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

Abstract

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.

Fig. 1.

Effects of nimodipine and enalapril treatments on resting [Ca²⁺]_i, amplitudes of depolarization-induced calcium transients and residual [Ca²⁺]_i after the depolarization. (A) Resting [Ca²⁺]_i levels in nimodipine- and enalapril-treated diabetic animals did not significantly differ from those found in non-treated diabetic animals. (B) Nimodipine treatment induced a significant decrease in amplitudes of depolarization-induced calcium transients in DRG and SG neurons of STZ-diabetic rats. Note that in neurons from nimodipine-treated diabetic rats this parameter was significantly (p < 0.05) different from that in enalapril-treated rats. (C) Nimodipine and enalapril treatment of diabetic animals do not affect residual [Ca²⁺]_i measured at 60th second after the termination of depolarization in both DRG and SG neurons. ^* p < 0.05 vs. control, ^# p<0.05 vs. diabetic animals. In all experiments n > 10.

Fig. 2.

Effects of nimodipine and enalapril treatments on amplitudes of caffeine- and ionomycin-induced Ca²⁺ release from the ER. (A) Both nimodipine and enalapril treatment significantly increased caffeine-induced calcium release from the ER of DRG and SG neurons of diabetic rats. Note that amplitudes of [Ca²⁺]_i transients in neurons from nimodipine-treated diabetic rats significantly (p < 0.05) differed from those in neurons of enalapril-treated diabetic animals. (B) Nimodipine and enalapril treatments significantly increased amplitudes of ionomycin-induced calcium release from the ER in neurons from diabetic rats. ^* p < 0.05 vs. control, ^# p < 0.05 vs. diabetic animals. In all experiments n > 10.

Fig. 3.

“Hot plate” test. Both nimodipine and enalapril treatment partially normalized thermal sensitivity of diabetic rats. (A) Dependence of the speed of nociceptive reaction from surface temperature. The speed of reaction was calculated as one over time to the beginning of a nociceptive response (licking of hindpaw). (B) Time to the beginning of a nociceptive response at 48°C. ^* p < 0.05 vs. control, ^# p < 0.05 vs. diabetic animals.

Fig. 4.

Formalin test. Nimodipine but not enalapril treatment was able to normalize in part the intensity of the acute phase of the formalin test. (Aa) A time course of the reaction of jerking the injected hindpaw. On Y-axis is the number of jerks per 5 min interval. (Ab) A time course of the reaction of licking the injected hindpaw. On Y-axis is the number of seconds spent licking the injected paw per 5 min interval. (Ba) An intensity of two behavioral reactions during the acute phase of formalin test, expressed as a total number of jerks (left group of diagrams) and seconds spent licking (right group of diagrams) during the acute phase. (Bb) The same as in Ba panel but for the tonic phase of the formalin test. ^* p < 0.05 vs. control, ^# p < 0.05 vs. diabetic animals.