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Functional effects of methylazoxymethanol-induced cerebellar hypoplasia in rats
S A Ferguson1, M G Paule, R R Holson
1Division of Reproductive & Developmental Toxicology, National Center for Toxicological Research, Jefferson, AR 72079, USA. sferguson@nctr.fda.gov
Abstract:
The behavioral effects of a series of methylazoxymethanol acetate (MAM) injections in neonatal rats were investigated. Pups were injected twice daily on days 5-8 after birth with 4 mg/kg MAM or saline. Similar treatment paradigms cause cerebellar hypoplasia, which is a result of a depletion of granule cells. MAM treatment reduced adult cerebellar weight to 92% that of control and was without effect on weight of other brain regions examined. Postweaning body weight gain in males was reduced. Nest odor preference and emergence (light/dark box) assessments indicated no significant effects. Complex maze assessments and performance in an operant test battery demonstrated no cognitive deficits. Indeed, MAM treated females performed better in a complex maze under lighted conditions than same-sex controls. Open field and running wheel activity levels in females were unaffected. Though not statistically significant, males exhibited a mild hyperactivity syndrome characterized by an increase in running wheel and open field activity, as well as a depressed startle response. Both sexes were hypersensitive to the locomotor-increasing effects of methamphetamine. These results suggest that the functional effects resulting from cerebellar hypoplasia produced by MAM treatment on PNDs 5-8 are milder than those resulting from MAM treatment beginning on the day of birth. The results are compared with other forms of cerebellar lesions and provide support for the hypothesis that early insult (day of birth or shortly after) produces hypoactivity whereas a later insult (day 4 or later after birth) produces a syndrome of hyperactivity.
Insights
Methylazoxymethanol acetate (MAM) injections in neonatal rats caused mild cerebellar hypoplasia. Behavioral testing revealed minimal cognitive deficits, suggesting later insult results in hyperactivity.
Area of Science:
- Neuroscience
- Developmental Neuroscience
- Behavioral Neuroscience
Background:
- Methylazoxymethanol acetate (MAM) exposure in early development can lead to cerebellar hypoplasia.
- Cerebellar hypoplasia is characterized by a depletion of granule cells, impacting motor function and behavior.
- The timing of MAM exposure influences the severity and type of neurodevelopmental effects.
Purpose of the Study:
- To investigate the behavioral consequences of methylazoxymethanol acetate (MAM) injections during a specific neonatal period (postnatal days 5-8) in rats.
- To determine if MAM-induced cerebellar hypoplasia affects cognitive function, motor activity, and behavioral responses.
- To compare the effects of later neonatal MAM exposure with those of earlier exposure.
Main Methods:
- Neonatal rats received twice-daily injections of MAM (4 mg/kg) or saline on postnatal days 5-8.
- Cerebellar and brain weights were measured in adulthood.
- Behavioral assessments included nest odor preference, light/dark emergence, complex maze, operant testing, open field, and running wheel activity.
- Startle response and locomotor activity following methamphetamine challenge were evaluated.
Main Results:
- MAM treatment significantly reduced adult cerebellar weight (92% of control) but did not affect other brain regions.
- No significant deficits were observed in nest odor preference, light/dark emergence, complex maze performance, or operant testing.
- MAM-exposed females showed enhanced performance in a complex maze under light conditions.
- Males exhibited a trend towards hyperactivity and a depressed startle response.
- Both sexes displayed hypersensitivity to methamphetamine-induced locomotor activity.
Conclusions:
- Neonatal MAM exposure on days 5-8 results in milder functional deficits compared to exposure on the day of birth.
- The findings support the hypothesis that the timing of early-life insult influences behavioral outcomes, with later insults (postnatal day 4+) potentially leading to hyperactivity.
- MAM-induced cerebellar hypoplasia in this paradigm does not cause severe cognitive impairment but may subtly alter motor activity and behavioral responses.