The effect of nimodipine on calcium homeostasis and pain sensitivity in diabetic rats.

1. The pathogenesis of diabetic neuropathy is a complex phenomenon, the mechanisms of which are not fully understood. Our previous studies have shown that the intracellular calcium signaling is impaired in primary and secondary nociceptive neurons in rats with streptozotocin (STZ)-induced diabetes. Here, we investigated the effect of prolonged treatment with the L-type calcium channel blocker nimodipine on diabetes-induced changes in neuronal calcium signaling and pain sensitivity. 2. Diabetes was induced in young rats (21 p.d.) by a streptozotocin injection. After 3 weeks of diabetes development, the rats were treated with nimodipine for another 3 weeks. The effect of nimodipine treatment on calcium homeostasis in nociceptive dorsal root ganglion neurons (DRG) and substantia gelatinosa (SG) neurons of the spinal cord slices was examined with fluorescent imaging technique. 3. Nimodipine treatment was not able to normalize elevated resting intracellular calcium ([Ca(2+)]( i )) levels in small DRG neurons. However, it was able to restore impaired Ca(2+) release from the ER, induced by either activation of ryanodine receptors or by receptor-independent mechanism in both DRG and SG neurons. 4. The beneficiary effects of nimodipine treatment on [Ca(2+)]( i ) signaling were paralleled with the reversal of diabetes-induced thermal hypoalgesia and normalization of the acute phase of the response to formalin injection. Nimodipine treatment was also able to shorten the duration of the tonic phase of formalin response to the control values. 5. To separate vasodilating effect of nimodipine Biessels et al., (Brain Res. 1035:86-93) from its effect on neuronal Ca(2+) channels, a group of STZ-diabetic rats was treated with vasodilator - enalapril. Enalapril treatment also have some beneficial effect on normalizing Ca(2+) release from the ER, however, it was far less explicit than the normalizing effect of nimodipine. Effect of enalapril treatment on nociceptive behavioral responses was also much less pronounced. It partially reversed diabetes-induced thermal hypoalgesia, but did not change the characteristics of the response to formalin injection. 6. The results of this study suggest that chronic nimodipine treatment may be effective in restoring diabetes-impaired neuronal calcium homeostasis as well as reduction of diabetes-induced thermal hypoalgesia and noxious stimuli responses. The nimodipine effect is mediated through a direct neuronal action combined with some vascular mechanism.

Diabetes-induced abnormalities in ER calcium mobilization in primary and secondary nociceptive neurons.

Development of diabetic sensory polyneuropathy is associated with alterations in intracellular calcium homeostasis in primary and secondary nociceptive neurons. We have shown previously that in a model of streptozotocin (STZ)-induced diabetes, the calcium signal is prolonged and calcium release from ryanodine-sensitive calcium stores down-regulated in neurons of the nociceptive system. The aim of the present study was a more detailed characterization of calcium homeostasis in primary (dorsal root ganglia, DRG) and secondary (dorsal horn, DH) nociceptive neurons in STZ-induced diabetes. Fluorescence video-imaging was used to measure free cytosolic [Ca2+] ([Ca2+]i) in lumbar nociceptive neurons of control and streptozotocin-diabetic rats. Resting [Ca2+]i rose progressively in these neurons with the duration of diabetes and calcium mobilization from the endoplasmic reticulum (ER) decreased during diabetes. The amplitude of calcium release from both ryanodine- and IP3-sensitive calcium stores induced by caffeine, ionomycin, ATP or glutamate was significantly (P<0.01) lower in DRG and DH neurons from 6-week STZ-diabetic rats. Diabetes-induced changes in the calcium homeostasis were similar in DRG and DH neurons indicating that they might be general for many types of neurons from the central and peripheral nervous systems.

Metabotropic purinoreceptors in rat dorsal horn neurones: predominant dendritic location.

Elevations of cytosolic free Ca2+ concentration ([Ca2+]i) induced by addition of ATP have been compared in rat dorsal horn neurones in slices and after their isolation. ATP application induced in neurones in situ a rise of [Ca2+]i by 201 +/- 12 nM. In Ca2+-free external solution the rise was 156 +/- 14 nM (n = 45 of 76), indicating the presence of active purinergic metabotropic receptors in about 59% of neurones. [Ca2+]i transients induced by 2MeSATP in Ca2+-free external solution were completely abolished by 10 microM PPADS, indicating that some of the corresponding receptors are of the P2Y1 type. In acutely isolated neurones which lost their dendrites, there were no metabotropic response. The results confirm the presence of metabotropic postsynaptic purinoreceptors located in the dendritic tree of dorsal horn neurones.

[Changes in intracellular mechanisms in sensory neurons in experimental diabetes mellitus]. / Porushennia vnutrishn'oklitynnykh mekhanismiv sensornykh neironiv pry eksperymental'nomu tsukrovomu diabeti.

Here we summarises the results of experimental investigation of changes in intracellular calcium homeostasis in sensory neurones of rats with streptozotocin-induced diabetes mellitus. Decrease in the calcium-accumulating function of both inositol-trisphosphate- as well as caffeine-sensitive endoplasmic reticulum has been detected both in primary sensory neurones of dorsal root ganglia and in secondary neurones of the spinal cord dorsal horn. Predominant depression in the functioning of metabotropic receptors of ligand-gated channels compared with those of ionotropic ones has been demonstrated. Changes in the pharmacological sensitivity of potential-operated calcium channels (predominantly of L-type), linked, probably, with alterations of functional connections between membrane channels and endoplasmic reticulum, are described. A predominant role of changes in the functioning of intracellular Ca(2+)-accumulating structures, leading to prolongation of depolarisation-induced Ca2+ transients in primary and secondary sensory neurones and corresponding changes in the transmission of nociceptive signals during diabetic neuropathy are discussed.