Neuroplasticity in Obesity
Original Editor - Srishti Banerjee
Introduction[edit | edit source]
Obesity leads to dysregulation of central neurocircuits which control autonomic, metabolic and cognitive functions. It has been established that in response to high fat diet (HFD) there is activation of astrocytes and neuroinflammation sets in. i.e. inflammatory responses that are centralised within the brain and spinal cord.
- Neuroinflammation causes dysregulation of developed central neurocircuits which are involved in energy homeostasis.
- Recent studies have shown that neuroinflammation sets in the areas regulating the food intake just after 1 day post exposure to HFD
Image 1: Neural signalling in the human brain.
Astroglial Neuroplasticity[edit | edit source]
Astroglial modulation is done by brain derived neurotrophic factors(BDNF) which plays an important role in neural plasticity. It does this by supporting motor neuron survival and promoting axonal growth of motor and sensory neurons.
Astrocytic modulation of excitatory synaptic transmission leads to neurodegeneration and excitotoxicity throughout the CNS. Excitotoxicity is a complex process triggered by glutamate receptor activation that results in the degeneration of dendrites and cell death.
Imaging studies have shown that obesity: Leads to reduced cortical thicknesswhich leads to neuronal reduction and impaired synaptic plasticity; Can lead to age related white matter changes, poor cognition , impaired memory and learning
- One of the mechanism which can be attributed to all of these changes is reduced levels and signalling of BDNF in obesity.
- Secondly, cognitive decline and impaired synaptic plasticity is attributed to systemic and central inflammation where pro-inflammatory cytokines, derived from the adipose tissues, suppresses the neuronal function in brain.
Image 2: Astrocyte
Changes in Brainstem[edit | edit source]
The brainstem undergoes neuroplastic changes in response to diet. It has been established that diet induced obesity (DIO) decreases the intrinsic excitability of vagal afferent and efferent neurons which also includes decrease in responsiveness of satiety peptides such as leptin , glucagon like peptide 1 and cholecystokinin.
- The decrease in afferent excitability reduces the stomach tone and increases gastric and fasting volume as the chemical and mechanical stimuli effects are reduced.
- All this eventually increases the volume of food required for satiation therefore leading to an increased food intake and meal size which develops and maintains obesity
Image 3: brainstem.
Changes in Hypothalamus[edit | edit source]
Leptin plays a pivotal role in food intake and energy balance. Involvement of leptin in astroglial modulation is essentially important in food intake behaviors. Leptin enters the CNS via a saturable transport system and it acts within the hypothalamus to induce satiation and reduces the food intake.
- Exposure to HFD leads to hyperleptinemia which causes leptin insensitivity in the brain stem and hypothalamus.
- This causes an energy imbalance, leading to increase in meal size , weight gain and obesity.
- Leptin resistance reduces the astrocytic release of ketone bodies from the hypothalamus leading to dysregulation of caloric intake and energy homeostasis
Image 4: Leptin (produced by adipocyctes) and Ghrelin, hunger hormones.
Basal Ganglia[edit | edit source]
Changes in Basal Ganglia
- Dorsal Striatum: Exposure to HFD causes changes in astrocytic signalling in dorsal striatum which leads to change in motor activity, food seeking behaviors and decision making. 
- Changes in Nucleus Accumbent (NAc): Following exposure to HFD, neuroplastic changes takes place in the reward pathways of NAc through modulation of neuronal morphology
Image 5: Basal Ganglia
Obesity Versus Addiction[edit | edit source]
Many imaging studies and literature reviews have found a link of similarity between DIO and addictive disorders. It has being established that the weight gain can be a result of addictive behaviours of food intake which include repeated food cravings , controlling food intake , inability to reduce the food intake despite of the negative consequences. Desire to lose weight has been compared to the attempts to quit addiction.
- Glutamatergic plasticity , dopamergic plasticity and opioid changes are the changes which are common in substance abuse and obesity.
- Drug abuse and HFD both activates the mesolimbic nuclei i.e. NAc which is responsible for behavioral adaptations.
In addition to the above mentioned alterations BDNF is associated with susceptibility to the drug seeking behavior. Exposure to HFD leads to increase in BDNF in the NAc lysates which is associated with alteration in the reward signalling pathways.Several imaging studies have shown that there is reduced prefrontal activity in obese individuals which is quite similar to hypofrontal activity in addiction. PET scan studies shows a reduced cerebral blood flow in pre frontal cortex in response to a meal after 36 hour fast in obese women as compared to control. However these results are heterogeneous in nature.
Dopaminergic changes occurring are similar when exposed to HFD as they are in addiction. Dopamine regulates the glutamatergic plasticity. However it is seen that when exposed to HFD and drugs like cocaine and alcohol , the dopamine transmission gets altered which leads to alteration in dopamine- dependent striatal glutamatergic plasticity. This includes increase in release of dopamine which over the time diminishes availability of dopamine receptors and dopamine reuptake is diminished. This leads to disruption of normal plasticity of the projections from the prefrontal cortex to NAc. This mechanisms form the basis of food choices and choice of physical inactivity in obesityApart from these changes, there is a different set of mechanism which is responsible for persistence of obesity. There is alteration in synaptic plasticity due to fat , free fatty acids and triglycerides from the HFD. This includes disruption of hippocampal long term potentiation(LTP). In addition to this obesity leads to inflammation of hippocampus causing impaired hippocampal- dependent memory due to the disruption of neural networks of hippocampusIn addition to this persistent effects of obesity include alteration in gene expression which is quite similar to the changes in drug abuse. Inflammation in hypothalamus associated with obesity is linked with coexistence of depression in obesity15. Also the persistent effects of obesity include leptin and insulin resistance which form the basis of sustained changes in dopaminergic systemTherefore it can be concluded that neuroplasticity plays a pivot role in explanation of food intake habits , inability to curb the need to eat and futile attempts of weight loss in DIO.
Exercise Induced Neuroplasticity in Obesity[edit | edit source]
Studies have shown that the exercises known to induce neuroplastic changes in obesity includes
- Warm up
- Cool down exercises as per the principles of exercise therapy
- Exercises such as biking, running, and individualized strength training.
It is important to enroll the individual for at least 60 minutes per session. However more evidence is required to generate a more detailed protocol to induce neuroplastic change.
Neuroplastic Changes in Response to Exercise[edit | edit source]
In response to exercise changes are seen in BMI, serum concentration of leptin, BDNF levels which is known to moderate neuroendocrine and metabotrophic processes which leads to a reduced food intake and improves glucose metabolism and insulin sensitivity and HDL-C.
- Changes in brain in response to exercise includes increase in gray matter density in left hippocampus , left insular cortex , and left inferior cerebellum 
- Exercise leads to generation of low oxidative stress which has positive effect on all the cell systems. In response to exercise a cascade of processes is initiated which is important in repair , remodelling and reshaping processes which improves the ability of cellular systems to handle the free radicals 
- Exercise has shown to reverse alteration in diffusion parameters which eventually restores obesity related regional white matter changes.
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