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PubMed1995

Immunohistochemical study of two phosphoinositide-linked metabotropic glutamate receptors (mGluR1 alpha and mGluR5) in the cat visual cortex before, during, and after the peak of the critical period for eye-specific connections.

The distribution of two phosphoinositide-linked metabotropic glutamate receptors (mGluR1 alpha and mGluR5) was studied immunohistochemically in area 17 before, during and after the peak of use-dependent modification of eye-specific connections. In the adult, mGluR1 alpha immunoreactivity is high in all layers except layer IV, where mGluR5 immunoreactivity is concentrated. This difference in distribution indicates different functions for these two receptor subtypes. The laminar pattern of mGluR1 alpha immunoreactivity is similar in all three ages, but the overall labeling intensity decreases after the peak (6 weeks of age) of the critical period. The laminar pattern of mGluR5 immunoreactivity changes with age. It is expressed in most layers at 2 days of age and is found mainly in layer IV in the adult. This laminar distribution and developmental pattern match the distribution and the development of the geniculocortical terminals. The change in mGluR1 alpha labeling intensity and mGluR5 laminar distribution over time is consistent with both of these mGluRs being involved in sensory-dependent plasticity for eye-specific connections in the visual cortex.

Animals↗
PubMed1998

Evidence of a critical period of airway instability during central apneas in preterm infants.

The timing and magnitude of airway narrowing in central apneas is unknown. We have developed a method of apnea classification that relies on the transmission of cardiac airflow oscillation to indicate airway patency. Using a theoretical model, we showed that the amplitude of the cardiac airflow oscillation is proportional to airway diameter for small lumens. While in the majority of central apneas the amplitude of the cardiac airflow oscillation remains nearly constant, in a subset of events the waveform decreases with time, suggesting airway narrowing. We hypothesized that this is not a random occurrence but reflects a critical period of airway instability during central apnea. To test this hypothesis we studied 41 preterm infants. Of 4,456 central apneas, 585 had a decrease in the amplitude of the cardiac oscillation. The amplitude of the cardiac airflow oscillation during an apnea was recorded to provide a dynamic measure of changes in airway diameter with time. To allow for comparisons between patients the amplitude of each cardiac airflow oscillation was expressed as a proportion of the maximum amplitude observed in each infant. We then compared the amplitude at multiple successive 0.5 s intervals with the amplitude of the cardiac airflow oscillation observed at the apnea outset using ANOVA. We found a significant decrease in cardiac airflow oscillation after only 1 s irrespective of the apnea duration (3 to 16 s). We conclude that airway narrowing during central apnea is not a random occurrence but appears shortly after the onset of the apnea. We speculate that the phenomenon is secondary to passive airway relaxation.

Apnea↗
PubMed1984

Alterations in receptor number, affinity and laminar distribution in cat visual cortex during the critical period.

The number, affinity, and laminar distributions of various receptors in cat visual cortex were examined during postnatal development using homogenate and in vitro autoradiographic techniques. For all receptor populations examined, the total number of receptors (Bmax) increased from relatively low early values to peak values during the first three months of postnatal life followed by a drop or plateau in the number of receptors. This peak in Bmax occurred during the physiologically-defined period for cortical plasticity. For most receptors examined, the affinity (KD) was also altered during postnatal development. Many of the receptor populations examined exhibited changes in their initial laminar distributions during the first three months of postnatal development, although other did not. The results show a more complex picture of receptor ontogenesis than previously reported, and suggest that the observed receptor modifications affect the synaptic efficacy and the basic chemical circuitry of the visual cortex during the critical period.

Animals↗
PubMedDate not supplied

[Detection of the critical period of pituitary-adrenal system development during postembryonic chicken ontogenesis].

Studies have been made on postembryonic development of the hypophyseal-adrenal system in domestic hens. Most significant functional rearrangements were observed at the 3rd week of life, when corticosteroid content of the blood plasma decreased threefold. Birds subjected to a series of cold stresses at this time, exhibited more rapid sexual maturation, more rapid reaction of the adrenals to exogenous corticotropin and higher resistance to unfavourable factors. Stress expositions in other days of the first month of life also resulted in trace phenomena, although these were less evident. It is suggested that the 3rd week after hatching is a critical period in the development of ihe hypophyseal-adrenal system in hens.

Adrenal Cortex Hormones↗
PubMed1987

Thyroid hormone modulates the expression of a neurofilament antigen in the cerebellar cortex: premature induction and overexpression by basket cells in hyperthyroidism and a critical period for the correction of hypothyroidism.

Neurofilament expression by basket cells of the cerebellar cortex is suppressed in hypothyroidism. By using a monoclonal antibody (mabN210) that selectively recognizes an epitope associated with the 210-kDa neurofilament subunit, we have explored the relationship between thyroid hormone levels and basket cell maturation. In animals rendered hypothyroid by inclusion of propylthiouracil in the maternal drinking water from embryo age E17, there is a complete absence of mabN210 immunoreactivity in the basket cell axons, while the other immunoreactive axons in the cerebellar cortex, primarily Purkinje cell axons and mossy fibers, are apparently unaffected. This deficit can be corrected by treatment with thyroid hormone but there seems to be a critical period for full recovery, for animals treated from birth recover normally whereas there is a gradual diminution in the efficacy of treatment the later it begins. Thyroid hormone therapy begun after postnatal day 30 (P30) leads only to very minor recovery. By contrast, animals on a hyperthyroid regime show premature mabN210-antigen induction in the basket cells and supranormal levels of expression at P25, despite the severe reduction in the number of basket cell somata. This suggests either abnormal compensatory sprouting of axon collaterals by the remaining basket cells or the occurrence, during normal cerebellar corticogenesis, of competition between basket cell axons for a limited number of Purkinje cell targets followed by the elimination of the excess collaterals.

Animals↗
PubMed2002

A critical period for right hemisphere recruitment in American Sign Language processing.

Signed languages such as American Sign Language (ASL) are natural languages that are formally similar to spoken languages, and thus present an opportunity to examine the effects of language structure and modality on the neural organization for language. Native learners of spoken languages show predominantly left-lateralized patterns of neural activation for language processing, whereas native learners of ASL show extensive right hemisphere (RH) and LH activation. We demonstrate that the RH angular gyrus is active during ASL processing only in native signers (hearing, ASL-English bilinguals) but not in those who acquired ASL after puberty (hearing, native English speakers). This is the first demonstration of a 'sensitive' or 'critical' period for language in an RH structure. This has implications for language acquisition and for understanding age-related changes in neuroplasticity more generally.

Adult↗
PubMed2005

Critical period for sensory experience-dependent survival of newly generated granule cells in the adult mouse olfactory bulb.

Granule cells in the olfactory bulb (OB) are continually produced and added into the neuronal circuit in the adult brain. Sensory input to the OB plays a key role in the survival of newly generated granule cells. Here, we examined in the adult mice whether there is a time window after the generation of new granule cells when their survival is strongly influenced by sensory input. New granule cells were labeled by BrdUrd injection, and the mice were deprived of sensory input unilaterally by naris cauterization. During the initial 14 days after BrdUrd labeling, the number of BrdUrd-positive granule cells was similar for deprived and undeprived OBs. At 28 days or later, the BrdUrd-positive cell number remarkably decreased in the deprived OB. Cauterization at days 14-28 effectively reduced the number of BrdUrd-positive granule cells, whereas 2-week cauterization before or after this period produced little effect. Administration of diazepam, a GABAA receptor modulator, decreased the number of BrdUrd-positive granule cells. The diazepam administration was most effective at days 14-28. Histochemical examination showed that activation of caspase-3 was accompanied by apoptotic cell death of granule cells that was induced by sensory deprivation or diazepam administration. Double labeling with activated caspase-3 and BrdUrd indicated that granule cells at days 14-20 were most susceptible to cell death. These results indicate that there is a critical period when the survival of new granule cells is determined in a sensory experience-dependent manner and that the pharmacological manipulation can mimic the effect of sensory deprivation.

Animals↗
PubMed1987

Indications for a critical period for synapse elimination in developing rat cerebral cortex cultures.

It was observed in an earlier study that chronic tetrodotoxin (TTX) blockade of spontaneous bioelectric activity (SBA) in rat cerebral cortex cultures prevented the large-scale elimination of synapses which normally occurs during the fourth week in vitro. This prompted us to study whether the persisting high synapse density during long-term TTX-treatment would still return to the "normal' low level after restoration of SBA. Therefore, cultures grown in TTX-supplemented medium for 5 weeks were switched to control medium for an additional week prior to fixation. Electron microscopic analysis showed that the numerical synapse density remained at a high level, thus suggesting the presence of a critical period whereafter bioelectrically controlled elimination of redundant connections no longer occurs. In contrast, the mean size of synaptic structures depended only on the functional state of the tissue at the moment of fixation, being larger in TTX-silenced cultures than in bioelectrically active ones regardless of treatment during the first 5 weeks in vitro.

Animals↗
PubMed1985

Postnatal development of alveoli. Regulation and evidence for a critical period in rats.

In many species, including humans, pulmonary alveoli are formed after birth by septal subdivision of the large gas-exchange saccules present at birth. In rats septation occurs mainly between the 4th and 14th postnatal days (Burri, P. H. 1974. Anat. Rec. 180:77-98), but little is known about the regulation of this process. We found that dexamethasone (0.1 micrograms daily) given to rats from age 4 to 13 d markedly impaired saccule septation to at least age 60 d and also diminished the extent of the increase of alveolar surface area (Sa). Underfeeding from birth to age 14 d did not diminish saccule septation but did result in diminished Sa. We conclude dexamethasone-treated rats have a critical period during which the gas-exchange saccules present at birth must be subdivided. Since Sa increased in dexamethasone-treated rats without a change in alveolar size, and, the enlargement of Sa was diminished in underfed rat pups without a deficit of saccule septation, we postulate new alveoli were formed by means other than septation of the large gas-exchange saccules present at birth. Furthermore, these various means of forming alveoli, and hence of increasing Sa, were differently regulated: dexamethasone decreased the enlargement of Sa brought about by both septation of the gas-exchange saccules present at birth and by other, as yet unidentified, means of forming alveoli; underfeeding did not diminish Sa increases produced by saccule septation but did decrease the extent of Sa enlargement due to the other means of forming alveoli.

Animals↗
PubMed1984

Sex differences in the serum concentrations of testosterone in mice and hamsters during their critical periods of neural sexual differentiation.

Male and female mice and hamsters were decapitated 1-5 days after birth and serum concentrations of testosterone determined by radioimmunoassay. In the two species studied, serum levels of testosterone in male pups were significantly (P less than 0.05) higher than those obtained in female neonates. This lends support to the hypothesis that circulating levels of testosterone play an important role in the process of neural sexual differentiation in rodents. Moreover, the sex differences in serum concentrations of testosterone in neonatal rodents together with the detectable levels of testosterone in female neonates may suggest that androgenization is a dose-dependent phenomenon. Alternatively, they may indicate that a minimum concentration of the steroid must be present for androgenization to occur during the critical period of neural sexual differentiation and that this 'threshold' is exceeded in male but not in female rodents.

Animals↗
PubMed1990

Critical period for the androgenic block of neuromuscular synapse elimination.

Juvenile androgen treatment during developmental synapse elimination changes the pattern of innervation in the adult levator ani (LA), an androgen-sensitive muscle (Jordan, Letinsky, and Arnold, 1989b). Most notably, such adult muscles contain an unusually high number of muscle fibers that are innervated by two or more axons indicating that these fibers are multiply innervated. Juvenile androgen treatment also increases the adult level of preterminal branching, the number of junctional sites per adult fiber, and the size of adult LA muscle fibers and motoneurons in the spinal nucleus of the bulbocavernosus (SNB). The present study was designed to determine when in development androgen treatment is most effective in maintaining multiple innervation in adulthood and whether there are different critical periods for the different effects of juvenile androgen treatment. Male rats were castrated on 7, 21, or 34 days after birth (roughly corresponding to the beginning, middle, and end of synapse elimination in the LA muscle) and treated daily with testosterone propionate for the next 2 weeks. All rats were sacrificed at 9 weeks and their spinal cords and LA muscles were stained and analyzed. Only during the first treatment period (7-20) did androgen treatment result in increased levels of multiple innervation at 9 weeks. During this period, androgen also increased the number of junctional sites per fiber and the size of SNB somata but did not influence the adult level of preterminal branching or the diameter of adult LA muscle fibers. Androgen treatment during the two later periods increased the level of preterminal branching and the size of LA muscle fibers without influencing the level of multiple innervation.(ABSTRACT TRUNCATED AT 250 WORDS)

Androgens↗
PubMed1994

Injection of an aromatase inhibitor after the critical period of sexual differentiation.

We have investigated the possible role of the second week of life in the differentiation of sexually dimorphic behaviours, dependent on the androgenic aromatization, and its possible relationship with the serotonergic systems. For this purpose, 5 mg/kg of a suspension of an aromatase inhibitor, LY43578, has been intraventricularly injected to males on day 12 of life. Studies have been made in adulthood on exploratory and motor activities, anxiety, sexual motivation, and sexual performance. Indoleamine levels in the hypothalamus and corpus striatum have been measured. Sexual behaviour, exploration, and serotonergic metabolism were not affected by the treatment. Sex partner preference and anxiety in the plus-maze showed a feminized tendency in the treated group that, however, did not reach statistical significance. From these results we have confirmed the restriction of the critical period of androgenic aromatization for the organization of reproductive and exploratory behaviour.

Animals↗
PubMed1979

Serotonin involvement in the control of phasic luteinizing hormone release in the rat: evidence for a critical period.

A marked surge of luteinizing hormone (LH) occurs daily at 18.00 h in oestrogen-treated ovariectomized rats maintained under regular lighting from 06.00 to 20.00 h. The administration of p-chlorophenylalanine (PCPA),p-chloramphetamine or 5,6-dihydroxytryptamine in treatments designed to cause a severe depletion of brain serotonin abolished this daily surge. Synthesis of serotonin may be temporarily restored in PCPA-treated animals by the administration of the serotonin precursor, 5-hydroxytryptophan. The effectiveness with which restoration of synthesis resulted in restoration of the LH surge varied according to the time at which the precursor was administered, the optimal time being 10.00 h. The results suggest that there is an essential, permissive function performed by serotonin in the production of the LH surge and that this function occurs during a critical period.

5,6-Dihydroxytryptamine↗
PubMed1986

Blockade of first ovulation in pubertal rats by delta-9-tetrahydrocannabinol: requirement for advanced treatment due to early initiation of the critical period.

Successful blockade of ovulation in pubertal rats by delta-9-tetrahydrocannabinol (THC) required earlier treatment during proestrus than was required in adults under the same conditions. Only 1 of 8 adult rats ovulated after treatment with THC (10 mg/kg body weight, i.p.) at 1400 h proestrus, whereas 77% of pubertal rats released full sets of ova following similar treatment during proestrus of the first or second vaginal cycle. When treatment of pubertal rats was advanced to 1300 h, only 2 of 10 THC-treated rats exhibited full ovulation, an incidence significantly lower than the 80% ovulation rate observed in vehicle-treated animals (p less than 0.05). To determine whether the requirement for earlier THC treatment in pubertal rats was related specifically to THC or reflected possible age-associated differences in timing of the critical period, the ovulation-blocking efficacy of atropine sulfate (ATR) was tested in pubertal rats for comparison with that of THC. The serum concentrations of luteinizing hormone (LH) during the first proestrus (1200-1900 h) were determined in pubertal rats that remained untreated. The incidence of ovulation in rats treated with ATR (350 mg/kg, s.c.) at 1400 h proestrus was not significantly reduced from that in vehicle-treated rats; however, after ATR treatment at 1300 h, only 2 of 11 animals released full sets of ova whereas all vehicle-treated rats ovulated (p less than 0.025). The mean serum LH concentration in untreated pubertal rats was not significantly increased over baseline at 1300 h proestrus, but was markedly elevated by 1400 h (1009 +/- 375 ng/ml; p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

[A study of functional plasticity of the brain in childhood. I. Critical period dislodging lateralization of language in the brain].

To investigate the functional plasticity of the brain in childhood, the intelligence and language tests, brain CT and dichotic listening test were performed on 34 patients with hemiplegia. All 13 patients with left hemiplegia, 8 to 37 years old, showed a right ear dominance suggesting the lateralization of language in the left hemisphere. Among 21 patients with right hemiplegia, 7 to 22 years old, 16 patients who had suffered from the left hemispheric damage before 6 years and 1 month of age, showed the left ear dominance suggesting the lateralization of language in right hemisphere except 2 patients. On the contrary, 3 patients with right hemiplegia who had suffered from the left hemispheric damage after 6 years and 6 months of age showed a right ear dominance. It was concluded that the critical period for the dislodging of language lateralization in the brain from the damaged left hemisphere to the intact right hemisphere is the first half of 6 years after birth. Five patients out of 8 patients with right hemiplegia showed significantly lower scores in performance than in verbal WISC IQ. The compensation of language function rather than performance function was considered to be characteristic for the left-hemispheric damage in childhood.

Adolescent↗
PubMed1990

Critical periods to ethanol exposure during early neuroembryogenesis in the chick embryo: cholinergic neurons.

The acute and chronic effects of ethanol on cholinergic neuronal expression were studied in the chick embryo during early neuroembryogenesis using choline acetyltransferase (ChAT) activity as a cholinergic marker. Ethanol administered to embryos in ovo on day 1 (E1) produced a 30% decrease in ChAT activity, while ethanol administration on day 3 elicited no significant change. Similar effects were produced by ethanol on ChAT activity in the spinal cord. The decrease in ChAT activity in both brain and spinal cord was not accompanied by a significant change in protein content. Of significance were our findings with chronic ethanol treatment: in embryos treated from E1 to E5 and sacrificed at E6, ChAT activity was decreased. In contrast, in embryos treated similarly but sacrificed at E8 ChAT activity was increased. These findings establish that the critical period of cholinergic neuronal sensitivity to ethanol is confined to E. Moreover, the increase in ChAT activity observed after chronic ethanol treatment indicates that the developing neurons have the capability to adapt to ethanol. This apparent adaptation results in overcompensation, as reflected by the increase in ChAT activity. Whether this overcompensation is at the expanse of another neuronal population remains to be investigated.

Animals↗
PubMed2003

Retinotopic map refinement requires spontaneous retinal waves during a brief critical period of development.

During retinocollicular map development, spontaneous waves of action potentials spread across the retina, correlating activity among neighboring retinal ganglion cells (RGCs). To address the role of retinal waves in topographic map development, we examined wave dynamics and retinocollicular projections in mice lacking the beta2 subunit of the nicotinic acetylcholine receptor. beta2(-/-) mice lack waves during the first postnatal week, but RGCs have high levels of uncorrelated firing. By P8, the wild-type retinocollicular projection remodels into a refined map characterized by axons of neighboring RGCs forming focal termination zones (TZs) of overlapping arbors. In contrast, in P8 beta2(-/-) mice, neighboring RGC axons form large TZs characterized by broadly distributed arbors. At P8, glutamatergic retinal waves appear in beta2(-/-) mice, and later, visually patterned activity appears, but the diffuse TZs fail to remodel. Thus, spontaneous retinal waves that correlate RGC activity are required for retinotopic map remodeling during a brief early critical period.

Action Potentials↗
PubMed1975

Patterns of body weight change in rats following neonatal hormone manipulation: a "critical period" for androgen-induced growth increases.

The development of sexual behaviour and gonadal function is largely determined by the early postnatal hormone environment in the rat; testosterone propionate (TP) treatment in the neonatal female will stimulate development of predominantly masculine functional characteristics. On the other hand, removal of the testes from neonatal males results in feminization of these characteristics. It has been shown that an optimal neonatal steroid hormone environment is also essential for normal growth. We now report the effects of different doses of TP (10, 30, 90, or 270 mug) given on postnatal days 2, 3, 4, or 5 on growth as measured primarily by body weight. Only treatment in the 30-270 mug range on days 2 or 3 was effective in causing significant growth changes, however, these same doses caused sterility and impaired female sexual behaviour when given on days 4 or 5. Therefore, there appears to be a "critical period" before the fourth postnatal day when TP can affect processes leading to increased growth. Removal of the neonatal testes retards growth to the levels of the androgenized females. The ovaries of the female TP treated rats still have a restraining influence on growth since their removal produces an increment in body weight similar to, though not as great as, that of the normal ovariectomized rat. These findings suggest that neonatal TP administration may possibly reduce the responsiveness of rats to the growth depressing effect of ovarian steroids by action at a site functionally different from that producing sterility and impaired sexual behaviour.

Age Factors↗