Some Characteristics of Neurons in the Reticular Nucleus of the Thalamus (Preliminary Data)

Our general understanding of the function of neurons is that dendrites receive information that is relayed to the axon, where action potentials are initiated and propagated to eventually trigger neurotransmitter release at synaptic terminals. Although for a number of neuron types in the mammalian brain, many neuron types do not follow this classical polarity pattern. In fact, dendrites may be the site of action potentials initiation and propagation. It should be noted that convincing evidence has been obtained for the existence of dendritic action potentials in hippocampal and neocortical neurons. With regard to the dendrite potentials of thalamic neurons in general and specifically the reticular nucleus of the thalamus, it has not yet been reported. The results of this study demonstrate, for the first time, that generation of spike potentials of different amplitudes was observed in the activity of the thalamic reticular nucleus neurons. The generation of one action potential does not interfere with the generation of another, and a spike potential of smaller amplitude can occur at the ascending or descending phase of the spike potential of large amplitude. It can be argued that the spike potentials of lower amplitudes arising in the thalamic reticular nucleus neuron are of dendrite origin. Given both the strategic position and the functional purpose of the TRN, it can be assumed that the neurons of this structure must each time be discharged with spike potentials in order to carry out their modulating effect on other areas of the nervous system of the brain without leakage.

Significance of the Thalamic Reticular Nucleus GABAergic Neurons in Normal and Pathological Activity of the Brain

The relationship between neurons of the thalamic reticular nucleus (TRN) and relay neurons of the thalamic nuclei was studied. Activation of the TRN neurons was shown to abate activity of relay neurons. This evidence was obtained by stimulation of the TRN and the neocortex and, by introduction of small doses of nembutal as well. Suppression of the relay neuronal activity by the TRN neurons is supposed to occur monosynaptically. It has been also revealed that neuronal activity of the TRN enhances in a clonic phase of seizures generated by stimulation of the hippocampus and as soon as the electroencephalographic seizure reactions disappear. The suppression of limbic motor seizures is obviously related to the process of potentiation in GABAergic synapses of thalamocortical networks. Thus, stimulation of the TRN appears to be a rather valuable methodical tool that can open up prospects in the development of new “anticonvulsive” strategies in the treated of temporal lobe epilepsy.

Pregnancy: Protection of the Hyperactive Brain, or a Risk Factor?

The main objective of the study was to determine the effect of gestation and postgestation periods on development and course of seizure activity. The data obtained in the study showed that i) there is a reduction of both EEG and behavioral seizures during gestation in the presence of hippocampal epileptogenic locus (kindling model);ii) suppression of audiogenic seizures occurs in gestation, and development of audiogenic kindling and decrease of seizure reactions occur in the pseudogestation state. It is assumed that in gestation period the intensification of the possible endogenous inhibitory mechanisms follows the course of seizure activity.

Interaction between Seizure and Theta Rhythm

Recently it was shown by us that combined stimulation of hippocampus and dorsomedial hypothalamus resulted in suppression of the electroencephalographic seizure reactions and, respectively, manifestations of behavioral seizures reduced. It is expected, that augmentation of inhibitory processes in hippocampal neurons in the course of dorsomedial hypothalamus stimulation can trigger mechanisms preventing the development of epileptiform activity. Because of two important characteristics of the hippocampus—theta rhythm and epileptogenesis—these appear to be interrelated in respect of their cellular substrates, and as far as theta rhythm may modulate hippocampal excitability, a study of the functional relationship between theta rhythm and seizure activity was endeavored. The purpose of this study is to test this proposal by determining the effects on seizures of induction or suppression of hippocampal theta activity. Our findings show that: 1) against background of strong unusual sound stimulation (in our case-sound) blockade of local seizure reactions induced by hippocampal stimulation occurred;2) the frequency of hippocampal interictal epileptiform dischargers increased with the transition from the awake state to drowsiness and a slow-wave sleep phase. After the animal came from slow-wave sleep to paradoxical sleep, epileptiform activity completely disappeared;3) at threshold stimulation of hypothalamus when electrohippocampogram shows augmentation of the theta rhythm there is a significant reduction of seizure durations. When at hypothalamus stimulation instead of theta rhythm the electrical activity is desynchronized, there occurs a considerable intensification of seizure activity. Therefore, seizure-theta antagonism in our experiments could be interpreted as an adjustment of the inhibitory mechanisms when the theta rhythm is evoked.

Mood Disorders: Protection of the Hyper-Excited Brain, or a Risk Factor?

Emotional disturbances such as anxiety, fear, depression and aggression are often experienced by patients with temporal lobe epilepsy. These psychiatric symptoms may occur during or just after (postictal) a seizure;however, in some patients, they occur interictally (i.e. between seizures) and may profoundly change the individual’s personality. Aside from confirming that a significant proportion of temporal lobe epileptics do suffer from interictal abnormal emotionality, there has been little progress to date in identifying the fundamental nature of these disturbances. There is a lack of evidences regarding the influence of activation of emotiogenic structures and emotional behavior on development of seizures. Kindling is a commonly used animal model for study of interictal emotionality and the effects of kindling and human epilepsy on emotional behavior are the primary focus of investigators and not vice versa. Respectively, the interrelation between emotional and seizure reactions was studied in Wistar albino rats. In our study we tried to elucidate: can emotional behavior evoked by stimulation of the emotiogenic zones of the hypothalamus or of induction of acute pain stress modify manifestations of generalized seizures within the period where a “full” epileptic syndrom has been stable formed earlier? Our leading hypothesis is as follow: the emotional disturbances can be considered as the emergence of instinctive behavior with an adaptive significance of defense and as a by-product of the inhibitory processes that build up to protect against the future occurrence of seizures.