Concept of functional neuroteratology and the importance of neurochemistry

'Fetal programming' and 'functional teratogenesis': on epigenetic mechanisms and prevention of perinatally acquired lasting health risks

Alterations of the intrauterine and early postnatal nutritional, metabolic, and hormonal environment may cause predispositions to the development of disorders and diseases in later life. Mechanisms responsible for this perinatally acquired 'malprogramming' still remain unclear. It has long been known, however, that hormones are environment-dependent organizers of the developing 'neuroendocrine-immune network', which regulates all fundamental processes of life. When present in nonphysiological concentrations during critical ontogenetic periods, hormones can therefore also act as 'endogenous functional teratogens'. Fetal and neonatal hyperinsulinism is a pathognomic feature in the offspring of diabetic mothers. Perinatal hyperinsulinism also occurs due to early postnatal overfeeding. Data obtained by our group indicate that elevated insulin concentrations during critical periods of perinatal life may induce a lasting 'malprogramming' of neuroendocrine systems regulating body weight, food intake, and metabolism. Similar characteristics may occur due to perinatal hyperleptinism, hypercortisolism etc. Since mechanisms of early 'programming' of obesity, diabetes, and the metabolic syndrome X are unclear, a complex 'neuroendocrine malprogramming' of the regulation of body weight and metabolism may provide a general etiopathogenetic concept in this context, exemplarily revealing critical new implications for chances and challenges of perinatal preventive medicine in the future.

Alterations in respiratory behavior, brain neurochemistry and receptor density induced by pharmacologic suppression of sleep in the neonatal period

The present study examined if drug suppression of active sleep (AS) in the neonate affected the development and expression of respiratory behavior. Secondly, we assessed brain neurochemistry and receptor density in specific supra-medullary brain regions to identify coincident biochemical alterations. Sprague-Dawley newborn rat pups were randomized and divided among six rat mothers (n=10/mother/group), each mother housed separately. Two untreated control (UC) groups received either no interventions or were fed milk vehicle twice daily and were handled similarly to the drug intervention animals. Pharmacological disruption of sleep was achieved by administration (2 groups of each) of either clonidine (CLO) 100 microm/kg, or scopolamine (SCO) 800 microm/kg, given orally twice daily for the first 7 days of life. On postnatal (P) days P10 and P19 of life, pups were assessed for metabolism, minute ventilation (VE), tidal volume (Vt) and frequency (f). On P21 (14 days after the end of drug exposure), pups from each condition were sacrificed and punch biopsies of the frontal cortex, hypothalamus, and hippocampus were examined for hydroxytryptophan (5-HT), and norepinepherine (NE) by HPLC. An equal number of pups were sacrificed and brains examined for muscarinic acetylcholine (mAch), alpha2-adrenergic and I1-imidazoline receptor density.

RESULTS

Both CLO and SCO exposed animals had a lower V(t) and respiratory quotient than UC animals (p<0.01). CLO animals exhibited a higher f (p<0.01) and both CLO and SCO exhibited a lower V(t) (p<0.05) than the UC groups; VE was reduced in the SCO groups, compared with CLO and UC groups (p<0.01). Pattern of breathing in response to brief hypoxia exposure was altered for CLO and SCO. The normal decline in VE during sleep was not observed in CLO rats. Both drug exposures resulted in a comparable reduction in hypothalamic NE and 5-HT levels (p<0.05), while in the frontal cortex, and the hippocampus variable changes in NE and 5-HT, occurred. In CLO and SCO rats mAch receptors were increased in cortex, and reduced in hypothalamus; I1-imidazoline receptors were increased in hypothalamus and decreased in hippocampus (p<0.05 for each). In contrast, alpha2-adrenergic receptors were increased in cortex for both CLO and SCO, decreased in hypothalamus for CLO, and decreased in hippocampus for SCO (p<0.05 for each).

CONCLUSIONS

these data show that drug-induced neonatal sleep suppression will alter ventilatory pattern, metabolism, and site-specific concentrations of adrenergic neurotransmitters and in receptor density, perhaps as a result of suppression of neonatal AS.

Behavioral and neuroendocrine assessment of ritanserin exposure in the developing chicken: lack of toxicity at effective doses

The 5-HT2 antagonist ritanserin (RIT) is undergoing Phase III clinical trials for the treatment of substance abuse disorders. RIT has also shown preclinical therapeutic potential for attenuating or blocking lethal and/or toxic effects of exposure to cocaine or the selective 5-HT2 agonist dimethoxyiodophenyl-aminopropane (DOI) in the developing chicken. To assess the potential toxicity ("side effects") of RIT itself during development, we exposed chicken embryos to 0, 0.1, 0.3, 0.9, or 2.7 mg RIT/kg egg by injecting the drug into eggs with 14-day-old embryos (E14). Voltage generated by spontaneous embryonic activity (motility) was measured on E15 to assess short-term effects of RIT; none were observed. There was no overall effect of these RIT doses on hatchability, though sample sizes were small (n = 13-15 per group). One to 2 weeks after hatching, chicks' acquisition of a detour learning response was tested. There were no observable effects of any RIT dose on detour learning. To assess potential effects of RIT on responsiveness to stress, some chicks were exposed to isolation stress approximately 3 weeks after hatching and killed 15 min later. Blood was assayed for serum corticosterone. There was no effect of any embryonic RIT dose on corticosterone concentrations in nonstressed subjects. Although corticosterone was elevated in all stressed groups, the group exposed to the highest embryonic RIT dose (2.7 mg/kg egg) showed a stress-induced elevation greater than other groups. Thus, except for the highest RIT dose (six to seven times greater than a therapeutically effective dose used in earlier work), embryonic RIT exposure on E14 had no effect on embryonic behavior, hatchability, posthatch learned behavior, and basal serum corticosterone concentrations. At a supraefficacious dose it appears to have modified the responsiveness of the neuroendocrine axis to mild stress.

Long-lasting effect of catecholamine deficiency on differentiating vasopressin and oxytocin neurons in the rat supraoptic nucleus

According to our earlier study, the catecholamine depletion in neonatal rats resulted in stimulation of the vasopressin and oxytocin gene expression in the neurons of the supraoptic nucleus. The present study extends this line, evaluating whether the catecholamine deficiency provides a long-lasting effect on the differentiating vasopressin and oxytocin neurons of the supraoptic nucleus. Catecholamines were depleted by daily injections of an inhibitor of the catecholamine synthesis, alpha-methyl-p-tyrosine, first, to pregnant rats from the 9th to the 21st day of gestation and, then, to their pups from the 2nd to the 10th postnatal day. The animals, injected with saline instead of drugs, served as controls. The pharmacologically-treated and control rats were kept for four months under normal laboratory conditions until processing the materials for semi-quantitative in situ hybridization and immunocytochemistry of vasopressin and oxytocin messenger RNAs and peptides, respectively. There were no differences in the vasopressin and oxytocin messenger RNA concentrations in the supraoptic nucleus in rats following preliminary catecholamine depletion compared to controls. Conversely, the catecholamine deficiency resulted in an increased content of the vasopressin-immunoreactive material in cell bodies and processes. This was also the case for the oxytocin-immunoreactive cell bodies but only in females, suggesting an interference of catecholamines with sexual steroids in their action. The number and size of vasopressin and oxytocin neurons did not change in pharmacologically-treated rats compared to the controls. Thus, the catecholamine deficiency in the course of the neuron differentiation resulted in a long-lasting augmentation of the intracellular content of vasopressin and oxytocin but did not influence the vasopressin and oxytocin gene expression. This might be explained rather by the reduced level of peptide release than by an increased level of the peptide production.

Vasopressin and oxytocin gene expression in intact rats and under catecholamine deficiency during ontogenesis

The development of the hypothalamic vasopressin (VP) and oxytocin (OT) systems has been studied in rats from the 16th embryonic day (E16) until the 11th postnatal day (P11). The VP and OT mRNA-producing neurons were identified on cryostat sections by in situ hybridization using oligonucleotide probes labeled by [35S], [3H] or digoxigenin. Moreover, VP and OT gene expressions were evaluated either at E21 or at P11 following chronic depletion of catecholamines (CA). For this purpose, pregnant rats were daily injected with alpha-methyl-m(p)-tyrosine from gestational day 13 to 20 while neonates were daily injected with alpha-methyl-m(p)-tyrosine and neurotoxin 6-hydroxydopamine from postnatal day 2 to 10. No VP mRNA- or OT mRNA-expressing cells were observed in the hypothalamus of intact fetuses at E16, while 2 days later rather numerous VP and OT neurons occupied the anterior hypothalamus. One major bilateral group of VP and OT neurons was located in the supraoptic nucleus (SON). Less numerous labeled cells were found in the developing paraventricular nucleus (PVN). Some VP and OT neurons were also spread along the ventrolateral surface of the hypothalamus from the level of the median eminence, caudally, to the level of the optic nerves, rostrally. From E18 until birth, the OT neurons were localized in the dorsal portion of the SON, while its ventral portion was occupied by the VP neurons. The VP mRNA- and OT mRNA-expressing cells seemed to increase both in size and in number over the perinatal period. Frequent relatively long neuronal processes contained VP and OT mRNAs in fetuses and in newborns. When performed during the second half of the fetal life, the chronic depletion of CA did not cause any change in the VP and OT mRNA concentrations in the SON and PVN of fetuses. By contrast, similar treatment of neonates resulted in a significant increase of both mRNA levels in the SON. These data suggest that at least in the SON VP and OT gene expression might be under the inhibitory control of CA during the neonatal period.

Effects of neonatal exposure to clonidine on basal and activated central noradrenaline metabolism and in vivo overflow

The occurrence of persistent effects of chronic neonatal exposure to the alpha 2-adrenoceptor agonist clonidine was investigated by determination of tissue concentrations of monoamines and metabolites and in vivo overflow of noradrenaline and its metabolites, in various rat brain regions during adulthood. Rat pups were treated with clonidine from postnatal day 10-20 and all measurements were carried out between postnatal day 40 and 58. Tissue concentrations of monoamines and metabolites of the early clonidine-treated rats did not differ significantly from the control group. A challenge with yohimbine did not reveal altered responses of monoaminergic systems, except for the failure of an increased serotonergic activity in the medulla pons. In vivo microdialysis measurements revealed an elevated basal extracellular noradrenaline level in amygdala, but not in frontal cortex and hippocampus. Pharmacological challenge in vivo with idazoxan did not reveal differences between clonidine- and saline-exposed rats. These results confirm previous findings that continuous activation of alpha 2-adrenoceptors during a particular period of rat brain development may result in long-lasting but small changes in monoaminergic activity. However, these alterations are not very consistent and may depend on the parameter chosen to reflect monoaminergic activity and are not revealed more clearly by activating (challenging) the noradrenaline system with alpha2-adrenoceptor antagonists.

Altered vasoactive intestinal polypeptide gene expression in the fetal rat suprachiasmatic nucleus following prenatal serotonin deficiency

This study has evaluated the possible role of serotonin, a potential morphogen, in the regulation of vasoactive intestinal polypeptide (VIP) gene expression in the target neurons of the suprachiasmatic nucleus (SCN) before and after the onset of the serotonin neurotransmitter function. VIP gene expression was quantified by in situ hybridization of the corresponding mRNA on cryostat sections with subsequent film autoradiography and densitometry. The content of VIP mRNA was measured in the SCN in fetuses at the 21st embryonic day (E21) and in postnatal rats at day 11 (P11) following chronic depletion of serotonin by p-chlorophenylalanine, an inhibitor of serotonin synthesis. This inhibitor was daily injected to pregnant rats for E13-20 or to postnatal animals for P2-10. Results of this study indicate that prenatal serotonin depletion caused a significant increase in VIP mRNA content in the SCN compared to control fetuses. On the contrary, the same treatment performed postnatally did not change VIP mRNA levels in the SCN. These data suggest that the VIP gene expression in differentiating target neurons of the SCN might be under serotonin inhibitory control during prenatal neurogenesis, prior to the onset of the serotoninergic neurotransmission.