Gloria J. Klapstein

Parkin-deficient Mice Exhibit Nigrostriatal Deficits but Not Loss of Dopaminergic Neurons

Loss-of-function mutations in parkin are the major cause of early-onset familial Parkinsons disease. To investigate the pathogenic mechanism by which loss of parkin function causes Parkinsons disease, we generated a mouse model bearing a germline disruption in parkin. Parkin–/– mice are viable and exhibit grossly normal brain morphology. Quantitative in vivo microdialysis revealed an increase in extracellular dopamine concentration in the striatum of parkin–/– mice. Intracellular recordings of medium-sized striatal spiny neurons showed that greater currents are required to induce synaptic responses, suggesting a reduction in synaptic excitability in the absence of parkin. Furthermore, parkin–/– mice exhibit deficits in behavioral paradigms sensitive to dysfunction of the nigrostriatal pathway. The number of dopaminergic neurons in the substantia nigra of parkin–/– mice, however, is normal up to the age of 24 months, in contrast to the substantial loss of nigral neurons characteristic of Parkinsons disease. Steady-state levels of CDCrel-1, synphilin-1, and α-synuclein, which were identified previously as substrates of the E3 ubiquitin ligase activity of parkin, are unaltered in parkin–/– brains. Together these findings provide the first evidence for a novel role of parkin in dopamine regulation and nigrostriatal function, and a non-essential role of parkin in the survival of nigral neurons in mice.

Enhanced sensitivity to N-methyl-D-aspartate receptor activation in transgenic and knockin mouse models of Huntington's disease

We used two mouse models of Huntingtons disease (HD) to examine changes in glutamate receptor sensitivity and striatal electrophysiology. One model, a transgenic, consisted of mice expressing exon 1 of the human HD gene and carrying 141–157 CAG repeat sequences (R6/2 line). The second model, a CAG repeat “knockin,” consisted of mice with different lengths of CAG repeats (CAG71 and CAG94 repeats). The effects of glutamate receptor activation were examined by visualizing neurons in brain slices with infrared videomicroscopy and differential interference contrast optics to determine changes in somatic area (cell swelling). Striatal and cortical neurons in both models (R6/2 and CAG94) displayed more rapid and increased swelling to N‐methyl‐D‐aspartate (NMDA) than those in controls. This effect was specific as there were no consistent group differences after exposure to α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionic acid (AMPA) or kainate (KA). Intracellular recordings revealed that resting membrane potentials (RMPs) in the R6/2 transgenics were significantly more depolarized than those in their respective controls. RMPs in CAG94 mice also were more depolarized than those in CAG71 mice or their controls in a subset of striatal neurons. Confirming previous results, R6/2 mice expressed behavioral abnormalities and nuclear inclusions. However, CAG71 and CAG94 knockins did not, suggesting that increased sensitivity to NMDA may occur early in the disease process. These findings imply that NMDA antagonists or compounds that alter sensitivity of NMDA receptors may be useful in the treatment of HD. J. Neurosci. Res. 58:515–532, 1999.

Morphological and electrophysiological characterization of abnormal cell types in pediatric cortical dysplasia.

The mechanisms responsible for seizure generation in cortical dysplasia (CD) are unknown, but morphologically abnormal cells could contribute. We examined the passive and active membrane properties of cells from pediatric CD in vitro. Normal‐ and abnormal‐appearing cells were identified morphologically by using infrared videomicroscopy and biocytin in slices from children with mild to severe CD. Electrophysiological properties were assessed with patch clamp recordings. Four groups of abnormal‐appearing cells were observed. The first consisted of large, pyramidal cells probably corresponding to cytomegalic neurons. Under conditions that reduced the contribution of K+ conductances, these cells generated large Ca2+ currents and influx when depolarized. When these cells were acutely dissociated, peak Ca2+ currents and densities were greater in cytomegalic compared with normal‐appearing pyramidal neurons. The second group included large, nonpyramidal cells with atypical somatodendritic morphology that could correspond to “balloon” cells. These cells did not display active voltage‐ or ligand‐gated currents and did not appear to receive synaptic inputs. The third group included misoriented and dysmorphic pyramidal neurons, and the fourth group consisted of immature‐looking pyramidal neurons. Electrophysiologically, neurons in these latter two groups did not display significant abnormalities when compared with normal‐appearing pyramidal neurons. We conclude that there are cells with abnormal intrinsic membrane properties in pediatric CD. Among the four groups of cells, the most abnormal electrophysiological properties were displayed by cytomegalic neurons and large cells with atypical morphology. Cytomegalic neurons could play an important role in the generation of epileptic activity.

Calbindin-D28k fails to protect hippocampal neurons against ischemia in spite of its cytoplasmic calcium buffering properties : Evidence from Calbindin-D28k knockout mice

Cytoplasmic calcium-binding proteins are thought to shield neurons against damage induced by excessive Ca2+ elevations. Yet, in theory, a mobile cellular Ca2+ buffer could just as well promote neuronal injury by facilitating the rapid dispersion of Ca2+ throughout the cytoplasm. In sharp contrast to controls, in mice lacking the gene for calbindin-D28k, synaptic responses of hippocampal CA1 pyramidal neurons which are normally extremely vulnerable to ischemia, recovered significantly faster and more completely after a transient oxygen-glucose deprivation in vitro, and sustained less cellular damage following a 12 min carotid artery occlusion in vivo. Other cellular and synaptic properties such as the altered adaptation of action potential firing, and altered paired-pulse and frequency potentiation at affected synapses in calbindin-D28k-deficient mice were consistent with a missing intraneuronal Ca2+ buffer. Our findings provide direct experimental evidence against a neuroprotective role for calbindin-D28k.

Pediatric Cortical Dysplasia: Correlations between Neuroimaging, Electrophysiology and Location of Cytomegalic Neurons and Balloon Cells and Glutamate/GABA Synaptic Circuits

Seizures in cortical dysplasia (CD) could be from cytomegalic neurons and balloon cells acting as epileptic ‘pacemakers’, or abnormal neurotransmission. This study examined these hypotheses using in vitro electrophysiological techniques to determine intrinsic membrane properties and spontaneous glutamatergic and GABAergic synaptic activity for normal-pyramidal neurons, cytomegalic neurons and balloon cells from 67 neocortical sites originating from 43 CD patients (ages 0.2–14 years). Magnetic resonance imaging (MRI), 18fluoro-2-deoxyglucose positron emission tomography (FDG-PET) and electrocorticography graded cortical sample sites from least to worst CD abnormality. Results found that cytomegalic neurons and balloon cells were observed more frequently in areas of severe CD compared with mild or normal CD regions as assessed by FDG-PET/MRI. Cytomegalic neurons (but not balloon cells) correlated with the worst electrocorticography scores. Electrophysiological recordings demonstrated that cytomegalic and normal-pyramidal neurons displayed similar firing properties without intrinsic bursting. By contrast, balloon cells were electrically silent. Normal-pyramidal and cytomegalic neurons displayed decreased spontaneous glutamatergic synaptic activity in areas of severe FDG-PET/MRI abnormalities compared with normal regions, while GABAergic activity was unaltered. In CD, these findings indicate that cytomegalic neurons (but not balloon cells) might contribute to epileptogenesis, but are not likely to be ‘pacemaker’ cells capable of spontaneous paroxysmal depolarizations. Furthermore, there was more GABA relative to glutamate synaptic neurotransmission in areas of severe CD. Thus, in CD tissue alternate mechanisms of epileptogenesis should be considered, and we suggest that GABAergic synaptic circuits interacting with cytomegalic and normal-pyramidal neurons with immature receptor properties might contribute to seizure generation.

Decreased sensitivity to Group III mGluR agonists in the lateral perforant path following kindling

The ability of the selective Group III mGluR agonist L-serine-O-phosphate (L-SOP) to inhibit lateral perforant path (LPP) evoked responses in the dentate gyrus was tested in hippocampal slices from commissurally-kindled rats 1-2 days after the last seizure, implanted controls, and fully-kindled rats rested for 28 days without stimulated seizures (28 days post-seizure, 28 dps). L-SOP was more potent in controls than kindled or 28 dps animals, decreasing the fEPSP slope with IC50s of 2.4 microM, 18.7 microM and 10.5 microM, respectively. Paired pulse facilitation (PPF, 50 ms) was comparable in control and kindled rats, but was markedly reduced in 28 dps rats, indicating increased release probability. Inhibition of the field excitatory postsynaptic potentials (fEPSP) by L-SOP was correlated with enhanced PPF in all groups, affirming a presynaptic site of action. At moderate levels of L-SOP-induced inhibition (20-60%), PPF showed significantly greater enhancement in 28 dps than in the other two groups. These results are interpreted as showing a functional reduction of the presynaptic inhibitory Group III mGluR (probably mGluR8) response in the LPP after kindling. Furthermore, PPF changes indicate that the kindled state may be associated with a long-lasting increase in the probability of release from LPP terminals, which may be temporarily masked or counterbalanced by recent seizures.

Presynaptic Inhibition Mediated by Neuropeptide Y in the Mammalian CNS: Possible Physiological Implications

Although numerous transmitters are released by central neurons to mediate conventional synaptic transmission, it has become evident recently that there are other substances that can alter conventional synaptic transmission on a prolonged time scale relative to conventional transmitters. These substances, broadly classified as neuromodulators, can include most conventional transmitters, peptides, and other substances released by neurons, which act through pre- or postsynaptic receptors to change the level of transmitter release, change the properties of postsynaptic ion channels, alter the response of the postsynaptic cell to the actions of conventional transmitters, or alter other aspects of the biochemical machinery of the nerve cell or presynaptic terminal. In most cases, neuromodulators activate G protein-coupled receptors, which can activate (or inhibit) all known second messenger cascades, as well as ion channels and other cell surface effectors. While the interpretation of conventional fast chemical neurotransmission has been relatively straightforward, understanding the purpose of modulatory transmitters within a particular physiological system has not always been intuitive. This chapter addresses the presynaptic actions of one modulatory substance, neuropeptide Y (NPY), and advances hypotheses regarding the action of NPY in two central systems: the hippocampus and the serotonergic dorsal raphe nucleus of the brain stem.