Gary M. Samoriski

Evidence that a protein-protein interaction ‘hot spot’ on heterotrimeric G protein βγ subunits is used for recognition of a subclass of effectors

To understand the requirements for binding to G protein βγ subunits, phage‐displayed random peptide libraries were screened using immobilized biotinylated βγ as the target. Selected peptides were grouped into four different families based on their sequence characteristics. One group (group I) had a clear conserved motif that has significant homology to peptides derived from phospholipase C β (PLC β) and to a short motif in phosducin that binds to G protein β subunits. The other groups had weaker sequence homologies or no homology to the group I sequences. A synthetic peptide from the strongest consensus group blocked activation of PLC by G protein βγ subunits. The peptide did not block βγ‐mediated inhibition of voltage‐gated calcium channels and had little effect on βγ‐mediated inhibition of Gs‐stimulated type I adenylate cyclase. Competition experiments indicated that peptides from all four families bound to a single site on βγ. These peptides may bind to a protein‐protein interaction ‘hot spot’ on the surface of βγ subunits that is used by a subclass of effectors.

Differential Spatial Patterns of Fos Induction Following Generalized Clonic and Generalized Tonic Seizures

The expression of generalized clonic and generalized tonic seizures has been suggested to result from the activation of different and independent neuronal circuits. Using the induction of the c-fos protein (Fos) as a marker of neuronal activity, we identified brain structures that are differentially associated with the expression of electroconvulsive shock-induced generalized clonic and generalized tonic seizures. Expression of either seizure phenotype resulted in a similar bilaterally symmetrical increase in Fos immunoreactivity in many forebrain structures, including the bed nucleus of the stria terminalis, hippocampal dentate gyrus, amygdala, and piriform cortex, compared to controls. However, following tonic hindlimb extension (THE), the degree of labeling in specific thalamic, hypothalamic, and brain stem areas was significantly greater than that of either controls or animals exhibiting clonic seizures. While a greater number of neurons in the hypothalamus (e.g., ventromedial nucleus), subparafascicular thalamic nucleus, peripeduncular area, deep medial superior colliculus, dorsal and lateral central gray, and paralemniscal nuclei were robustly labeled following THE, noticeably fewer cells were immunoreactive following face and forelimb clonic seizure behaviors. These differences were also found to be independent of the stimulus magnitude. In animals stimulated with the same current intensity but expressing either of the two seizure phenotypes, the pattern of Fos induction was consistent with the seizure phenotype expressed. These results demonstrate that specific subsets of neurons are differentially activated following the expression of different generalized seizure behaviors and that activity in discrete mesencephalic and diencephalic structures is more frequently associated with the expression of generalized tonic seizures than with the expression of generalized clonic seizures.

Effects of Valproate, Phenytoin, and MK‐801 in a Novel Model of Epileptogenesis

Summary: Purpose: We have developed and characterized a novel model of epileptogenesis based on the convulsive actions of flurothyl in mice. The hallmark feature of this model is a reliable change in the type of seizure expressed in response to flurothyl from generalized clonic to generalized tonic seizures. The purpose of our study was to evaluate the effects of chronic administration of valproate (VPA), phenytoin (PHT), and MK‐801 on the change in seizure phenotype observed in our model system.

Enhancement of Antidepressant-Like Effects but Not Brain-Derived Neurotrophic Factor mRNA Expression by the Novel N-Methyl-d-aspartate Receptor Antagonist Neramexane in Mice

Improved efficacy in the treatment of depression may be achieved by the combined use of several antidepressants. In the present study, acute administration of the novel N-methyl-d-aspartate (NMDA) receptor antagonist neramexane, as well as the representative antidepressants imipramine, fluoxetine, and venlafaxine, shortened the duration of immobility in the mouse tail suspension test with a minimal effective dose of 5 mg/kg. When tested in combination, the antidepressant-like effects of 5 mg/kg imipramine, 20 mg/kg fluoxetine, and 5 mg/kg venlafaxine were potentiated by neramexane (2.5 mg/kg), a dose that alone did not produce a significant effect on the duration of immobility. These effects seemed to be specific, because they were not accompanied by significant effects on locomotor activity. The enhanced antidepressant-like activity produced with the different combinations was not synergistic as determined by comparing the theoretical and observed ED50 values for each combination. In separate experiments, Northern blot analysis showed that a 14-day treatment with imipramine (10 mg/kg b.i.d.) increased brain-derived neurotrophic factor (BDNF) mRNA expression in the cortex, whereas neramexane (5 mg/kg b.i.d.) decreased it. Combined treatment produced no effect on BDNF mRNA expression. Mice treated with imipramine or neramexane for 14 days and tested shortly after the last dose demonstrated significant shortening of immobility, and the combined treatment produced an even greater antidepressant-like effect. Together, these data support the view that NMDA receptor antagonists enhance the potency of antidepressants, but they leave open the question as to whether enhanced BDNF expression is a necessary feature of the antidepressant-like effect.

Evidence for the interaction of brainstem systems mediating seizure expression in kindling and electroconvulsive shock seizure models

Amygdala kindling was observed to increase significantly the proportion of rats that exhibited tonic hindlimb extension in response to corneal electroshock stimulation. Mechanical brainstem lesions which abolished electroshock-induced tonic hindlimb extension failed to alter either the expression of fully generalized kindled seizures or the development of amygdala kindled seizures. Results suggest that while kindling can alter the sensitivity of brainstem systems involved in the expression of tonic hindlimb extension, these same systems are not necessary for either the development or expression of amygdala kindled seizures.

The Role of Rhinencephalic Networks in Early Stage Kindling

Kindling is a powerful model of epilepsy.11,29 It is the best model we have of mesial temporal lobe epilepsy (MTLE), which is the most devastating type of human epilepsy, accounting for the vast majority of people with medically intractable epilepsy. More than 0.5 million people in the USA are afflicted with MTLE and continue to have seizures despite the best available medical treatment. It has been the hope of investigators that understanding the mechanisms underlying kindling will provide insight into treatment for MTLE. In nearly thirty years of research, however, kindling has yet to give up its most fundamental secrets.

Mechanisms of kindling: an evaluation of single trial seizure induction procedures for use as controls

Investigations into the neurochemical or molecular biological mechanisms underlying the kindled state require a seizure induction procedure for eliciting generalized tonic-clonic seizures in naive animals. Such seizure controls are necessary for dissociating the influence of ictal motor events on measures of interest from the influence of the kindling process on these same measures. In this study three procedures for inducing seizures in naive animals were evaluated against a set of criteria considered ideal.