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into three bilaterally symmetrical clusters in the two lobes of the brain, with the
remaining DAneurons distributed singly along the length of the ventral ganglion.
These DA neurons are retained in the central nervous system of adult flies and are
primarily grouped together into six major clusters (Nassel and Elekes, 1992).
These six clusters are arranged symmetrically about the midline with the neuronal
cell bodies residing at the periphery of the brain and their axons projecting toward
the center (Fig. 1.1). There are four additional clusters of DA neurons on the
posterior side of the brain: two medial clusters, designated the protocerebral
posterior medial (PPM) 2 and 3, and two lateral clusters, named the protocerebral
posterior lateral (PPL) 1 and 2. On the anterior side of the brain, there is a small
cluster of approximately five DA neurons, designated the protocerebral anterior
lateral (PAL), and a larger cluster of approximately 60 DA neurons with charac-
teristically small cell bodies, designated the protocerebral anterior medial
(PAM). In addition to these clusters, there are also a small number of DA neurons
that are separated from the clusters, such as the PPM1, the deutocerebral 1 (D1),
and the ventral unpaired medial (VUM) neurons.
To date, most studies of DA neuron integrity in Drosophila models of
PD have used antisera against tyrosine hydroxylase (TH), an enzyme required for
DA biosynthesis, to image DA neurons. However, several different immunocy-
tochemical methods have been utilized to conduct these imaging studies. Early
studies used thick sections of paraffin-embedded CNS samples in conjunction
with light microscopy to analyze DA neuron integrity, but the sensitivity of this
technique has been questioned (Auluck
, 2005). Confocal
microscopy of whole-mount brain samples is now more commonly used. Another
potential advantage of confocal microscopy relative to the methodology employ-
ing paraffin sections is that this technique allows a better visualization of the
three-dimensional arrangement of the neurons within the intact brain. Thus,
this methodology might facilitate studies of more subtle aspects of DA neuron
dysfunction, such as axonal projection and synaptic defects (e.g., see Fig. 1.1C).
In contrast, some studies have assessed the presence of DA neurons by monitor-
ing the expression of a reporter transgene such as green fluorescent protein (GFP)
under the influence of GAL4 expressed from the endogenous
et al.
, 2005; Pesah
et al.
promo-
ters. The reliability of this method is questionable however, since it is clear that
TH
TH
or
ddc
-GAL4-induced reporters do not exactly mirror the pattern of
TH protein abundance detected by anti-TH immunostaining (see Fig. 1.1B;
Pesah
-GAL4- or
ddc
, 2005). In general, anti-TH immunostaining analyzed by whole-
mount confocal microscopy is considered the preferential technique.
In addition to the spatial distribution of DA neurons in Drosophila, the
functional effects of DA depletion and DA neuron perturbation have also been
studied. Genetic or pharmacologic depletion of DA in Drosophila results in a
variety of characteristic phenotypes. Mutations affecting the
et al.
dopa decarboxylase
gene result in decreased learning ability, while mutations in the TH encoding
