Time-dependent neurobiological effects of colchicine administered directly into the hippocampus of rats

Targeting NLRP3-Mediated Neuroinflammation in Alzheimer’s Disease Treatment

Alzheimer’s disease (AD) is the most common cause of dementia in the general population and, to date, constitutes a major therapeutic challenge. In the pathogenesis of AD, aggregates of amyloid β (Aβ) and neurofibrillary tangles (NFTs) containing Tau-microtubule-associated protein (tau) are known to trigger a neuroinflammatory response with subsequent formation of an inflammasome. In particular, the NOD-like receptor pyrin domain-containing 3 (NLRP3) inflammasome is thought to play a crucial role in AD-related pathology. While the mechanisms for NLRP3 activation are not fully understood, it has been demonstrated that, after detection of protein aggregates, NLRP3 induces pro-inflammatory cytokines, such as interleukin 18 (IL-18) or interleukin 1β (IL-1β), that further potentiate AD progression. Specific inhibitors of NLRP3 that exhibit various mechanisms to attenuate the activity of NLRP3 have been tested in in vivo studies and have yielded promising results, as shown by the reduced level of tau and Aβ aggregates and diminished cognitive impairment. Herein, we would like to summarize the current state of knowledge on NLRP3 inflammasome priming, activation, and its actual role in AD pathogenesis, and to characterize the NLRP3 inhibitors that have been studied most and their impact on AD-related pathology.

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

An analysis of dentate gyrus function (an update)

In this review there will be a description of the dentate gyrus (DG) neural circuitry that mediates the operation of a variety of mnemonic processes associated with dorsal and ventral DG function in rats. Dysfunction of the dorsal DG can be shown to mediate mnemonic processing of spatially based information including a) the operation of conjunctive encoding of multiple sensory inputs to determine spatial representations, b) pattern separation based on reducing interference between similar spatial locations and spatial contexts for horizontal distance between objects, vertical distance for height of objects, slope or angle of motor movements, c) importance of spatial context in object recognition and processing of shades of grey associated with the walls of the box d) temporal integration in the creation of remote memory based in part on DG neurogenesis and function of the CA3 subregion of the hippocampus. Dysfunction of the ventral DG can be shown to mediate mnemonic processing of odor and reward value based information including a) pattern separation for odors and reward value, and b) social recognition.

Rodent models of neuroinflammation for Alzheimer's disease

Alzheimer's disease remains incurable, and the failures of current disease-modifying strategies for Alzheimer's disease could be attributed to a lack of in vivo models that recapitulate the underlying etiology of late-onset Alzheimer's disease. The etiology of late-onset Alzheimer's disease is not based on mutations related to amyloid-β (Aβ) or tau production which are currently the basis of in vivo models of Alzheimer's disease. It has recently been suggested that mechanisms like chronic neuroinflammation may occur prior to amyloid-β and tau pathologies in late-onset Alzheimer's disease. The aim of this study is to analyze the characteristics of rodent models of neuroinflammation in late-onset Alzheimer's disease. Our search criteria were based on characteristics of an idealistic disease model that should recapitulate causes, symptoms, and lesions in a chronological order similar to the actual disease. Therefore, a model based on the inflammation hypothesis of late-onset Alzheimer's disease should include the following features: (i) primary chronic neuroinflammation, (ii) manifestations of memory and cognitive impairment, and (iii) late development of tau and Aβ pathologies. The following models fit the pre-defined criteria: lipopolysaccharide- and PolyI:C-induced models of immune challenge; streptozotocin-, okadaic acid-, and colchicine neurotoxin-induced neuroinflammation models, as well as interleukin-1β, anti-nerve growth factor and p25 transgenic models. Among these models, streptozotocin, PolyI:C-induced, and p25 neuroinflammation models are compatible with the inflammation hypothesis of Alzheimer's disease.

Dentate gyrus and spatial behaviour

This article reviews evidence from studies employing colchicine-induced granule cell loss in the adult rat brain, and irradiation-induced hypoplasia of the neonatal dentate gyrus, on the performance of spatial and non-spatial behavioral tasks. The general picture emerging from this analysis reveals that the dentate gyrus granule cells are critically involved in spatial behavior, particularly when this requires the adoption of place strategies. This notion also provides an explanation for the behavioral effects of dentate gyrus granule cell loss seen in apparently non-spatial tasks.

The role of the CA3 hippocampal subregion in spatial memory: a process oriented behavioral assessment

Computational models, behavioral data, and electrophysiological data suggest that the CA3 subregion of the hippocampus may support multiple mnemonic processes critical to the formation and subsequent retrieval of spatial memories. Multiple researchers have proposed that the CA3 subregion contains an autoassociative network in which synaptic connections between CA3 neurons that represent different components of a memory are strengthened via recurrent collateral connections. As a result, it has been suggested that the CA3 autoassociative network may support multiple processes including the formation of spatial arbitrary associations, temporary maintenance of spatial working memory, and spatial pattern completion. In addition, the CA3 subregion has been suggested to be involved in spatial pattern separation. The separation of patterns is hypothesized to be accomplished based on the low probability that any two CA3 neurons will receive mossy-fiber input synapses from a similar subset of dentate gyrus cells. The separation of patterns also may be enhanced by competitive inhibition within CA3 and dentate gyrus. This review will focus on the mnemonic processes supported by CA3 neurons and how these processes may facilitate the encoding and retrieval of spatial information. Although there is growing evidence indicating that the hippocampus plays a role in the processing of nonspatial information as well, the scope of the present review will focus on the role of the CA3 subregion in spatial memory.

A behavioral analysis of dentate gyrus function

Computational models of the dentate gyrus (DG) have suggested based on anatomical, electrophysiological, and computer simulation data that the DG plays an important role in learning and memory by processing and representing spatial information on the basis of conjunctive encoding, pattern separation, and encoding of spatial information in conjunction with the CA3. Behavioral evidence supports a role for the DG in mnemonic processing of spatial information based on the operation of conjunctive encoding of multiple sensory inputs, pattern separation of spatial (especially metric) information, and subsequent encoding in cooperation with CA3. A potential role of the DG in mediating processes, such as recall of sequential information and short-term memory as well as temporal order for remote memory, are also discussed.

Dentate gyrus-selective colchicine lesion and performance in temporal and spatial tasks

The effects of multiple-site, intradentate, colchicine injections on the performance of a temporal, 'differential reinforcement of low rates of responding' (DRL-20s) task and a spatial, 'delayed non-matching-to-place' (DNMTP) task in a plus-maze were investigated in rats trained in both tasks prior to the lesion. Quantitative analysis revealed a greater than 86% reduction in the dentate gyrus (DG) of the colchicine-injected rats compared to the sham-operated controls. Dentate gyrus damage rendered rats less efficient than sham-operated controls in the performance of the DRL-20s task. The DRL inter-response time (IRT) distribution for the DG-lesioned rats and the sham-operated controls was similar; however, while the distribution peak for the control rats was 20s, it was 16s for the DG-lesioned rats, indicating that the latter rats underestimated time. Performance of the DG-lesioned rats was also disrupted in the DNMTP task. However, DG-lesioned rats recovered control levels of performance during repeated training with an intertrial interval equal to 3s. An increase in intertrial interval in lesioned and sham-operated controls disrupted performance in both groups; however, while DG-lesioned rats performed at chance levels when the intertrial interval was increased to 4min or longer, the sham-operated controls performed at chance levels only when the intertrial interval was increased to 16min. These results seem most parsimoniously interpreted following the cognitive map theory of hippocampal function.

A Behavioral Assessment of Hippocampal Function Based on a Subregional Analysis

The purpose of this review is to determine whether specific subregions (dentate gyrus [DG], CA3, and CA1) of the hippocampus provide unique contributions to specific processes associated with intrinsic information processing exemplified by novelty detection, encoding, pattern separation, pattern association, pattern completion, retrieval, short-term memory and intermediate-term memory. Based on anatomical neural network organization, electrophysiology of cellular activity, lesions, early gene activation, and computational modeling, it can be shown that there exists extensive cooperation among the three subregions of the hippocampus, but there also exists reliable specificity of function for each of the subregions of the hippocampus. The primary process supported by the DG subregion of the hippocampus can be characterized by orthogonalization of sensory inputs to create a metric spatial representation. Furthermore the DG participates in conjunction with CA3 in supporting spatial pattern separation. The CA3 subregion of the hippocampus supports processes associated with spatial pattern association, spatial pattern completion, novelty detection, and short-term memory. The CA1 subregion of the hippocampus supports processes associated with temporal pattern association, temporal pattern completion, and intermediate-term memory. Furthermore, the CA3 in conjunction with CA1 supports temporal pattern separation. All the above-mentioned processes are assumed to reflect intrinsic processing of information within the hippocampus. The diversity of functions associated with the different subregions of the hippocampus suggests that one should not treat the hippocampus as a single entity, but rather that one should concentrate on elucidating further the functions of both dorsal and ventral subregions of the hippocampus and pathways that directly connect each of the subregions as well as their connections with the entorhinal cortex.

Transplantation of neurospheres after granule cell lesions in rats: cognitive improvements despite no long-term immunodetection of grafted cells

EGF-responsive C17 murine-derived neural stem cells (neurospheres) were grafted into the dentate gyrus of adult male rats after dentate granule cells lesions produced by colchicine injections. Behavioural performance was evaluated over two post-grafting periods, using tests sensitive to hippocampal dysfunctions. The first period began 1 month after grafting and testing conducted in the water maze and the radial maze distinguished working- and reference-memory performance. The second period began 9 months after grafting and learning performance was also evaluated in a Hebb-Williams maze, in addition to both other tests. The lesions induced lasting deficits in all tests. During the first period, the grafts had no effect in either test. Conversely, during the second period, grafted rats showed a weak improvement in the water maze and a significant increase of reference memory performance in the radial maze. In the Hebb-Williams maze, performance of grafted rats was close to normal. Strengthening the idea that dentate gyrus granule cells play an important role in the acquisition of new (perhaps more configural than only spatial) information, our results, moreover, suggest that neurosphere grafts may foster recovery after damage to point-to-point connection systems in the adult brain.

Involvement of microtubule integrity in memory impairment caused by colchicine

In order to fully evaluate the effects of colchicine treatment on learning ability in rats, colchicine was administered, and both Morris water maze (MWM) and step-through type passive avoidance (PA) learning tests were conducted. In both learning tests, infusion of colchicine into the rat dentate gyrus, at two distinct bilateral rostrocaudal locations, potently impaired memory function in a dose-dependent manner (0.01-2.0 microg/site), whereas systemic injection of colchicine (50-300 microg/kg) did not. In the MWM test, memory impairment was observed even at doses where there was no evidence of any histological changes in the dentate granule cells. This suggests that functional deterioration, that is, learning impairment was induced by the dysfunction of microtubules and/or axons, was caused by colchicine. Moreover, ameliorated learning behavior was observed with chronic treatment of beta-estradiol 3-benzoate, which has been suggested to have an important role as an adjuvant treatment for younger Alzheimer's disease (AD), immediately after colchicine infusion (0.3 microg). These results indicate that the animal model accompanying the colchicine-induced functional defect showing early tau pathology, but not neuronal cell degeneration, may well mimic comparatively early stage of AD.

Cognitive performances and locomotor activity following dentate granule cell damage in rats: role of lesion extent and type of memory tested

Intradentate injection of colchicine is one of the techniques used to destroy granule cells. This study compared the behavioral effects of various amounts of colchicine (1.0, 3.0, and 6.0 microg; Col 1, Col 3, and Col 6, respectively) injected into the dentate gyrus of adult Long-Evans male rats. Starting 10 days after lesion surgery, behavioral testing assessed home-cage and open-field locomotion, alternation in a T-maze, water-maze, and radial-maze learning according to protocols placing emphasis on reference, and working memory. All of these tasks are sensitive to hippocampal disruption. Histological verifications showed that the extent of the lesions depends on the dose of colchicine (index of dentate gyrus shrinkage: -33% in Col 1, -54% in Col 3, and -67% in Col 6 rats). Colchicine dose-dependently increased nocturnal home cage activity (an effect found 10 days but not 5 months after surgery), but had no significant effect on open-field locomotion or T-maze alternation. A dose-dependent reference memory impairment was found during the acquisition of spatial navigation in the water maze; Col 3 and Col 6 rats were more impaired than Col 1 rats. During the probe trial (platform removed), control rats spent a longer distance swimming over the platform area than all rats with colchicine lesions. In the working memory version of the test, all rats with colchicine lesions showed significant deficits. The deficits were larger in Col 3 and Col 6 rats compared to Col 1 rats. The lesions had no effect on swimming speed. In the radial-maze test, there was also a dose-dependent working memory impairment. However, reference memory was disrupted in a manner that did not differ among the three groups of lesioned rats. Our data are in line with the view that the dentate gyrus plays an important role in the acquisition of new information and is an integral neural substrate for spatial reference and spatial working memory. They also suggest that damage to granule cells might have more pronounced effects on reference than on working memory in the radial maze. Finally, they demonstrate that part of the variability in the conclusions from previous experiments concerning the role of granule cells in cognitive processes, particularly in spatial learning and memory, may be due to the type of tests used and/or the extent of the damage produced.

Hippocampus and contextual fear conditioning: recent controversies and advances

Dorsal hippocampal (DH) lesions produce a severe deficit in recently, but not remotely, acquired contextual fear without impairing memory of discrete training stimuli, i.e., DH lesions produce an anterograde and time-limited retrograde amnesia specific to contextual memory. These data are consistent with the standard model which posits temporary involvement of the hippocampus in recent memory maintenance. However, three recent controversies apparently weaken the case for a selective mnemonic role for the hippocampus in contextual fear. First, although retrograde amnesia (from posttraining lesions) is severe, anterograde amnesia (from pretraining lesions) may be mild or nonexistent. Second, a performance, rather than mnemonic, account of contextual freezing deficits in hippocampal-lesioned animals has been offered. Third, damage to the entire hippocampus, including the ventral hippocampus, can produce a dramatic and temporally stable disruption of context and tone fear. These data are reviewed and explanations are offered as to why they do not necessarily challenge the standard model of hippocampal memory function in contextual fear. Finally, a more complete description of the hippocampus' proposed role in contextual fear is offered, along with new data supporting this view. In summary, the data support a specific mnemonic role for the DH in the acquisition and consolidation of contextual representations.

Is the hippocampus necessary for contextual fear conditioning?

The hippocampus is widely believed to be essential for learning about the context in which conditioning occurs. This view is based primarily on evidence that lesions of the dorsal hippocampus disrupt freezing to contextual cues after fear conditioning. However, lesions that disrupt freezing produce no effect on fear-potentiated startle, a second measure of contextual fear. Moreover, hippocampal lesions also do not disrupt the contextual 'blocking' phenomenon, which provides an indirect measure of contextual fear. In these paradigms, at least, it appears that hippocampal lesions disrupt the expression of freezing, rather than contextual fear itself. This interpretation is supported by the finding that rats showing preserved contextual blocking after hippocampal lesions show deficits not only in contextual freezing, but also in unconditioned freezing. These findings are consistent with a growing body of data from other conditioning paradigms that contextual learning is spared after lesions of the dorsal hippocampus. Nonetheless, there remain some reports of impaired contextual fear conditioning after hippocampal lesions that cannot be attributed easily to a disruption of freezing. Thus, it is concluded that the hippocampus may be involved in contextual learning under certain--as yet, unspecified--circumstances, but is not critical for contextual learning in general.

Contextual fear, gestalt memories, and the hippocampus

This review examines the relationship between exploration and contextual fear conditioning. The fear acquired to places or contexts associated with aversive events is a form of Pavlovian conditioning. However, an initial period of exploration is necessary to allow the animal to form an integrated memory of the features of the context before conditioning can take place. The hippocampal formation plays a critical role in this process. Cells within the dorsal hippocampus are involved in the formation, storage and consolidation of this integrated representation of context. Projections from the subiculum to the nucleus accumbens regulate the exploration necessary for the acquisition of information about the features of the context. This model explains why electrolytic but not excitotoxic lesions of the dorsal hippocampus cause enhanced exploratory activity but both cause deficits in contextual fear. It also explains why retrograde amnesia of contextual fear is greater than anterograde amnesia.

Testing neural network models of memory with behavioral experiments

In recent years, a number of computational neural networks have been proposed aimed at describing memory functions associated with different subregions of the hippocampus, namely dentate gyrus, CA3 and CA1. Recent evidence suggests that indeed specific subregions of the hippocampus may subserve different computational functions, such as spatial and temporal pattern separation, short-term or working memory, pattern association, and temporal pattern completion.

Dentate gyrus-selective colchicine lesion and disruption of performance in spatial tasks: difficulties in "place strategy" because of a lack of flexibility in the use of environmental cues?

The effects of intradentate colchicine injections on the performance of tasks requiring spatial working and reference memory are controversial. Multiple-site colchicine injections (7 microg/microl; via a drawn micropipette) throughout the dentate gyrus (DG) of rats (nine sites in each hemisphere, 0.06 microl at each site) selectively destroy about 90% of the DG granule cells, as revealed by quantitative stereological estimates; stereology also revealed minor neuronal losses in the CA4 (33%) and CA1 (23%) subfields, but lack of damage to the CA3 hippocampal subfield. Spatial reference and working memory were assessed in Morris' water maze; in the reference memory task, the rats were required to learn a single, fixed location for the platform over several days of training; in the working memory task, animals were required to learn a new platform location every day, in a matching-to-place procedure. Compared to sham-operated controls, lesioned rats showed significant disruption in acquisition of the reference memory water maze task; however, the data reveal that these rats did acquire relevant information about the task, probably based on guidance and orientation strategies. In a subsequent probe test, with the platform removed, lesioned rats showed disruption in precise indexes of spatial memory (e.g., driving search towards the surroundings of the former platform location), but not in less precise indexes of spatial location. Finally, the lesioned rats showed no improvement in the match-to-place procedure, suggesting that their working memory for places was disrupted. Thus, although capable of acquiring relevant information about the task, possibly through guidance and/or orientation strategies, DG-lesioned rats exhibit a marked difficulty in place strategies. This is particularly evident when these rats are required to deal with one-trial place learning in a familiar environment, such as in the working memory version of the water maze task, which requires flexibility in the use of previously acquired information.

Temporally graded retrograde amnesia of contextual fear after hippocampal damage in rats: within-subjects examination

We have shown previously that electrolytic lesions of the dorsal hippocampus (DH) produce a severe deficit in contextual fear if made 1 d, but not 28 d, after fear conditioning (). As such, the hippocampus seems to play a time-limited role in the consolidation of contextual fear conditioning. Here, we examine retrograde amnesia of contextual fear produced by DH lesions in a within-subjects design. Unlike our previous reports, rats had both a remote and recent memory at the time of the lesion. Rats were given 10 tone-shock pairings in one context (remote memory) and 10 tone-shock pairings in a distinct context (with a different tone) 50 d later (recent memory), followed by DH or sham lesions 1 d later. Relative to controls, DH-lesioned rats exhibited no deficit in remote contextual fear, but recent contextual fear memory was severely impaired. They also did not exhibit deficits in tone freezing. This highly specific deficit in recent contextual memory demonstrated in a within-subjects design favors mnemonic over performance accounts of hippocampal involvement in fear. These findings also provide further support for a time-limited role of the hippocampus in memory storage.

Basolateral amygdala lesions block the disruptive effects of long-term adrenalectomy on spatial memory

The present study examined, in rats with N-methyl-D-aspartate-induced lesions of the basolateral amygdala, the effects of long-term adrenalectomy (i.e. 12-13 weeks) on memory for spatial and cued learning in a water maze. In sham amygdala-lesioned rats, adrenalectomy induced impairments in acquisition and retention performance for the spatial, but not the cued water-maze task. The adrenalectomized rats sustained selective degeneration and death of granule cells in the dentate gyrus dorsal blade. Continuous supplementation of the animals' drinking water with an extremely low dose of corticosterone (20 microg/ml) did not block the retention deficit, but blocked the acquisition deficit and the dentate gyrus neurodegenerative changes. The finding that the memory impairments and dentate gyrus neurodegeneration are dissociable supports the view that the adrenalectomy-induced memory effects are due to the loss of activational effects of circulating adrenal hormones at the time of learning. In adrenalectomized rats which received corticosterone as well as those which did not, lesions of the basolateral amygdala blocked the impairing effects of adrenalectomy on spatial learning and memory. However, the basolateral amygdala lesions did not affect the neurodegenerative changes in the dentate gyrus. In conclusion, the present findings provide further evidence that the basolateral amygdala is involved in regulating stress hormone effects on learning and memory.

Protective effect of a low dose of colchicine on the delayed cell death of hippocampal CA1 neurons following transient forebrain ischemia

Transient forebrain ischemia in rats preferentially causes neuron death in the striatum and hippocampal CA1 area. Prior injection of a low dose (1 microl) of colchicine (1 microM) into the unilateral hippocampus prevented ischemic damage of CA1 neurons in both hippocampal hemispheres, whereas no protection against ischemic damage was seen in the striatum. These results suggest that disconnection of hippocampal neurons by blockade of axoplasmic transport with colchicine specifically protects ischemic damage of CA1 neurons.

Retrograde degeneration and colchicine protection of basal forebrain cholinergic neurons following hippocampal injections of an immunotoxin against the P75 nerve growth factor receptor

Intracerebroventricular injection of 192 IgG antibody against the p75LNGFR rat low affinity nerve growth factor receptor conjugated with saporin, a ribosome inactivating protein, has been shown to destroy the p75LNGFR-expressing cholinergic neurons of the basal forebrain. We injected this immunotoxin into the hippocampus and studied its retrograde effect upon the cholinergic neurons of the medial septum and the vertical limb of the diagonal band of Broca. Seven days after injection, there was a nearly total depletion of cholinergic axons within the hippocampus. This depletion was associated with a marked and significant decrease in the number of cholinergic neurons of the ipsilateral medial septum and the vertical limb of the diagonal band of Broca. At longer survival times, these changes were more pronounced. Parvalbumin-positive, GABAergic neurons within the same areas of the basal forebrain were not affected by immunotoxin injections. Injections of saporin alone had no effect upon cholinergic neurons. Simultaneous injection of colchicine with the immunotoxin resulted in a significant reduction of retrograde degeneration of cholinergic neurons and relative preservation of hippocampal cholinergic axons. These observations suggest that 192 IgG-saporin is transported retrogradely from the hippocampus to the cholinergic neurons in the medial septum and the vertical limb of the diagonal band of Broca and provide a model for retrograde degeneration of basal forebrain cholinergic neurons following cortically based toxic-pathologic processes.

Effects of adrenalectomy on spatial memory performance and dentate gyrus morphology

Adrenalectomy (ADX) causes neuronal degeneration and cell loss in the dentate gyrus (DG) of the hippocampus. Since chemical or mechanical lesions of the DG are associated with impairments of spatial memory in rats, the effects of ADX on radial arm maze performance were evaluated. During 15 trials, where all 8 arms of the maze were baited, ADX rats were significantly impaired compared to sham operated controls (Shams). These trials were conducted 21-42 days post-ADX. Following these trials, time delays were instituted between the 4th and 5th choices, and ADX rats continued to show impaired performance. Daily intake of 3% saline was monitored in all rats and serum corticosterone (Cort) was measured. Saline consumption (ml/day) was higher in the ADX group (16.9 +/- 1.6 in ADX vs. 1.3 +/- 0.3 in Shams) and was negatively correlated with Cort level. Serum Cort (% microgram) differed between groups (0.6 +/- 0.4 vs. 15.0 +/- 2.3) and was negatively correlated with a greater number of maze errors, a measure of impaired performance. Cross sectional DG area was not reduced in ADX rats, and pyknotic cell number did not differ significantly between ADX and Sham animals. Moreover, pyknotic cell counts did not correlate with behavioral measures. These results lead to two conclusions: First, the recovery of accessory adrenal tissue in ADX rats, as indicated by the low levels of Cort, appears sufficient to suppress dentate granule neuron pyknosis, but may not be sufficient to suppress salt appetite.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular recordings in the colchicine-lesioned rat dentate gyrus following transplants of fetal dentate gyrus and CA1 hippocampal subfield tissue

Grafts of fetal dentate gyrus (DG) and CA1 hippocampal subfield tissue were extruded into the dentate gyri of adult male Sprague-Dawley rats, 7-10 days after lesioning the granule cells with colchicine (0.06 microliter of 7 mg/ml solution at each of 5 sites/hippocampus). Graft area-host and host-graft area connectivities were investigated 4-6 months post-transplantation by recoding extracellular evoked response in hippocampal slice preparations. Following stimulation of the host mid-molecular layer, evoked field potential responses, showing considerable variation, were recorded in both types of graft. Evoked responses in the lesioned DG without grafts were recorded in very few slices. Stimulation of the area of DG tissue grafts occasionally evoked responses in the host CA3/CA4 and there was no evidence for CA1 graft area-CA3/CA4 connectivity; stimulation of DG and CA1 graft areas occasionally evoked responses in the host CA1. Responses in the area of both DG and CA1 grafts supported short-term potentiation following stimulation of the host mid-molecular layer but only DG graft areas supported long-term potentiation of the population spike amplitude. In the area of both types of transplant a tonic bicuculline-sensitive inhibition was present and paired-pulse stimulation paradigms provided some evidence for inhibition. It is possible that responses recorded within the area of grafted tissue to stimulation of the host are attributable to host-graft connectivity and similarly, responses recorded in the host to stimulation of the area of the graft may be attributable to graft-host connectivity. Only DG graft areas received host inputs which were capable of sustaining a long-term potentiation and establishing efferent contacts with the host CA3/CA4 subfield, suggesting that these would be more likely than CA1 grafts to reinstate normal functional circuitry.

Serial injections of MK 801 (Dizocilpine) in neonatal rats reduce behavioral deficits associated with X-ray-induced hippocampal granule cell hypoplasia

MK 801 (NMDA antagonist) has been shown to protect newborns from hypoxia-induced brain damage. Here, we determined if (+)-5-methyl-10,11-dihydroxy-5h-dibenzo (a,d)cyclohepten-5,10-imine (MK 801) could attenuate behavioral deficits associated with early radiation-induced hypoplasia of fascia dentata granule cells. We pretreated neonatal rats (n = 20) with MK 801 (0, 0.1, or 0.2 mg/kg, IP) before each of eight fractionated, head-only doses of X-rays (13 Gy total) administered during the first 16 days postpartum. Other rats (n = 18) received the same drug treatments but were sham irradiated. At age 16 months, water-escape latencies to a submerged platform were measured in a water maze. Irradiated rats with hippocampal damage exhibited impaired learning (longer latencies to find the platform) than did sham-irradiated subjects. Moderate doses of MK 801 (0.1 mg/kg) facilitated the learning of the water maze by irradiated subjects but did not enhance the number of their fascia dentata granule cells. Higher doses (0.2 mg/kg) of MK 801 provided no behavioral benefits. In fact, this dose significantly impaired the learning of the water maze by sham-irradiated rats and potentiated the granule cell hypoplasia observed in irradiated subjects. Thus, early MK 801 treatment produces dose-dependent behavioral protection for rats with radiation-induced hippocampal damage. Future studies may reveal the neurophysiological and neuroanatomic substrates of this behavioral recovery.

The neurobiological effects of colchicine: modulation by nerve growth factor

To study the effects of exogenously applied nerve growth factor (NGF) on colchicine-induced neurodegeneration in the dentate gyrus of the rat hippocampal formation, colchicine (COLCH) or artificial cerebrospinal fluid (ACSF) was infused into the dorsal hippocampus (HPC) followed by unilateral infusion of either purified beta-NGF (in ACSF) or cytochrome C. One week later, animals were tested in activity chambers when NGF treatment was found to reduce the COLCH-induced hyperactivity. Animals were sacrificed 3 or 12 weeks postlesion for neurochemical or morphological analysis. Carbachol-stimulated phosphatidyl inositol (PI) turnover performed in hippocampal slices was not affected by any treatment at 3 weeks. However, 12 weeks after the lesion, CARB stimulation of PI hydrolysis was increased in the COLCH/ACSF group. NGF treatment significantly reduced the hyperstimulation in COLCH-treated rats. Morphological analysis showed that COLCH treatment increased AChE staining in the hippocampus, whereas NGF treatment had no effect on AChE staining. There was no difference in the number of septal ChAT immunoreactive cell bodies of controls or colchicine-treated rats at either time point examined. However, NGF treatment resulted in a significant increase in the number of ChAT immunoreactive cell bodies 3 weeks postlesion. Results from this study indicate that NGF can modify colchicine-induced compensatory changes in hippocampal signal transduction and has transitory influences on cholinergic cells in the medial septum.

Compensatory alterations in receptor-stimulated phosphoinositide hydrolysis in the hippocampus vary as a function of dose of colchicine

The stimulation of inositol phospholipid (PI) hydrolysis by various receptor agonists was measured in the hippocampus of rats 12 weeks after various concentrations (0.5-2.0 microgram/site) of colchicine were infused into the dentate gyrus. Colchicine produced a dose-related decrease in the average width and length of the granule cell line; the pyramidal cells in CA1 and CA3 regions of the hippocampus were affected only at higher concentrations of colchicine. Compensatory increases in receptor-mediated hydrolysis of phosphoinositides (PI) in hippocampal slices were seen at 100 microns carbachol and ibotenic acid in rats receiving 1.5-3.5 microgram colchicine/site. Compensatory increases in norepinephrine (100 microns) and N-methyl-D-aspartate (100 microns) stimulated PI were seen at 2.5-3.5 and 3.5 micrograms colchicine/site, respectively. Compensatory increases in PI hydrolysis were not seen in slices from animals receiving 0.5 microgram colchicine/site. These data support the hypothesis that the signal transduction system in the hippocampus undergoes a compensatory change following experimentally induced destruction of dentate gyrus granule cells. In addition, these changes occur for more than one neurotransmitter and the alterations vary as a function of the size of the lesion.

High doses of aspartame have no effects on sensorimotor function or learning and memory in rats

Acute or repeated (14 days) intragastric administration of L-d-aspartyl-L-phenylalanine methyl ester (aspartame) suspended in saline and Tween-80 in doses of up to 1,000 mg/kg had no significant effect in male Fischer-344 rats on routine measures of sensorimotor function, including spontaneous motor activity, acoustic startle reflex and prepulse inhibition. Other experiments found that aspartame (500 or 1,000 mg/kg) had no significant effect on acquisition of passive or active avoidance or a spatial, reference memory task in the Morris water maze. A series of separate studies found that aspartame had no effects in rats fasted 24 hours prior to testing, or if it was suspended in carboxymethylcellulose or administered by the intraperitoneal route. Under the conditions of these experiments, large doses of aspartame have no significant neurobiological effects in adult rats as measured by procedures known to be sensitive to the neurobiological effects of neurotoxicants, including convulsants, organochlorine insecticides and heavy metals.

Pretreatment effects on nitrite-induced methemoglobinemia: saline and calcium channel antagonists

This study was undertaken to evaluate the effects of pretreatment with calcium channel antagonists (nimodipine or verapamil) on the formation of methemoglobin produced by sodium nitrite. Unexpectedly, the pretreatment of animals with control injections of physiological saline 2 hours before the nitrite administration reduced the amount of methemoglobin found in the blood 25 minutes later. When either of the calcium channel antagonists was given 2 hours before the administration of sodium nitrite, the saline effect was eliminated. When the injections of physiological saline or either of the calcium channel blockers were divided 24 hours before the nitrite administration, all reduced the amount of methemoglobin formed relative to rats that received no pretreatment. A tentative hypothesis is that the reduction of the nitrite-induced methemoglobin can be induced by the stress of handling and intraperitoneal injection and that this stress effect can last at least 24 hours. It is likely that whatever stress-related mechanism is involved in reducing methemoglobin levels, this effect can be reduced by the presence of "L channel" voltage-sensitive calcium antagonists or their active metabolites.

Potentiation of inputs from the posterolateral amygdala to the dentate gyrus and resistance to stress ulcers formation in rats

Physical restraint was found to increase the activity of a number of multiple units in the lateral amygdala of rats. High-frequency electrical stimulation of units in the posterolateral amygdala increased the amplitudes of granule cell potentials in the dentate gyrus. This bilateral long-term potentiation (LTP) of inputs from posterior areas of the lateral amygdala also attenuated the severity of stress ulcers produced by physical restraint. This effect was reversed by intraventricular injections of the selective N-methyl-D-aspartate receptor blocker, aminophosphonovaleric acid. LTP in this pathway also reduced "struggling" behaviour during restraint. The data were interpreted to indicate that LTP in this temporal lobe pathway increased the coping ability because of faster habituation to stressors.

MK-801 impedes the acquisition of a spatial memory task in rats

Several studies have reported that MK-801 impairs the acquisition of various learning and memory tasks, while others suggest that MK-801 may interfere with performance rather than having a specific effect on memory. To characterize further the effects of MK-801 on learning and memory, MK-801 (0.05 mg/kg, SC) was administered prior to or immediately after learning trials in a trial-independent water maze task. Since MK-801 may affect nonassociative variables that may influence learning and memory, motor activity and general reactivity measures were also determined for 0.0125, 0.025, or 0.05 mg/kg of MK-801 administered SC. Since MK-801 may also be used to treat children with epilepsy, we investigated the possible persistent cognitive effects on neonates. MK-801 (0.02 mg/kg, SC) was administered at postnatal days 9-15 and tested in the same task as above starting at day 36 of age. There were no persistent effects of neonatal treatment. However, in adult rats, MK-801 impaired the acquisition of the water maze task but did not affect performance during a recall task in the same apparatus. At doses affecting learning, there were no effects on motor activity or general reactivity in adult rats. These results are consistent with the conclusion that MK-801 interferes with acquisition of spatial learning in the rat.

Differential effects of colchicine lesions of dentate granule cells on wet dog shakes and seizures elicited by direct hippocampal stimulation

Direct electrical stimulation of either the dorsal or ventral hippocampal formation elicits wet dog shakes and overt seizures. Destruction of dentate granule cells in the dorsal hippocampal formation does not significantly reduce the number of wet dog shakes elicited by ventral hippocampal stimulation. However, destruction of dentate granule cells in the ventral hippocampus virtually eliminates wet dog shaking elicited by dorsal hippocampal stimulation. Destruction of either dorsal or ventral dentate granule cells lowers the threshold for eliciting forelimb clonus with rearing. These results suggest that dentate granule cells in the ventral hippocampus are essential for wet dog shakes elicited by intrahippocampal stimulation. However, dentate granule cells throughout the hippocampal formation appear to play an important inhibitory role in the spread of seizure activity within the hippocampus.

Behavioral indices of neurotoxicity

There are many classes of chemicals widely used in a number of commercial and industrial processes having a potential to affect adversely the nervous system. Because of their relative sensitivity to some agents and general noninvasive characteristics, functional measurements of neurotoxicity are being used with greater frequency, especially at the level of hazard identification. The neurobehavioral test battery used by the National Toxicology Program (NTP) includes motor activity, fore- and hindlimb grip strength, acoustic startle response, responsiveness to an adverse thermal stimulus and general health and clinical measures (body weight, autonomic signs, tremor, convulsions). These tests have been used in nearly 40 studies involving various dosing regimens with rats and mice. The NTP battery shows several salient features of an effective screen, including the ability to differentiate known neurotoxicants from nonneurotoxicants, identify certain types of neurological sequelae or profiles of neurotoxic effects and construct dose- and time-response data in a screening context. The extent to which the NTP battery has predictive validity is still being evaluated.

Colchicine-induced alterations of reference memory in rats: role of spatial versus non-spatial task components

Male, Fischer-344 rats received bilateral injections of 2.5 micrograms of colchicine per site in the dorsal and ventral hippocampus. Intradentate colchicine preferentially destroyed dentate granule cells. Subsequent behavioral studies showed that 3 weeks after dosing, colchicine impaired the acquisition of a spatial, reference memory task in the Morris water maze. In a second group of rats, which were trained in the water maze prior to dosing, intradentate colchicine impaired retention of this task when rats were tested 3 weeks later. The acquisition of non-spatial reference memory task, an autoshape of a lever touch response in an operant chamber, was facilitated by prior administration of colchicine. Facilitative effects of colchicine were seen if delays of 0, 4, or 6 s were interposed between response and presentation of food reinforcement. If rats were trained to lever-touch with either a 0- or 4-s delay between response and reinforcement, intradentate colchicine had no effect on retention of the response 3 weeks later. These data are in accord with the conclusion that the dentate gyrus plays an important role in the acquisition of new information and retrieval of previously learned material and is an integral neural substrate for reference memory involving a spatial component.

Characterization of receptor-coupled phosphoinositide hydrolysis in the rat hippocampus after intradentate colchicine

Lesions produced by intradentate hippocampal administration of colchicine have been reported to produce several time-dependent behavioral and neurochemical changes, including a possible change in the signal transduction process for the cholinergic muscarinic receptor. To characterize further the effects of colchicine on receptor-coupled hydrolysis of phosphoinositides, colchicine was injected stereotaxically into the dentate gyrus of rats at a dose of 2.5 micrograms/site. The animals were killed 1, 3, or 12 weeks after injection and the hippocampi removed and sliced. [3H]Inositol was incorporated into slices, and various receptor agonists known to stimulate inositol phosphate (IP) metabolism were studied. Colchicine administration altered agonist-stimulated turnover in the hippocampus in a time-dependent manner. This hyperstimulation was receptor-mediated, because it was blocked by pirenzepine. The hyperstimulation of turnover was observed also with norepinephrine and serotonin. Colchicine had no effect on IP turnover in vitro. The effect of the colchicine lesion was observed only in the hippocampus, because no change in cholinergic muscarinic receptor-stimulated phosphatidylinositol turnover was observed in the cortex. These studies indicate that intradentate administration of colchicine produces a compensatory change in the signal transduction process in the hippocampus detectable 12 weeks after the lesion.

Functional deficits after sustained stimulation of the perforant path

Several reports have implicated the overactivity of hippocampal glutaminergic systems in neurodegenerative conditions including Senile dementia of the Alzheimer's type (SDAT). The neurobiological effects of hippocampal glutaminergic hyperactivity were studied by perforant pathway stimulation. Forty-five minutes of sustained perforant pathway stimulation produced a 50% or greater increase in motor activity 1, 2, and 3 weeks after stimulation. Robust retention deficits in a 48-h step-through passive avoidance task were evident 2 weeks post-stimulation. Furthermore, animals receiving stimulation were impaired in the acquisition of a spatial task in the Morris water maze. Stimulated animals exhibited little reduction in their escape latencies over the testing period. The learning and memory deficits were associated with a loss of CA1 and CA3 pyramidal cells and pretreatment with the N-methyl-D-aspartate antagonist MK-801 reduced this cell loss, particularly in the CA1 region of the hippocampus. These results suggest that sustained stimulation of the perforant pathway may be useful in studying neurological deficits associated with glutaminergic hyperfunction.

Selective destruction of mossy fibers and granule cells with preservation of the GABAergic network in the inferior region of the rat hippocampus after colchicine treatment

Lesions induced by colchicine injection into the rat hippocampus were investigated by means of electron microscopy and GABA immunocytochemistry. Granule cells were nearly completely destroyed 3 days after colchicine injection; since the necrosis of their axonal endings was delayed, an anterograde degeneration of the mossy fibers had probably taken place. The selectivity of the lesions was not limited to granule cells, for some pyramidal neurons in CA1 pyramidal layer were damaged. It was, however, striking to observe that throughout the hippocampal structure GABAergic neurons were spared from the effects of colchicine. For instance, GABAergic neurons were found in the vicinity of the completely destroyed granule cell layer. GABAergic neurons and terminals were also present in the CA3 region where the GABA-containing terminals formed a dense network of synapses with somata and dendrites of pyramidal cells. It was interesting to note that, consistent with previous studies, the GABAergic neurons in CA3 are innervated by mossy fibers. We conclude that after colchicine treatment the destruction of the granule cells was not associated with a lesion of the GABAergic network. This selective lesion provides a useful model with which to study the properties of CA3 neurons deprived of their major excitatory input but with an intact inhibitory network.

Enhanced acoustic startle responding in rats with radiation-induced hippocampal granule cell hypoplasia

Irradiation of the neonatal rat hippocampus reduces the proliferation of granule cells in the dentate gyrus and results in locomotor hyperactivity, behavioral perseveration and deficits on some learned tasks. In order to address the role of changes in stimulus salience and behavioral inhibition in animals with this type of brain damage, irradiated and normal rats were compared in their startle reactions to an acoustic stimulus. A portion of the brain of 10 rats was exposed to a fractionated total dose of 13 Gy during the first 16 days post partum. This procedure produced selective hypoplasia (91% reduction) of the granule cells in the hippocampal dentate gyrus. Other rats (N = 10) were sham irradiated. Sudden tones were presented to each adult rat at a rate of 1 every 30 s (spaced trials) during an initial 10-min session and 1 every 15 s (massed trials) during a subsequent session. Irradiated rats startled with a consistently higher amplitude than controls and were more likely to exhibit startle responses. These animals with hippocampal damage also failed to habituate to the startle stimulus and, under certain circumstances, showed potentiated startle responses after many tone presentations.

Colchicine-induced alterations in receptor-stimulated phosphoinositide hydrolysis in the rat hippocampus

Intradentate administration of colchicine has been reported to affect the cholinergic muscarinic system in the hippocampus, causing a reduction in quinclidinylbenzilate binding sites and an increase in choline acetyltransferase activity. Since the cholinergic muscarinic system is coupled to the formation of inositolphosphates in the brain, the effect of intradentate administration of colchicine on the agonist-induced turnover of inositollipid was studied in rats. The animals were sacrificed 12 weeks after injection, and the hippocampi removed and sliced. [3H]Inositol was incorporated into slices in the presence of lithium, and carbachol, a cholinergic muscarinic receptor agonist, was used to study the stimulated turnover of inositollipids. Hippocampal slices taken from colchicine-treated rats showed an increased carbachol-stimulated accumulation of inositolmonophosphate. Thus, intradentate colchicine appears to alter the signal transduction process for the muscarinic cholinergic receptor in the hippocampus, a change that may be associated with compensatory processes following damage to the hippocampus.

Intradentate colchicine impairs acquisition of a two-way active avoidance response in a Y-maze

Male, Fischer-344 rats received bilateral injections of colchicine (2.0 micrograms/site) into the dorsal and ventral hippocampus to destroy dentate gyrus granule cells. Colchicine impaired the ability of rats to learn a brightness discrimination, active avoidance task in a Y-maze. Acquisition was measured in 120-trial sessions given once weekly for 3 or 4 weeks. In a second experiment, colchicine-treated rats were tested in a modified procedure in which only the tone warning stimulus and a light directional cue were presented; shock was not presented. Colchicine-treated rats showed an enhanced response bias under these conditions, selecting to go to the unlighted arm with a greater frequency than the control rats whenever the tone and light were activated. These data are consistent with other reports indicating that rats with hippocampal lesions are impaired in the acquisition of a simultaneous discrimination if a response bias exists to one of the choices.

Fetal hippocampal cell suspensions ameliorate behavioral effects of intradentate colchicine in the rat

Colchicine, a neurotoxin that preferentially destroys dentate gyrus granule cells and mossy fibers, was injected into the hippocampus of adult rats. Three weeks later, the rats were tested for colchicine-induced hypermotility after which they received fetal hippocampal explants. Locomotor activity was retested three weeks later, after which the rats were trained over a period of four weeks on a food-reinforced, spatial, working memory task in an 8-arm radial maze. Fetal hippocampal explants were found to attenuate significantly the colchicine-induced hypermotility and spatial learning deficits. Histological observations showed the presence of surviving hippocampal explants in both the lesioned and the control rat brains, suggesting that the presence of viable implants facilitates the recovery of behavioral function in rats with spatial memory deficits.

Radial-arm maze deficits produced by colchicine administered into the area of the nucleus basalis are ameliorated by cholinergic agents

Rats were given bilateral injections of colchicine into the area of the nucleus basalis. Colchicine produced dose-dependent alterations in the acquisition of a food-reinforced working-memory task. Colchicine-induced deficits in maze performance were attenuated by cholinergic agents, including physostigmine, RS-86 (2-ethyl-8-methyl-2,8-diazospiro-(4,5)-decan-1,3-dione-hydro bromide) and nicotine. Naloxone and vasopressin did not affect radial-arm maze performance of colchicine-treated rats. Subsequent neurochemical analysis showed that colchicine decreased choline acetyltransferase (ChAT) activity and levels of norepinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, serotonin and 5-hydroxyindoleacetic acid in the neocortex. However, ChAT activity and other neurochemical measures were not altered in the hippocampus or corpus striatum. Histological assessment indicated damage limited to the injection in the area of the nucleus basalis and enlarged cerebrolateral ventricles. These data suggest the possible utility of the colchicine model in the study of cognitive deficits associated with neurodegenerative diseases.

Some behavioral effects of short-term exposure of rats to 2.45 GHz microwave radiation

Rats were tested for neurobehavioral alterations immediately after exposure to 2.45-GHz (CW) microwave radiation at 10 mW/cm2 for 7 h. Behavioral tests used were locomotor activity, startle to an acoustic stimulus and acquisition and retention of a shock-motivated passive avoidance task. Both horizontal and vertical components of locomotor activity were assessed in 5-min epochs for a period of 30 min using photoelectric detectors. Microwave-exposed animals exhibited less activity than sham-exposed animals. This was most evident during the last 10-15 min of the 30-min test session. Twenty identical acoustical stimuli (8 KHz, 110 dB) were delivered to each rat at 40-s intervals. The microwave-exposed animals were less responsive to the stimuli than sham-exposed animals. Microwave exposure had no effect on the retention of a passive avoidance procedure when tested at 1 week after training. Both the locomotor activity and acoustic startle data demonstrate that, under the conditions of this experiment, microwave exposure may alter responsiveness of rats to novel environmental conditions or stimuli.

Potential interactions between GABAb and cholinergic systems: baclofen augments scopolamine-induced performance deficits in the eight-arm radial maze

Sixteen male Fischer-344N rats were trained on a eight-arm radial maze task for food reinforcement. The effects of various doses of baclofen (1.25 or 2.50 mg/kg) and scopolamine (0.188, 0.375, and 0.750 mg/kg) were determined alone and in combination. Relative to vehicle controls, baclofen alone did not affect performance in the radial arm maze (number correct in the first eight responses, total errors) or the time required to complete the maze. Scopolamine alone decreased the number of correct responses in the first eight arm choices, while increasing both the number of errors and the time necessary to complete the maze. When the two drugs were co-administered, baclofen had no effect on the number of errors or time required to complete the maze in the presence of scopolamine; however, in combination with the high dose of scopolamine, it significantly increased the number of errors made during the first eight choices. Baclofen thus can exacerbate some radial arm maze dificits produced by an anticholinergic drug. In a subsequent experiment to test the interaction between scopolamine and baclofen using a nonlearned behavior, baclofen (1.25 and 2.5 mg/kg) did not affect motor activity, whereas all doses of scopolamine (0.188-0.75 mg/kg) increased activity. The higher dose of baclofen attenuated scopolamine-induced hypermotility by 50%, but the lower dose of baclofen was not effective. These data demonstrate pharmacological interactions between baclofen, a drug used clinically for spaticity, and a drug having anticholinergic properties.

Colchicine as an investigative tool in neurobiology

Neurotoxicants are being used with increasing frequency in neurobiology as investigative tools to study the structure and function of the nervous system. Colchicine administered directly into the hippocampus produces preferential destruction of dentate gyrus granule cells and mossy fibers and affects conditioned behavior. Studies from our laboratory have demonstrated the dose- and time-dependent loss of dentate gyrus granule cells and stimulation of motor activity following intradentate administration of colchicine. Preferential degeneration of pyramidal cells and other morphological changes observable at the light microscopic level were not seen in colchicine-treated rats. Other studies showed that intradentate colchicine produces specific damage to granule cells in both the dorsal and ventral hippocampus and that this damage is associated with depletion of dynorphin, a neuropeptide preferentially localized in the granule cells and mossy fibers of the hippocampus. Finally, it was noted that there are compensatory changes in the nervous system following treatment with colchicine and that the behavioral effects of colchicine can be modified by factors such as handling. One potentially important compensatory change occurring after intradentate colchicine is an alteration in cholinergic function. Pharmacological studies suggest that colchicine-treated rats may be less sensitive to the behavioral effects of scopolamine. These experiments support the conclusion that given at the appropriate dose into the hippocampus, colchicine may be a useful investigative tool to study the function of the dentate gyrus granule cells and mossy fibers, as well as the compensatory changes in the nervous system that follow chemical-induced neurodegeneration.