Dopaminergic neurons intrinsic to the primate striatum

Abstract

Intrinsic, striatal tyrosine hydroxylase-immunoreactive (TH-i) cells have received little consideration. In this study we have characterized these neurons and their regulatory response to nigrostriatal dopaminergic deafferentation. TH-i cells were observed in the striatum of both control and 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-treated monkeys; TH-i cell counts, however, were 3.5-fold higher in the striatum of MPTP-lesioned monkeys. To establish the dopaminergic nature of the TH-i cells, sections were double-labeled with antibodies to dopamine transporter (DAT). Immunofluorescence studies demonstrated that nearly all TH-i cells were double-labeled with DAT, suggesting that they contain the machinery to be functional dopaminergic neurons. Two types of TH-i cells were identified in the striatum: small, aspiny, bipolar cells with varicose dendrites and larger spiny, multipolar cells. The aspiny cells, which were more prevalent, corresponded morphologically to the GABAergic interneurons of the striatum. Double-label immunofluorescence studies using antibodies to TH and glutamate decarboxylase (GAD67), the synthetic enzyme for GABA, showed that 99% of the TH-i cells were GAD67-positive. Very few (<1%) of the TH-i cells, however, were immunoreactive for the calcium-binding proteins calbindin and parvalbumin. In summary, these results demonstrate that the dopaminergic cell population of the striatum responds to dopamine denervation by increasing in number, apparently to compensate for loss of extrinsic dopaminergic innervation. Moreover, this population of cells corresponds largely with the intrinsic GABAergic cells of the striatum. This study also suggests that the adult primate striatum does retain some intrinsic capacity to compensate for dopaminergic cell loss.

Fig. 1.

Schematic diagrams of representative coronal sections through a monkey brain illustrating the striatal areas (dark gray areas) analyzed for TH-i cell counts. Two sections at different levels anterior to and two sections at the level of the anterior commissure were analyzed from each animal.

Fig. 2.

TH-i cells in the striatum of a control monkey (A) and an MPTP-treated monkey (B). Note the increase in TH-i cell density in the dopamine-depleted striatum (B). Because of the presence of intact nigrostriatal dopaminergic fibers, the neuropil in the control striatum (A) is more intensely stained than in the dopamine-denervated striatum (B). Nevertheless, TH-i cells are easily discerned in control tissue (A). All the TH-i cells in this field are indicated by arrows (A). Scale bar, 100 μm.

Fig. 3.

TH-immunoreactive cell counts in the striatum of control and MPTP-treated monkeys. Bars represent the mean ± SE total cell counts from four anatomically matched sections of striatum from each of three control and four MPTP-treated monkeys. Cell counts in MPTP-treated monkeys were 3.5-fold greater than in control monkeys (*p < 0.05).

Fig. 4.

Both aspiny and spiny TH-i neurons were identified in the striatum of MPTP-treated monkeys. A, Photomicrograph of an aspiny, oval-shaped neuron with smooth dendrites. These TH-i neurons were observed much more frequently than the spiny neurons after MPTP treatment. B, Photomicrograph of a spiny TH-i neuron with a number of primary dendrites that are densely covered with spines. The insets in A andB show a magnified image of a portion of the dendrite denoted by the arrows. Scale bar, 30 μm.

Fig. 5.

Top left. Examples of striatal aspiny TH-immunoreactive cells with varicose dendritic processes in MPTP-treated monkeys. The images are three-dimensional reconstructions of optical sections through TH-i cells that were obtained by confocal microscopy in 50 μm sections of monkey brain.

Fig. 9.

Colocalization of TH and parvalbumin in striatal cells. Fluorescent photomicrograph of a TH-immunnoreactive cell (arrow) in the striatum of an MPTP-treated monkey, which is also immuoreactive for PV. Less than 1% of the striatal TH-i cells double-stained for parvalbumin. Scale bar, 20 μm.