Neurons, Chromaffin Cells and Membrane Fusion - Fusion of Biological Membranes and Related Problems - page 325

content only at higher frequencies of stimulation (Bartfai et al., 1988).
Although this concept has now been generalized, it has, at least for nora-
drenergic neurons, been challenged recently. By using different approaches,
it is demonstrated that the release of the neurotransmitters from symp-
athetic neurons ultimately occurs from the LDVs (De Potter et al., 1995;
1997).
Since the original demonstration of the co-storage of chromogranins
and NA in secretory granules in chromaffin cells, many studies revealed
that most hormones and also neurotransmitters are stored in specialized
organelles, called endocrine secretory granules in various endocrine tissues
and LDVs in the nervous tissue. For a long time it was assumed that all
these vesicles, storing various hormones and transmitters, were different
from each other in their biochemical composition but evidence accumulates
that the opposite is true. Various proteins (chromogranins) and neuro-
peptides originally confined to chromaffin granules and LDVs are found
in endocrine secretory granules. It is now generally established that LDVs,
chromaffin granules and endocrine secretory granules, apart from their
respective hormones and transmitters, exhibit similar properties (for review
see Winkler and Fischer-Colbrie, 1990). In this review we will discuss the
properties of their membrane proteins, with respect to their role in exocy-
tosis, based on studies on chromaffin cells, the human neuroblastoma cell
SH-SY5Y and primary cultures of noradrenergic neurons.
Because of their common ontology with peripheral noradrenergic
neurons and their large yield, primary cultures from bovine chromaffin cells
have been used extensively as a model to study the physiological and mor-
phological aspects of sympathetic adrenergic neurons.
The neuroblastoma clone SH-SY5Y derived from a human sympa-
thetic ganglion (Ross and Biedler, 1985) is becoming of increasing impor-
tance as a model for a sympathetic neuron. Thus previous studies have
shown that SH-SY5Y can be differentiated to express many properties of
a mature sympathetic neuron (Pahlman et al., 1990). For example treatment
of SH-SY5Y cells for several days with low concentrations (16 nM) of TPA,
resulted in an increased synthesis of NA leading to a higher content of NA
compared to DA. In addition, preliminary studies demonstrated that TPA
treated SH-SY5Y cells expressed high affinity uptake and depolarisation-
evoked Ca 2+ -dependent release of NA.
A primary culture of peripheral noradrenergic neurons, obtained from
the porcine ganglion superior cervicalis (GSC), was introduced a few years
ago (Wang et al., 1995; Wang and De Potter, 1998). The larger yield of this
culture provides the possibility to study noradrenergic neurons not only
microscopically but also biochemically.