Sensitive periods

Amphibians. Amphibians are highly susceptible to endocrine disruption during development of the larval form and during metamorphosis. The action of metamorphosis is triggered and controlled by the thyroid gland via an increase in triiodothyronine and a decrease in thyroxine, and differs greatly between oviparous and viviparous species. Experimentally, it has been shown that disruption during this sensitive period can lead to malformations and adverse impacts on immune and reproductive functions. 

Hormonal actions occur during sensitive periods in development, in adult life during natural endocrine cycles and in response to experience as well as during the aging process (see Ch. 30). As a result of their fundamental actions on cellular processes and genomic activity and of the control of their secretion by environmental signals, steroid and thyroid hormone actions on the brain provide unique insights into the plasticity of the brain and behavior (see also Ch. 50). 

Lasch-Loquire and coworkers [45] evaluated the contact hypersensitivity reaction against picryl chloride (0.5 mg/Kg Pb s.c.) in mice with Pb administered just before or during the sensitization period. Pb exposure suppressed the DTH response regardless of the window (before or during sensitization) in which it had been administered. 

Hexachloroethane did not act as a sensitizer in guinea pigs when a challenge dose was given 2 weeks after the end of a 3-week sensitization period (Weeks et al. 1979). Accordingly, it did not stimulate antibody formation during sensitization. The NOAEL for dermal sensitization is reported in Table 2-3. 

Coureaud, G., Montigny, D., Dewas, M., Moncomble, A.S., Patris, P. and Schaal, S. (2005) A sensitive period for pheromone-induced odour learning in newborn rabbits. 29th International Ethological Conference, Budapest, Hungary, 20-27 August. 

In a classical experiment Mainardi (1965) exposed male mouse pups in their nest to an artificial odor, Chanel No. 5. As adults, these mice preferred females treated with Chanel No. 5 over those with regular mouse odor. Mammals differ from birds in that learning plays a critical role in acquiring social preferences the sensitive period is more drawn out than a narrow critical period, and effects are not necessarily permanent, and they can be overridden more easily by subsequent relearning. 

Rabbit pups learnt an odor of their mother in one trial. This odor emanates from the belly of the dam and releases suckling. This rapid learning is considered a form of olfactory imprinting tied to an early sensitive period (Hudson and Altbacker, 1994). 

Morin, P. P., Dodson, J. J., and Dore, F. Y. (1987). Laboratoiy identification of a sensitive period for olfactory imprinting in young Atlantic salmon. American Zoologist 17, 77h. 

Most drugs and chemicals pose a threat to the developing fetus. An estimated 4 to 5% of developmental defects in humans result from prenatal exposure to drugs or environmental chemicals. This is particularly important, since women with irregular menstrual cycles may be exposed to teratogens and enter the sensitive period of organogenesis before pregnancy is suspected. 

Teeth are most susceptible to the adverse effects of excess fluoride at the transition and early maturation stages of enamel development [87]. The timing of these stages varies with the type of tooth [88]. For the most prominent teeth, the upper central incisors, the most sensitive period appears to be 13-24 months, with some indication that the timing varies slightly for boys and girls [87,88]. 

Children with PTSD may be more likely to have comorbid conditions because traumatic insults occur in developmentally sensitive periods. Early life trauma is particularly toxic in its effects on development. Adults with severe sexual abuse histories exhibit high rates of debilitating disorders such as depression, anxiety disorders, alcoholism, substance abuse, and personality disorders (Herman and Van der Kolk, 1987 Putnam and Trickett, 1993). 

Although embryos, fetuses, and nursing infants may be exposed to relatively high amounts of PBBs and PBDEs during sensitive periods of development, it is not known if the susceptibility of children to the health effects of these chemicals differs from that of adults. Younger children may be particularly... 

During early fetal development, there are critical or sensitive periods when malformations are most likely to be induced. These critical periods are short windows of time during the phase of organogenesis. Teratological calendars have been drawn up for humans, laboratory animals, and target animals identifying the critical time periods. 

The sensitive period for induction of malformations is the 5- to 14-day period in the rat and mouse and the third week to the third month in humans. This is illustrated in Figure 6.22 for the teratogen actinomycin D. The later period of fetal development, like the initial proliferative stage, is less susceptible to specific effects and an "all-or-none" type of response is usually seen, such as either death or no gross effect. 

The two most effective times insulin was utilized was during the two highest insulin sensitivity periods (Which are also high cortisol level periods) ... 

In rodents, placental transfer of CDDs to the fetus is relatively limited, but transfer during sensitive periods of organogenesis is biologically important as evidenced by effects on fetuses or offspring exposed in utero. Excretion into milk represents a major pathway for maternal elimination of CDDs and, therefore, for exposure to offspring. In C57BL/6N mice administered a single oral dose of 30 g 14C-... 

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