Destruction of dopaminergic neurons in the midbrain by 6-hydroxydopamine decreases hippocampal cell proliferation in rats: reversal by fluoxetine

Abstract

Background: Non-motor symptoms (e.g., depression, anxiety, and cognitive deficits) in patients with Parkinson disease (PD) precede the onset of the motor symptoms. Although these symptoms do not respond to pharmacological dopamine replacement therapy, their precise pathological mechanisms are currently unclear. The present study was undertaken to examine whether the unilateral 6-hydroxydopamine (6-OHDA) lesion to the substantia nigra pars compacta (SNc), which represents a model of long-term dopaminergic neurotoxicity, could affect cell proliferation in the adult rat brain. Furthermore, we examined the effects of the selective serotonin reuptake inhibitor (SSRI) fluoxetine and the selective noradrenaline reuptake inhibitor maprotiline on the reduction in cell proliferation in the subgranular zone (SGZ) by the unilateral 6-OHDA lesion.

Methodology/principal findings: A single unilateral injection of 6-OHDA into the rat SNc resulted in an almost complete loss of tyrosine hydroxylase (TH) immunoreactivity in the striatum and SNc, as well as in reductions of TH-positive cells and fibers in the ventral tegmental area (VTA). On the other hand, an injection of vehicle alone showed no overt change in TH immunoreactivity. A unilateral 6-OHDA lesion to SNc significantly decreased cell proliferation in the SGZ ipsilateral to the 6-OHDA lesion, but not in the contralateral SGZ or the subventricular zone (SVZ), of rats. Furthermore, subchronic (14 days) administration of fluoxetine (5 mg/kg/day), but not maprotiline significantly attenuated the reduction in cell proliferation in the SGZ by unilateral 6-OHDA lesion.

Conclusions/significance: The present study suggests that cell proliferation in the SGZ of the dentate gyrus might be, in part, under dopaminergic control by SNc and VTA, and that subchronic administration of fluoxetine reversed the reduction in cell proliferation in the SGZ by 6-OHDA. Therefore, SSRIs such as fluoxetine might be potential therapeutic drugs for non-motor symptoms as well as motor symptoms in patients with PD, which might be associated with the reduction in cell proliferation in the SGZ.

Figure 1. Experimental paradigms.

(A) Proliferation paradigm. Animals received intra-SNc injection of 6-OHDA or vehicle. Seven days after the surgery, animals were injected with fluoxetine, maprotiline, or saline for 14 days. A day after the last injection, animals were given BrdU and sacrificed. (B) Differentiation paradigm. Animals were given intra-SNc injection of 6-OHDA. Seven days after the surgery, animals were injected with fluoxetine or saline for 14 days and given BrdU. Twenty-four days after the injection of BrdU, animals were given Morris water maze (MWM) tasks for 3 days, and then sacrificed.

Figure 2. Reduced TH immunoreactivity in the striatum, midbrain, and hippocampus following 6-OHDA lesioning.

(A, B) Coronal sections of the striatum immunostained for TH in representative animals from the sham group (A) and 6-OHDA group (B). Almost complete loss of TH immunoreactivity is observed in the right striatum (dotted area) of the 6-OHDA group animal. (C, D) Coronal sections of the midbrain immunostained for TH in animals from the sham group (C) and 6-OHDA group (D). (E–H) Sections of the SNc (E, F) and ventral tegmental area (G, H) at higher magnification of C and D. is The 6-OHDA group animal (F, H) has fewer TH-positive dopaminergic cells than the sham-operated animal (E, G). (I–L) Coronal sections of the hippocampus in animals from the sham group (I, K) and 6-OHDA group (J, L). TH-positive fibers are observed in the hilus (h) as well as in the granular cell layer (g) of the dentate gyrus in the sham group animal (I, K), which is completely lost in the 6-OHDA lesioned animal (J, L).

Figure 3. Cell proliferation in the SVZ and SGZ.

(A, B) Coronal sections of the SVZ of the lateral ventricle contralateral (A) or ipsilateral (B) to the 6-OHDA lesioning in an animal from the 6-OHDA group. BrdU-positive nuclei are clearly observed. (C, D) Sections of the dentate gyrus of the hippocampus contralateral (C) or ipsilateral (D) to the 6-OHDA lesioning. BrdU-positive cells (arrows) are observed in the SGZ of the dentate gyrus.

Figure 4. Cell proliferation in the SGZ after antidepressant treatment.

Reduction of cell proliferation in animals given 6-OHDA lesioning was rescued by 2 weeks of treatment with fluoxetine (5 mg/kg) to a level comparable to that of contralateral side. *Significantly lower than the corresponding value in the contralateral side. ^†Significantly higher than corresponding values of 6-OHDA group animals treated with saline.

Figure 5. Mean escape latency in the Morris water maze task.

There was no significant difference between the treatments.

Figure 6. Differentiation of BrdU-positive cells in the dentate gyrus in rats 28 days after the subchronic treatment with fluoxetine.

BrdU-positive cells (green) were co-labeled with a neuronal marker, NeuN (A), but not GFAP (B). Scale bars: 400 µm in lower magnification and 40 µm in higher magnification.