Yoga has become a proven therapy to treat depression, stress and pain. It has a therapeutic effect on neurological diseases as well. Therapeutically changes may be brought about by promoting brain plasticity. Brain plasticity can be influenced by several factors including neurotrophic factors, neurotransmitters, endocrines, cytoskeleton proteins and neuronal electrical activity to name a few. This review will focus on the effect of Yoga on neurotrophic factors.
Neurotrophic Factors are key molecules which are involved in the development and differentiation of the nervous system and play a major role in neuronal plasticity. The practice of yoga produces brain plasticity leading to enhancement in cognitive performance and alleviation of symptoms like depression and Post Traumatic Stress Disorder (PTSD). It has great potentials in treating neurological diseases as well. Accordingly, it is critical to evaluate yoga induced changes in the expression of neurotrophic factors and their receptors. The neurotrophic factors which are thought to be important are: Brain Derived Neurotrophic Factor (BDNF), Vascular Endothelial Growth factor (VEGF), Insulin like Growth factor -1(IGF-1), Glial Cell Derived Neurotrophic Factor (GDNF), Nerve Growth Factor (NGF) and Fibroblast Growth Factor (FGF2). This review will focus on BDNF, VEGF, IGF and GDNF.
BDNF is a key member of the neurotrophin family which also includes nerve growth factor, neurotrophin 3, and neurotrophin 4 . BDNF has a high expression levels in the brain and it plays an important role in the survival, differentiation and synaptic formation of several classes of neurons [2,3]. It also influences synaptic function by regulating the activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP), which is the cellular basis of learning and memory . The main receptor involved is TrkB . The immature form of BDNF (proBDNF) activates p75NTR receptor, which has an opposing effect on the TrkB induced action. This may result in long term depression (LTD) . BDNF in addition to its role in hippocampus also has a profound effect on spinal cord neurons .
BDNF is known to be an essential factor for preventing the motor neuron cell death and is an important skeletal muscle derived trophic factor for motor neurons [8-10]. BDNF knockout mice display a lack of coordination of movements and balance along with excessive loss of neurons in peripheral sensory ganglia . Further, BDNF offers a significant neuroprotection against excitotoxicity and also mediates anti-apoptotic effect [12,13].
In a study aimed at the treatment of depression with Yoga, it was found that patients who undertook yoga therapy were better than drugs-only group with respect to reduction in Hamilton Depression Rating Scale (HDRS) scores. There was a statistically significant positive correlation between a fall in HDRS and rise in serum BDNF levels in yoga-only group, but not in those receiving yoga and antidepressants or antidepressants- alone over a three month time period. The authors suggest that
synaptic neuroplasticity induced by BDNF may be responsible for the therapeutic benefit of yoga in depression. Another study also showed a reduction in cortical level along with an increased BDNF content following treatment with Yoga in patients with depression [14-16].
With regards to Yoga in the modulation of pain, it has been shown that the decrease of low chronic back pain in women following practice of Yoga was associated with an increase in the level of BDNF. The authors propose that BDNF may be one of the key factors mediating beneficial effects of yoga on chronic low back pain .
Insulin, the insulin-like growth factors (IGF1, IGF2) and their receptors also play an important role in the development and function of central nervous system. These factors can regulate neuronal survival, neurogenesis, angiogenesis, excitatory and inhibitory neurotransmission, food intake, and cognition . There is a linkage between brain insulin/IGF1 and certain neuropathology's, such as Alzheimer's, Motor Neuron Disease . IGF-1 is also an important skeletal muscle derived trophic factor for motor neurons that has a role during development as well as during recovery from injury .
A study was conducted to examine the effects of specific Mindfulness-Based Stress Reduction (MBSR) activities (yoga, sitting and informal meditation, body scan) on immune function, circulating IGF-1 concentrations, and positive affect among older adults. A committed practice of yoga was associated with higher IGF-1 levels and greater improvement in positive affect. The authors also found that sitting meditation was positively associated with post-treatment IGF-1 level .
VEGF promotes neurogenesis, neuronal patterning, neuroprotection and glial growth . VEGF guides neuronal migration in the embryonic brain and supports axonal and arterial co-patterning in the developing skin . VEGF can be considered to be a promising tool to promote neuronal health and nerve repair. VEGF has potent neurotrophic and mitogenic activity as it induces axonal out growth and increases the survival of neurons, satellite cells and proliferating Schwann cells .
VEGF appears to induce neurogenesis in-vitro as well as in- vivo  During CNS development, VEGF stimulates neuronal growth and maturation, mediated by the VEGFR2 receptor via the MAPK (Mitogen-activated protein kinases) pathway [24,25]. In addition, VEGF is shown to be unregulated in the skeletal muscle after exercise as well as after chronic motor neuron stimulation [26,27]. Recently it has been shown that VEGF promotes reinnervation by inducing axonal regeneration after ischemic injury by promoting the expression of NGF/ GDNF in the skeletal muscle  suggesting the influence of VEGF on other tropic factors. The role of VEGF in Yoga/Meditation still needs to be explored.
GDNF is known to regulate the functioning of the brain's dopaminergic system. BDNF and GDNF interact with the 5-hydroxytryptamine (5-HT) system of the brain to regulate complex behaviours. GDNF, as well as BDNF, stimulates 5-HT neurons and in turn, 5-HT affects the expression of genes that control BDNF and GDNF in brain structure .
GDNF is the most potent neurotrophic factor for the motor neurons [30-32]. Yoga and exercise can increase the level of GDNF which can be beneficial in diseases like Parkinson's disease.
The study on the effect of Yoga on neurotrophic factors offers immense potentials for further investigation. A comprehensive understanding of this area will enable us to define the beneficial role of yoga in treating not only mental health disorders but also neurological disorders such as Parkinson's, Alzheimer's, Motor Neuron diseases, Multiple sclerosis and stroke since it is well known that neurotrophic factors confer potent neuroprotection.