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In the hybrid striped bass (
Morone saxatilis
×
M. chrysops
), diazinon exposure inhibited
brain AChE activity at all concentrations tested, whereas only the medium and high treat-
ment groups showed impairment of prey capture. Gaworecki et al. (2009) concluded that
sublethal exposure to AChE-inhibiting substances may decrease the ecological fitness of
hybrid striped bass, a situation that has been described for another species (
Fundulus het-
eroclitus
) in field conditions (Weis et al. 2001). It may be also noted that the more sensitive
response of the biochemical marker provides a predictive assessment of the potential risks
associated with diazinon exposure.
In
Daphnia magna
, Duquesne (2006) observed that above a threshold concentration of 2.2
μg L
-1
paraoxon-methyl, inhibition of ChE activity was accompanied by effects on survival,
reproduction, and body size, and a reduced population growth rate was also reported. In
a complementary study, Duquesne and Küster (2010) showed that ChE and swimming
activities were significantly affected at lower exposure concentrations (1.0 and 0.7 μg L
-1
,
respectively) than filtration activity, which had the same response threshold (1.5 μg L
-1
) as
physiological responses (use of energy reserves and body size). Despite a high potential
for the affected parameters to recover, these authors consider that “the effects of pesticides
can propagate through biological systems and possibly induce long-term effects at higher
levels of biological organisation.”
The pesticides currently used have been preferred to OC pesticides particularly because
they are less persistent. Pollution incidents in the aquatic environment often occur as
pulses. Thus, it is important to integrate into risk assessments the influence of exposure
duration on the effects of pesticides. In
D. magna
, it seems that the longer the exposure,
the weaker the recovery (Andersen et al. 2006; Duquesne 2006). A review by Sánchez-
Hernandez (2001) indicates that recovery duration varies from 3 to 28 days in different
vertebrate and invertebrate species. In the copepod
Tigriopus brevicornis
, recovery from
pesticide exposure was nearly complete within 14 days (Forget et al. 2003).
Sparling et al. (2001) evoked a link between the contamination of aquatic media by pesti-
cides and the decline of numerous amphibian populations across the world. They mention
that when AChE activity was >2 μmol min
-1
g
-1
, populations of the frog
Hyla regilla
showed
good health status, whereas the health status turned bad when AChE activity was <1.7
μmol min
-1
g
-1
. However, although neurotoxicity of pesticides can contribute to amphibian
decline, other causes may be also involved (Chapter 9).
4.2.5 Retinol
Retinol (vitamin A) and its biologically active derivatives [retinoids, most notably reti-
noic acids (RAs)] are essential compounds for several basic physiological functions such
as growth, cellular differentiation, and reproduction. Food provides a regular amount of
retinoids by means of precursors such as β carotene. Cellular and tissue needs are fulfilled
by retinol, which is the major form present in blood. Because an excess of retinol in tis-
sues may be toxic, surplus amounts are stored in the liver as retinol esters. A schematic
representation of the metabolism of retinoids has been proposed by Inoue et al. (2010). In
the nucleus of target cells, retinoids bind to RA receptors (RAR) and retinoid X receptors
(RXRs). In the basal state, the RAR/RXR heterodimer is bound to a nuclear receptor core-
pressor; then, binding to the ligand results in the release of corepressors and recruitment
of coactivators (Figure 4.1). This permits the transcriptional activation of target genes via
specific RA response elements as described by Inoue et al. (2010).
Different classes of environmental pollutants have been shown to interfere with reti-
noid physiology through effects on retinoid content and gene transcription level, retinoid
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