It is well established that NOS-containing myenteric neurons are curiously susceptible to diabetic injuries
[26][27][28] and impaired nitrergic innervation is accompanied by motility dysfunction
[29]. Moreover, the nitrergic neurons located in different gut segments display strictly region-specific responsiveness to the diabetic state and also to immediate insulin replacement
[27]. In the jejunum, ileum and colon of diabetic rats, both the nitrergic and total number of myenteric neurons were decreased assuming diabetes-related cell loss in these segments. However, in the duodenum of diabetic rats, the decreased number of nitrergic neurons was not accompanied by changes in the total number of myenteric neurons, presuming region-specific neurochemical modification of neurons here
[27]. Several other studies confirm diabetes-associated decreases of nitrergic myenteric neurons. nNOS neurons are reduced in the antrum and jejunum in spontaneously diabetic Bio-breeding rats
[28], in the ileum of diabetic dogs
[30], and in the human appendix
[31], and reduced nNOS protein and mRNA expression were observed in the stomach of mice
[29][32] with P2×7 receptor-mediated diabetic damage of nitrergic neurons
[32]. Besides nitrergic neuronal damage, the diabetic animals represented delayed gastric emptying
[29] and faster small intestinal and colonic transit compared to controls
[27][33].
In addition, in the microenvironment of enteric neurons, the number of eNOS-labelling gold particles was increased in the capillary endothelium of different gut segments
[35], suggesting that the microvessels supplying to the myenteric ganglia are targets of diabetic damage in a regional manner and may contribute to developing neuropathy in diabetes.
Sex dependency on the diabetic nitrergic dysfunction was also observed
[36]; females seem to have greater vulnerability to diabetes-related gastric impairments than males
[37]. A decreased level of tetrahydrobiopterin, a major cofactor for NO synthesis, contributes to delayed gastric emptying, reduced pyloric nitrergic relaxation and nNOS-α protein expression in female diabetic rats
[38].
NOS-containing neurons go through a two-phase degeneration process. The decrease in axonal nNOS expression as the hallmark of the first phase is reversible, however, causing irreversible changes, as apoptotic loss of nitrergic neurons occurs in the second one
[26][39]. In addition, progressive accumulation of advanced glycation end products (AGEs) during diabetes seems to enhance this apoptotic process
[40]. AGEs significantly reduce the expression of nNOS and NO release in myenteric neurons via their receptor
[41]. Prevention of AGE formation by certain drugs precedes the decrease of nNOS in diabetic rats
[42] and may also help to protect against nitrergic nerve dysfunction. Furthermore, the endogenous antioxidant defense of the gut can also be protective for nitrergic myenteric neurons. An extensive increase in the ratio of nNOS-immunoreactive neurons colocalizing with heme oxygenases was revealed in the ileum and colon of diabetic rats, though the nitrergic neuronal number decreased
[43], suggesting that those NOS neurons which do not colocalize with heme oxygenases are the most damaged by diabetes
[43][44].