Age-dependent inhibition of long-term potentiation by ethanol in immature versus mature hippocampus.

The goal of this study was to assess the effects of ethanol on the induction of long-term potentiation (LTP) in hippocampal slices from immature versus mature rats. Population excitatory postsynaptic potentials (pEPSPs) were recorded from stratum radiatum of area CA1 of hippocampal slices using electrical stimulation of the Schaffer collateral/commissural fiber pathway. The slices were prepared from rats aged 15 to 25 or from 70 to 100 days. Long-term potentiation (LTP) of the pEPSP slope was induced using a single, theta-burst stimulus train in the presence or absence of 60 mM ethanol. Under control conditions, the stimulus train induced LTP in slices from both immature and mature animals. However, the magnitude of LTP was greater in slices from immature rats. When ethanol was present during the stimulus train, the magnitude of LTP in slices from mature animals did not differ significantly from the magnitude of LTP in control slices. However, ethanol virtually blocked the induction of LTP in slices from immature animals. These results indicate that memory-related synaptic plasticity in the hippocampus is attenuated by ethanol to a greater degree in immature versus mature animals.

Ethanol inhibition of AMPA and kainate receptor-mediated depolarizations of hippocampal area CA1.

Longitudinal hippocampal slices were prepared from adult female rats. The excitatory amino acids, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainic acid, were applied to area CA1, and the resulting depolarizations were measured using the grease-gap electrophysiological technique. Agonist dose-response curves were generated in the presence and absence of various concentrations of ethanol. Ethanol (25-200 mM) significantly attenuated the depolarizations that were produced by each agonist. In addition, we found that ethanol potently antagonized kainate-induced depolarizations across the agonist concentration-response curve, whereas it significantly suppressed only AMPA responses that were induced with moderate-to-high agonist concentrations. These results indicate that ethanol has potent antagonist actions against non-N-methyl-D-aspartate (NMDA) excitatory amino acid-induced neuronal depolarizations in hippocampal area CA1. Moreover, the relative potency of ethanol depends on the specific excitatory agonist tested and the concentration of that agonist. This suggests that, in addition to the known effects of ethanol on NMDA receptor-mediated activity, it may also potently attenuate ongoing "fast" glutamatergic synaptic activity in the hippocampus.

Differential sensitivity of NMDA receptor-mediated synaptic potentials to ethanol in immature versus mature hippocampus.

Pharmacologically isolated, NMDA receptor-mediated population EPSPs (pEPSPs) were evoked from area CA1 of hippocampal slices using electrical stimulation of the Schaffer collateral/commissural fiber pathway. Slices were prepared from rats aged 20-25 or 80-100 days. The inhibitory effects of a range of ethanol concentrations were assessed. While ethanol antagonized NMDA-mediated pEPSPs in slices from both age groups, it was significantly more potent against pEPSPs from immature versus mature hippocampi. In slices from mature animals, significant and consistent reduction of pEPSPs was observed only with the highest ethanol concentration (100 mM), whereas 10, 30, or 100 mM significantly reduced the amplitude of pEPSPs in slices from immature animals. These results indicate that NMDA-mediated synaptic activity in the hippocampus is more sensitive to the effects of ethanol in immature versus mature animals.

Glucocorticoids and the development of neuronal function: effects of prenatal dexamethasone exposure on central noradrenergic activity.

Although glucocorticoids slow the development of most cell types, they have been hypothesized to promote the differentiation of catecholaminergic cells. In the current study, pregnant rats were given dexamethasone on gestational days 17, 18 and 19, and the functional state of noradrenergic synaptic activity was assessed throughout postnatal development by measurements of transmitter levels and turnover, and receptor binding capabilities. Despite growth inhibition caused by dexamethasone, the steroid treatment had little or no effect on transmitter levels or receptor binding and accelerated the maturation of norepinephrine turnover in a regionally selective manner. Effects were most notable in the midbrain and brainstem, where turnover rose to maximum levels 1-2 weeks in advance of controls. Turnover also leveled off prematurely in the dexamethasone group, leading to deficits in the postweaning period and into young adulthood. Although similar patterns were obtained in other, later-developing regions, the effects were less consistent and robust; the smaller effects also extended to dopamine turnover. These results suggest that glucocorticoids have a specific promotional effect on the development of central catecholaminergic activity and that administration of exogenous steroids during critical periods of development can lead to lasting functional abnormalities.

Dose-dependent glucocorticoid effects on noradrenergic synaptogenesis in rat brain: ontogeny of [3H]desmethylimipramine binding sites after fetal exposure to dexamethasone.

Glucocorticoid administration slows the development of many types of cells, but may selectively accelerate differentiation of catecholaminergic cells. In the current study, pregnant rats were given dexamethasone on gestational days 17, 18 and 19 and noradrenergic synaptogenesis assessed in the offspring by measurements of binding capabilities for [3H]desmethylimipramine (DMI), a radioligand probe for noradrenergic presynaptic terminals. After treatment with 0.05 mg/kg of dexamethasone, a dose that did not suppress body or brain region growth, [3H]DMI was initially enhanced in midbrain + brainstem and in cerebellum; the former region also displayed a secondary phase of augmented [3H]DMI binding during the ontogenetic peak occurring in the second to third postnatal week. At a higher dose (0.2 mg/kg) that elicited moderate growth impairment, fetal dexamethasone exposure produced biphasic effects on [3H]DMI binding: initial enhancement was still apparent in cerebral cortex and cerebellum, but there were subsequent deficits in binding and the peak in midbrain + brainstem was shifted to later stages. At the highest dose (0.8 mg/kg), profound growth impairment was evident and only the cerebellum showed unequivocal evidence of enhanced [3H]DMI binding. All changes were associated with alterations in the maximum [3H]DMI binding capacity (Bmax) not in the binding affinity (Kd). These results suggest that low doses of dexamethasone that do not suppress general growth, enhance noradrenergic synaptogenesis in a regionally-selective and age-selective manner; at higher, growth-suppressing doses, this effect is intermixed with general delays in maturation, likely contributing to the variable effects of glucocorticoids on neurobehavioral development.

Chronic prenatal nicotine exposure sensitizes rat brain to acute postnatal nicotine challenge as assessed with ornithine decarboxylase.

Prenatal exposure to nicotine has been shown to produce postnatal up-regulation of central nervous system nicotinic receptors and to alter subsequent differentiation of neural tissues. In the current study, pregnant rats received nicotine infusions of 6 mg/kg/day throughout gestation, administered by osmotic minipump implants; the postnatal development of cholinergic receptor reactivity was examined through measurements of the ability of acute nicotine administration to stimulate midbrain + brainstem ornithine decarboxylase (ODC) activity, a key regulatory enzyme in neural cell differentiation and growth. In control rats, the ODC response to nicotine was absent at birth and developed during the second postnatal week in parallel with the known ontogenetic rise of nicotinic receptors. Offspring of the nicotine-infused dams exhibited hyper-reactivity of ODC to postnatal acute nicotine challenge: the response developed earlier than in controls and subsequently the magnitude of the effect was 2-3 times greater. Since the development of cholinergic transmission influences differentiation of target cells, alterations in cholinergic nicotinic receptor mediated responses likely explain the delayed appearance of abnormal cell differentiation associated with prenatal nicotine.

Prenatal nicotine exposure impairs beta-adrenergic function: persistent chronotropic subsensitivity despite recovery from deficits in receptor binding.

Gestational exposure to nicotine has been shown to interfere with biochemical markers of development of central and peripheral noradrenergic activity. The current study examines the development and function of cardiac beta-adrenergic receptors in the offspring of pregnant rats given nicotine infusions of 6 mg/kg/day from gestational days 4 through 20, administered by subcutaneously implanted osmotic minipumps. Prenatal nicotine exposure delayed the development of beta-adrenergic receptor binding capabilities, as assessed with [125I]pindolol in membrane preparations from heart and kidney. The deficits in receptor binding were associated with marked subsensitivity of chronotropic responses to administration of a beta-adrenergic agonist, isoproterenol. Although the effects on receptor binding resolved after weaning, functional deficiencies in responsiveness to isoproterenol or to preganglionic electrical stimulation of sympathetic nerves to the heart persisted into adulthood. These results indicate that prenatal exposure to nicotine produces long-term alterations in adrenergic responsiveness of sympathetic target tissues.

Fetal terbutaline exposure causes selective postnatal increases in cerebellar alpha-adrenergic receptor binding.

beta-Adrenergic agonists used in therapy of premature labor and asthma cross the placenta and can affect development of the fetal nervous system. In the current study, pregnant rats were given 10 mg/kg of terbutaline on gestational days 17, 18 and 19 and adrenergic receptor binding capabilities examined in brain regions of the offspring. Despite the absence of body or brain growth impairment, selective increases were seen postnatally in cerebellar alpha 1- and alpha 2-receptor subtypes, whereas the same receptor populations were decreased by small amounts in cerebral cortex and midbrain + brainstem. beta-Adrenergic receptors showed little or no change in any region. The regional and subtype selectivity are compatible with primary deficits in the development of noradrenergic projections to the cerebellum identified in previous studies and provide further evidence that therapeutic use of beta-adrenergic agonists may produce neurobehavioral teratology.

A head-only exposure system for controlled exposures of small rodents.

We have designed a low-cost, compact, head-only exposure system which is easy to use and allows exposure of up to 8 or 16 small rodents depending on the chamber used with the system. Animals are exposed without anesthesia or extreme restraint. Deflection tubes at the chamber inlet and outlet serve to provide turbulent flow within the chamber. Performance of the exposure system was evaluated by analyzing data which was collected during investigations of the kinetics of respirable nickel chloride (NiCl2) and cobalt chloride (CoCl2) aerosols in Sprague-Dawley rats. Acute 2 h exposure to NiCl2 at concentrations ranging from 129 to 1208 micrograms Ni/m3 (MMAD 0.7-0.9 micron, sigma g 1.2-1.5) had a mean run-to-run coefficient of variation (cv) in chamber concentration of 11.6%, and a mean within-run cv of 8.5%. A 26-day repeated exposure of 3 groups of rats to 18.4 micrograms Ni/m3 gave a mean run-to-run cv of 20.9%, mean within-run cv of 10.2%, and within-day cv of 14.5%. Acute exposure studies with CoCl2 at concentrations ranging from 298 to 1371 micrograms Co/m3 (MMAD 0.9, sigma g 1.4) produced a mean run-to-run cv of 8.4% and a mean within-run cv of 7.8%. The repeated exposure of 2 groups of rats for 5 days to CoCl2 at a concentration of 583 micrograms Co/m3 had a mean run-to-run cv of 10.1%, a within-run cv of 7.4% and a within-day cv of 6.8%. The distribution of aerosol within the chamber is shown to have been uniform, and chamber performance was linear over the range tested. This system provides an inexpensive and uniform means of conducting inhalation exposure studies with selected airborne contaminants that might represent a potential health hazard.

Pharmacokinetic modeling of the lung burden from repeated inhalation of nickel aerosols.

The saturable nature of the clearance of soluble nickel compounds from the lung was studied by repeated exposures of rats to respirable submicron-size nickel aerosols. Using Michaelis-Menten type kinetics for removal of nickel lung burdens and a constant rate of deposition, the lung nickel burdens were simulated by computer. The computer simulation was used to design a repeated exposure regimen to test further the hypothesis of saturable clearance. Male Sprague-Dawley rats were exposed for 2 h/day to nickel chloride aerosols at either 90 or 400 micrograms Ni/m3 for up to 14 days. During the 22 h between exposures and up to 3 days post-exposure rats were kept in clean air. The particle size of the aerosol ranged from 0.7 to 0.9 micron mass median aerodynamic diameter with a geometric standard deviation of 1.2-1.4. A steady-state nickel lung burden was observed at 90 micrograms/m3, as predicted from computer modeling, while lung burdens continued to increase with repeated exposure to 400 micrograms Ni/m3. The best fit for the experimental data was obtained with a maximum clearance velocity (Vmax) of 34.6 ng Ni/g X h and a Michaelis-Menten constant for transport (Kt) of 1380 ng Ni/g. The percentage of submicron nickel chloride aerosols retained in the lung was 6.9%. These data support the hypothesis of a saturable clearance mechanism for soluble nickel and provide physiological constants useful for estimating human health risks from nickel inhalation.