Clifford L. Mitchell

A simple recording grip strength device

Abstract A simple-to-construct, inexpensive recording strain gauge device which may be used to obtain graded measurements of forelimb grip strength in rats and mice is described.

Proconvulsant action of diethyldithiocarbamate in stimulation of the perforant path

The ability of diethyldithiocarbamate (DEDTC) to prolong electrical afterdischarge (AD) and lower the threshold for behavioral seizures elicited by stimulation of the perforant path (PPS) was examined. DEDTC was given in doses of 25, 50, and 100 mg/kg, IP. The effects of DEDTC on the threshold for wet dog shakes (WDS) and the number of WDS elicited by PPS were inconsistent. It had no effect on the duration of AD accompanied with WDS. However, DEDTC, at both 50 and 100 mg/kg, significantly lowered the threshold for rearing accompanied with forelimb clonus. At 100 mg/kg, it also prolonged the duration of AD occurring with these seizures. The effects of DEDTC were transitory and coincided with the time course for its ability to chelate the mossy fiber intravesicular pool of zinc (i.e., that which is released by activation of dentate granule cells). It is suggested that release of zinc from the mossy fibers may serve to protect the hippocampus from paroxysmal seizure activity.

Repeated electroconvulsive shock downregulates the opioid receptors in rat brain

Ten consecutive daily electroconvulsive shocks (ECSs), which produce maximal tonic and clonic convulsions, caused reductions of mu- and delta-opioid receptor binding in the hypothalamus, hippocampus and caudate nucleus, but not in the frontal cortex and brainstem. These changes of opioid receptor binding were not observed in rats receiving a single ECS. Scatchard analysis revealed that ECS-induced reduction of mu- and delta-receptor binding was due to a decrease in the binding sites but not to a change in the binding affinity. Time course studies showed that 7 days after the end of 10 consecutive daily ECSs, both mu- and delta-receptor binding remained lower than those of sham controls. However, the effects of ECS on the opioid receptor binding disappeared in 2-3 weeks. These observations are consistent with the hypothesis that ECS treatments increase the release of opioid peptides in certain brain regions which in turn down-regulate the opioid receptors.

Diethyldithiocarbamate and dithizone augment the toxicity of kainic acid

Male Fischer-344 rats were injected i.p. with diethyldithiocarbamate or dithizone 15 min after kainic acid (KA), s.c. Diethyldithiocarbamate and dithizone reduced both the number of wet dog shakes and the latency to onset of seizures induced by KA. Moreover, they increased the severity of seizures. These compounds may be useful tools for investigating the role of zinc in central nervous system excitatory transmission and/or convulsive phenomena.

Functional deficits after sustained stimulation of the perforant path

Several reports have implicated the overactivity of hippocampal glutaminergic systems in neurodegenerative conditions including Senile dementia of the Alzheimers type (SDAT). The neurobiological effects of hippocampal glutaminergic hyperactivity were studied by perforant pathway stimulation. Forty-five minutes of sustained perforant pathway stimulation produced a 50% or greater increase in motor activity 1, 2, and 3 weeks after stimulation. Robust retention deficits in a 48-h step-through passive avoidance task were evident 2 weeks post-stimulation. Furthermore, animals receiving stimulation were impaired in the acquisition of a spatial task in the Morris water maze. Stimulated animals exhibited little reduction in their escape latencies over the testing period. The learning and memory deficits were associated with a loss of CA1 and CA3 pyramidal cells and pretreatment with the N-methyl-D-aspartate antagonist MK-801 reduced this cell loss, particularly in the CA1 region of the hippocampus. These results suggest that sustained stimulation of the perforant pathway may be useful in studying neurological deficits associated with glutaminergic hyperfunction.

Regional Variation in the Response of Cerebral Ornithine Decarboxylase to Electroconvulsive Shock

Levels of ornithine decarboxylase activity were measured in brain regions and in adrenal glands of adult male rats exposed to electroshock. Five hours after shock at levels causing transient loss of consciousness and fore and hindlimb tonic extensor seizures, major increases in ornithine decarboxylase activity were found in adrenals, hippocampus, brain stem, frontal cortex, and cerebellum, but striatal levels were unchanged. These increases were reversed by 24 h after electroshock. When lower levels of shock, which caused no loss of consciousness, were also used, a clear dose-response relationship of shock intensity and ornithine decarboxylase activity was found for hippocampus and brain stem. The ornithine decarboxylase response in brain increased with higher shock levels. However, the changes of ornithine decarboxylase in adrenal glands were maximal at intermediate, and diminished at maximal shock values, as were levels of circulating testosterone. These data suggest a differing role for cerebral and adrenal ornithine decarboxylase in the mature rat. The brain enzyme may be primarily related to metabolic repair processes, whereas adrenal ornithine decarboxylase may function in the activation of secretion.

A glutamate antagonist blocks perforant path stimulation-induced reduction of dynorphin peptide and prodynorphin mRNA levels in rat hippocampus

Stimulation of the perforant path elicits a behavioral response, wet dog shakes (WDS), and reduction in hippocampal dynorphin A(1-8) immunoreactivity (DYN-IR) and prodynorphin mRNA (DYN mRNA) in rats. This study examined whether glutamate, the proposed endogenous transmitter released by perforant fibers, mediated the above responses. A glutamate antagonist, gamma-D-glutamylglycine (DGG, 25 micrograms/0.5 microliters), or artificial cerebrospinal fluid (ACSF, 0.5 microliters) was injected into the ventral hippocampus 10-20 min prior to acute or daily stimulation of the left perforant path in rats. In acute stimulation experiments, 4 consecutive stimulation trials elicited a total of 73 +/- 4 WDS at an average threshold intensity of 0.46 +/- 0.03 mA in ACSF-treated rats. The hippocampal DYN-IR in these animals decreased by more than 40% in both dorsal and ventral hippocampus relative to sham-stimulated rats. DGG injections significantly elevated the threshold for WDS (0.78 +/- 0.05 mA, P less than 0.01), reduced the number of WDS (45 +/- 6, P less than 0.01), and partially antagonized stimulation-induced reduction of DYN-IR in the ventral, but not dorsal, hippocampus. In daily stimulation experiments, rats received a single trial of stimulation once per day for 6 days. Daily DGG pretreatment almost completely abolished WDS at control threshold intensities, and significantly inhibited stimulation-induced decrease of DYN-IR in both dorsal and ventral hippocampus. In situ hybridization using a 35S-labeled oligodeoxyribonucleotide probe demonstrated a clear depletion of DYN mRNA signal in the dentate granule cell layer of ACSF-treated animals. This depletion was completely prevented in DGG-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Granule cells in the ventral, but not dorsal, dentate gyrus are essential for kainic acid-induced wet dog shakes

Intrahippocampal injections of colchicine selectively destroy dentate granule cells. Wet dog shaking elicited by systemic administration of kainic acid is eliminated by bilateral destruction of ventral dentate granule cells but unaffected by bilateral destruction of dorsal dentate granule cells. This implies that ventral dentate granule cells are essential for the generation of kainic acid-induced wet dog shakes.

Perforant path stimulation differentially alters prodynorphin mRNA and proenkephalin mRNA levels in the entorhinal cortex-hippocampal region

The regulatory effect of the perforant path on opioid gene expression in the entorhinal cortex-hippocampal region was investigated. The left perforant path was electrically stimulated at the angular bundle under conditions which elicit wet dog shakes but no motor seizures in rats. Animals were given either an acute stimulation composed of several consecutive stimulation trials, or daily stimulations with a single trial every day for 6 days. Rats were then sacrificed at 24 h or 6 days after the last trial. The amounts of prodynorphin mRNA (DYN mRNA) and proenkephalin A mRNA (EK mRNA) in the hippocampus and entorhinal cortex were measured by RNA blot analysis. Dynorphin A(1-8) and [Met5]enkephalin immunoreactivities were determined by radioimmunoassay. A decrease in DYN mRNA level of approximately 50-80% was found on both sides of the hippocampus 24 h after both acute and daily stimulation. Hippocampal dynorphin A(1-8) immunoreactivity was also reduced at 24 h, and persisted for at least 6 days. In contrast, bilateral increases in EK mRNA level were observed in the hippocampus (54-101%) and entorhinal cortex (97-165%) 24 h after the acute stimulation. Also, [Met5]enkephalin immunoreactivity in the hippocampus tended to be increased at this time. These results indicate that activation of the perforant path inhibits the gene expression of prodynorphin, but enhances that of proenkephalin in the entorhinal cortex-hippocampal region.

Stimulation of the perforant path alters hippocampal levels of opioid peptides, glutamine and GABA

This investigation demonstrates that stimulation of the perforant path under conditions which elicit wet dog shakes in rats produces a significant decrease in hippocampal levels of methionine-enkephalin, dynorphin A(1-8) and glutamine, and an increase in gamma-aminobutyric acid (GABA). Levels of these substances are not altered by stimulus parameters insufficient to elicit wet dog shakes. These results lend support to the notion that endogenous opioid peptides play a role in regulation of hippocampal excitability but may only be released under relatively intense stimulus conditions. The increase in GABA levels could be due to an increase in synthesis, an increase in reuptake or a reduction in release. The latter possibility is consistent with reports that iontophoretically applied enkephalin exerts its apparent excitatory effects via an inhibitory action on inhibitory neurons in the hippocampus.