Biomedical subjects

M D Shumate

Publications and source records attributed to M D Shumate.

7 recordsLinked to original sources

PubMed2001

gamma-Aminobutyric acid(A) receptor subunit expression predicts functional changes in hippocampal dentate granule cells during postnatal development.

Profound alterations in the function of GABA occur over the course of postnatal development. Changes in GABA(A) receptor expression are thought to contribute to these differences in GABAergic function, but how subunit changes correlate with receptor function in individual developing neurons has not been defined precisely. In the current study, we correlate expression of 14 different GABA(A) receptor subunit mRNAs with changes in the pharmacological properties of the receptor in individual hippocampal dentate granule cells over the course of postnatal development in rat. We demonstrate significant developmental differences in GABA(A) receptor subunit mRNA expression, including greater than two-fold lower expression of alpha1-, alpha4- and gamma2-subunit mRNAs and 10-fold higher expression of alpha5-mRNA in immature compared with adult neurons. These differences correlate both with regional changes in subunit protein level and with alterations in GABA(A) receptor function in immature dentate granule cells, including two-fold higher blockade by zinc and three-fold lower augmentation by type-I benzodiazepine site modulators. Further, we find an inverse correlation between changes in GABA(A) receptor zinc sensitivity and abundance of vesicular zinc in dentate gyrus during postnatal development. These findings suggest that developmental differences in subunit expression contribute to alterations in GABA(A) receptor function during postnatal development.

Animals↗

PubMed2000

Regionally selective blockade of GABAergic inhibition by zinc in the thalamocortical system: functional significance.

The thalamocortical (TC) system is a tightly coupled synaptic circuit in which GABAergic inhibition originating from the nucleus reticularis thalami (NRT) serves to synchronize oscillatory TC rhythmic behavior. Zinc is colocalized within nerve terminals throughout the TC system with dense staining for zinc observed in NRT, neocortex, and thalamus. Whole cell voltage-clamp recordings of GABA-evoked responses were conducted in neurons isolated from ventrobasal thalamus, NRT, and somatosensory cortex to investigate modulation of the GABA-mediated chloride conductance by zinc. Zinc blocked GABA responses in a regionally specific, noncompetitive manner within the TC system. The regional levels of GABA blockade efficacy by zinc were: thalamus > NRT > cortex. The relationship between clonazepam and zinc sensitivity of GABA(A)-mediated responses was examined to investigate possible presence or absence of specific GABA(A) receptor (GABAR) subunits. These properties of GABARs have been hypothesized previously to be dependent on presence or absence of the gamma2 subunit and seem to display an inverse relationship. In cross-correlation plots, thalamic and NRT neurons did not show a statistically significant relationship between clonazepam and zinc sensitivity; however, a statistically significant correlation was observed in cortical neurons. Spontaneous epileptic TC oscillations can be induced in vitro by perfusion of TC slices with an extracellular medium containing no added Mg(2+). Multiple varieties of oscillations are generated, including simple TC burst complexes (sTBCs), which resemble spike-wave discharge activity. A second variant was termed a complex TC burst complex (cTBC), which resembled generalized tonic clonic seizure activity. sTBCs were exacerbated by zinc, whereas cTBCs were blocked completely by zinc. This supported the concept that zinc release may modulate TC rhythms in vivo. Zinc interacts with a variety of ionic conductances, including GABAR currents, N-methyl-D-aspartate (NMDA) receptor currents, and transient potassium (A) currents. D-2-amino-5-phosphonovaleric acid and 4-aminopyridine blocked both s- and cTBCs in TC slices. Therefore NMDA and A current-blocking effects of zinc are insufficient to explain differential zinc sensitivity of these rhythms. This supports a significant role of zinc-induced GABAR modulation in differential TC rhythm effects. Zinc is localized in high levels within the TC system and appears to be released during TC activity. Furthermore application of exogenous zinc modulates TC rhythms and differentially blocks GABARs within the TC system. These data are consistent with the hypothesis that endogenously released zinc may have important neuromodulatory actions impacting generation of TC rhythms, mediated at least in part by effects on GABARs.

2-Amino-5-phosphonovalerate↗

PubMed1999

Human neuronal gamma-aminobutyric acid(A) receptors: coordinated subunit mRNA expression and functional correlates in individual dentate granule cells.

gamma-Aminobutyric acid(A) receptors (GABARs) are heteromeric proteins composed of multiple subunits. Numerous subunit subtypes are expressed in individual neurons, which assemble in specific preferred GABAR configurations. Little is known, however, about the coordination of subunit expression within individual neurons or the impact this may have on GABAR function. To investigate this, it is necessary to profile quantitatively the expression of multiple subunit mRNAs within individual cells. In this study, single-cell antisense RNA amplification was used to examine the expression of 14 different GABAR subunit mRNAs simultaneously in individual human dentate granule cells (DGCs) harvested during hippocampectomy for intractable epilepsy. alpha4, beta2, and delta-mRNA levels were tightly correlated within individual DGCs, indicating that these subunits are expressed coordinately. Levels of alpha3- and beta2-mRNAs, as well as epsilon- and beta1-mRNAs, also were strongly correlated. No other subunit correlations were identified. Coordinated expression could not be explained by the chromosomal clustering of GABAR genes and was observed in control and epileptic rats as well as in humans, suggesting that it was not species-specific or secondary to epileptogenesis. Benzodiazepine augmentation of GABA-evoked currents also was examined to determine whether levels of subunit mRNA expression correlated with receptor pharmacology. This analysis delineated two distinct cell populations that differed in clonazepam modulation and patterns of alpha-subunit expression. Clonazepam augmentation correlated positively with the relative expression of alpha1- and gamma2-mRNAs and negatively with alpha4- and delta-mRNAs. These data demonstrate that specific GABAR subunit mRNAs exhibit coordinated control of expression in individual DGCs, which has significant impact on inhibitory function.

Adolescent↗

PubMed1998

GABA(A) receptor function in epileptic human dentate granule cells: comparison to epileptic and control rat.

Using patch clamp recording techniques in dentate granule cells (DGCs) isolated from patients undergoing temporal lobectomy for intractable epilepsy, we investigated basic properties of GABA(A) receptors (GABA(A)Rs) and pharmacological sensitivity of GABA-evoked currents to modulation by zinc and benzodiazepines (BZ). Properties of human DGC GABA(A)Rs were compared to DGC GABA(A)R properties in control and epileptic rats. Blockade of GABA evoked currents by zinc was significantly enhanced in epileptic human relative to control rat DGCs. Augmentation of the GABA(A)R current by the non-subunit selective BZ agonist, clonazepam (CNZ) and by the BZ1 specific agonist, zolpidem (ZOL), were not significantly different in human DGCs relative to control or epileptic rat. GABA potency was significantly higher in epileptic human DGCs than in control or epileptic rat DGCs. The significantly enhanced efficacy of zinc in blocking GABA currents in epileptic human DGCs mirrors that seen in epileptic rat DGCs, and was coupled with mossy fiber sprouting evident in both epileptic human and rat dentate gyrus. The aberrant mossy fibers provide a novel zinc delivery system within the epileptic dentate gyrus. The mossy fiber release of zinc onto DGCs coupled with the enhanced zinc sensitivity of GABA(A)Rs in epileptic DGCs, may lead to 'dynamic disinhibition' which could compromise inhibitory efficacy in the epileptic rat and human hippocampus.

Animals↗

PubMed1998

Selective changes in single cell GABA(A) receptor subunit expression and function in temporal lobe epilepsy.

Temporal lobe epilepsy is the most prevalent seizure disorder in adults. Compromised inhibitory neurotransmitter function in the hippocampus contributes to the hyperexcitability generating this condition, but the underlying molecular mechanisms are unknown. Combining patch-clamp recording and single-cell mRNA amplification (aRNA) techniques in single dentate granule cells, we demonstrate that expression of GABA(A) receptor subunit mRNAs is substantially altered in neurons from epileptic rats. These changes in gene expression precede epilepsy onset by weeks and correlate with profound alterations in receptor function, indicating that aberrant GABA(A) receptor expression and function has an essential role in the process of epileptogenesis.

Animals↗

PubMed1997

Differential epilepsy-associated alterations in postsynaptic GABA(A) receptor function in dentate granule and CA1 neurons.

Alterations in GABAergic function associated with the development of temporal lobe epilepsy (TLE) were examined with the use of patch-clamp recording techniques in dentate granule (DG) and CA1 neurons acutely isolated from control and spontaneously epileptic rats in which TLE was elicited by pilocarpine injection 3-17 wk before use. The maximal efficacy of gamma-aminobutyric acid (GABA) in activating whole cell GABA currents increased significantly in epileptic DG neurons relative to controls. This efficacy increase was due to a 78% enhancement in the functional capacitance-normalized GABA(A) receptor (GABA(A)R) current density in epileptic DG neurons. Increased DG GABA(A)R current density was not accompanied by alterations in GABA potency (EC50). However, the maximal sensitivity of DG GABA-evoked currents to blockade by zinc increased 190% in epileptic neurons. Augmentation of epileptic DG neuron GABA-evoked currents by the broad-spectrum anticonvulsant benzodiazepine clonazepam (100 nM) was enhanced 114% relative to controls, whereas augmentation by the benzodiazepine, (BZ1)-selective agonist zolpidem (100 nM) was decreased by 66%. In contrast to DG neurons, maximal efficacy of GABA in activating GABA currents decreased in epileptic CA1 neurons relative to controls, due to a 52% decrease in functional capacitance-normalized GABA(A)R current density. This altered efficacy of GABA was accompanied by an increased GABA potency (GABA EC50 was 35.8 and 24.5 microM in control and epileptic neurons, respectively). Sensitivity of GABA-evoked currents to blockade by zinc was unchanged in epileptic CA1 neurons, whereas clonazepam (100 nM) augmentation of CA1 GABA-evoked currents decreased 81% relative to controls. These regionally distinct epilepsy-associated modifications in hippocampal GABAergic function may be due to discrete structural alterations in postsynaptic GABA(A)Rs accompanying epileptogenesis, could be therapeutically important, and undoubtedly could contribute to the enhanced limbic excitability underlying TLE.

Animals↗

PubMed1995

Opioid peptide expression in models of chronic temporal lobe epilepsy.

Expression of the opioid peptides dynorphin and enkephalin is altered within the first 24 h after acutely induced seizures in certain experimental models of epilepsy. Using in situ hybridization, we examined the expression of prodynorphin and preproenkephalin messenger RNA acutely following induction of kindling with recurrent seizures and in two models of chronic temporal lobe epilepsy: (i) rats fully kindled with rapidly recurring hippocampal seizures; and (ii) rats surviving after self-sustaining limbic status epilepticus induced with focal electrical stimulation of the hippocampus. In naive animals, a ventral-dorsal gradient was identified in the expression of both prodynorphin and preproenkephalin messenger RNA in the dentate gyrus and expression of prodynorphin message was demonstrated for the first time in the ventral portion of cornu Ammonis regio superior. After stimulation producing rapidly recurring hippocampal seizures, acute decreases in prodynorphin messenger RNA were seen in the dentate gyrus and cornu Ammonis regio superior at 24 h after the last seizure. In contrast, increases in preproenkephalin messenger RNA expression were seen acutely in the dentate gyrus, with a decrease seen in the entorhinal cortex. The change in prodynorphin message expression in cornu Ammonis regio superior persisted in kindled animals that were studied after one month seizure-free period. There were no changes in preproenkephalin message in kindled animals studied after the one month seizure-free interval. No statistically significant changes were found for either prodynorphin or preproenkephalin message in the post-self-sustaining limbic status epilepticus group at one month following induced seizures. Acute changes in peptide expression may contribute to increased excitation in the dentate gyrus during induction of kindling, while the chronic change identified in cornu Ammonis regio superior may contribute directly to persistently increased excitability in this region.

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