Methods of induction
STZ. STZ is a glucosamine–nitrosourea compound targeting insulin-producing β-cells in the pancreas to mimic diabetogenic properties in humans [
5]. There are more than a hundred analogues known. STZ occurs in nature as a 50–50 combination of (α and β anomers) [
6]. The fundamental STZ structure has been kept in approximately one-third of the analogues, although alterations include acetylation, alkylation, and nitroso group substitution. Acetyl derivatives have the same activity as their parent molecule [
6]. In an early report in 1963, STZ administration at 65 mg/kg was reported to induce type 1 DM (T1DM) in rats [
7]. A higher dose of STZ is required for mice, which ranges from 150 to 400 mg/kg of STZ [
5]. STZ administration also has been reported to induce type 2 DM (T2DM) through multiple low-dose injections, in combination with other chemicals (such as nicotinamide), or with dietary manipulations for the induction of diabetes in rodents and dogs [
5‐
9]. In general, STZ-induced onset hyperglycaemia occurs within 48 h post-STZ administration regardless of the dosage and the hyperglycaemic condition can be maintained for up to 22–24 months [
10]. STZ direct uptake to the pancreatic β-cell is aided by the glucose transporter 2 (GLUT2) receptor [
11]. STZ executes its cytotoxic effects on the pancreatic islets of Langerhans through its nitrosourea moiety, where it causes DNA alkylation [
3]. Other than that, DNA damage by STZ occurs due to increased production of reactive oxygen species (ROS) and nitric oxide (NO) [
11]. STZ can be administered through intraperitoneal, intravenous, or subcutaneous injection resulting in prolonged hyperglycaemia and other characteristics of diabetes, such as polyuria and polydipsia [
12].
Other than rodents, several animal species such as zebrafish, rabbits, dogs, pigs, beagles, and monkeys, are sensitive to the pancreatic β-cell cytotoxic effects of STZ [
12‐
16]. Zebrafish were usually induced with multiple STZ doses, whereas huge animals such as rabbits, dogs, and nonhuman primates were usually induced with a single dose protocol. STZ-induced zebrafish model remained in a hyperglycaemic state around 80 days after induction of diabetes [
15]. In contrast, huge animals may stay in a hyperglycaemic state for more than four years post-induction [
16].
Alloxan. In 1942, before STZ was discovered, alloxan was employed to induce diabetes in animal models [
17]. Alloxan is a uric acid derivative directly targeting pancreatic cells [
18]. It was first created by Wöhler and Liebig by combining uric acid and nitric acid [
19]. McLetchie and his colleagues discovered that intravenous injection of alloxan resulted in hypoglycaemia owing to necrosis in the pancreas islets of Langerhans while researching kidney problems in rabbits [
17]. The death of cells resulted in the release of insulin reserves, resulting in hypoglycaemia, followed by the establishment of diabetes within 24 h. Similar to STZ, alloxan can also be administered via intraperitoneal and subcutaneous injection [
19]. While rabbits induced with alloxan usually did not change their food intake frequency, rodents induced with alloxan usually have polydipsia, polyuria, glycosuria, and hyperglycaemia, all of which are common symptoms of diabetes [
17]. The suppression of glucokinase, an enzyme implicated in the glucose-insulin regulation system and expressed in the liver and pancreas, is triggered by a direct cell death associated with alloxan. The compound can be toxic to liver and kidney cells, although toxicity can be prevented with careful dosage. Owing to the mode of action of alloxan being specific to pancreatic β-cells, the appropriate dosage is required to avoid possible toxicity accumulated in liver and kidney cells [
20]. Nevertheless, a diabetic induce agent by alloxan is challenging to administer to animals due to its low stability in water at room and body temperature [
17,
20]. All diabetic animal models administered with alloxan, including mice, rats, dogs, rabbits, and pigs, have experienced damage to their pancreatic β-cells and lead to DR due to retina-induced lesions [
21‐
25].
STZ vs. Alloxan. Due to the higher ease of use and efficacy, STZ has become the gold standard agent over alloxan in inducing diabetes in vivo. This is due to more excellent stability of the STZ compound and extremely fast result in disease progression [
26]. Conversely, alloxan is less favourable as it produces unpredictable and inconsistent results. Therefore, it could be more efficacious [
27]. However, unsuccessful hyperglycaemia induction in experimental animals happens. It is associated with resistance to STZ or alloxan, which may be related to hormones or genetic indifference of different species or gender [
27,
28].
Effects on retina
STZ. The induction of this chemical has been shown to induce DR in a wide range of animals, as mentioned above, and various dose regimens have been developed, depending on the type of animals. Nevertheless, rodents remain the most preferred animal model for the characterisation of the disease and therapeutic drug investigations. These animals are favoured even more than the non-human primates owing to their ability to develop hyperglycaemia one week after a single dose of STZ administration. They have been reported to exhibit various non-proliferative DR (NPDR) features.
In the STZ-induced mice model, the retinal morphologic alterations observed were thinning of the ganglion cell layer (GCL), inner plexiform layer (IPL), outer plexiform layer (OPL), and total retinal thickness as early as 3 to 4 weeks post-STZ induction [
29]. Other than that, there were reduced retinal ganglion cells (RGCs) with increased astrocytes number, accompanied by reactive gliosis within 4 to 6 weeks post-STZ induction [
30]. Meanwhile, at ten weeks post-STZ induction there was thinning of the inner nuclear layer (INL) and outer nuclear layer (ONL) [
3]. For the microvascular changes, enhanced vascular permeability, pericyte loss, and microglial changes were reported at eight weeks [
31,
32], neovascularisation (formation of new vessels) at 16 weeks [
33], thickening of the capillary basal lamina at 17 weeks [
34], and increased of acellular capillaries and pericyte ghosts within 6 to 9 months of STZ induction [
10,
35,
36]. Continuous inflammation that occurs due to chronic hyperglycaemia leads to leukostasis (leukocytes plug within vessels) [
35‐
38] and is associated with increased acellular capillary formation [
35]. Hemodynamic and electroretinogram (ERG) instability were also reported within four weeks post-STZ induction including a decrease of arteriolar and venular flow velocity with decreased diameter and shear rates [
39,
40], a reduced total oscillatory potential (OP) and OP3 amplitudes with prolonged of implicit times OP2-3 [
41,
42]. In contrast to Kurihara et al. [
41] and Sasaki et al. [
42], the majority of studies failed to demonstrate the same declined trend for amplitudes of a-/b-wave, highlighting a conflicting outcome of mice as DR animal models [
36]. Although multiple studies have recognised STZ as a diabetogenic chemical agent, researchers have yet to demonstrate consistent evidence on the effect of STZ diabetic-induced apoptotic and loss of RGC, thus remaining the subject of concern in recent times. It is worth noting that few studies demonstrated different numbers of RGC post-diabetic induction, such as an increased RGC apoptosis after two weeks [
43] and decreased RGC numbers within 6 to 10 weeks after STZ administration [
43,
44], supporting the notion of inconsistent results. Contrary to the aforementioned studies, others found no profound RGC apoptosis or GCL cell loss within ten months of post-hyperglycaemia [
36,
38,
45]. These contrasting observations may be due to different strains of mice or regimens of STZ administration.
As opposed to mice, lower doses of STZ are required to induce diabetic in rats [
5]. Several strains of rats were used to develop the DR model, including
Sprague Dawley (SD),
Wistar, and
Lewis. Changes in the retinal morphology observed in rats were similar to those in mice. Thinning of the IPL and ONL were reported to be as early as four weeks post-STZ induction in SD rats [
46,
47]. However, some studies also reported an increased total retinal layer thickness, which has been suggested to be due to retinal oedema [
48,
49]. In fact, several authors reported retinal oedema as a retinal morphological feature of DR [
50,
51]. Retinal oedema usually occurs after 12 to 16 weeks of STZ administration.
Wistar and
Lewis rats displayed degeneration of capillaries with the formation of pericyte ghosts within eight months of hyperglycaemia [
52]. At the same time,
Lewis and SD rats exhibit neuronal loss in GCL. Although the onset of retinal lesions varies with different rat strains, numerous phenotypes have been documented, including blood-retinal barrier (BRB) breakdown two weeks after diabetes onset [
53], increased acellular capillaries containing degenerated intramural pericytes and apparent proliferation of endothelial cells after eight weeks [
54], and thickening of basement membrane after one year [
55]. Throughout the literature, studies have shown that DR development in rats has been linked to neuronal and glial damage before prominent vascular changes. It is identical to the mice model whereby progressive apoptosis and thinning of retinae were evident after 3 to 4 weeks of post-induction hyperglycaemia. Likewise, prominent gliosis with increased glial fibrillary acidic protein (GFAP) expression was observed at an early stage of four weeks of hyperglycaemia [
56‐
58]. In addition, multiple studies also revealed a reduction of cells within GCL [
52,
58 ,
205] and astrocytes [
53,
57] and an increase of microglia [
52,
58] at 4 to 6 weeks of post-STZ induction. Subsequently, other vascular events include increased acellular capillaries [
52,
59], leukostasis [
60], thickening of capillary basement membrane [
61], and pericyte loss [
58,
61] were apparent at a more extended period (4 to 8 months) post-STZ induction. Other than the morphological changes, retinal dysfunction was also observed through reduced visual-behaviour responses [
62], reduced a-/b-wave [
56,
59,
63] and OP amplitude [
56,
59] with delayed OP [
63],increased retinal vessel diameter[
203 ,
204], retinal oxidative stress markers [
64], retinal pro-inflammatory cytokines level [
65] and retinal pro-angiogenic markers [
64,
65].
Besides rodents, animals of different species have been examined with various results on disease development. For instance, 3 to 4 weeks post-STZ administration, zebrafish displayed thinning of IPL and photoreceptor segment layer, damaged cone receptor, and neuronal loss [
15]. In contrast, rabbits induced with a single dosage of STZ acquired retinal and retinal haemorrhages, vascular lesions, venous thrombosis, and proliferative retinopathy after 4.5 months of induction [
66]. STZ-induced DR dogs exhibit retinal lesions similar to those of human DR. In an extensive study of the DR dogs model, STZ administration caused a thickening capillary basement membrane (BM) after one year, pericyte and smooth muscle cell loss after four years, and microaneurysms, acellular capillaries, and intraretinal microvascular abnormalities (IRMAs) after seven years of hyperglycaemia [
16]. Numerous studies have shown that loss of pericytes and microaneurysm formation depend on age, with younger animals exhibiting the features faster than older ones [
67]. STZ-induced diabetic pigs displayed enhanced BRB permeability, INL, and GCL thinning and thickening capillary BM thickening [
68,
69]. Accordingly, the tight junctions in the retina were weakened [
68]. Monkeys induced with STZ remain diabetic for 6 to 15 years. However, no significant ocular changes were observed [
70]. They usually developed hypertension and ischemic retinopathy with cotton-wool spots, and hyperfluorescent patches usually occur once they were hypertensive.
Alloxan. FOT_FB mice induced with alloxan acquired pericyte and RGCs loss post one week of induction, along with microaneurysms within three weeks post induction [
29]. In another study using C57/Bl6 mice, alloxan induction caused microglial changes (thicker cell bodies and shorter dendrites) in the retina [
31]. However, Gaucher et al. [
31] failed to observe neuronal apoptosis, glial activation, microaneurysm, and haemorrhage formation, as demonstrated in comparable models. Nevertheless, they observed a significant reduction in the b-wave/a-wave amplitude ratio and an increment in the OP latency period in ERG after three months of alloxan induction [
31].
Contrariwise to STZ, studies of alloxan-induced DR in rat models were limited. Neovascularisation and cataracts were observed between two to nine months and a year, respectively, post-alloxan administration [
32,
71]. Pericyte ghosts, acellular capillaries, and thicker capillary BM were observed 15 months after alloxan induction [
26,
71]. BRB breakdown, Müller glia growth, and endothelial enlargement were also reported with this induction model in rats [
31,
72]. Five doses of alloxan within five weeks in dogs causes retinopathy comparable to DR in humans. However, dogs only acquired DR 53 to 69 months after the start of hyperglycaemia [
22]. Alloxan-induced dogs developed haemorrhages, acellular capillaries, pericyte loss, and microaneurysms once DR appeared, making this a feasible model of PDR. Alloxan-induced pigs developed Müller cell contraction-promoting activity, which may be detected as early as 30 days following alloxan administration and can last up to 90 days [
73]. These pigs developed cataracts [
73], BRB breakdown, capillary collapse, and pericyte ghosts 60 days after receiving alloxan [
44]. In contrast to dogs, the alloxan-induced pig model displayed several changes in NPDR.
The comparison of the chemical structure, mechanism of action, advantages, and disadvantages between STZ- and alloxan-induced models was described in Table
1.
Table 1
Comparison of the chemical structure, mechanism of action, advantages and disadvantages between STZ- and alloxan-induced model
Source | Glucosamine–nitrosourea compound derived from Streptomyces achromogenes | Synthesised from uric acid oxidation |
Mechanism of Action | ● Selective pancreatic β-cell uptake via GLUT2 ● Generates ROS ● Causes DNA fragmentation ● Acts as a NO donor ● Generates Adenosine triphosphate (ATP) de-phosphorylation ● β-cell necrotic death | ● Selective pancreatic β-cell uptake via the GLUT2 ● Inhibit glucokinase ● Generates ROS ● β-cell necrotic death |
Advantages | ● Rapid simulation of natural T2DM disease progression ● Induced diabetes remains longer ● Cost-effective ● Easy handling (stable at 37 °C within one hour) | ● High selective loss of pancreatic β-cell due to its inhibitory effect on glucokinase |
Disadvantages | ● Poor standardisation ● Carcinogenic to human | ● Can be toxic to liver and kidney cells ● May cause spontaneous regeneration of β-cells ● Less stable in water at room and body temperature ● High variability ● High mortality |