Arun K. Agrawal

Differential effects of neonatally administered glutamate on the ultradian pattern of circulating growth hormone regulating expression of sex-dependent forms of cytochrome P450

Neonatal male rats were treated with monosodium glutamate (MSG) at either 0.5, 1.0, 2.0, 3.0 or 4.0 mg/g body weight on alternate days during the first 9 days of life. As adults, rats were catheterized to obtain unstressed, serial blood samples for the determination of ultradian patterns of circulating growth hormone. In addition, the levels of drug-metabolizing enzymes (i.e. hexobarbital hydroxylase, cytochromes P450 and b5, NADPH-cytochrome P450 reductase and ethoxyresorufin O-deethylase) as well as sex-dependent forms of cytochrome P450 [i.e. male-dependent cytochromes P450 2c (IIC11), 2a (IIIA2) and RLM2 (IIA2) and female-dependent cytochromes P450 2d (IIC12) and 3 (IIA1)] and/or their catalytic activities were measured in the hepatic microsomes of the treated rats. The results demonstrated a dose-dependent, graded response to MSG treatment. As the dose of MSG increased from 0.5 to 4.0 mg, there was a concurrent decline in the amplitudes of the characteristically masculine, episodic bursts of growth hormone, until at the highest dose (4 mg), the pulses were no longer detectable. Associated with this dose-dependent alteration in the ultradian pattern of growth hormone secretion was a measurable change in the activities of the sex-dependent hepatic enzymes. As the pulse heights of the hormone declined to 10-20% of their normal amplitudes, the levels of the male-dependent enzymes (i.e. the drug-metabolizing enzymes, as well as the male forms of cytochrome P450 and their specific steroid hydroxylases) were maintained, and in some cases, exceeded the levels normally found in males. However, as the hormone pulse heights declined, there appeared an accompanying increase in the activities of some of the female-dependent enzymes. Finally, with the loss of all detectable levels of circulating growth hormone, the normal masculine profile of hepatic enzymes was reversed to an apparently normal (with the exception of cytochrome P450 2d) feminine profile. Summarizing, the results indicate that (1) neonatal administration of MSG can produce dose-dependent, graded, long-term developmental defects in the ultradian rhythm of circulating growth hormone and associated sex-dependent hepatic enzymes, and (2) while the male-dependent hepatic enzymes can be maintained at normal or even higher levels in the face of an up to 90% reduction in the pulse heights of plasma growth hormone, the activities of the female-dependent enzymes may begin to increase.

Neonatal phenobarbital imprints overexpression of cytochromes P450 with associated increase in tumorigenesis and reduced life span

Perinatal exposure to phenobarbital produces a range of permanent reproductive, growth, locomoter, and learning dysfunctions in animals as well as humans. In addition, the affected individuals exhibit latently expressed (i.e., postpuberal) above normal activity levels of hepatic multicytochrome P450‐dependent drug metabolizing enzymes. We report that in spite of apparent normal health for the better part of their lives, daily administration of therapeutic‐like doses of phenobarbital to male and female rat pups during the first postpartum week reduced life expectancy by ∼20%. Necropsy at the time of natural death revealed an associated two‐ to threefold increase in the incidence of tumors in barbiturate‐exposed rats of both sexes and a three‐ to fourfold increase in urinary tract pathologies in male rats. At 2 yr of age, in agreement with an overexpression of hepatic CYP2C6 and CYP2C7, both in vitro and in vivo drug metabolism was more rapid in the phenobarbital‐imprinted male and female animals. Moreover, when the senescent rats were rechallenged with a nominal dose of the barbiturate, males and females neonatally exposed to phenobarbital exhibited a dramatic overinduction of multicytochrome P450‐dependent drug metabolizing enzymes as well as an overexpression of individual isoforms of cytochrome P450 implicated in enhanced susceptibility to tumorigenesis. Our findings support the growing realization that many adult diseases have their origins in early life by emphasizing that unlike adults, the new born is “plastic”, and even therapeutic drugs may produce “silent” programming defects that subtly, but irrevocably, jeopardize life‐long well‐being.

Sex- and dose-dependent effects of neonatally administered aspartate on the ultradian patterns of circulating growth hormone regulating hexobarbital metabolism and action

Rats, neonatally treated with monosodium aspartate (MSA), exhibited developmental defects through adulthood that were characterized by stunted growth, obesity, and reduced size of the liver, kidney, adrenals, and pituitary. Adult male and female rats treated with 4 mg of MSA had no detectable plasma growth hormone as determined from serial blood samples taken every 15 min for 8 consecutive hr. Associated with this loss of circulating growth hormone in the males was a dramatic decline in in vivo and in vitro hexobarbital metabolism and hepatic cytochrome P450 to female levels. The loss of plasma growth hormone in the females had no effect on the already low levels of hepatic monooxygenases. At 2 mg/g body weight, MSA produced both sex- and dose-dependent effects that were far more subtle than the full-blown obesity and growth retardation associated with the larger 4-mg dose. While the mean concentration of circulating growth hormone was reduced 70 to 90% in 2-mg-MSA-treated rats, the sexually dimorphic, ultradian patterns of growth hormone secretion were undisturbed. Affected males continued to secrete a pulse of growth hormone every 3 hr, albeit at greatly reduced amplitudes, interposed by normally undetectable baselines. Similarly, 2-mg-MSA-treated females had greatly reduced mean levels of plasma growth hormone, but with the usual secretion of multiple pulses never dropping to baseline. Surprisingly, the sex-dependent, hepatic monooxygenases, which are normally regulated by the ultradian secretions of growth hormone, were unaffected by the 2-mg MSA treatment. Our results suggest that while an ultradian pulse of circulating growth hormone is necessary for the characteristically male profile of hepatic monooxygenases, neither the amplitude of the secretory peaks nor their total growth hormone content is critical.

Phenobarbital-Imprinted Overinduction of Adult Constituent CYP Isoforms

Newborn male and female rats were treated with therapeutic-like levels of phenobarbital to determine whether early exposure to the barbiturate permanently alters (i.e., imprints) mechanisms regulating induction of constituent CYP (cytochrome P-450) isoforms. When the rats were 65 and 150 days old, they were rechallenged with phenobarbital at doses reflecting either the possible inducing activities of environmental agents (1 mg/kg) or at the minimal anticonvulsant therapeutic dose for the rat (10 mg/kg). The expression levels (mRNA and protein) of constituent, gender-dependent CYP2C6, CYP2C7, CYP2C11, CYP2C12, CYP2C13, and CYP3A2 and nonconstitutive CYP3A1 were monitored at various times (i.e., 0.1–136 h) during the rechallenge period. The major female-specific CYP2C12 and male-specific CYP2C13 were unresponsive to induction, whereas the major male-specific CYP2C11 responded to phenobarbital administration with a 100% increase in transcript levels that were not translated into new protein. The expression of the other isoforms was significantly elevated by both doses of phenobarbital (10 mg >1 mg), though CYP2C6, CYP3A1, and CYP3A2 levels were increased an additional 30–50% when the animals were neonatally exposed to the barbiturate, demonstrating for the first time that mechanisms regulating induction of constitutive CYPs in adults are imprintable at birth. This overinduction response appears to result, at least in part, from phenobarbital-programmed alterations in the sexually dimorphic plasma growth hormone profiles that normally regulate expression of the isoforms. The long-term health consequences of the overinduction response and possible clinical significance are discussed.