Chronic exposure to alcohol during development alters the membrane properties of cerebellar Purkinje neurons in culture

Impacts on prenatal development of the human cerebellum: a systematic review

PURPOSE

The cerebellum is essential for normal neurodevelopment and is particularly susceptible for intra-uterine disruptions. Although some causal prenatal exposures have been identified, the origin of neurodevelopmental disorders remains mostly unclear. Therefore, a systematic literature search was conducted to provide an overview of parental environmental exposures and intrinsic factors influencing prenatal cerebellar growth and development in humans.

MATERIALS AND METHODS

The literature search was limited to human studies in the English language and was conducted in Embase, Medline, Cochrane, Web of Science, Pubmed and GoogleScholar. Eligible studies were selected by three independent reviewers and study quality was scored by two independent reviewers.

RESULTS

The search yielded 3872 articles. We found 15 eligible studies reporting associations between cerebellar development and maternal smoking (4), use of alcohol (3), in vitro fertilization mediums (1), mercury (1), mifepristone (2), aminopropionitriles (1), ethnicity (2) and cortisol levels (1). No studies reported on paternal factors.

CONCLUSIONS

Current literature on associations between parental environmental exposures, intrinsic factors and human cerebellar development is scarce. Yet, this systematic review provided an essential overview of human studies demonstrating the vulnerability of the cerebellum to the intra-uterine environment.

Controlling Ca2+-activated K+ channels with models of Ca2+ buffering in Purkinje cells

Intracellular Ca(2+) concentrations play a crucial role in the physiological interaction between Ca(2+) channels and Ca(2+)-activated K(+) channels. The commonly used model, a Ca(2+) pool with a short relaxation time, fails to simulate interactions occurring at multiple time scales. On the other hand, detailed computational models including various Ca(2+) buffers and pumps can result in large computational cost due to radial diffusion in large compartments, which may be undesirable when simulating morphologically detailed Purkinje cell models. We present a method using a compensating mechanism to replace radial diffusion and compared the dynamics of different Ca(2+) buffering models during generation of a dendritic Ca(2+) spike in a single compartment model of a PC dendritic segment with Ca(2+) channels of P- and T-type and Ca(2+)-activated K(+) channels of BK- and SK-type. The Ca(2+) dynamics models used are (1) a single Ca(2+) pool; (2) two Ca(2+) pools, respectively, for the fast and slow transients; (3) detailed Ca(2+) dynamics with buffers, pump, and diffusion; and (4) detailed Ca(2+) dynamics with buffers, pump, and diffusion compensation. Our results show that detailed Ca(2+) dynamics models have significantly better control over Ca(2+)-activated K(+) channels and lead to physiologically more realistic simulations of Ca(2+) spikes and bursting. Furthermore, the compensating mechanism largely eliminates the effect of removing diffusion from the model on Ca(2+) dynamics over multiple time scales.

Homer proteins control neuronal differentiation through IP(3) receptor signaling

Neurons expand, sustain or prune their dendritic trees during ontogenesis [Cline, H.T. (2001). Dendritic arbor development and synaptogenesis. Curr. Opin. Neurobiol. 11, 118-126; Wong, W.T. and Wong, R.O.L. (2000) Rapid dendritic movements during synapse formation and rearrangement. Curr. Opin. Neurobiol. 10, 118-124] which critically depends on neuronal activity [Wong, W.T., Faulkner-Jones, B.E., Sanes, J.R. and Wong, R.O.L. (2000) Rapid dendritic remodeling in the developing retina: dependence on neurotransmission and reciprocal regulation by Rac and Rho. J. Neurosci. 20, 5024-5036; Li, Z., Van Aelst, L. and Cline, H.T. (2000) Rho GTPases regulate distinct aspects of dendritic arbor growth in Xenopus central neurons in vivo. Nat. Neurosci. 3, 217-225; Wong, W.T. and Wong, R.O.L. (2001) Changing specificity of neurotransmitter regulation of rapid dendritic remodeling during synaptogenesis. Nat. Neurosci. 4, 351-352.] and sub-cellular Ca(2+) signals [Lohmann, C., Myhr, K.L. and Wong, R.O. (2002) Transmitter-evoked local calcium release stabilizes developing dendrites, Nature 418, 177-181.]. The role of synaptic clustering proteins connecting both processes is unclear. Here, we show that expression levels of Vesl-1/Homer 1 isoforms critically control properties of Ca(2+) release from intracellular stores and dendritic morphology of CNS neurons. Vesl-1L/Homer 1c, an isoform with a functional WH1 and coiled-coil domain, but not isoforms missing these features were capable of potentiating intracellular calcium signaling activity indicating that such regulatory interactions function as a general paradigm in cellular differentiation and are subject to changes in expression levels of Vesl/Homer isoforms.

Chronic ethanol exposure enhances AMPA-elicited Ca2+ signals in the somatic and dendritic regions of cerebellar Purkinje neurons

Intracellular Ca2+ signals produced by the glutamate receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA; 5 microM) were measured in the somatic and dendritic regions of cerebellar Purkinje neurons in mature cerebellar control cultures (> or = 20 days in vitro) and cultures chronically treated with 32 mM ethanol (146 mg%; 8-11 days). Recordings were made in physiological saline without ethanol. The mean peak amplitude of the Ca2+ signal elicited by AMPA (applied by brief 1-s microperfusion) in the somatic region was enhanced 38% in chronic ethanol-treated Purkinje neurons compared with control neurons. In contrast, Ca2+ signals evoked by AMPA in the dendritic region were similar in magnitude between control and chronic ethanol-treated Purkinje neurons. When tetrodotoxin (TTX; 500 nM) was included in the bath saline to block spike activity and synaptically-generated events, the mean peak amplitude of the Ca2+ signal elicited by AMPA was enhanced 60% in both the somatic and dendritic regions of chronic ethanol-treated Purkinje neurons compared with control neurons. Thus, TTX-sensitive mechanisms (i.e., spike or synaptic activity) appear to play a role in normalizing neuronal functions involved in Ca2+ signaling in the chronic ethanol-treated neurons. In parallel current clamp experiments, the resting membrane potential of chronic ethanol-treated neurons was slightly depolarized compared with control neurons. However, no differences were found between control and chronic ethanol-treated Purkinje neurons in input resistance or the peak amplitude or duration of the depolarizations or hyperpolarizations elicited by AMPA. AMPA receptors mediate fast excitatory neurotransmission in the majority of neurons in the central nervous system (CNS) and Ca2+ signals in response to AMPA receptor activation contribute to synaptic function. Thus, our results suggest that modulation of Ca2+ signals to AMPA receptor activation (or other cellular inputs) may provide an important mechanism contributing to the actions of prolonged ethanol exposure in the CNS.

Chronic ethanol treatment alters AMPA-induced calcium signals in developing Purkinje neurons

Cerebellar Purkinje neurons developing in culture were treated chronically with 30 mM (140 mg%; 3-11 days in vitro) ethanol to study the actions of prolonged ethanol exposure on responses to exogenous application of AMPA, a selective agonist at the AMPA subtype of ionotropic glutamate receptors. There was no consistent difference between control and chronic ethanol-treated neurons in resting membrane potential, input resistance, or the amplitude or duration of the membrane responses to AMPA (1 or 5 microM applied by brief microperfusion) as measured using the nystatin patch method of whole cell recording. In additional studies, the Ca2+ signal to AMPA was examined using the Ca2+ sensitive dye fura-2. The mean peak Ca2+ signal elicited by 5 microM AMPA was enhanced in the dendritic region (but not the somatic region) of chronic ethanol-treated Purkinje neurons compared to control neurons. In contrast, there was no difference between control and chronic ethanol-treated neurons in the peak amplitude of the Ca2+ signal to 1 microM AMPA, whereas the recovery of the Ca2+ signals was more rapid in both somatic and dendritic regions of ethanol-treated neurons. Resting Ca2+ levels in the somatic and dendritic regions were similar between control and ethanol-treated neurons. These data show that the membrane and Ca2+ responses to AMPA in Purkinje neurons are differentially affected by prolonged ethanol exposure during development. Moreover, chronic ethanol exposure produces a selective enhancement of AMPA-evoked dendritic Ca2+ signals under conditions reflecting intense activation (i.e., 5 microM AMPA), whereas both somatic and dendritic Ca2+ signals are attenuated with smaller levels of activation (i.e., 1 microM AMPA). Because Ca2+ is an important regulator of numerous intracellular functions, chronic ethanol exposure during development could produce widespread changes in the development and function of the cerebellum.

Chronic alcohol reduces calcium signaling elicited by glutamate receptor stimulation in developing cerebellar neurons

The effect of chronic alcohol (33 mM ethanol) on Ca2+ signals elicited by glutamate receptor agonists (quisqualate and NMDA) was examined in developing cerebellar Purkinje and granule neurons in culture. The neurons were exposed to alcohol during the second week in culture, the main period of morphological and physiological development. The Ca2+ signals were measured with fura-2 based microscopic video imaging. Chronic exposure to alcohol during development significantly reduced the peak amplitude of the Ca2+ signals to quisqualate (1 microM; Quis) in both the somatic and dendritic regions of the Purkinje neurons. The dendritic region was affected to a greater extent than the somatic region. Granule neurons also showed a reduced somatic Ca2+ signal to Quis (dendrites not measured) in the alcohol-treated cultures, indicating that the effect was not limited to Purkinje neurons. In addition to the effects on in the response to Quis, the peak amplitude of the Ca2+ signals to NMDA (100 microM) was reduced by chronic alcohol exposure during development in both the cultured Purkinje and granule neurons. Resting Ca2+ levels were not consistently affected by alcohol treatment in either neuronal type. These results indicate that Ca2+ signaling linked to glutamate receptor activation is an important target of alcohol in the developing nervous system and could be a contributing factor in the altered CNS function and development observed in animal models of fetal alcohol syndrome.

Perturbation of target-directed neurite outgrowth in embryonic CNS co-cultures grown in the presence of ethanol

Studies were conducted to determine the influence of ethanol on target-directed fiber outgrowth in culture, using embryonic chick spinal cord-muscle, and fetal rat septal-hippocampal co-cultured explants. Process extension from the spinal cord and septal explants in control cultures was selectively oriented toward the appropriate target tissue. Ethanol in the culture medium (500 mg/dl) eliminated this target-oriented outgrowth in both systems, although the overall extent of neurite outgrowth was not affected. In an effort to further characterize the source of this disruption, target explants were grown alone, with and without ethanol, and the target-conditioned culture media was subsequently harvested and placed on newly plated spinal cord or septal explants, to determine whether ethanol decreased the target production of soluble substances. To determine whether deposition of substrate-bound materials by the target tissue was affected by ethanol, spinal cord or septal explants were plated in wells which had previously been occupied by the appropriate target tissue. These studies revealed that ethanol significantly inhibited production of soluble and substrate-bound materials by muscle explants, but not by hippocampal explants. It was concluded that the ethanol-induced loss of target-directed neurite outgrowth in the spinal cord explants could be accounted for primarily by the attenuated production of neurotropic/neurotropic substances by the muscle tissue. The loss of target-directionality in the septal explants appeared to be due to other factors, possibly related to ethanol-induced compromise of the capacity of the septal neurons to respond appropriately to target-derived neurotrophic/neurotropic substances. The implications of these results for the fetal alcohol syndrome are considered.

In vitro comparison of the effects of ethanol and acetaldehyde on dorsal root ganglion neurons

Results of previous experiments designed to investigate the role of acetaldehyde, the primary metabolite of ethanol, have been contradictory. Experiments have provided evidence that supports and refutes the idea that acetaldehyde is responsible for the teratogenic effects observed in fetal alcohol syndrome. In the present study, cell culture techniques were used to examine the effects of acetaldehyde, both independently and in conjunction with ethanol. The purpose was to determine whether acetaldehyde had any effect on survival and process outgrowth of dorsal root ganglion (DRG) neurons cultured in vitro. This study revealed that acetaldehyde was as toxic to DRG survival as is ethanol, but had a lesser effect on neurite outgrowth than ethanol. Also, acetaldehyde and ethanol do not act synergistically to damage neurons in culture. The results indicate that, although acetaldehyde is probably not solely responsible for ethanol neurotoxicity, it does exhibit a secondary toxicity that could be the subject of future studies.

Alterations in responsiveness to ethanol and neurotrophic substances in fetal septohippocampal neurons following chronic prenatal ethanol exposure

Pregnant Long-Evans rats were maintained on three diets: a liquid diet in which ethanol accounted for 35-39% of the total calories, a similar diet with the isocaloric substitution of sucrose for ethanol, and a lab chow control diet. At gestation day 18, the fetuses were taken and cultures of septal and hippocampal neurons prepared. Neuronal survival and neurite outgrowth were compared in cultures from the three diet groups, using the following media supplements: ethanol (1.2, 1.8 or 2.4 g/dl), neurotrophic factors (nerve growth factor [NGF] with the septal cultures, basic fibroblast growth factor [bFGF] with the hippocampal cultures), or ethanol plus neurotrophic factors. Both the septal and hippocampal neurons responded to ethanol in a dose-dependent manner. The neurons from both populations from fetuses which had been exposed prenatally to ethanol, however, tolerated considerably higher ethanol concentrations before decreases in survival or outgrowth were seen. These ethanol-exposed neuronal populations were also less responsive to neurotrophic factors: in hippocampal cultures, process outgrowth was significantly enhanced by bFGF in control but not ethanol-derived cultures, and in septal and hippocampal cultures, the neurotrophic factors significantly ameliorated ethanol neurotoxicity in control cultures, but not in those from the ethanol-exposed fetuses. The possible relevance of these observations to the fetal alcohol syndrome is discussed.

Calcium signals elicited by quisqualate in cultured Purkinje neurons show developmental changes in sensitivity to acute alcohol

The effect of acute alcohol (33 mM ethanol) on calcium signaling evoked by glutamate receptor activation was studied in cultured cerebellar Purkinje and granule neurons at different stages of development. Calcium signals were measured by microscopic imaging using the calcium sensitive dye fura-2. At an early stage in development (10 days in vitro), acute alcohol enhanced the calcium signals evoked in Purkinje neurons by exogenous application of quisqualate, an agonist at ionotropic and metabotropic glutamate receptors. In contrast, in mature cultured Purkinje neurons (21-24 days in vitro) the calcium signals produced by quisqualate were reduced by alcohol. At an intermediate stage of development (14 days in vitro) reflecting the main period of morphological and physiological maturation, alcohol had no significant effect on the response to quisqualate. Alcohol's actions were significantly altered by manipulation of the intracellular stores with caffeine, implicating intracellular stores in alcohol's actions. Calcium signals produced by quisqualate in the cultured granule neurons were also altered by acute alcohol, in a manner similar to that observed in the Purkinje neurons. These data demonstrate that calcium signaling pathways are a site of alcohol action in developing CNS neurons and that the cellular consequences of alcohol exposure can change with development. Such actions of alcohol could have significant effects on the immature nervous system, where the precise timing of appropriate signaling levels are important aspects of the maturation process.

Ethanol influences on the chick embryo spinal cord motor system: analyses of motoneuron cell death, motility, and target trophic factor activity and in vitro analyses of neurotoxicity and trophic factor neuroprotection

A series of in vivo and in vitro experiments were conducted to determine the influence of prenatally administered ethanol on several aspects of the developing chick embryo spinal cord motor system. Specifically, we examined: (1) the effect of chronic ethanol administration during the natural cell death period on spinal cord motoneuron numbers; (2) the influence of ethanol on ongoing embryonic motility; (3) the effect of ethanol exposure on neurotrophic activity in motoneuron target tissue (limb bud); and (4) the responsiveness of cultured spinal cord neurons to ethanol, and the potential of target-derived neurotrophic factors to ameliorate ethanol neurotoxicity. These studies revealed the following: Chronic prenatal ethanol exposure reduces the number of motoneurons present in the lateral motor column after the cell death period [embryonic day 12 (E12)]. Ethanol tends to inhibit embryonic motility, particularly during the later stages viewed (E9-E11). Chronic ethanol exposure reduces the neurotrophic activity contained in target muscle tissue. Such diminished support could contribute to the observed motoneuron loss. Direct exposure of spinal cord neurons to ethanol decreases neuronal survival and process outgrowth in a dose-dependent manner, but the addition of target muscle extract to ethanol-containing cultures can ameliorate this ethanol neurotoxicity. These studies demonstrate ethanol toxicity in a population not previously viewed in this regard and suggest a mechanism that may be related to this cell loss (i.e., decreased neurotrophic support).

Responsiveness of cultured septal and hippocampal neurons to ethanol and neurotrophic substances

Dissociated septal and hippocampal neurons from E18 fetal rats were cultured with varying concentrations of ethanol (0.6-2.4 g/dl) and in cultures containing ethanol plus nerve growth factor (NGF) or basic fibroblast growth factor (bFGF). These substances have been shown to provide neurotrophic support for these populations and to afford neuroprotection against certain toxic substances or conditions applied to some neuronal populations. Both the septal and hippocampal neurons responded to ethanol in a dose-dependent manner. Survival of septal neurons was generally unaffected by initial ethanol concentrations of 0.6 and 1.2 g/dl but was considerably impaired by higher concentrations (1.8 and 2.4 g/dl), while neurite outgrowth was compromised by all ethanol concentrations except the lowest one applied. The hippocampal neurons survived ethanol concentrations up to 2.4 g/dl, although process extension was decreased in concentrations of 1.2 g/dl and higher. NGF or bFGF in the culture medium (in cultures without ethanol) did not affect neuronal survival or process outgrowth in either population, probably owing to the relatively high plating densities of the cultures. NGF did tend to have a moderate ameliorative effect on the ethanol neurotoxicity in the septal cultures, however, and was slightly effective in this regard in hippocampal cultures at intermediate ethanol concentrations (1.8 g/dl). High concentrations of ethanol (2.4 g/dl) reduced the proportion of cholinergic cells in the septal preparations by approximately 50%. This neuronal loss could be reversed by inclusion of high concentrations of NGF in the culture medium (100 ng/ml) but not by a lower concentration (20 ng/ml). bFGF provided some protection against ethanol cytotoxicity with respect to both populations. The implications of these results for studies of fetal alcohol effects are discussed, as well as their relation to prior reports of trophic factor neuroprotection.

Chronic exposure to alcohol during development alters the calcium currents of cultured cerebellar Purkinje neurons

The effect of chronic exposure to alcohol during development on the calcium currents of rat cerebellar Purkinje neurons was studied in a culture model system using voltage clamp techniques. The neurons were exposed to 30 mM alcohol (ethanol) during the main period of morphological and physiological development. The calcium currents were measured at the end of the treatment period, which lasted for 8-10 days. The currents were evoked by a series of depolarizing test commands from holding potentials of -62 mV and -90 mV. The evoked currents were qualitatively similar in control and alcohol-treated neurons and were comprised of a high threshold slowly inactivating calcium current and a low threshold rapidly inactivating calcium current. The low threshold current could be observed in isolation at test potentials ranging from -50 to -30 mV. The mean peak amplitude of this current was significantly smaller in the alcohol-treated neurons compared to controls. At more depolarized test potentials, the high threshold current dominated the current response, which was characterized by an initial peak that slowly declined to a smaller relatively sustained level. The mean amplitude of the high threshold current at both peak and sustained levels was significantly larger in the alcohol-treated neurons compared to controls. Measurement of cell size indicated that alcohol-treated neurons were approximately 25% smaller than control neurons, a difference that could contribute to the smaller low threshold current observed in these neurons. These data show that chronic exposure to alcohol during the development can significantly influence the amplitude of calcium currents of the cultured Purkinje neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic exposure to alcohol during development alters the responses to excitatory amino acids in cultured Purkinje neurons

The effects of chronic alcohol exposure during development on the responses evoked by glutamate and the selective excitatory amino acid receptor agonists quisqualate (Quis) and kainate were studied in cultured cerebellar Purkinje neurons. The cultures were treated with 22 mM or 44 mM ethanol continuously for one or two weeks during the main period of morphological and physiological development. Extracellular recordings used for most studies characterized the responses to all 3 agonists as initial increase in simple spike firing, usually including a period of burst activity, followed by reduced activity or total inhibition, then return to control firing pattern. Analysis of these responses and background spontaneous activity showed several significant differences between control and ethanol treated Purkinje neurons. Background spontaneous firing, agonist evoked firing, the initial period of activity of the response to Quis, and the inhibitory period of the response to glutamate were all significantly reduced in the chronically treated neurons; the inhibitory period of the response to kainate was significantly increased. In contrast to the effects of chronic ethanol exposure, acutely administered ethanol significantly increased background spontaneous firing and the inhibitory period of the response to Quis. Thus, administering both acute and chronic ethanol altered the responses evoked by excitatory amino acids in the developing Purkinje neurons. The effect of chronic ethanol exposure on some response components was similar for all agonists tested and may be linked to changes in intrinsic membrane properties. However, alterations in the inhibitory component of the agonist responses were agonist specific, indicating that receptor-linked actions of ethanol were involved.(ABSTRACT TRUNCATED AT 250 WORDS)