Behavioral biomarkers include neuropathic pain, which is characterized by mechanical/chemical hyperalgesia, tactile allodynia in sensory large fibers, thermal nociception in sensory small fibers and sensory motor deficit in large sensory fibers.
a) Neuropathic pain: Neuropathic phenotyping in animal models begins with the evaluation of sensory loss by a quantitative assessment of neuropathic pain. The hallmark of this type of pain is allodynia or the elicitation of pain by a stimulus that is not normally considered noxious, such as light touch, pressure or mild temperature changes and hyperalgesia or pain sensation to less pain stimulation .
Biomarkers of neuropathic pain in diabetic rodents
Neuropathic pain in diabetic rodents can be assessed by using thermal, mechanical and chemically induced pain,[8
] which are enlisted in . There are several reports examining duration-dependent thermal, chemical, mechanical hyperalgesia and allodynia induced in experimental diabetic animal models. Short-term diabetes has shown thermal, chemical and mechanical hyperalgesia,[15
] whereas long-term diabetes has shown thermal and mechanical hypoalgesia.[8
] These duration-dependent changes in the thermal and mechanical nociceptive threshold reflect the symptoms in human DN. Thermal hyperalgesia is well observed in the early stage of DM in human subjects,[16
] and longer term of DM shows an increased thermal threshold, which is caused by the loss of all types of peripheral nerve fibers. Thermal allodynia to warm stimulus of 42°C[18
] and cold allodynia to cold stimulus of 10°C[7
] were also observed from the second week of diabetes.
Tests for assessment of neuropathic pain in rodent models of diabetes
Heating the plantar surface of the paw to evoke withdrawal responses in diabetic rodents also show both hypoalgesia and hyperalgesia.[19
] In STZ-diabetic rats, thermal hypoalgesia develops in the hind paw after approximately 8 weeks.[21
In addition to thermal allodynia, diabetic rats show a marked tactile allodynia when submitted to light touch to the plantar hind paw as well as chemical allodynia to paw injection of lower concentration of formalin (0.2%), while normal rats are insensitive to these stimuli. Tactile allodynia are notable within the first week of diabetes and remain up to 4 weeks, with worsening of the trouble from the second week onwards.[15
] Allodynia in response to a 0.2% formalin stimulus is detectable after 1 week of hyperglycemia, and maximum worsening of symptoms occurs from 4 weeks to 8 weeks.[15
b) Motor incoordination:
The sensorimotor deficits resulting from large-fiber diabetic peripheral neuropathy (DPN) can lead to significant impairment. Numerous human studies report that patients with DPN are at an increased risk of falls due to decreased postural control and altered gait and balance.[22
] Muscle spindles are involved in many sensorimotor behaviors such as the regulation of proprioception, balance, gait and the postural response, and spindle damage can lead to deficits such as motor incoordination.
The beam-walk apparatus has been used to assess sensorimotor deficits following brain injury and other conditions, resulting in altered gait, balance and/or proprioception.[23
] The beam-walk apparatus was found to be a sensitive measure for the evaluation of diabetes-induced sensorimotor changes.[9
] In this method, the number of hind paw slips as mice crossed the beam-walk apparatus was used to access the sensorimotor ability. For a slip to be counted, the foot had to lose during walking on the beam-walk apparatus. After 10 weeks of hyperglycemia, the diabetic mice showed a significantly greater number of foot slips than the nondiabetic mice, which is a marker of diabetes-induced large fiber damage.[9