Helmut Segner Fish Nociception and pain A biological perspective

nociceptors as well as slow and fast adapting mechanical receptors
(Sneddon et al. 2003a, Ashley et al. 2007). All receptor types except
mechanothermal receptors showed an increase in peak firing
frequency with increased strength of stimulation, with evidence of
response saturation at higher intensities (Ashley et al. 2007). The
mechanical thresholds were higher in the polymodal than in the
mechanical receptors, but generally they were very low compared
to the thresholds of mechanoreceptors in the skin of mammals.
Sneddon (2003a) speculated that this is a compensatory feature to
the more easily damageable nature of fish skin. Thermal nociceptors
of trout showed a threshold that was about 10 °C lower than the
thermal threshold of mammalian thermal nociceptors, and none
of the tested receptors gave any response to water temperatures
below 7 °C, indicating an absence of cold receptors (Ashley et al.
2007). Both properties might be explained by the poikilothermic
physio logy of fish. Overall, the results from this series of studies
provide good evidence for the presence of nociception in teleostean
fish, and that the properties of the nociceptors are adapted to
the specific biological properties of fish. This conclusion agrees
with the conclusion from the EFSA Report (2009) which says
that “there is good scientific evidence that fish possess the sensory
equipment for detecting potentially painful stimuli”.
In mammals, the next step after peripheral nociception is
the processing of the nociceptive signals at the spinal cord level
(see above). Fish, in general, show an organization of major
spinal pathways that is similar to mammals, including spino-thalamic,
spino-mesencephalic, spino-reticular and spino-limbic
tracts (Chandroo et al. 2004a). This similarity suggests that spinal
tracts of fish function in carrying nociceptive signals to the
brain, although to date actual experimental confirmation of this
assumption is not available. What, however, has been shown is the
presence of neuronal activities in the brain of fish after cutaneous
noxious stimulation (Dunlop and Lamig 2005), indicating that
peripheral nociceptive signals reach the brain. This observation
provides indirect evidence for the functioning of the spinal cord
of fish in nociceptive transmission.
3.2 Pain perception: neuroanatomical evidence
3.2.1 The fish brain
The brain anatomy of teleostean fish is rather diverse, reflecting
their diverse habitats and modes of life (Butler and Hodos 1996,
Kotrschal et al. 1998, Nieuwenhuys et al. 1998, Wulliman 1998).
As emphasized by Rose (2002):
“Fish brains are not merely simpler versions of mammalian or amphibian
or reptilian brains, but they can have highly diverse, unique
adaptations due to their diverse evolutionary paths.”
For instance, electro-sensing mormyrid fish have a specific structure,
the valucla, for the central processing of electric stimuli.
Independent of species variations, however, the fish brain comprises
the principal elements of the vertebrate brain, as also found
in human brain (see above): telencephalon (with the olfactory
lobe), diencephalon, mesencephalon (with the optic tectum), metencephalon
(with the cerebellum), and myelencephalon (Figure 5).
The teleost telencephalon receives input from the olfactory bulbs,
but there are large areas of the pallium which are not devoted to
olfactory input and are involved in other functions (see below).
The telencephalon shows many reciprocal connections with the
other parts of the brain. Advanced teleost fish like the percids tend
to enlarge the midbrain and develop a prominent optic tectum in
order to process the sensory information from the visual system
as well as from the lateral line system, which provides the fish
Figure 5: External morphology and the
anatomical location of the fish brain.
(Modified from www.britannica.com/
EBchecked/topic/452101/perch.)
Cerebellum
Cerebrum
Optic Lobe
Hypothalamus and Pituitary
Olfactory bulb
34 Fish. Nociception and pain | Contributions to Ethics and Biotechnology
Fish. Nociception and pain | Contributions to Ethics and Biotechnology
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