Pain cells (nociceptors) are difficult to examine. Their most
interesting parts – the sensory endings – are very thin fibers,
embedded in a tough layer of skin tissue. Conventional methods for
physiological experimentation like calcium imaging or
electrophysiology can hardly be applied to these fibers. Consequently,
there is little direct physiological information about what happens
when painful stimuli hit the skin. One approach to this problem is to
isolate the somata of pain cells out of the dorsal root ganglia, to
keep them in primary culture, and to study transduction proteins in
these cultured neurons. The basic assumption is that the cultured pain
cells express the same proteins that mediate pain transduction the
sensory endings in vivo.
dorsal root ganglia (DRG) contain the cell bodies of nociceptors. A
bifurcated axon emanates from each cell body. At its central end this
axon forms a synapse within the spinal cord, the peripheral, sensory
ending lies in the skin or in other pain-sensitive tissues. Primary
cultures from dorsal root ganglia grow well on a surface covered with
laminin, an extracelluar matrix protein. Non-neuronal cells grow a dense
carpet over the entire surface and form a support on which pain cells
(large, round cells) can live for several weeks. Pain cells often grow
neurites which connect several cells, a process which is promoted by
neuronal growth factors.
Which transduction proteins can
be studied in primary cultures of pain cells? A good way to detect the
proteins is to use specific antibodies, labeled with a fluorescent dye.
The image on the right shows a pain cell culture stained with an
antibody that was raised against the heat-sensitive ion channel TRPV1.
This channel mediates the perception of noxious heat; it is openend by
tempereatures above 40 oC. Neurons expressing TRPV1
(“transient receptor potential vanilloid receptor type 1”) are labeled
red on the image, the blue stain (DAPI) shows cell nuclei.
TRPV1 channels can not only be
opened by heat, but also by capsaicin, the pungent component of chilli
peppers. Capsaicin opens TRPV1 channels in sensory endings of
heat-sensitive nociceptors and, thereby, produces a fake “hot”
sensation. This effect can be studied in pain cell cultures using the
patch-clamp technique. A microelectrode is used to record the membrane
potential of a pain cell. The resting, unstimulated cell displays
membrane potentials in the range of -60 to -80 mV. Applying 1 µM
capsaicin depolarizes the neuron and causes it to fire action
potentials. Thus, the cultured DRG neuron responds to a painful stimulus
by generating an electrical signal.