osteoarthritis - associated knee pain accompanied by restricted joint function occurs as the 
 joint cartilage is exfoliated from the joint after degenerative changes and the production 
 of osteophytes , which result in mechanical stimulation of nerves in the subchondral bone 
 during weight loading1 , 2 .
it is also characterized by an increase in inflammatory cytokines 
 in the soft tissue and cartilage surrounding the joint3 , 4 .
the latter causes a local 
 inflammatory reaction , thereby increasing the expression of diverse pain signaling molecules 
 such as bradykinin , serotonin , and histamine5 .
these pain signals subsequently activate the peripheral nociceptors 
 surrounding the joint , causing pain6 .
if the pain signal continuously flows into the spinal cord due to the activation of 
 peripheral invasive sensory receptors , a local inflammatory reaction occurs in the dorsal 
 horn part of the spinal cord . at the same time
, the activation of glia cells , such as 
 astrocytes and microglia , leads to the secretion of large amounts of cytokines , and this 
 further accelerates spinal cord inflammation7 .
this inflammatory reaction damages the nerve cells of the spine , 
 increasing their sensitivity and stimulating the secretion of inflammation - inducing 
 peptides . as a result , neuropathic pain , such as hyperalgesia , occurs8 , 9 .
nerve growth factor ( ngf ) is a neurotrophic factor that controls the differentiation and 
 survival of sensory nerves and sympathetic nerves , growth of the neuraxis , and transfer of 
 neural signals in general10 . in the early 
 phase of arthritis ,
ngf plays a role in providing protection against inflammation - related 
 neuronal damage caused by proinflammatory cytokines and neuronal regeneration11 . in an albino rat model of induced 
 osteoarthritis of the ankle joint , pezet et al .
reported that the expression of the ngf 
 receptors tyrosine kinase a and p75 ( or ngfrp75 ) , which are closely related to neuronal 
 plasticity , increased in laminae 35 of the dorsal horn at the l3l4 level of the spinal 
 cord12 .
ngf plays a crucial role in the 
 growth , proliferation , and differentiation of nerve cells , and an increase of this indicates 
 the existence of nerve damage13 . although surgery and medical treatments such as painkillers are widely used for pain 
 reduction in osteoarthritis , it is important that patients take steps to prevent pain by 
 controlling pain - related risk factors . in particular , exercise can help prevent and treat 
 the disease without the adverse effects of medicine . as exercise is effective in improving 
 joint function by reinforcing the soft tissue in the knee joint and improving the 
 regeneration power of the joint cartilage
, exercise can be expected to be extremely 
 beneficial in osteoarthritis treatment1 , 14 , 15 . in the present study
, we examined the expression pattern of ngf in the spinal cord of a rat 
 model of induced osteoarthritis of the knee joint to clarify the impact of exercise on ngf 
 expression .
all experimental protocols were performed in accordance with the guidelines of daegu 
 university , based on the national institutes of health guidelines for the care and use of 
 laboratory animals ( 1996 ) .
forty 810-week - old male sprague - dawley rats , weighing 250300  g , 
 were housed at a temperature of 25  2  c and maintained in a 12-h light - dark cycle .
the 
 animals had free access to food and water . for the induction of monosodium iodoacetate ( mia ; sigma ,
i2512)-induced arthritis , rats were anesthetized with 2  ml / kg of a 50% zoletil/50% xylazine 
 hydrochloride mixture and given 3  mg/50  l mia through the interarticular space of the right 
 knee2 , 16 .
three weeks after mia injection , the rats were subjected to a 
 treadmill exercise according to previously described methods17 . briefly
, the treadmill velocity was set at 8 m / min per 20 minutes 
 for 2 days and at 16 m / min per 30 minutes for 4 weeks .
the rats were randomly divided into four groups : rats sacrificed at 3 weeks after 0.9% 
 saline solution injection ( shame group , sg , n = 10 ) , rats sacrificed at 3 weeks after mia 
 injection ( control group , cg , n = 10 ) , rats with 4 weeks rest from 3 weeks after mia 
 injection ( no exercise group , neg , n = 10 ) , and rats with 4 weeks treadmill training from 3 
 weeks after mia injection ( exercise group , eg , n = 10 ) . at the end of the exercise period , the rats were sacrificed and their spinal cords were 
 removed . the lumbar spinal cord region was selected for immunohistochemistry analysis 
 performed according to a previously described method18 .
briefly , rats were anesthetized with 2  ml / kg of a 50% zoletil/50% 
 xylazine hydrochloride mixture , then intracardially perfused with phosphate - buffered saline 
 ( pbs ) followed by 4% paraformaldehyde .
serial coronal sections ( 30 m ) of the lumbar spine 
 ( l4s1 ) were cut by using a microtome ( bright5040 ) on a cryostat , and processed for 
 immunohistochemistry .
tissue sections were incubated at 4  c overnight in a solution 
 containing an anti - ngf antibody ( diluted 1:500 in pbs ) and added with rabbit polyclonal 
 anti - ngf antisera ( vector , usa ) , which specifically recognizes rat ngf .
differences between groups 
 were tested by using one - way anova , followed by a post - hoc test ( duncan test ) when a 
 difference was detected .
the results of the present study , which examined the impact of exercise on ngf expression , 
 showed that ngf was expressed in all groups of rats .
ngf showed a statistically significant 
 increase in rats with osteoarthritis induction ( cg ) compared with those without 
 osteoarthritis induction ( sg ) ( p < 0.05 ) .
the expression of ngf was also significantly 
 elevated in the neg compared with that in the sg ( p < 0.05 ) ; however , no statistically 
 significant difference was observed when compared with that of the cg ( p > 0.05 ) . on the 
 other hand ,
ngf expression increased significantly in the eg compared with that in the sg , 
 cg , and neg ( p < 0.05 ) ( table 1table 1.comparison of ngf expression in the spinal cord between the four groups ( unit : 
 pixels)expression of ngf ( mean  sd)group(n = 40)sg ( n = 10)cg ( n = 10)neg ( n = 10)eg ( n = 10)11,152.1671.714,315.3805.8 16,069488.5 20,257747.5 significant difference from sg , p < 0.05 .
ngf : nerve growth factor , sg : shame 
 group , cg : control group , neg : no exercise group , eg : exercise group ) .
ngf : nerve growth factor , sg : shame 
 group , cg : control group , neg : no exercise group , eg : exercise group
osteoarthritis pain can be linked to central nervous system diseases that can affect many 
 factors , from peripheral nerve function to central nervous system mechanisms19 .
recently , a number of researchers have 
 argued that neurological - related factors should be included in considerations of 
 osteoarthritis treatment strategies20 .
the primary cause of chronic pain in cases of peripheral tissue damage is increased 
 irritability of peripheral nociceptors located near the damaged part .
a secondary cause is 
 increased irritability of nerve cells within the central nervous system21 .
this is because inflammatory pain in peripheral areas , 
 stimulation of peripheral primary afferent sensory nerves , and inflammation - related 
 neuropathic pain in the spinal dorsal horn are all interrelated7 , 9 .
neurotrophins ( nts ) are essential for the growth , proliferation , differentiation , and 
 survival of nerve cells , which include ngf , brain - derived neurotrophin factor , nt-3 , and 
 nt-413 . among these ,
ngf is a 
 metabolically active peptide that controls the survival and differentiation of sensory nerve 
 cells and sympathetic nerves , the growth of axons , and the transfer of neural signals . 

combined with the low - affinity ngf receptor p75ngfr , ngf plays a central role in the 
 recovery , regeneration , and growth of nerve cells damaged by inflammation induced by 
 proinflammatory cytokines11 .
examined the expression pattern of the ngf receptor in the spinal dorsal horn 
 over time after inducing arthritis in albino rats . in a control group without arthritis 
 induction ,
the ngf receptor was expressed mainly on the surface of laminae 12 , with some 
 expression near the edge of laminae 35 .
the expression pattern did not change over time . in 
 the experimental group with arthritis induction ,
the expression increased near laminae 35 
 until the acute phase ( i.e. , 3 weeks after the injection with an arthritis - inducing drug ) . 

in particular , the expression of the ngf receptor dramatically increased in the deep part . 

in the postacute phase , starting from the 8th week , the expression pattern began to decrease 
 in laminae 35 , and it decreased to a level similar to that of the control group in the 
 recovery phase of the 12th week12 .
in the present study , ngf significantly increased between laminae 3 and 5 in the cg , where 
 osteoarthritis was induced 3 weeks after the mia injection , compared with that of the normal 
 group ( sg ) .
the expression of ngf increased in the neg exposed to an additional 4 weeks of 
 treatment compared with that of the cg , although the increase was not significant . taking into consideration the results of previous studies , the stimulation of ngf 
 expression during the active period of arthritis in the cg and neg was likely due to a 
 natural healing response to inflammation - related nerve cell damage . during this response , 

molecules are induced to protect and regenerate nerve cells , and these eventually increase 
 the levels of ngf9,10,11,12 . in our study , we observed a relatively higher expression of ngf in the eg that underwent 
 exercise training compared with the other groups . with osteoarthritis induction ,
we surmise that 
 exercise further stimulated the ngf expression , affecting the regeneration of nerve cells in 
 the neuraxis .
many previous studies have reported that exercise stimulated the regeneration of nerve 
 cells by increasing the expression of ngf . according to chae and kim , treadmill exercise of 
 moderate intensity stimulated the expression of ngf , which controls the growth , 
 differentiation , and apoptosis of nerve cells22 .
they argued that exercise restricted apoptosis , thereby preventing 
 nerve cell damage . in an experiment with albino rats with ischemic stroke induced by 
 surgery , chung et al . reported that 2 weeks of treadmill exercise was effective in 
 increasing ngf expression in the brain compared with a control group that did not undergo 
 exercise training
matsuda et al . found 
 that ngf expression increased in an albino rat with ischemic stroke after 20 minutes of 
 treadmill exercise for 28 days24 .
they 
 also reported that the area of brain tissue destroyed by ischemia decreased and that 
 behavioral function improved after the exercise .
the results of the present study suggest that exercise - induced stimulation of spinal cord 
 nerve cells , damaged by continuous osteoarthritis - related pain due to increased ngf 
 expression in the spinal cord , can be effective in treating chronic pain , which , thus far , 
 has proved difficult to treat .
such treatment will contribute not only in improving the 
 joint function of patients with chronic pain but also their quality of life .
 [ purpose ] we examined the impact of exercise on the expression pattern of nerve growth 
 factor in the spinal cord of rats with induced osteoarthritis of the knee joint . 
 [ subjects 
 and methods ] to produce monosodium iodoacetate - induced arthritis , rats were administered 
 3  mg/50  l monosodium iodoacetate through the interarticular space of the right knee . 
 the 
 animals were randomly divided into four groups : rats sacrificed 3 weeks after 0.9% saline 
 solution injection ( shame group , n = 10 ) , rats sacrificed 3 weeks after monosodium 
 iodoacetate injection ( control group , n = 10 ) , rats with 4 weeks rest from 3 weeks after 
 monosodium iodoacetate injection ( no exercise group , n = 10 ) , and rats with 4 weeks 
 treadmill training from 3 weeks after monosodium iodoacetate injection ( exercise group , n 
 = 10 ) . 
 serial coronal sections of the lumbar spine were cut and processed for 
 immunohistochemistry . [ results ] 
 the expression of nerve growth factor was significantly 
 increased in the eg compared with the sg , cg , and neg . 
 [ conclusion ] increased nerve growth 
 factor expression in the spinal cord due to exercise - induced stimulation can be effective 
 in treating chronic pain . 
 such treatment will contribute not only to improving the joint 
 function of patients with chronic pain but also their quality of life . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION