Pramipexole attenuates the dopaminergic cell loss induced by intraventricular 6-hydroxydopamine.

The D3 preferring dopamine agonist pramipexole has been shown to attenuate the cell loss induced by levodopa in vitro. Pramipexole was herein evaluated in the 6-hydroxydopamine lesion model to determine its in vivo effect. Rats were treated with pramipexole or saline before and after an intracerebroventricular 6-hydroxydopamine injection. In the preliminary study, 6-hydroxydopamine produced a 68% reduction in striatal dopamine and a 62% loss in tyrosine hydroxylase immunoreactive (THir) cell counts in the substantia nigra. Pramipexole treated animals exhibited a 29% and a 27% reduction in striatal dopamine and THir cell counts, respectively. THir cell counts and striatal dopamine were significantly correlated. In the stereological study, 6-hydroxydopamine reduced THir cell counts by 47% in saline treated animals and 26% in pramipexole treated animals. These data demonstrate that pramipexole attenuates the biochemical and THir cell changes normally produced by 6-hydroxydopamine consistent with its neuroprotective actions in vitro.

Striatal trophic activity is reduced in the aged rat brain.

Our previous studies demonstrated that the survival of a mesencephalic graft was reduced in aged animals suggesting an age-related decline in target-derived neurotrophic activity. We tested this hypothesis by examining dopamine (DA) and trophic activities from the striatum of intact or unilateral 6-hydroxydopamine (6-OHDA) lesioned rats of increasing age. Fisher 344 rats were 4, 12, 18, and 23 months old (m.o.) at sacrifice. Half the animals had received unilateral 6-OHDA lesions of the mesostriatal DA pathway 8 weeks earlier. Striatal tissue punches were analyzed for DA, homovanillic acid (HVA), and DA activity (HVA/DA) using HPLC. The remainder of the striatal tissue was homogenized to generate tissue extracts which were added to E14.5 ventral mesencephalic cultures to test trophic activity. In the non-lesioned animals, striatal DA was reduced and striatal DA activity was increased in the 18 and 23 m.o. animals relative to the 4 and 12 m.o. animals. Striatal trophic activity was inversely related to age. In the lesioned animals, striatal DA ipsilateral to 6-OHDA infusion was below detection limits while the contralateral striatum exhibited age-related changes in DA similar to those seen in the non-lesioned animals. In 4 m.o. lesioned rats, striatal trophic activity ipsilateral to 6-OHDA infusion was elevated by 26% relative to the contralateral side. The ipsi/contra-lateral differences in striatal trophic activity were reduced in 12 m.o. animals and absent in the 18 and 23 m.o. groups. These data suggest that advancing age is associated with a reduction in striatal DA as well as trophic activity. Moreover, the aged striatum loses its ability to biochemically and trophically compensate for DA reduction and therefore may represent a more challenging environment for the survival, growth, and function of a fetal graft.

Prenatal cocaine exposure reduces glial cell line-derived neurotrophic factor (GDNF) in the striatum and the carotid body of the rat: implications for DA neurodevelopment.

Glial cell line-derived neurotrophic factor (GDNF) is a glycosylated, disulfide-bonded homodimer, and a member of the transforming growth factor-beta superfamily. GDNF has been shown to promote the survival and morphological differentiation of dopamine (DA) neurons and increase their high-affinity dopamine uptake. In order to determine whether the mechanism for our previously observed cocaine-induced DA reductions in brain and carotid body were GDNF-mediated, we exposed Sprague-Dawley rat fetuses to cocaine via maternal subcutaneous injections (30 mg/kg b.i.d., E7-E19). Brains and carotid bodies of fetuses were excised and processed for assessment of GDNF levels using an Enzyme-Linked ImmunoadSorbent Assay (ELISA). ANOVA indicated that cocaine reduced carotid body GDNF by 36% (F((1,5))=28. 11, p<0.05) and striatal GDNF by 41% (F((1,5))=41.77, p<0.01). Although there was no interaction between drug exposure and fetal uterine position, post-hoc pairwise comparisons indicated that reductions in GDNF in the cocaine groups were due to differences at more distal positions (positions 4-8). The magnitude of the reductions in striatal GDNF (but not carotid body GDNF) in both cocaine-exposed and control fetuses followed a cervical (smallest GDNF reductions) to ovarian (greatest GDNF reductions) uterine position gradient. This pattern was similar to that which we observed in prior studies examining DA reductions in brain following prenatal cocaine exposure. The finding that cocaine reduces GDNF levels in striatum and carotid body support the hypothesis that cocaine's ability to reduce striatal and carotid body DA may be indirect through its ability to reduce GDNF. These data along with previous findings support the hypothesis that cocaine's effects on DA neurons are at least partially due to its indirect effects on trophic activity. The possible mechanisms whereby cocaine affects trophic activity are discussed.

Both the antioxidant and D3 agonist actions of pramipexole mediate its neuroprotective actions in mesencephalic cultures.

Pramipexole (PPX) is a full intrinsic activity, direct-acting dopamine (DA) agonist possessing 7-fold higher affinity for D3 than for D2 receptors. It also is a potent antioxidant. PPX was previously shown to be neuroprotective because it dose dependently attenuated the DA neuron loss produced by levodopa in mesencephalic cultures. Several different drugs with properties similar to PPX were studied here to better understand the mechanism or mechanisms responsible for this neuroprotective effect. The D3-preferring agonist 7-hydroxy-diphenylaminotetralin (7-OH-DPAT) and the D3 antagonist U99194, respectively, increased and decreased the neuroprotective effects of PPX in a dose-dependent fashion. Addition of the selective D2 agonist U95666 or the D2/D3 antagonists domperidone or raclopride did not affect PPX's neuroprotective effect. Interestingly, 7-OH-DPAT by itself did not attenuate the DA neuron loss produced by levodopa. However, when 7-OH-DPAT was combined with a low dose of the antioxidants U101033E or alpha-tocopherol, the toxic effects of levodopa were attenuated. Similar results were observed when the D3-preferring agonist PD128, 907 was studied. In addition, media conditioned by exposure of mesencephalic cultures incubated with all D3-preferring agonists studied was shown to enhance the growth of DA neurons in freshly harvested recipient cultures implicating a D3-mediated trophic activity in the neuroprotective effect. These data suggest that PPX's neuroprotective actions in the levodopa toxicity model are a consequence of its combined actions as a D3 receptor agonist and an antioxidant.

The magnitude of brain dopamine depletion from prenatal cocaine exposure is a function of uterine position.

Cocaine's teratogenicity remains equivocal in the literature. The variance in cocaine-induced teratogenic data led us to consider that the intrauterine exposure to cocaine is not homogeneous and that sampling methods presently utilized in the literature lead to inconsistent results. Cocaine's vasoconstrictive actions, in concert with regional variance in the uterine milieu of the rodent, were postulated to differentially reduce the distribution of cocaine to fetal brains as a function of uterine position. Fetuses in positions with the highest levels of cocaine exposure were also hypothesized to have the most pronounced deficits in whole brain dopamine (DA). The results indicated that whole brain cocaine levels vary significantly in relation to a fetus' position in the uterine horn following a single SC injection of 30 mg/kg cocaine HCI as measured by GC/MS. Brains of fetuses from the most proximal uterine position (in relation to the cervix) received an average of 329% of the cocaine of fetuses from the most distal uterine position, whereas no such relationship existed for amniotic fluid cocaine levels. Following exposure to cocaine from embryonic days 7 to 21, brain DA levels were significantly reduced in distal fetuses relative to proximal fetuses and to distal controls. Contrary to the initial hypothesis, the results indicated that the magnitude of cocaine exposure was inversely related to the magnitude of DA reduction. Based upon findings in the literature related to the uterine gradient of placental progesterone distribution in the rat, cocaine's ability to lower brain DA levels was attributed primarily to its vasoconstrictive actions. Recommendations on how to statistically treat littermates, when foreknowledge of uterine position exists, are discussed.