhibitory input to the axon initial segment of these pyramidal neurons, we determined the density of parvalbumin-immunoreactive axon terminals belonging to the chandelier class of local circuit neurons. The density of these distinctive axon terminals (cartridges) exhibited a temporal pattern of change that exactly paralleled the changes in dendritic spine density. These results suggest that subpopulations of cortical neurons may be regulated by dynamic interactions between excitatory and inhibitory inputs during development and, in concert with other data, they emphasize the cellular specificity of postnatal refinements in cortical circuitry.

5,7-Dihydroxytryptamine and Gonadal Steroid Manipulation Alter Spine Density in Ventromedial Hypothalamic Neurons

Golgi impregnation was used to examine changes in dendritic spine density in the ventromedial hypothalmic nucleus (VMN) of intact and gonadectomized (GDX) rats treated with 5,7-dihydroxytryptamine (5,7-DHT) and/or estrogen. In intact rats of both sexes 5,7-DHT injection resulted in a large increase in dendritic spine density in the VMN. The combination of 5,7-DHT and estrogen treatment in GDX females resulted in a further increase in dendritic spine density. In male rats gonadectomy increased dendritic spine density on VMN neurons, and estrogen treatment partially reversed this effect. When GDX males were treated with 5,7-DHT and estrogen, there was no effect on dendritic spine density. These results complement a previous study in which it was demonstrated that dendritic spine density in the VMN was greatest during proestrus and after estrogen administration to ovariectomized females. The relevance of these findings to gonadal-steroid-mediated lordosis is discussed.

Naturally occurring fluctuation in dendritic spine density on adult hippocampal pyramidal neurons

We have used Golgi-impregnated tissue to demonstrate that apical dendritic spine density in CA1 hippocampal pyramidal cells undergoes a cyclic fluctuation as estradiol and progesterone levels vary across the estrous cycle in the adult female rat. We observed a 30% decrease in apical dendritic spine density over the 24-hr period between the late proestrus and the late estrus phases of the cycle. Spine density then appears to cycle back to proestrus values over a period of several days. In contrast, no significant changes in dendritic spine density across the estrous cycle occur in CA3 pyramidal cells or dentate gyrus granule cells. These results demonstrate rapid and ongoing dendritic plasticity in a specific population of hippocampal neurons in experimentally unmanipulated animals.

Training induced dendritic spine density changes are specifically related to memory formation processes in the chick, Gallus domesticus

The density of dendritic spines on large, multipolar, projection neurons in the intermediate medial hyperstriatum ventrale (IMHV) of 1-day-old chicks was examined after training on a one-trial passive avoidance task. Chicks trained on the task were given a brief, subconvulsive transcranial electroshock 5 min after training, a procedure which rendered about half of them amnesic. The spine density was found to be significantly higher 24–26 h after training in the left IMHV in chicks that remembered the response compared to chicks rendered amnesic. These data, taken together with our earlier observations on spine density changes following training, argue strongly in favour of a specific role for dendritic spines in memory formation in the chick.

The effects of protein deprivation on the nucleus locus coeruleus: A morphometric golgi study in rats of three age groups

In a previous morphometric Golgi study of the nucleus locus coeruleus we identified in rats fed a 25% casein diet 3 cell types, fusiform, multipolar and ovoid, and compared their age-related changes from 30 to 90 days and 90 to 220 days. In the present study we investigated the effects of an 8% casein diet, initiated prenatally and continued postnatally in the pups, using the same morphometric parameters at the same 3 ages. In these rats the majority of significant age-related changes were in primary and secondary dendritic spine density. On all 3 cell types between 30 and 90 days there was a decreased spine density followed by an increase between 90 and 220 days. These age-related changes closely followed those in controls and, as a result, when the two diet groups were compared at each age they showed only 4 significant differences out of 18 comparisons. In control rats these age-related changes in dendritic spine density in the nucleus locus coeruleus were diametrically out-of-phase with those found in the nucleus raphe dorsalis in a previous study. This is consistent with the postulated mutual inhibitory relationship between the nucleus locus coeruleus and nucleus raphe dorsalis. Comparison of these same events in the nucleus locus coeruleus and nucleus raphe dorsalis in the 8% casein diet rats show no evidence of an out-of-phase relationship. Thus, the relationship between these two closely related nuclei appears to be fundamentally altered by the 8% casein diet. In these comparisons of dendritic spine density and other parameters the 8% casein deit rats have shown in both the nucleus locus coeruleus and nucleus raphe dorsalis deficits and increases as compared to controls. In contrast, in morphometric Golgi studies of pyramidal and Purkinje cells undernutrition generally results in deficits or little change in the various parameters. This suggests that the adaptation of the non-pyramidal neurons in the present study is different from that shown by pyramidal and Purkinje cells.

Effects of chronic crowding stress on midbrain development: changes in dendritic spine density and morphology in jewel fish optic tectum.

This study investigates the effects of a biologically relevant stressor, crowding, on the development of neurons in the major teleostean brain area, the optic tectum. Adult jewel fish were reared for approximately 4 years under conditions of moderate density (3.3 liter/fish), or under uncrowded control conditions (25 liter/fish). Quantitative morphometric measures of Golgi-stained tissue were used to test whether long-term crowding at moderate density produced developmental deficits lasting beyond the juvenile period. Chronic crowding did not affect body size or gross tectal growth. However, crowding significantly decreased the density of dendritic spine formation on apical dendrites of pyriform interneurons in the basal region of the tectum (stratum album centrale). Additionally, the shapes of spines on this segment of the apical dendrite were altered by crowding: relative frequencies of overall spine length and spine stem length changed significantly. These results are interpreted in the context of cytoarchitectural changes produced by more extreme developmental stressors in previous studies.

Changes in dendritic spine density in the preoptic area of the female rat at puberty

Preoptic area (POA) neurons stained by a modification of the Golgi-Kopsch method were studied in developing female rats. Particular attention was focused on dendritic spine densities as they might be altered in relation to the onset of puberty. Neurons in the anterior superior and posterior superior portions of the POA showed an increase in mean spine density at the age of vaginal opening. The increase in dendritic spine density in the posterior inferior quadrant of POA occurred prior to vaginal opening. The entire posterior POA maintained peak levels of spine numbers through 55 days of age, but then exhibited a dramatic drop at 75 days. Thus, modification of the neuropil may well continue at least to this age. The increases in spine densities at vaginal opening for the superior POA quadrants, followed by relatively stable values, appear to implicate these areas in the timing of the onset of puberty.

Crowded jewel fish show changes in dendritic spine density and spine morphology

This study demonstrates that neuronal development can be adversely affected by short-term crowding. Compared with uncrowded siblings (15 fish/73-litre), juvenile jewel fish (15 fish/3.6-litre) crowded for 60 days exhibited 50–58% fewer dendritic spines and spines with narrower heads on pyriform interneurons in deep tectal strata. These findings indicate that crowded environments can produce aberrant neuronal development similar to that seen in socially deprived jewel fish.