The chemotherapeutic agent paclitaxel selectively impairs reversal learning while sparing prior learning, new learning and episodic memory

Replay of incidentally encoded novel odors in the rat

Although events are not always known to be important when they occur, people can remember details about such incidentally encoded information using episodic memory. Sheridan et al. (2024) argued that rats replayed episodic memories of incidentally encoded information in an unexpected assessment of memory. In one task, rats reported the third-last item in an explicitly encoded list of trial-unique odors. In a second task, rats foraged in a radial maze in the absence of odors. On a critical test, rats foraged in the maze, but scented lids covered the food. Next, memory of the third-last odor was assessed. The rats correctly answered the unexpected question. Because the odors used in the critical test were the same as those used during training, automatically encoding odors for the purpose of taking an upcoming test of memory (stimulus generalization) may have been encouraged. Here, we provided an opportunity for incidental encoding of novel odors. Previously trained rats foraged in the radial maze with entirely novel odors covering the food. Next, memory of the third-last odor was assessed. The rats correctly answered the unexpected question. High accuracy when confronted with novel odors provides evidence that the rats did not automatically encode odors for the purpose of taking an upcoming test, ruling out stimulus generalization. We conclude that rats encode multiple pieces of putatively unimportant information, and later replayed a stream of novel episodic memories when that information was needed to solve an unexpected problem.

Effect of vitamin E on doxorubicin and paclitaxel-induced memory impairments in male rats

PURPOSE

In addition to peripheral neuronal dysfunction, conventional chemotherapy can be associated with other neurological treatment-limiting adverse effects, including cognitive dysfunction, memory impairment, and anxiety, which are referred to as "chemobrain". This study aimed to investigate the effects of doxorubicin (DOX) and paclitaxel (PAC) on learning and memory in rats using radial arm water maze (RAWM) and investigated a potential beneficial effect of vitamin E (Vit. E).

METHODS

Adult male rats were injected with four doses of 2 mg/kg/week DOX, or 2 mg/kg PAC every other day intraperitoneally. Vit. E was co-administered with these drugs in other groups to study its antioxidative effects. Using the RAWM, each rat was assessed for learning and memory performance through two sets of six trials separated by a 5-min rest period evaluating both short- and long-term effects on memory.

RESULTS

There was no deficit in learning or long-term memory in both drug groups compared to control. However, rats in both drug groups made significantly more errors in all short-term memory trials. This effect was mitigated when Vit. E was co-administered with either drug. Moreover, PAC (but not DOX) induced hippocampal lipid peroxidation by increasing the levels of standard biomarker thiobarbituric acid reactive substances (TBARS). Interestingly, Vit. E prevented PAC-induced hippocampal oxidative stress. Furthermore, both DOX and PAC were correlated with reduction in Brain-Derived Neurotrophic Factor (BDNF) expression levels in the hippocampus, which was overcome by the co-administration of Vit. E.

CONCLUSION

There is a potential role of Vit. E in alleviating short-term memory impairment in rats exposed to chemotherapy, possibly by reducing hippocampal oxidative stress and neurodegeneration.

Paclitaxel induces cognitive impairment via necroptosis, decreased synaptic plasticity and M1 polarisation of microglia

CONTEXT

Paclitaxel (PTX) leads to chemotherapy brain (chemo-brain) which is characterised by cognitive impairment. It has been reported that necroptosis is associated with cognitive impairment in some neurodegenerative diseases, but it is not clear whether it is related to the development of chemo-brain.

OBJECTIVE

To investigate the role of necroptosis and related changes in PTX-induced cognitive impairment.

MATERIALS AND METHODS

C57bl/6n mice were randomly divided into five groups: control, vehicle, and different concentrations of PTX (6, 8, 10 mg/kg). Two additional groups received pre-treatment with Gdcl3 or PBS through Intracerebroventricular (ICV) injection before PTX-treatment. Cognitive function, necroptosis, synaptic plasticity and microglia polarisation were analysed.

RESULTS

PTX (10 mg/kg) induced significant cognitive impairment, accompanied by changes in synaptic plasticity, including decreased density of PSD95 (0.65-fold), BDNF (0.44-fold) and dendritic spines (0.57-fold). PTX induced necroptosis of 53.41% (RIP3) and 61.91% (MLKL) in hippocampal neurons, with high expression of RIP3 (1.58-fold) compared with the control group. MLKL (1.87-fold) exhibited the same trend, reaching a peak on the 14th day. The increased expression of iNOS (1.63-fold) and inflammatory factors such as TNF-α (1.85-fold) and IL-β (1.89-fold) compared to the control group suggests that M1 polarisation of microglia is involved in the process of cognitive impairment. Pre-treatment with Gdcl3 effectively reduced the number of microglia (0.50-fold), inhibited the release of TNF-α (0.73-fold) and IL-β (0.56-fold), and improved cognitive impairment.

CONCLUSION

We established a stable animal model of PTX-induced cognitive impairment and explored the underlying pathophysiological mechanism. These findings can guide the future treatment of chemo-brain.

Episodic Memory and Recollection Network Disruptions Following Chemotherapy Treatment in Breast Cancer Survivors: A Review of Neuroimaging Findings

Simple Summary

Memory disturbances are amongst the most common and disruptive symptoms of chemotherapy-related cognitive impairment. Chemotherapy treatments commonly cause neurotoxicity within the hippocampus, creating a vulnerability to memory impairment. Most clinical assessments of long-term memory in breast cancer survivors assess basic verbal and visual memory processing, and do not capture the complexities of everyday event memories, including episodic and autobiographical memory. This review focuses on structural and functional neuroimaging studies identifying disruptions in the hippocampus and recollection network, and related episodic memory impairments in chemotherapy-treated breast cancer survivors. We argue for the need to better characterize memory dysfunction following chemotherapy treatments. Given the importance of episodic and autobiographical memory to a person’s personal history and quality of life, an under-appreciation of how this memory domain is impacted by standard cancer treatments potentially diminishes the negative experiences of breast cancer survivors, and neglects cognitive problems that could benefit from intervention strategies.

Abstract

Long-term memory disturbances are amongst the most common and disruptive cognitive symptoms experienced by breast cancer survivors following chemotherapy. To date, most clinical assessments of long-term memory dysfunction in breast cancer survivors have utilized basic verbal and visual memory tasks that do not capture the complexities of everyday event memories. Complex event memories, including episodic memory and autobiographical memory, critically rely on hippocampal processing for encoding and retrieval. Systemic chemotherapy treatments used in breast cancer commonly cause neurotoxicity within the hippocampus, thereby creating a vulnerability to memory impairment. We review structural and functional neuroimaging studies that have identified disruptions in the recollection network and related episodic memory impairments in chemotherapy-treated breast cancer survivors, and argue for the need to better characterize hippocampally mediated memory dysfunction following chemotherapy treatments. Given the importance of autobiographical memory for a person’s sense of identity, ability to plan for the future, and general functioning, under-appreciation of how this type of memory is impacted by cancer treatment can lead to overlooking or minimizing the negative experiences of breast cancer survivors, and neglecting a cognitive domain that may benefit from intervention strategies.

Sex Differences in Taxane Toxicities

Simple Summary

Clinically observed sex differences in acute and long-term taxane chemotherapy-induced normal tissue toxicity are routinely documented but remain poorly understood despite the significant impact such toxicities have on treatment tolerance and quality of life outcomes in cancer survivors. This review draws from pre-clinical and clinical literature to highlight sex-specific mechanisms of action in taxane drug toxicity and proposes hypotheses for sex-specific clinical discrepancies in taxane-induced acute and long-term toxicities. To our knowledge, this is the first review exploring how sex as a biological variable impacts taxane-mediated mechanisms of action and clinical outcomes. In doing so, we have provided a novel framework to investigate and understand common sex differences observed in clinical and pre-clinical research.

Abstract

The taxane family of microtubule poisons and chemotherapeutics have been studied for over 50 years and are among the most frequently used antineoplastic agents today. Still, limited research exists characterizing taxane-induced sex-specific mechanisms of action and toxicities in cancer and non-cancerous tissue. Such research is important to advance cancer treatment outcomes as well as to address clinically observed sex-differences in short- and long-term taxane-induced toxicities that have disproportionate effects on female and male cancer patients. To gain more insight into these underlying differences between the sexes, the following review draws from pre-clinical and clinical paclitaxel and taxane oncology literature, examines sex-discrepancies, and highlights uncharacterized sex-dependent mechanisms of action and clinical outcomes. To our knowledge, this is the first literature review to provide a current overview of the basic and clinical sex dimorphisms of taxane-induced effects. Most importantly, we hope to provide a starting point for improving and advancing sex-specific personalized chemotherapy and cancer treatment strategies as well as to present a novel approach to review sex as a biological variable in basic and clinical biology.

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.

Chemotherapy-Induced Cognitive Impairment and Hippocampal Neurogenesis: A Review of Physiological Mechanisms and Interventions

A wide range of cognitive deficits, including memory loss associated with hippocampal dysfunction, have been widely reported in cancer survivors who received chemotherapy. Changes in both white matter and gray matter volume have been observed following chemotherapy treatment, with reduced volume in the medial temporal lobe thought to be due in part to reductions in hippocampal neurogenesis. Pre-clinical rodent models confirm that common chemotherapeutic agents used to treat various forms of non-CNS cancers reduce rates of hippocampal neurogenesis and impair performance on hippocampally-mediated learning and memory tasks. We review the pre-clinical rodent literature to identify how various chemotherapeutic drugs affect hippocampal neurogenesis and induce cognitive impairment. We also review factors such as physical exercise and environmental stimulation that may protect against chemotherapy-induced neurogenic suppression and hippocampal neurotoxicity. Finally, we review pharmacological interventions that target the hippocampus and are designed to prevent or reduce the cognitive and neurotoxic side effects of chemotherapy.

Memantine Protects against Paclitaxel-Induced Cognitive Impairment through Modulation of Neurogenesis and Inflammation in Mice

Chemotherapy-induced cognitive impairment (CICI) is an adverse side effect of cancer treatment with increasing awareness. Hippocampal damage and related neurocognitive impairment may mediate the development of CICI, in which altered neurogenesis may play a role. In addition, increased inflammation may be related to chemotherapy-induced hippocampal damage. Memantine, an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist that may enhance neurogenesis and modulate inflammation, may be useful for treating CICI. To test this hypothesis, paclitaxel was administered to eight-week-old male B6 mice to demonstrate the relationship between CICI and impaired neurogenesis, and then, we evaluated the impact of different memantine regimens on neurogenesis and inflammation in this CICI model. The results demonstrated that both the pretreatment and cotreatment regimens with memantine successfully reversed impaired neurogenesis and spatial memory impairment in behavior tests. The pretreatment regimen unsuccessfully inhibited the expression of peripheral and central TNF-α and IL-1β and did not improve the mood alterations following paclitaxel treatment. However, the cotreatment regimen led to a better modulatory effect on inflammation and restoration of mood disturbance. In conclusion, this study illustrated that impaired neurogenesis is one of the mechanisms of paclitaxel-induced CICI. Memantine may serve as a potential treatment for paclitaxel-induced CICI, but different treatment strategies may lead to variations in the treatment efficacy.

An adolescent rat model of vincristine-induced peripheral neuropathy

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Vincristine treatment in adolescent rat induces significant mechanical and cold allodynia and muscle weakness.

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Voluntary exercise prevents vincristine-induced peripheral neuropathy.

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Vincristine treatment during early adolescence produces more severe peripheral neuropathy than treatment during late adolescence.

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Peripheral neuropathy induced by vincristine during adolescence persists into early adulthood.

Childhood acute lymphoblastic leukemia (ALL) is a significant clinical problem that can be effectively treated with vincristine, a vinca alkaloid-based chemotherapeutic agent. However, nearly all children receiving vincristine treatment develop vincristine-induced peripheral neuropathy (VIPN). The impact of adolescent vincristine treatment across the lifespan remains poorly understood. We, consequently, developed an adolescent rodent model of VIPN which can be utilized to study possible long term consequences of vincristine treatment in the developing rat. We also evaluated the therapeutic efficacy of voluntary exercise and potential impact of obesity as a genetic risk factor in this model on the development and maintenance of VIPN. Out of all the dosing regimens we evaluated, the most potent VIPN was produced by fifteen consecutive daily intraperitoneal (i.p.) vincristine injections at 100 µg/kg/day, throughout the critical period of adolescence from postnatal day 35 to 49. With this treatment, vincristine-treated animals developed hypersensitivity to mechanical and cold stimulation of the plantar hind paw surface, which outlasted the period of vincristine treatment and resolved within two weeks following the cessation of vincristine injection. By contrast, impairment in grip strength gain was delayed by vincristine treatment, emerging shortly following the termination of vincristine dosing, and persisted into early adulthood without diminishing. Interestingly, voluntary wheel running exercise prevented the development of vincristine-induced hypersensitivities to mechanical and cold stimulation. However, Zucker fa/fa obese animals did not exhibit higher risk of developing VIPN compared to lean rats. Our studies identify sensory and motor impairments produced by vincristine in adolescent animals and support the therapeutic efficacy of voluntary exercise for suppressing VIPN in developing rats.

Effects of taxol on neuronal differentiation of postnatal neural stem cells cultured from mouse subventricular zone

Taxol (paclitaxel), a chemotherapeutic agent for several cancers, can adversely affect the peripheral nervous system. Recently, its negative impact on cognitive function in cancer patients has become evident. In rodents, taxol impaired learning and memory, with other possible negative effects on the brain. In this study, we investigated the effects of taxol on cultured neural stem cells (NSCs) from the mouse neurogenic region, the subventricular zone (SVZ). Taxol significantly decreased both proliferation and neuronal differentiation of NSCs. Transient treatment with taxol for one day during a 4-day differentiation greatly decreased neurogenesis along with an abnormal cell cycle progression. Yet, taxol did not kill differentiated Tuj1+ neurons and those neurons had longer neurites than neurons under control conditions. For glial differentiation, taxol significantly reduced oligodendrogenesis as observed by immunostaining for Olig2 and O4. However, differentiation of astrocytes was not affected by taxol. In contrast, differentiated oligodendrocytes were extremely sensitive to taxol. Almost no Olig2-positive cells were observed after three days of treatment with taxol. Taxol has distinct effects on neurons and glial cells during their production through differentiation from NSCs as well as post-differentiation. Thus, we suggest that taxol might interfere with neurogenesis of NSCs possibly through a disturbance in the cell cycle and may eliminate differentiated oligodendrocytes.

Nicotinamide riboside relieves paclitaxel-induced peripheral neuropathy and enhances suppression of tumor growth in tumor-bearing rats

Nicotinamide riboside (NR) is a vitamin B3 precursor of NAD that blunts diabetic and chemotherapy-induced peripheral neuropathy in preclinical models. This study examined whether NR also blunts the loss of intraepidermal nerve fibers induced by paclitaxel, which is associated with peripheral neuropathy. The work was conducted in female rats with N-methyl-nitrosourea (MNU)-induced tumors of the mammary gland to increase its translational relevance, and to assess the interaction of NR with paclitaxel and NR's effect on tumor growth. Once daily oral administration of 200 mg/kg NR p.o. beginning with the first of 3 i.v. injections of 6.6 mg/kg paclitaxel to tumor-bearing rats significantly decreased paclitaxel-induced hypersensitivity to tactile and cool stimuli, as well as place-escape avoidance behaviors. It also blunted the loss of intraepidermal nerve fibers in tumor-bearing rats, as well as a separate cohort of tumor-naive rats. Unexpectedly, concomitant administration of NR during paclitaxel treatment further decreased tumor growth; thereafter, tumor growth resumed at the same rate as vehicle-treated controls. Administration of NR also decreased the percentage of Ki67-positive tumor cells in these rats. Once daily administration of NR did not seem to alter tumor growth or the percentage of Ki67-positive tumor cells in rats that were not treated with paclitaxel and followed for 3 months. These results further support the ability of NR to play a protective role after nerve injury. They also suggest that NR may not only alleviate peripheral neuropathy in patients receiving taxane chemotherapy, but also offer an added benefit by possibly enhancing its tumor-suppressing effects.

Animal models of chemotherapy-induced cognitive decline in preclinical drug development

RATIONALE

Chemotherapy-induced cognitive impairment (CICI), chemobrain, and chemofog are the common terms for mental dysfunction in a cancer patient/survivor under the influence of chemotherapeutics. CICI is manifested as short/long term memory problems and delayed mental processing, which interferes with a person's day-to-day activities. Understanding CICI mechanisms help in developing therapeutic interventions that may alleviate the disease condition. Animal models facilitate critical evaluation to elucidate the underlying mechanisms and form an integral part of verifying different treatment hypotheses and strategies.

OBJECTIVES

A methodical evaluation of scientific literature is required to understand cognitive changes associated with the use of chemotherapeutic agents in different preclinical studies. This review mainly emphasizes animal models developed with various chemotherapeutic agents individually and in combination, with their proposed mechanisms contributing to the cognitive dysfunction. This review also points toward the analysis of chemobrain in healthy animals to understand the mechanism of interventions in absence of tumor and in tumor-bearing animals to mimic human cancer conditions to screen potential drug candidates against chemobrain.

RESULTS

Substantial memory deficit as a result of commonly used chemotherapeutic agents was evidenced in healthy and tumor-bearing animals. Spatial and episodic cognitive impairments, alterations in neurotrophins, oxidative and inflammatory markers, and changes in long-term potentiation were commonly observed changes in different animal models irrespective of the chemotherapeutic agent.

CONCLUSION

Dyscognition exists as one of the serious side effects of cancer chemotherapy. Due to differing mechanisms of chemotherapeutic agents with differing tendencies to alter behavioral and biochemical parameters, chemotherapy may present a significant risk in resulting memory impairments in healthy as well as tumor-bearing animals.

Cancer Chemotherapy Related Cognitive Impairment and the Impact of the Alzheimer's Disease Risk Factor APOE

Cancer related cognitive impairment (CRCI) is a serious impairment to maintaining quality of life in cancer survivors. Cancer chemotherapy contributes to this condition through several potential mechanisms, including damage to the blood brain barrier, increases in oxidative stress and inflammation in the brain, and impaired neurogenesis, each of which lead to neuronal dysfunction. A genetic predisposition to CRCI is the E4 allele of the Apolipoprotein E gene (APOE), which is also the strongest genetic risk factor for Alzheimer's disease. In normal brains, APOE performs essential lipid transport functions. The APOE4 isoform has been linked to altered lipid binding, increased oxidative stress and inflammation, reduced turnover of neural progenitor cells, and impairment of the blood brain barrier. As chemotherapy also affects these processes, the influence of APOE4 on CRCI takes on great significance. This review outlines the main areas where APOE genotype could play a role in CRCI. Potential therapeutics based on APOE biology could mitigate these detrimental cognitive effects for those receiving chemotherapy, emphasizing that the APOE genotype could help in developing personalized cancer treatment regimens.

Inhaled Cannabis Suppresses Chemotherapy-Induced Neuropathic Nociception by Decoupling the Raphe Nucleus: A Functional Imaging Study in Rats

BACKGROUND

Efficacy of inhaled cannabis for treating pain is controversial. Effective treatment for chemotherapy-induced neuropathy represents an unmet medical need. We hypothesized that cannabis reduces neuropathic pain by reducing functional coupling in the raphe nuclei.

METHODS

We assessed the impact of inhalation of vaporized cannabis plant (containing 10.3% Δ⁹-tetrahydrocannabinol/0.05% cannabidiol) or placebo cannabis on brain resting-state blood oxygen level-dependent functional connectivity and pain behavior induced by paclitaxel in rats. Rats received paclitaxel to produce chemotherapy-induced peripheral neuropathy or its vehicle. Behavioral and imaging experiments were performed after neuropathy was established and stable. Images were registered to, and analyzed using, a 3D magnetic resonance imaging rat atlas providing site-specific data on more than 168 different brain areas.

RESULTS

Prior to vaporization, paclitaxel produced cold allodynia. Inhaled vaporized cannabis increased cold withdrawal latencies relative to prevaporization or placebo cannabis, consistent with Δ⁹-tetrahydrocannabinol-induced antinociception. In paclitaxel-treated rats, the midbrain serotonergic system, comprising the dorsal and median raphe, showed hyperconnectivity to cortical, brainstem, and hippocampal areas, consistent with nociceptive processing. Inhalation of vaporized cannabis uncoupled paclitaxel-induced hyperconnectivity patterns. No such changes in connectivity or cold responsiveness were observed following placebo cannabis vaporization.

CONCLUSIONS

Inhaled vaporized cannabis plant uncoupled brain resting-state connectivity in the raphe nuclei, normalizing paclitaxel-induced hyperconnectivity to levels observed in vehicle-treated rats. Inhaled vaporized cannabis produced antinociception in both paclitaxel- and vehicle-treated rats. Our study elucidates neural circuitry implicated in the therapeutic effects of Δ⁹-tetrahydrocannabinol and supports a role for functional imaging studies in animals in guiding indications for future clinical trials.

Taxane-induced neurotoxicity: Pathophysiology and therapeutic perspectives

Taxane-derived drugs are antineoplastic agents used for the treatment of highly common malignancies. Paclitaxel and docetaxel are the most commonly used taxanes; however, other drugs and formulations have been used, such as cabazitaxel and nab-paclitaxel. Taxane treatment is associated with neurotoxicity, a well-known and relevant side effect, very prevalent amongst patients undergoing chemotherapy. Painful peripheral neuropathy is the most dose-limiting side effect of taxanes, affecting up to 97% of paclitaxel-treated patients. Central neurotoxicity is an emerging side effect of taxanes and it is characterized by cognitive impairment and encephalopathy. Besides impairing compliance to chemotherapy treatment, taxane-induced neurotoxicity (TIN) can adversely affect the patient's life quality on a long-term basis. Despite the clinical relevance, not many reviews have comprehensively addressed taxane-induced neurotoxicity when they are used therapeutically. This article provides an up-to-date review on the pathophysiology of TIN and the novel potential therapies to prevent or treat this side effect.

Voluntary exercise reduces both chemotherapy-induced neuropathic nociception and deficits in hippocampal cellular proliferation in a mouse model of paclitaxel-induced peripheral neuropathy

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Paclitaxel treatment did not alter voluntary running activity.

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Voluntary running reduced mechanical and cold allodynia induced by paclitaxel.

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Voluntary running reduced paclitaxel-induced deficits in hippocampal cellular proliferation.

Chemotherapy-induced peripheral neuropathy (CIPN) is a common dose-limiting side-effect of all major chemotherapeutic agents. Here, we explored efficacy of voluntary exercise as a nonpharmacological strategy for suppressing two distinct adverse side effects of chemotherapy treatment. We evaluated whether voluntary running would suppress both neuropathic pain and deficits in hippocampal cell proliferation in a mouse model of CIPN induced by the taxane chemotherapeutic agent paclitaxel. Mice were given free access to running wheels or were housed without running wheels during one of three different intervention phases: 1) during the onset (i.e. development phase) of paclitaxel-induced neuropathy, 2) prior to dosing with paclitaxel or its vehicle, or 3) following the establishment (i.e. maintenance phase) of paclitaxel-induced neuropathy. Paclitaxel treatment did not alter running wheel behavior relative to vehicle-treated animals in any study. Animals that engaged in voluntary running during the development phase of paclitaxel-induced neuropathy failed to display mechanical or cold hypersensitivities relative to sedentary control animals that did not have access to running wheels. A prior history of voluntary running delayed the onset of, but did not fully prevent, development of paclitaxel-induced neuropathic pain behavior. Voluntary running reduced already established mechanical and cold allodynia induced by paclitaxel. Importantly, voluntary running did not alter mechanical or cold responsivity in vehicle-treated animals, suggesting that the observed antinociceptive effect of exercise was dependent upon the presence of the pathological pain state. In the same animals evaluated for nociceptive responding, paclitaxel also reduced cellular proliferation but not cellular survival in the dentate gyrus of the hippocampus, as measured by immunohistochemistry for Ki67 and BrdU expression, respectively. Voluntary running abrogated paclitaxel-induced reductions in cellular proliferation to levels observed in vehicle-treated mice and also increased BrdU expression levels irrespective of chemotherapy treatment. Our studies support the hypothesis that voluntary exercise may be beneficial in suppressing both neuropathic pain and markers of hippocampal cellular function that are impacted by toxic challenge with chemotherapeutic agents.

Alterations in brain neurocircuitry following treatment with the chemotherapeutic agent paclitaxel in rats

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Imaging the reorganization of pain neural circuitry within 8 days of chemotherapy.

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Using rat model of neuropathy with multimodal MRI.

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Showing loss of anticorrelation between prefrontal cortex and PAG.

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Identifying the interaction between periaqueductal gray and brainstem raphe.

Human and animal studies suggest that both traumatic nerve injury and toxic challenge with chemotherapeutic agents involves the reorganization of neural circuits in the brain. However, there have been no prospective studies, human or animal, using magnetic resonance imaging (MRI) to identify changes in brain neural circuitry that accompany the development of chemotherapy-induced neuropathic pain (i.e. within days following cessation of chemotherapy treatment and without the confound cancer). To this end, different MRI protocols were used to ascertain whether a reorganization of brain neural circuits is observed in otherwise normal rats exposed to the taxane chemotherapeutic agent paclitaxel.

We conducted an imaging study to evaluate the impact of a well-established paclitaxel dosing regimen, validated to induce allodynia in control rats within eight days of treatment, on brain neural circuitry. Rats received either paclitaxel (2 mg/kg/day i.p; cumulative dose of 8 mg/kg) or its vehicle four times on alternate days (i.e. day 0, 2, 4, 6). Following the cessation of treatments (i.e. on day 8), all rats were tested for responsiveness to cold followed by diffusion weighted magnetic resonance imaging and assessment of resting state functional connectivity. Imaging data were analyzed using a 3D MRI rat with 173 segmented and annotated brain areas.

Paclitaxel-treated rats were more sensitive to a cold stimulus compared to controls. Diffusion weighted imaging identified brain areas involved in the emotional and motivational response to chronic pain that were impacted by paclitaxel treatment. Affected brain regions included the prefrontal cortex, amygdala, hippocampus, hypothalamus and the striatum/nucleus accumbens. This putative reorganization of gray matter microarchitecture formed a continuum of brain areas stretching from the basal medial/lateral forebrain to the midbrain. Resting state functional connectivity showed reorganization between the periaqueductal gray, a key node in nociceptive neural circuitry, and connections to the brainstem.

Our results, employing different imaging modalities to assess the central nervous system effects of chemotherapy, fit the theory that chronic pain is regulated by emotion and motivation and influences activity in the periaqueductal gray and brainstem to modulate pain perception.

Proinflammatory Factors Mediate Paclitaxel-Induced Impairment of Learning and Memory

The chemotherapeutic agent paclitaxel is widely used for cancer treatment. Paclitaxel treatment impairs learning and memory function, a side effect that reduces the quality of life of cancer survivors. However, the neural mechanisms underlying paclitaxel-induced impairment of learning and memory remain unclear. Paclitaxel treatment leads to proinflammatory factor release and neuronal apoptosis. Thus, we hypothesized that paclitaxel impairs learning and memory function through proinflammatory factor-induced neuronal apoptosis. Neuronal apoptosis was assessed by TUNEL assay in the hippocampus. Protein expression levels of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) in the hippocampus tissue were analyzed by Western blot assay. Spatial learning and memory function were determined by using the Morris water maze (MWM) test. Paclitaxel treatment significantly increased the escape latencies and decreased the number of crossing in the MWM test. Furthermore, paclitaxel significantly increased the number of TUNEL-positive neurons in the hippocampus. Also, paclitaxel treatment increased the expression levels of TNF-α and IL-1β in the hippocampus tissue. In addition, the TNF-α synthesis inhibitor thalidomide significantly attenuated the number of paclitaxel-induced TUNEL-positive neurons in the hippocampus and restored the impaired spatial learning and memory function in paclitaxel-treated rats. These data suggest that TNF-α is critically involved in the paclitaxel-induced impairment of learning and memory function.

Comparative Cognition: Rats Pay Back Quid Pro Quo

Humans engage in exchanges of commodities or services, often paying back a commodity with a different service. New research suggests that rats can reciprocally trade food for allogrooming, and vice versa.