Calcitriol promotes augmented dopamine release in the lesioned striatum of 6-hydroxydopamine treated rats

Abstract

Current therapies for Parkinson's disease (PD) offer symptomatic relief but do not provide a cure or slow the disease process. Treatments that could halt progression of the disease or help restore function to damaged neurons would be of substantial benefit. Calcitriol, the active metabolite of vitamin D, has been shown to have significant effects on the brain. These effects include upregulating trophic factor levels, and reducing the severity of some central nervous system lesions. While previous studies have shown that calcitriol can be neuroprotective in 6-hydroxydopamine (6-OHDA) rodent models of PD, the present experiments were designed to examine the ability of calcitriol to promote restoration of extracellular dopamine (DA) levels and tissue content of DA in animals previously lesioned with 6-OHDA. Male Fischer-344 rats were given a single injection of 12 µg 6-OHDA into the right striatum. Four weeks later the animals were administered vehicle or calcitriol (0.3 or 1.0 µg/kg, s.c.) once a day for eight consecutive days. Three weeks after the calcitriol treatments in vivo microdialysis experiments were conducted to measure potassium and amphetamine evoked overflow of DA from both the left and right striata. In control animals treated with 6-OHDA and vehicle there were significant reductions in both potassium and amphetamine evoked overflow of DA on the lesioned side of the brain compared to the contralateral side. In animals treated with 6-OHDA followed by calcitriol there was significantly greater potassium and amphetamine evoked overflow of DA from the lesioned striatum compared to that from the control animals. The calcitriol treatments also led to increases in postmortem tissue levels of DA in the striatum and substantia nigra. These results suggest that calcitriol may help promote recovery of dopaminergic functioning in injured nigrostriatal neurons.

Fig. 1

Dialysate levels of DA from the left and right striata of animals injected in the right striatum with 6-OHDA four weeks prior to treatment with vehicle or calcitriol. Microdialysis experiments were performed three weeks after the last vehicle or calcitriol injection. Excess potassium (100 mM) was included in the perfusate for 20-min starting at 0 min (horizontal bar above K⁺), and 100 µM amphetamine was included in the perfusate for 20-min starting at 120 min (horizontal bar above Amphetamine). Values shown are mean ± SEM from 8 animals per group. * p < 0.001 vs. left striatum, # p < 0.05 vs. right striatum of vehicle treated group (mixed ANOVA with side of brain and time of dialysis sample collection as within factors, and treatment group as a between factor; followed by Newman-Keuls post-hoc comparisons)

Fig. 2

Potassium-evoked (top panel) and amphetamine-evoked (bottom panel) overflow of DA from the striatum of animals injected in the right striatum with 6-OHDA four weeks prior to treatment with vehicle (0.0 µg dose of calcitriol) or calcitriol. Microdialysis experiments were performed three weeks after the last vehicle or calcitriol injection. Values shown are mean ± SEM from 8 animals per group. * p < 0.05 vs. left side of same group, # p < 0.05 vs. right side of vehicle treated group (mixed ANOVA with side of brain as a within factor and treatment as a between factor; followed by Newman-Keuls post-hoc comparisons)

Fig. 3

Dialysate levels of DOPAC from the left and right striata of animals injected in the right striatum with 6-OHDA four weeks prior to treatment with vehicle or calcitriol. Microdialysis experiments were performed three weeks after the last vehicle or calcitriol injection. Excess potassium (100 mM) was included in the perfusate for 20-min starting at 0 min (horizontal bar above K⁺), and 100 µM amphetamine was included in the perfusate for 20-min starting at 120 min (horizontal bar above Amphetamine). Values shown are mean ± SEM from 8 animals per group. * p < 0.001 vs. left striatum, # p < 0.05 vs. right striatum of vehicle treated group (mixed ANOVA with side of brain and time of dialysis sample collection as within factors, and treatment group as a between factor; followed by Newman-Keuls post-hoc comparisons)

Fig. 4

Dialysate levels of HVA from the left and right striata of animals injected in the right striatum with 6-OHDA four weeks prior to treatment with vehicle or calcitriol. Microdialysis experiments were performed three weeks after the last vehicle or calcitriol injection. Excess potassium (100 mM) was included in the perfusate for 20-min starting at 0 min (horizontal bar above K⁺), and 100 µM amphetamine was included in the perfusate for 20-min starting at 120 min (horizontal bar above Amphetamine). Values shown are mean ± SEM from 8 animals per group. * p < 0.001 vs. left striatum, # p < 0.05 vs. right striatum of vehicle treated group (mixed ANOVA with side of brain and time of dialysis sample collection as within factors, and treatment group as a between factor; followed by Newman-Keuls post-hoc comparisons)

Fig. 5

Postmortem tissue levels of DA from the striatum and substantia nigra of animals injected in the right striatum with 6-OHDA four weeks prior to treatment with vehicle (0.0 µg dose of calcitriol) or calcitriol. Tissue was harvested three weeks after the last vehicle or calcitriol injection. Values shown are mean ± SEM from 8 animals per group. * p < 0.05 vs. left side of same group, # p < 0.05 vs. right side of vehicle treated group (mixed ANOVA with side of brain as a within factor and treatment group as a between factor; followed by Newman-Keuls post-hoc comparisons)

Fig. 6

Tissue levels of GDNF from the striatum and substantia nigra of animals injected in the right striatum with 6-OHDA four weeks prior to eight days of treatment with vehicle (0.0 µg dose of calcitriol) or calcitriol. Tissue was harvested 2–3 hours after the last injection of vehicle or calcitriol. Data from all groups are expressed as a percentage of the left side of the animals treated with vehicle (0.0 µg dose of calcitriol). Values shown are mean ± SEM from 12 animals per group. * p < 0.05 vs. left side of all groups and right side of vehicle treated group (mixed ANOVA with side of brain as a within factor and treatment group as a between factor; followed by Fisher’s LSD post-hoc comparisons)