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.
At present, a number of forms of epileptic attacks are intractable (not sensitive to pharmacological treatment). The search for alternative possibilities for therapy of such disorders motivated ones to study such “antiseizure” approaches as electrical stimulation of afferent nerves and/ or profound structures of the brain. Stimulations of the cerebellum [
Several studies have indicated that thalamic nuclei are involved in seizure development and expression [
Despite its strategic position in thalamocirtical neural circuity, it is unknown whether TRN is involved in the generalization and expression of limbic seizures. The main objectives were twofold: first, to determine the relations between neurons of the TRN and relay nuclei of the thalamus; because our recent data [
Experiments were carried out on adult cats (n = 17). Under ether anesthesia craniotomy was performed and steel bipolar electrodes with factory insulation were used to stimulate TRN or the neocortex. TRN and neocortex was stimulated by single or short series of stimuli. After insertion of the electrodes the administration of ether was stopped and the animals were immobilized by intravenous injection of tubocurarine and artificially ventilated. The experiments began 1.0 - 1.5 h after administration of ether had ceased. Small doses of nembutal (5 to 8 mg/kg) were additionally injected when necessary. All wound surfaces were periodically treated with a 2% lidocaine solution. Unit responses both from thalamic ventrolateral nucleus (VL) and lateral geniculate body (LGB) and from TRN were derived by glass (tip diameter 1.0 to 1.5 µm; resistance about 8 ± 1, 5 - 2 MΩ) or metal microelectrodes (tipdiameter 10 to 15 µm, with a resistance of between 15 and 40 MΩ. The glass microelectrodes werefilled with potassium or sodium citrate solution (3M). The microelectrodes were directed stereotaxically as they were passed through the neocortex. In some experiments unit activity was recorded simultaneously from TRN and VL or lateral geniculate body (LGB). In the latter case the microelectrode was inserted into TRN at an angle.
Simultaneous recordings were made expracellularly in the VL and TRN.
In experiments, with an aim to record the TRN neuronal activity, the surface of the anterior suprasylvian gyrus and the white matter were removed by suction until exposing the head of the caudate nucleus. The microelectrodes were then lowered through the head of the caudate nucleus in order to reach the rostral section of the TRN. Pentobarbital, in a dose of 0.5 - 12 mg/kg, was injected intravenously. Relay units of the VL and/or LGB were identified by generation of antidromic spikes after stimulation of the neocortex; these spikes were characterized by a stable latency and reproduced high (300 - 400 Hz) stimulation frequencies. In some experiments, we used a multilevel amplitude discriminator, which allowed us to separately record spike activity by a single microelectrode from a few single neurons. The location of the insulated tips of the stimulating electrodes was also established in frontal sections.
Data analysis was performed with GNU PSPP software version 0.6.2 using the methods of descriptional statistics, paired 2-tailed t-test (α < 0.05) and corellation computing (Pearson r).
Experiments were carried out on adult cats (n = 3). Activity of 67 VL neurons (14 intracellularly, three quasiintracellularly, the rest extracellularly) and LGB neurons (nine intracellularly, the rest extracellularly) was studied. Under the influence of stimulation of TRN 39% of the 67 VL neurones were inhibited, 31% were excited, and the remainder did not respond to stimulation. The single stimulation of TRN led to the appearance of inhibitory postsynaptic potentials (IPSPs) of varied duration (80 to 400 ms) and with different latent periods (1 - 2 to 12 ms) in LGB neurons. Similar results were obtained when unit activity was recorded in VL. The IPSP arising in VL and LGB neurons in response to single stimulation of TRN was sometimes repeated several times and became, as it were, rhythmic in character,
Experiments were carried out on adult cats (n = 4). Activity of 78 pairs of TRN and VL neurons and of 39 pairs of TRN and LGB neurons was studied in this series of experiments. In the first case reciprocity between TRN and VL neurons was observed in 42 pairs (53.8%), in the second case (TRN and LGB) in 19 pairs (48.7%). In 16 TRN-VL neuron pairs and in 8 TRN-LGB neuron pairs no visible signs of interaction were found. 13 TRN-VL neuron pairs and seven TRN-LGB neuron pairs were simultaneously excited, whereas activity of seven TRNVL and five TRN-LGB neuron pairs was simultaneously inhibited.
The interaction of 21 pairs of TRN and VL neurons in respond to different frequency cortical stimulation is shown in
When single and low-frequency afferent or cortical stimulation (in most cases cortical stimulation was used, as it is know [
Experiments were carried out on adult cats (n = 3). Responses of single units in the TRN and VL nuclei were studied in aucte experiments on curarized cats before and after intravenous injection of small (5 - 8 mg/kg) doses of pentobarbital, with simultaneous derivation of activity by two microelectrods,
The study of spontaneous activity of TRN and VL neurons showed desynchronisation of the EEG accompanied by periods of asynchronous activation. The mean spontaneous discharge frequency of TRN neurons was a little higher than that of VL neurons,
Experiments were carried out on adult cats (n = 3). Hippocampal induced epileptiform activity may be a more suitable model for the epilepsy studies because of its prolonged and progressive development period without severe clinical manifestations and with only focal electrical discharges in evidence as well as with development of epilepsy and secondary generalization [
Recently, we showed that the development of kindling induced by hippocampal stimulation can be blocked by simultaneous stimulation of the TRN. The latter stimulation was capable of blocking generalization of the seizures when two different paradigms of kindling were used. Thus, it is believed that the inhibitory influence of stimulation of the TRN does not depend on the type of stimulation of the hippocampus. When discussing other possibilities, we postulated that blocking of the seizure reactions induced by stimulation of the thalamic TRN can result from potentiation of the activity of GABAergic inhibitory neurons of the latter nucleus.
Experiments were carried out on adult cats (n = 4). We studied plastic changes in the activity of VL thalamic neurons after stimulations of the TRN. Those neurons (VL) whose activity was inhibited after single TRN stimulations were examined.
Testing was performed in the following way. First, we applied single-pulse stimulations of the TRN, which evoked clear suppresion of the background activity of the VL neurons (
Among 112 examined neurons of the VL, the effects of facilitation of the inhibitory reactions induced by TRN stimulation were preserved for a rather long time interval in 97 units (86.6%). The time segment where potentialtion of inhibition was obvious varied in different VL neurons from 5 to 35 min, and in 15 neurons was this effect weak or absent (
Intraor quasi-intracellular recordings from relay neurons of the VL showed that inhibitory postsynaptic potentials (IPSPs) evoked in these neurons by single stimulations of the TRN are siglificantly facilitated both in the course of conditioning tetanic stimulation of the above nucleus by high-frequency series and for a rather long time after such stimulation. Single stimulation of the TRN with a near-threshold intensity evoked low-amplitude IPSPs in the studied neurons, and their background spike activity was suppressed for 200 to 400 msec (
was several times greater than that at single-pulse TRN stimulations (
Relationships between neurons of the TRN and specific nuclei of the thalamus were studied. Under the influence of stimulation of the TRN unit activity in the thalamic relay nuclei was found to be considerably modulated. Cases of the appearance of IPSPs (possibly of monosynaptic nature), evoked by stimulation of TRN, in neurons of the VL and LGB are described. During simultaneous recording of unit activity in TRN and VL or LGB by means of two electrodes interaction of several types was found: first, inhibition of discharges of VL or LGB neurons accompanied by excitation of TRN neurons; second, alternation of excitation-inhibition in neuron pairs in TRN and VL or LGB during low-frequency afferent or cortical stimulation. In this case excitation of TRN neurons is associated with inhibition of VL or LGB neurons.
The results are in agreement with those of early investtigations by other workers who found that TRN neurons have an inhibitory action on neurons in other parts of the thalamus. Purpura and Cohen [
The experimental results showed that after intravenous injections of small doses of pentobarbital the appearance of spindle activity in the EEG was accompanied by activation of most TRN neurons; this was manifested as the appearance of high-frequency grouped (52.5%) or highfrequency prolonged continuous (30%) discharges throughout the period of recording of spindles. Generation of high-frequency discharges by TRN neurons could coincide with an inhibitory pause in activity of VL neurons, and on that basis it might be supposed that inhibition of activity of VL neurons was caused by discharges from TRN neurons.
What are the mechanisms of the increased activity of TRN neurons?
General anesthetics are known [18,19] to inhibit synaptic transmission at all levels in the central nervous system. It is claimed [
Desynchronization of the EEG, caused by stimulation of mesencephalic reticular formation (MRF), is known to be accompanied by facilitation of the transmission of excitation through thalamic nuclei. Intracellular studies [
Our experiments support the hypothesis that the activity of inhibitory neurons of the TRN is potentiated in the course of stimulation of this structure. This effect is longlasting, and inhibition of generalization of the seizures can be based on this phenomenon.
Long-lasting plastic changes in the efficacy of synaptic transmission were earlier studies in most cases in excitatory synaptic connections in the central nervous system. Analogous studies of the inhibitory synaptic transmission are rather less numerous despite the fact that such a phenomenon per se (potentiation in inhibitory central synapses) was demonstrated long ago by Sherrington [
Our findings, of course, do not allow us to propose strict single-valued conclusions on synaptic mechanisms of blocking of motor seizures provided by stimulation of the TRN. At the same time, results of the above-described experiments allow us to believe that such a stimulation induces long-lasting plastic modifications in GABAergic synapses formed by axons of the TRN neurons, in particular, in synapses on neurons of the relay thalamic structures.
The stimulation of the profound cerebral structures has been extensively used in attempts to influence pathological central phenomena both in experiments and clinics. It is obvious that such stimulation excites all neuronal elements (axon, axon terminals and/or cell somata), which are localized at a sufficiently short distance from the electrode. As is known, the excitability of the axons and axon terminals is in general higher than that of the neuronal somata [
Our experiments have shown that stimulation of the TRN effectively suppresses limbic motor seizures. This suppression is obviously related to the process of potentiation in GABAergic neurons of the TRN. 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 treatment of temporal lobe epilepsy.
The data generated in this study indicated: 1) The stimulation of the TRN evoked monosynaptic IPSPs in the thalamic relay neurons; 2) Cortical single and/or rhythmmic stimulation evoked axcitation of the TRN neurons and inhibition of the thalamic relay neurons; 3) After injection of pentobarbital excitation of the TRN neurons corresponded to a period of silence of the thalamic relay neurons; 4) The activity of TRN neurons was significantly enhanced during the silence within the seizure episodes; 5) This suppresion is obviously related to the process of potentiation in GABAergic synapses of thalamo-cortical neuronal networks.
In addition, our data raise the possibility that stimulation of TRN might be of value as a new anticonvulsive approach in patients with intractable temporal lobe epilepsy.