Age-related impairment of long-term depression in area CA1 and dentate gyrus of rat hippocampus following developmental lead exposure in vitro

Patch-Clamp Recording of Low Frequency Stimulation-induced Long-Term Synaptic Depression in Rat Hippocampus Slices During Early and Late Neurodevelopment

BACKGROUND

Studying synaptic plasticity in the rat hippocampus slice is a well-established way to analyze cellular mechanisms related to learning and memory. Different modes of recording can be used, such as extracellular field excitatory post-synaptic potential (EPSP) and diverse patch-clamp methods. However, most studies using these methods have examined only up to the juvenile stage of brain maturation, which is known to terminate during late adolescence/early adulthood. Moreover, several animal models of human diseases have been developed at this late stage of brain development. To study the vulnerability of adolescent rat to the cognitive impairment of alcohol, we developed a model of binge-like exposure in which ethanol selectively abolishes low frequency stimulation (LFS)-induced, field EPSP long-term depression (LTD) in the rat hippocampus slice.

METHODS

In the present study, we sought to use whole-cell patch-clamp recording in the voltage-clamp mode to further investigate the mechanisms involved in the abolition of LFS-induced LTD in our model of binge-like exposure in adolescent rat hippocampus slices. In addition, we investigated LFS-induced NMDAR-LTD and mGluR-LTD at different ages and changed several parameters to improve the recordings.

RESULTS

Using patch-clamp recording, LFS-induced NMDAR-LTD and mGluR-LTD could be measured until 4 weeks of age, but not in older animals. Similarly, chemical mGluR-LTD and a combined LFS-LTD involving both N-Methyl-D-Aspartate Receptor (NMDAR) and mGluR were not measured in older animals. The absence of LFS-LTD was not due to the loss of a diffusible intracellular agent nor the voltage mode of recording or intracellular blockade of either sodium or potassium currents. In contrast to voltage-clamp recordings, LFS-induced LTD tested with field recordings was measured at all ages and the effects of EtOH were visible in all cases.

CONCLUSIONS

We concluded that whole-cell patch-clamp recordings are not suitable for studying synaptic LFS-induced LTD in rats older than 4 weeks of age and therefore cannot be used to explore electrophysiological disturbances, such as those induced by alcohol binge drinking during adolescence, which constitutes a late period of brain maturation.

Cellular and Molecular Differences Between Area CA1 and the Dentate Gyrus of the Hippocampus

A distinct feature of the hippocampus of the brain is its unidirectional tri-synaptic pathway originating from the entorhinal cortex and projecting to the dentate gyrus (DG) then to area CA3 and subsequently, area CA1 of the Ammon's horn. Each of these areas of the hippocampus has its own cellular structure and distinctive function. The principal neurons in these areas are granule cells in the DG and pyramidal cells in the Ammon's horn's CA1 and CA3 areas with a vast network of interneurons. This review discusses the fundamental differences between the CA1 and DG areas regarding cell morphology, synaptic plasticity, signaling molecules, ability for neurogenesis, vulnerability to various insults and pathologies, and response to pharmacological agents.

Lead Exposure and (n-3) Fatty Acid Deficiency during Rat Neonatal Development Affect Subsequent Spatial Task Performance and Olfactory Discrimination

Docosahexaenoic acid [22:6(n-3), DHA] is important for optimal infant central nervous system development, and lead (Pb) exposure during development can produce neurological deficits. Long-Evans strain rats were fed either an (n-3) deficient [(n-3) Def] diet to produce brain DHA deficiency, or an adequate [(n-3) Adq] diet through 2 generations. At the birth of the 2nd generation, the dams were subdivided into 4 groups and supplied drinking water containing either 5.27 mmol/L (Pb) or sodium (Na) acetate until weaning. Rats were killed at 3 wk (weaning) and 11 wk (maturity) for brain Pb and fatty acid analysis. Spatial task and olfactory-cued behavioral assessments were initiated at 9 wk. Rats in the (n-3) Def group had a 79% lower concentration of brain DHA compared with the (n-3) Adq group with no effect of Pb exposure. At weaning, Pb concentrations were 7.17 +/- 0.47 nmol Pb/g of brain (wet weight) in the (n-3) Adq-Pb group and 6.49 +/- 0.63 nmol Pb/g of brain (wet weight) in the (n-3) Def-Pb group. At maturity, the brains contained 1.30 +/- 0.22 and 1.07 +/- 0.12 nmol Pb/g (wet weight), respectively. In behavioral testing, significant effects of both Pb and DHA deficiency were observed in the Morris water maze probe trial and in 2-odor olfactory discrimination acquisition and olfactory-based reversal learning tasks. Both lactational Pb exposure and (n-3) fatty acid deficiency led to behavioral deficits with additive effects observed only in the acquisition of 2-odor discriminations.

Protection by a taurine supplemented diet from lead-induced deficits of long-term potentiation/depotentiation in dentate gyrus of rats in vivo

Previous studies have demonstrated that synaptic plasticity, which includes long-term potentiation (LTP) and depotentiation (DP) in hippocampus, is important for learning and memory. The purpose of this study is to evaluate the effect of taurine via drinking water on the lead-induced impairments of LTP and DP in rat dentate gyrus (DG) in vivo. The experiments were carried out in four groups of rats (control, lead-exposed, control and lead-exposed with a taurine-supplement diet, respectively). The input-output (I/O) function, excitatory postsynaptic potential (EPSP) and population spike (PS) amplitude were measured in the DG area of adult rats (60-90 days) in response to stimulation applied to the lateral perforant path. The results show that: 1. chronic lead exposure impaired LTP/DP measured on both EPSP slope and PS amplitude in DG area of the hippocampus; 2. in control rats, taurine had no effect on LTP/DP; 3. the amplitudes of LTP/DP of lead-exposed group were significantly increased by applying taurine. These results suggest that dietary taurine supplement could protect rats from the lead-induced impairments of synaptic plasticity and might be a preventive medicine to cure the cognitive deficits induced by lead.

Effects of Pb2+ on the transient outward potassium current in acutely dissociated rat hippocampal neurons

Modulation of the voltage-dependent transient outward potassium current (IA) by Pb2+ was studied in acutely dissociated rat hippocampal pyramidal cells from the CA1 region at postnatal ages 7-14 days using the conventional whole-cell patch-clamp technique. In the presence of different concentrations of external Pb2+, the initial delay and activation time of IA were concentration-dependently lengthened. In particular, the initial delay was even longer in 1 mM Pb2+, showing no signs of saturation. Pb2+ also slowed the inactivation of IA, for decay time constants in the presence of Pb2+ were increased under the same experimental protocols. The activation curves, which were reasonably fitted by a single Boltzmann function, illustrated that Pb2+ increased the voltage threshold of IA and shifted the normalized activation current-voltage curves to more depolarizing voltage commands. Moreover, Pb2+ significantly affected the steady-state inactivation of IA. The application of Pb 2+ made the curves of the steady-state inactivation of IA shift to more depolarizing voltages with little change in the slopes factors. In brief, the results demonstrated that Pb2+ is a dose- and voltage-dependent, reversible blocker of IA currents of hippocampal CA1 neurons. The observations were fitted by the revised "Kuo and Chen type model", which postulates a Pb2+-selective site near the pore of the IA channel and that modulation of the IA channel by Pb2+ is the result of the competitive influences of Pb2+ on opening and inactivating different pathways.

Age-related morphological impairments in the rat hippocampus following developmental lead exposure: an MRI, LM and EM study

Lead is one of the most common neurotoxic metals present in our environment. Chronic developmental lead exposure is known to be associated with cognitive dysfunction in children. Functional and morphological impairment of the rat brain has also been reported in the hippocampus (Hi) following developmental lead exposure. The present study was carried out to further investigate age-related morphological impairments in the rat Hi following developmental lead exposure with three methods: (1) magnetic resonance imaging (MRI); (2) light microscopy (LM); and (3) electron microscopy (EM) techniques. Neonatal Wistar rats were exposed to lead from parturition to weaning via milk of dams drinking a 0.2% lead acetate solution. Age-related morphological alternations were investigated in the Hi of lead-exposed rats at various postnatal ages: postnatal day (PND) 17, 30 and 90. The MRI signal intensities (SIs) in the left, right, superior and inferior hippocampal regions of control and lead-exposed rats were analyzed. Compared with controls, the SIs of the four hippocampal regions of interest were significantly increased in lead-exposed rats at PND 17, 30 and 90. Moreover, the lead-induced impairment of the Hi showed an age-related decline and a specific topographical pattern. The impairment of inferior hippocampal regions was more severe than that of superior regions in lead-exposed rats at PND 17 and 30, while no significant difference of SIs was observed between left and right hippocampal regions in the three age groups, and between superior and inferior regions in the PND 90 lead-exposed rats. The LM observations indicated that the morphological injury of hippocampal neurons in lead-exposed rats was also age-related. The EM observations revealed that the endoplasmic reticular, Golgi complex and mitochondria of hippocampal CA1 and dentate gyrus neurons in lead-exposed rats were damaged. These results demonstrate that lead-induced morphological impairments of the rat Hi follow a specific age- and site-related pattern.

Molecular targets of lead in brain neurotoxicity

The detrimental effects of lead poisoning have been well known since ancient times, but some of the most severe consequences of exposure to this metal have only been described recently. Lead [Pb(II)] affects the higher functions of the central nervous system and undermines brain growth, preventing the correct development of cognitive and behavioral functions. As an established neurotoxin, Pb(II) crosses the blood-brain barrier rapidly and concentrates in the brain. The mechanisms of lead neurotoxicity are complex and still not fully understood, but recent findings recognized that both Ca(II) dependent proteins and neurotransmitters receptors represent significant targets for Pb(II). In particular, acute and chronic exposure to lead would predominantly affect two specific protein complexes: protein kinase C and the N-methyl-D-aspartate subtype of glutamate receptor. These protein complexes are deeply involved in learning and cognitive functions and are also thought to interact significantly with each other to mediate these functions. This review outlines the most recent hypotheses and evidences that link lead poisoning to impairment of these protein functions, as well as the in vitro experimental approaches that are most likely to provide information on basic mechanicistic processes.

Lead exposure activates nuclear factor kappa B, activator protein-1, c-Jun N-terminal kinase and caspases in the rat brain

How lead manifests its neurotoxicity is not well understood. The hypothesis that lead may activate nuclear transcription factors NF-kappaB, activator protein-1 (AP-1), c-Jun N-terminal kinase (JNK), mitogen-activated protein kinase kinase (MAPKK) and caspases in the rat brain leading to the manifestation of its neurotoxic effects, was tested in 21-day-old male Long-Evans rats exposed to 50 ppm Pb in drinking water for 90 days. After the 90-day exposure, blood lead levels of the rats in control group were 4+/-0.2 microg/dl, while those of the Pb-exposed group were 18+/-0.3 microg/dl (n=50). Similarly, at the end of the exposure period, the Pb-exposed group showed significantly higher accumulation of Pb in brain regions such as, frontal cortex (FC), brain stem (BS), striatum (ST), and hippocampus (HIP) (338.6+/-7.7, 391.6+/-3.8, 288.3+/-6.7, and 382.3+/-3.3 ng/g wet tissue, respectively, in FC, BS, ST, and HIP) than the control group (126.6+/-2.7, 127.6+/-1.8, 201.3+/-9.4, and 180.3+/-4.4 ng/g wet tissue, respectively, in FC, BS, ST, and HIP). There was a 3-4-fold increase in NF-kappaB and AP-1 level in all the four regions of the brain of lead-treated animals. All four regions showed 4-10-fold activation of JNK and a 5-6-fold activation of MAPKK. As indicated by poly(ADP ribose) polymerase cleavage, lead exposure induced the activation of caspases in all four regions. Overall our results indicate that lead exposure induces the activation of NF-kappaB, AP-1, JNK, MAPKK, and caspases in the brain, which may contribute to its neurotoxic effects.

Effects of lead exposure on long-term potentiation induced by 2-deoxy-D-glucose in area CA1 of rat hippocampus in vitro

Chronic developmental lead exposure is known to be associated with cognitive dysfunction in children. Previous studies have demonstrated that chronic lead exposure could impair the induction and maintenance of long-term potentiation induced by high-frequency stimulation (HFS-LTP). In area CA1 of rat hippocampus, long-term potentiation could also be induced following temporary replacement of 10 mM 2-deoxy-D-glucose (2-DG) for 10 mM glucose in the normal perfusate (artificial cerebrospinal fluid). The present study was carried out to investigate whether chronic lead exposure affected long-term potentiation induced by 2-DG (2-DG-LTP). Neonatal Wistar rats were exposed to lead from parturition to weaning via milk of dams whose drinking water contained 0.2% lead acetate. Field excitatory postsynaptic potentials (EPSPs) in area CA1 of hippocampus were recorded on postnatal days 25-30. 2-DG application was followed by an increase in EPSP slopes in a time-course-dependent manner in both control and lead-exposed rats, while the amplitude of 2-DG-LTP in the lead-exposed rats (225.9+/-19.0%, n=12) was significantly greater than that in controls (155.2+/-9.8%, n=12). In contrast to the effects of lead exposure on 2-DG-LTP, the amplitude of HFS-LTP in the lead-exposed rats (121.5+/-13.7%, n=12) was significantly less than that in controls (183.9+/-18.6%, n=12). These results indicate that chronic lead exposure had opposite effects on the two types of LTP induced by HFS and 2-DG. This would suggest that the effects of lead on HFS-LTP and 2-DG-LTP are the result of different sites of lead toxicity.