Prenatal nicotine exposure affects the development of the central serotonergic system as well as the dopaminergic system in rat offspring: involvement of route of drug administrations

Abstract

The present study was undertaken to examine the effects of prenatal nicotine exposure by two different routes of drug administration, injection and infusion, on the development of monoaminergic systems and open field behavior in the neonatal and juvenile rat. The nicotine administration to pregnant Sprague–Dawley rats was carried out by subcutaneous injection (3 mg/kg twice daily) or infusion via implanted osmotic minipumps (6 mg/kg/day) from gestational day 4 (GD4) until GD20. At postnatal day 7 (PD7), 15 and 22, the contents of the neurotransmitters and their metabolites, including noradrenaline (NA), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanilic acid (HVA), serotonin (5-HT) and 5-hydroxy-3-indolacetic acid (5-HIAA) were measured in the midbrain+pons−medulla (M+P−M), forebrain and cerebellum. Prenatal nicotine exposure caused a persistent reduction of DA turnover in the forebrain at PD15 and PD22. In addition, the 5-HT system was also affected by prenatal nicotine, and reductions of 5-HT turnover in the M+P−M at PD15 and in the forebrain and the cerebellum at PD22 were found. Although there was no effect of prenatal nicotine on NE contents, the involvement of this system remains uncertain since we measured only NE contents without metabolites. In the present study, we also found significant route-related changes in the contents of the monoamines and metabolites in the NA, DA and 5-HT systems in all brain regions in rat offspring besides the effects of prenatal nicotine. In addition, the difference in administration route reflected the results of the open field test and the number of ambulations in the injection-group was less than that in the infusion-groups with no relation to nicotine administration. Therefore, such effects of “prenatal stress” accompanied by drug administration are not negligible in considering the risk assessment of prenatal nicotine exposure.

Introduction

Tobacco smoking during pregnancy has been reported to be associated with behavioral abnormalities in children, e.g., attention deficit, hyperactivity and retardation in reading and writing 3, 14, 16. Several animal studies support the idea that nicotine is a “behavioral teratogen” 6, 8, 20, 25, 26, 31, 40. The neurochemical alterations induced by prenatal nicotine exposure have also been well documented by several investigators, indicating that prenatal nicotine affects the development of central noradrenergic 12, 13, 18, 22, 30, 32, 36, 37, dopaminergic 12, 13, 18, 30, 37and cholinergic systems 12, 13, 17, 36, 39, 45in the rat brain. However, regarding the effect of nicotine on the development of the serotonergic system, there are only a few reports 9, 29.

With regard to the prenatal nicotine administration, some different routes of administration have been employed, e.g., injection, infusion and via drinking water. Among these administration routes, subcutaneous injection of nicotine is thought to be a potent stressor because an acute increase in nicotine concentration to the dams causes transient ischemia and hypoxia to both mother and fetus. This explanation is supported by the evidence that there are behavioral and biochemical similarities between the effects of maternally injected nicotine and experimental hypoxia [36]. In addition, the subcutaneous injection itself, whether nicotine is included or not, might be a potent factor affecting the brain functions 4, 10, 21, 23, 24, e.g., central serotonergic functions and responsiveness to stressful stimuli. In contrast to the nicotine injection into the dams, the infusion of nicotine via subcutaneously implanted osmotic minipumps induces neither ischemia nor hypoxia 13, 36and is not accompanied by a stressful situation such as injection.

In the present study, we examined the effect of prenatal nicotine exposure by two different routes of administration (injection and infusion) on the development of the central monoaminergic systems and discussed the effect of “prenatal stress” accompanied by nicotine administrations.

With respect to behavioral changes induced by prenatal treatments, Peters indicated hypersensitivity to novel environment using open field test in the rat given stressful manipulation prenatally and also the involvement of serotonergic dysfunctions in these rats [24]. On the other hand, prenatal nicotine exposure was reported to result in the changes in spontaneous activity related to dopaminergic dysfunctions 6, 25. Clinical study also reported that maternal nicotine or smoking induce some adverse effects to behavioral parameters in children including short attention span or hyperactivity [12]. We performed open field test to clarify interrelationship between stress and nicotine exposure during prenatal period on spontaneous activity and stress responsiveness in the neonatal and juvenile rat in this experiment.

We chose the daily dose (6 mg/kg/day) in this animal study, which total daily dose has been compared to those obtained in humans who smoke 2–3 packages of cigarettes/day 13, 36.