Cardiovascular responses and central catecholamines in streptozocin-diabetic rats

Diabetic Goto-Kakizaki rats display pronounced hyperglycemia and longer-lasting cognitive impairments following ischemia induced by cortical compression

Hyperglycemia has been shown to worsen the outcome of brain ischemia in several animal models but few experimental studies have investigated impairments in cognition induced by ischemic brain lesions in hyperglycemic animals. The Goto-Kakizaki (GK) rat naturally develops type 2 diabetes characterized by mild hyperglycemia and insulin resistance. We hypothesized that GK rats would display more severe cerebral damage due to hyperglycemia-aggravated brain injury and, accordingly, more severe cognitive impairments. In this study, recovery of motor and cognitive functions of GK and healthy Wistar rats was examined following extradural compression (EC) of the sensorimotor cortex. For this purpose, tests of vestibulomotor function (beam-walking) and combined tests of motor function and learning (locomotor activity from day (D) 1 to D5, operant lever-pressing from D14 to D25) were used. EC consistently reduced cerebral blood flow in both strains. Anesthesia-challenge and EC resulted in pronounced hyperglycemia in GK but not in Wistar rats. Lower beam-walking scores, increased locomotor activity, impairments in long-term habituation and learning of operant lever-pressing were more pronounced and observed at later time-points in GK rats. Fluoro-Jade, a marker of irreversible neuronal degeneration, revealed consistent degeneration in the ipsilateral cortex, hippocampus and thalamus at 2, 7 and 14 days post-compression. The amount of degeneration in these structures was considerably higher in GK rats. Thus, GK rats exhibited marked hyperglycemia during EC, as well as longer-lasting behavioral deficits and increased neurodegeneration during recovery. The GK rat is thus an attractive model for neuropathologic and cognitive studies after ischemic brain injury in hyperglycemic rats.

Increased endogenous noradrenaline and neuropeptide Y release from the hypothalamus of streptozotocin diabetic rats

Noradrenaline and neuropeptide Y (NPY) in the hypothalamus regulate a number of important endocrine and autonomic functions. Alterations in brain neurotransmitter content have been described in type 1 diabetes but there is little understanding of whether these changes affect neurotransmitter release. This study examined for the first time, region-specific co-release of NPY and noradrenaline from the hypothalamus of male Sprague-Dawley rats treated intravenously with 48 mg/kg streptozotocin (STZ) or vehicle. Five weeks later, the release of endogenous noradrenaline and NPY was monitored by in vitro superfusion of ventral and dorsal hypothalamus slices under basal and potassium-stimulated conditions. STZ-diabetes induced significant increases in basal noradrenaline and NPY overflow from the ventral hypothalamus (P<0.05); only NPY overflow was increased in the dorsal hypothalamus (P<0.05). Noradrenaline overflow increased similarly to potassium depolarisation in vehicle and STZ-diabetic rats, whereas diabetic rats showed a significantly increased NPY overflow response to potassium depolarisation compared to vehicle rats. These region-specific increases in endogenous noradrenaline and NPY overflow from the hypothalamus in diabetes suggest increased neuronal activity at rest and enhanced responses under some conditions. Increased hypothalamic NPY and noradrenaline overflow most likely contributes to diabetic hyperphagia.

Potential mechanisms of neuropathic pain in diabetes

Abnormal sensations and pain are features of approximately 10% of all cases of diabvetic neuropathy and can cause marked diminution in the quality of life for these patients. The quality and distribution of pain are variable, although descriptions of burning pain in the hands and feet are commonly reported. Like other neuropathic pain states, painful diabetic neuropathy has an unknown pathogenesis and, in many cases, is not alleviated by nonsteriodal anti-inflammatory drugs or opiates. In the last decase, a number of behavioral and physiologic studies have revealed indices of sensory dysfunction in animal models of diabetes. These include hyperalgesia to mechanical and noxious chemical stimuli and allodynia to light touch. Animal models of painful diabetic neuropathy have been used to investigate the therapeutic potential of a range of experimental agents and also to explore potential etiologic mechanisms. There is relatively little evidence to suggest that the peripheral sensory nerves of diabetic rodents exhibit spontaneous activity or increased responsiveness to peripheral stimuli. Indeed, the weight of eveidence suggests that sensory input to the spinal cord is decreased rather than increased in diabetic rodents. Aberrant spinal or supraspinal modulation of sensory processing may therefore be involved in generating allodynia and hyperalgesia in these models. Studies have supported a role for spinally mediated hyeralgesia in diabetic rats that may reflect either a response to diminished peripheral input or a consequence of hyperglycemia on local or descending modulatory systems. Elucidating the affects of diabetes on spinal sensory processing may assist development of novel therapeutic strategies for preventing and alleviating painful diabetic neuropathy.

Impaired distribution of alpha-methyl-L-p-tyrosine in diabetic rats

Although alpha-methyl-L-p-tyrosine (alpha-MPT), an inhibitor of catecholamine synthesis, has been used to study catecholamine turnover in diabetic animals, effects of diabetes on metabolism of the drug have not been investigated. In this study, administration of a standard dose of alpha-MPT (250 mg/kg initially and 125 mg/kg at 2 h intervals) resulted in lower plasma and tissue levels of alpha-MPT and its metabolites in streptozocin-diabetic rats than in controls. Two to six hours after the initial dose of alpha-MPT, concentrations of alpha-MPT were 2-8-fold lower in the hypothalamus, medulla/pons, and plasma of diabetic animals than in controls. Brain and plasma levels of the alpha-MPT metabolite, alpha-methyl DOPA (alpha-MD) were 2-10-fold lower in tissues of diabetic animals. Levels of the alpha-MPT metabolite alpha-methyl norepinephrine (alpha-MNE), measured only in the hypothalamus, were 4-fold lower in diabetic rats than in controls. There were no differences in the ratio of free/conjugated alpha-MPT in plasma. Treatment of diabetic rats with insulin restored alpha-MPT and alpha-MD to control levels. These findings indicate that i.p. administration of alpha-MPT does not result in equivalent levels of the drug in diabetic and control rats and suggest caution in the use of alpha-MPT to compare catecholamine turnover in diabetic and healthy animals.