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.

Decreased seizure threshold after intrahippocampal colchicine injection in rats.

Wet dog shaking elicited by perforant path stimulation is little affected by bilateral injection of colchicine into the dorsal hippocampal formation but virtually eliminated by bilateral injection into the ventral hippocampal formation. Injection of colchicine into either dorsal or ventral hippocampal formation lowers the threshold for eliciting forelimb clonus with rearing. This effect is more pronounced 8 weeks postinjection than 2 weeks postinjection when colchicine is injected in the ventral hippocampal formation. This suggests that inappropriate reactive synaptogenesis and/or neuronal degeneration continues for an extended period after intrahippocampal injection of colchicine, especially in the ventral hippocampal formation.

Effect of destruction of dentate granule cells on kindling induced by stimulation of the perforant path.

Bilateral destruction of dentate granule cells in both the dorsal and ventral hippocampal formation had no effect on the threshold for hippocampal afterdischarge before or after kindling. Neither did it affect the number of stimulations required to attain kindling. However, the duration of afterdischarge was significantly longer in colchicine-lesioned animals compared to those receiving artificial cerebrospinal fluid. This was true for the threshold for afterdischarge as well as at the first and last kindling trials. These results suggest that dentate granule cells may inhibit the duration of afterdischarge induced by perforant path stimulation or that other postlesion changes occur which result in a prolongation of hippocampal afterdischarge.

Opioid mu and delta receptor antagonists reduce wet dog shaking elicited by perforant path stimulation.

Stimulation of the perforant path, a major input to the hippocampal formation, produced significant decreases in the hippocampal levels of methionine enkephalin, dynorphin A(1-8) and an increase in the hippocampal level of gamma-aminobutyric acid. In addition, it was also observed that both mu and delta opioid receptor antagonists reduce wet dog shakes elicited by perforant path stimulation. The antagonists did not affect the changes in hippocampal levels of methionine enkephalin, dynorphin A(1-8) or gamma-aminobutyric acid. The results demonstrate that endogenous opioids are involved in the wet dog shakes elicited by perforant path stimulation. Since electrographic seizure activity occurs in the hippocampus in conjunction with perforant path stimulation-induced wet dog shakes, these data provide further evidence that endogenous opioid peptides play an important role in regulation of limbic system epileptogenic phenomena.

Differential effects of colchicine lesions of dentate granule cells on wet dog shakes and seizures elicited by direct hippocampal stimulation.

Direct electrical stimulation of either the dorsal or ventral hippocampal formation elicits wet dog shakes and overt seizures. Destruction of dentate granule cells in the dorsal hippocampal formation does not significantly reduce the number of wet dog shakes elicited by ventral hippocampal stimulation. However, destruction of dentate granule cells in the ventral hippocampus virtually eliminates wet dog shaking elicited by dorsal hippocampal stimulation. Destruction of either dorsal or ventral dentate granule cells lowers the threshold for eliciting forelimb clonus with rearing. These results suggest that dentate granule cells in the ventral hippocampus are essential for wet dog shakes elicited by intrahippocampal stimulation. However, dentate granule cells throughout the hippocampal formation appear to play an important inhibitory role in the spread of seizure activity within the hippocampus.

Colchicine lesions of ventral, but not dorsal, dentate granule cells attenuate wet dog shakes elicited by perforant path stimulation.

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

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 Alzheimer's 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.

Superior colliculus lesions and flash evoked potentials from rat cortex.

It is generally assumed that the primary response of the rat flash evoked potential (FEP) is activated by a retino-geniculate path, and that the secondary response reflects input to the cortex by way of the superior colliculus (SC) or other brainstem structures. In the present study, male Long-Evans rats were implanted with monopolar screw electrodes placed over the left visual cortex, and a pair of twisted monopolar depth electrodes, which were used to produce electrolytic lesions, were placed in each SC. One half of the animals did not receive the electrolytic treatment (controls). FEP waveforms were obtained from all animals prior to treatment, and 2 and 5 days after treatment. Histological analysis was performed to verify electrode placement and determine lesion size. Electrolytic lesions resulting in massive destruction of the SC produced no decrement in any portion of the rat FEP but did produce an increase in amplitude of the N2P3 component. The data show that the secondary response is not generated by SC in rats, but that SC may modulate amplitude of the response.