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/92/ Plenum Publishing Corporation NEUROBIOLOGY OF INTEGRATIVE BRAIN ACTIVITY

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NEUROBIOLOGY OF INTEGRATIVE BRAIN ACTIVITY LAYER-BY-LAYER ANALYSIS OF THE COMPONENTS OF THALAMOCORTICAL RESPONSES OF THE SENSORIMOTOR CORTEX IN THE RABBIT DURING ONTOGENESIS I. A. Shimko UDC /
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NEUROBIOLOGY OF INTEGRATIVE BRAIN ACTIVITY LAYER-BY-LAYER ANALYSIS OF THE COMPONENTS OF THALAMOCORTICAL RESPONSES OF THE SENSORIMOTOR CORTEX IN THE RABBIT DURING ONTOGENESIS I. A. Shimko UDC / The complex dynamics of the changes in the spatial-temporal disposition of the heterocomponent thalamocortical responses (TCR ) when traversing the sensorimotor cortex (SMC), are governed by the characteristics of the electrogenesis of each of the components of the TCR, the age of the animal, and the frequency of stimulation of the ventroposterolateral (VPL) nucleus of the thalamus. At the same time, the transformation of the electrical profile of the second positive component (PC-2) of the TCR during traversal of the SMC may suggest the algebraic summation in this component of the bioelectrical processes of several sources of generation (of the inverted and noninverted PC-2). The characteristics of the ontogenetic dynamics of the profiles of the biological fields of the third negative component (NC-3) of the TCR which were found may be the result of age-related stages of development and of a change in the embryonal mechanisms of electrogenesis to the definitive mechanisms in the presence of outwardly similar negativities. Data were obtained in our preceding investigations regarding a significant increase in the amplitude of various components of the thalamocortical responses (TCR) of the sensorimotor cortex (SMC) in the critical period commencing after the animals become able to see [11]. At that age (rabbits in the second to third weeks of postnatal life), a greater degree of potentiation of the TCR was observed during the activation of emotiogenic zones of the hypothalamus [12], by which means the substantial expansion of adaptive possibilities of the developing organism which are realized in the process of the formation of various behavioral acts [3, 5, 6] is determined; a greater functional sensitivity of the thalamocortical connections in the system of the motor analyzer to modulator influences of psychotropic agents [13] was also observed. There are a substantial number of fundamental ontogenetic studies in the literature devoted to the investigation of the establishment and functional development of the central element of the motor analyzer by means of the technique of the recording of evoked potentials (EP) of the SMC during the electrical stimulation of peripheral nerves or the stimulation of receptor apparatuses [2, 4, 7]. At the same time, the methodologically more laborious, but more informative investigations involving the recording of the TCR of the SMC are few in number [15-17]. This relates in particular to studies in which a layer-by-layer analysis of the TCR or the EP of the SMC in ontogenesis was carried out [14, 17]. At the same time, such an analysis of the component composition of the TCR of the SMC in ontogenesis appears to be most urgent, since it makes it possible to follow the age-related dynamics of the laminar profiles of the bioelectrical fields of the TCR of the SMC, which is important for the understanding of the electrogenesis of certain phases of the TCR of the SMC and its age-related patterns. METHODS The experiments were carried out on 17 chinchilla rabbits, aged from 14 days to two months after birth, which were subdivided into four age groups: days (five animals), (four), days (three), and two months (five). The removal of the soft tissues of the skull of the infant rabbits was carried out under n~wocaine anesthesia. Bipolar, finely honed electrodes of Nichrome wire, 100 I~m in diameter, and at an interelectrode distancc of 0.5 mm, were implanted according to coordinates of stereotaxic atlases for young [1] and adult [9] rabbits into the venlxoposterolateral (VPL) nucleus of the thalamus (AP ; L ; H , taking age into account). The correctness of the position of the stimulating electrodes in the VPL was monitored by photographic prints of frozen sections of the brain {Fig. 1). The contact of a Institute of the Brain, Russian Academy of Medical Sciences, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyaternosti imeni I. P. Pavlova, Vol. 41, No. 3, pp , May-June, Original article submitted April 23, 1990; revision submitted January 9, /92/ Plenum Publishing Corporation Fig. 1. Layer-by-layer analysis of thalamocortical responses in a rabbit at the age of 14 days. I) Change in the configuration of the TCR at different levels of the SMC at a frequency of stimulation of the VPL of 0.2 Hz. II) Rhythm assimilation reaction (RAR) of a heterocomponent TCR under conditions of traversal of the SMC. wh~... lower track on the O.~hlogram~, ~,-.'l ~ TCR on the surface of the cortex; upper track, at the depth of advancement. Numbers on the left, the depth of advancement of the pickup electrode from the surface of the cortex, mm. Frequency of stimulation of the VPL in A, 0.2 Hz; in B, 80 Hz. (+)1, PC-l; (-)2, NC-2; (+)3, PC-3; (+)3i, inverting PC-3; (-)3, (NC-3); (-)3i, inverting NC-3; (+)2, noninverting PC-2; (+)2i, inverting PC-2. In the inset below AVSh, map of atlas [1] for rabbits at the age of days; 14, 17) morphocontrol in rabbits at the age of 14 and 17 days, respectively. Calibration: 400 ~tv, 10 msec. 270 Fig. 2. Layer-by-layer analysis of thalamocortical responses in a two-month old rabbit. Changes in the TCR as the electrode is advanced into the depths of the SMC at a frequency of stimulation of the VPL of 0.2 Hz. The upper track on the oscillograms, TCR on the surface of the cortex; lower track, at the depth of advancement. Numbers on the left, the depth of advancement of the pickup electrode from the surface of the cortex, mm. Calibration: 600 ~tv, 40 msec. / (+) 2 E Z u VPL Fig. 3. Diagram of the possible organization of the cortical generators of additional components of the TCR of the SMC. Roman numerals on the right, layers of the neocortex (explanation in the text). Other designations as in Fig. 1. monopolar steel electrode (tip diameter 50 gm) with the inner table of the parietal bone was accomplished through an opening placed above: the focus of maximal activity in the SMC according to the following coordinates: AP -3; AP +3; L , depending upon age and individual variability. The implanting of the microfeeds (interval of 1--2/am) of the second monopolar electrode into the SMC (insulated except for the face, finely honed Nichrome wire, 100 gm in diameter), was accomplished through an opening placed 1 mm medial to the first pickup electrode. The indifferent electrodes were attached to the frontal bone. Solitary rectangular impulses of current, duration 0.1 msec, exceeding the threshold by a factor of 4 (2--6 V; ma), with a stimulation frequency ranging from 0.2 to 80 Hz, were delivered from an I~SU-1 stimulator for the stimulation of the VPL. A stimulus eliciting the minimal TCR was taken as threshold. The EP were recorded 4-5 h following the operation by means of an S1-18 oscillograph through a pre-amplifier with a 10 to 800 Hz pass band. The data obtained were analyzed by memas of parametric statistical techniques. INVESTIGATION RESULTS The facts established indicate that the complex dynamics of the spatial-temporal picture of the bioelectrical fields creating the TCR of the SMC depend at least on three factors: the electrogenesis of each of the components of the TCR, the age of the animal and the frequency of stimulation of the VPL. The first positive component (PC-l) of the TCR as it traverses the SMC increases its amplitude in the rabbits of all of the age groups (Fig 2). If the maximal value of the PC-1 is (p 0.001) gv in 14- to 15-day-old animals in the range of implantation depth of gm (p 0.001), it reaches gv (p 0.001) at the lower boundary of the SMC at a depth of _ gm (p 0.001). The PC-1 is the shortest-latency component of the TCR which is capable of assimilating a very high rhythm of stimulation of the VPL, 80 Hz (Fig. 1B). Such characteristics of the first positive component of the TCR confirm the well-known notions regarding the fact that the PC- 1 reflects action potentials arising in rapidly conducting thalamocortical fibers below the lower boundary of the cortex (Fig. 3). 271 The gradual advancing of the reference electrode into the depths of the SMC is coupled with a progressive decline in the amplitude of the PC-2: IxV (p 0.001) at the surface of the SMC; ~tv (p 0.002) at a depth of p.m (p 0.001) (Fig. 1). However, this decrease is accompanied not only by an increase in the amplitude of the second negative component NC-2 from ~tv (p 0.001) at the surface of the SMC to p.v at a depth of ~tm (p 0.001), but also by an elevation above the isoline by ~tv (p 0.05) without inversion of the PC-2 peak at a depth of ~tm (p 0.001, Fig. 1). The complex picture of the transformation of the electrical profile of the second positive component, PC-2, of the TCR as it traverses the SMC apparently may suggest the algebraic summation in this component of the bioelectrical processes of several sources of generation. The possibility that the PC-2 consists of two potentials coinciding in time, an inverting and a noninverting PC-2, cannot be excluded. In fact, the combination of the process of the decrease in the amplitude of the PC-2 during traversal of the SMC with a rise without inversion of the PC-2 peak above the isoline, as well as with a synchronous increase in the amplitude of the NC- 2, may be associated with a summation, starting at a depth of ~tm (p 0.001), of the independent effect of a decrease in the amplitude of a noninverting PC-2 with another effect, autonomous with respect to electrogenesis, of a gradual increase in the amplitude of the inverting PC-2. The validity of such a point of view is confirmedby a specially conducted series of experiments involving variation of the frequency of stimulation of the VPL (Fig. 1). It can be seen in Fig. 1 in the B columns of the oscillograms that an increase in the frequency of stimulation of the VPL up to 80 Hz leads to the disappearance (depression) of all components of the TCR except the PC-1 and the PC-2 (the PC-l, as noted above, is especially well seen at the lower limit of the cortex). At the same time, the monophasic surface-positive PC-2 (the lower track of the oscillograms, column B) inverts its polarity during stimulation of the VPL at 80 Hz at a depth of 690 ~tm from the surface of the SMC (upper track of the oscillograms, column B). However, a noninverting PC-2 is still clearly visible at this implantation depth during stimulation of the VPL at 0.2 Hz (upper track of the oscillograms, column A), and disappears only at a depth of 710 ~m (Fig. 1). Also, the fact of the recording of a noninverting PC-2 (0.2 Hz) at a depth of 690 ~tm in combination with the inverting PC-2 (80 Hz) may be an additional argument suggesting the existence of two PC-2 coinciding in time and summated. The third positive component PC-3 decreases in all of the investigated age groups with respect to amplitude during the advancement of the pickup electrode into the SMC, and inverts its polarity at a depth of _ ~tm (p 0.001) from the surface of the SMC (Figs. 1 and 2). The PC-3 assimilates the frequency of the stimulation of the VPL only in the range of Hz. When the frequency of stimulation of the VPL is further increased, depression and disappearance of the PC-3 is observed. The electrogenesis of the surface PC-3 is apparently associated, according to the belief of the majority of investigators, with a dipole inversion of the EPSP of the neurons of layers III-IV of the SMC. The dynamics of the changes in the pattern of the third negative component (NC-3) of the TCR of the SMC during its passage in animals of the second to third weeks of life are not identical with that found in adult animals. The NC-3 of the TCR in animals of the second to third weeks of life when the pickup electrode is advanced into the depths of the SMC inverts its polarity; the degree of inversion in 14- to 15-day-old animals is greater than in the 23- to 24-day-old animals. The latent period of the NC-3 does not change in the process. By contrast with this, the corresponding component of the TCR in adult animals under the conditions of traversal gradually decreases the latent period without inversion by msec (p 0.001), until it fuses with the inverting component (PC-3) at a depth of (p 0.001) (Fig. 2). The identified age-related characteristics of the dynamics of the profiles of the bioelectrical fields of the NC-3 of the TCR are apparently associated with a change in the mechanisms of the electrogenesis of outwardly similar negativities at various age-related stages. DISCUSSION OF RESULTS As was shown above, only the PC-1 and the PC-2 of the TCR of the SMC assimilate a high frequency of stimulation of the VPL (80 Hz). This fact may attest to their presynaptic electrogenesis, i.e., the generation of these phases of the TCR by action potentials of the conducting fibers. At the same time, the results of our investigations prompt the thought that the PC-2 of the TCR is not a potential of one source of generation, but evidently represents the algebraic sum of inverting and noninverting PC-2 (Fig. 1). The inverting PC-2 is readily seen during the stimulation of the VPL at a frequency of 80 Hz; it decreases in amplitude as it traverses the SMC, and at a depth of ~tm (p 0.001) from the surface of the cortex, it inverts its polarity. All of the above-indicated characteristics of the inverting PC-2, as well as the fact of the absence of inversion of PC-2 at a depth 272 up to ,.19 lain (p 0.001) may indicate that the inverting PC-2 is generated by small rapidly conducting branches of the thalamocorfical fibers which terminate in layers III-IV of the SMC (the external striae of Baillarger, the Kaes-Bekhterev striae) (Fig. 3). A noninve~rting PC-2 is clearly recorded at a frequency of stimulation of the VPL of 0.2 Hz; it assimilates a maximal frequency of stimulation of Hz, and disappears at a higher frequency. It is decreased in amplitude as it traverses the cortex as the amp]litude of the inverting PC-2, which is visible during the stimulation of the VPL at a frequency of 80 Hz, decreases. However, the noninverting PC-2 can be seen also at the depth of the SMC where the inverting PC-2 changed its polarity, which may also attest to the existence of two components coinciding in time and summating. Based on the above, it can be assumed that the noninverting PC-2 may be generated, like the Forbes response, by a focus of hyperpolarization in the inhibitory neurons of the afferent input which are located in the upper layers of the cortex [8, 10] (Fig. 3). The age-related differences in the dynamics of the changes of the character of the NC-3 of the TCR are of particular interest. A pronounced inversion of the NC-3, without changes in latency, is clearly observed in two to three week old animals during travers~ of the SMC. The ratio of the amplitude of such an inverting potential to the magnitudes of the NC-3 at the surface of the cortex decreased by the end of the first month of life of the rabbits. Another type of changes in this negativity has now been observed in adult animals: the absence of inversion and a shortening of the latency of the NC-3 which led to a gradual summation of the NC-3 with the inverting PC-3 (Fig. 2). These data are in agreement with the results of investigations [14] which describe the inversion of the surface-negative component of the primary response of the SMC during the stimulation of a peripheral nerve in seven-day old rats, and an inversion of this component was not observed in thirty-day old animals even at a depth of 2,000 lim. The authors explain this phenomenon of ontogenesis by an age-related change in the mechanisms of the electrogenesis of outwardly similar negativities; however, they do not provide an answer to the question as to which structures that generate the NC-3 of the SMC may participate in this change. In our opinion the embryonal mechanism of the electrogenesis of the NC-3, in which the embryonal systems of the Martinotti and Cajal-Retsius cell type may participate, operates and gradually disappears in early postnatal ontogenesis along with the establishment and the development of the definitive mechanism of generation of NC-3. It is known that the Martinotti cells, the earliest maturing neurons of the neocortex, are located in the depths of the cortical lamina and have very well developed dendrites with spinous outgrowths, and axons which rise to layer I of the cortex and which form branches in layers IV-V as well. These cells gradually disappear after two to three weeks of postnatal life. Moreover, the hypotheses have been advanced regarding the contribution of EPSP in the soma and dendrites of these neurons to the electrogenesis of the summated negative potential on the surface of the cortex, regarding their corrective role at the early stages of postnatal ontogenesis in the formation of neuronal ensembles, the columnar structures of the projection zones of the neocortex, etc. [4]. CONCLUSIONS 1. The complex dynamics of the layer-by-layer spatial-temporal character of the heterocomponent thalamocortical responses (TCR) of the sensorimotor cortex (SMC) depends on the electrogenesis of each of the components of the TCR, the age of the animal, and the frequency of stimulation of the ventroposterolateral (VPL) nucleus of the thalamus. 2. Only the first (PC-l) and second (PC-2) positive components of the TCR of the SMC assimilate a high frequency of stimulation of the VPL (80 Hz); this may suggest their presynaptic electrogenesis. 3. The complex character of the transformation of the electrical profile of the PC-2 of the TCR during traversal of the SMC may suggest an algebraic summation in this component of the bioelectrical processes of several sources of generation (inverting and noninverting PC-2). 4. The sw, cific characteristics of the ontogenetic dynamics of the profiles of the bioelectrical fields of the third negative component ~FC-3) of the TCR of the SMC which were found may be the result of a change from the embryonal mechanisms of electrogenesis to definitive mechanisms in outwardly similar negativities. LITERATURE CITED 1. A. A. Volokhov and N. N. Shilyagina, Stereotaxic atlas of the brain of young rabbits, Zh. Vyssh. Nervn. Deyat., 16, No. 1, (1966). 2. A. A. Volokhov and I. A. Shimko, The significance of the activation of various groups of muscle afferents in the formarion of evoked potentials of the motor cortex of rabbits in ontogenesis, Zh. Vyssh. Nervn. Deyat., 22, No. 1, (197:2). 273 3. A. A. Volokhov, The development of the nervous system in early age, in: Handbook of Physiology. Age-Related Physiology [in Russian], Nauka, Leningrad (1975), pp E. V. Maksimova, The Functional Maturation of the Neocortex in Prenatal Ontogenesis [in Russian], Nauka, Moscow (1979). 5. G. M. Nikitina, The Formation of lntergal Activity of the Organism in Ontogenesis [in Russian], Meditsina, Moscow (1971). 6. G. A. Obraztsova, Problems of the Ontogenesis of Higher Nervous Activity [in Russian], Nauka, Moscow, Leningrad (1964). 7. V. V. Raevskii, The Formation of the Cortical Component of the Alimentary Functional System in Early Ontogenesis, Abstract of Dissertation, Candidate of Medical Sciences, In-t Fiziologii Detei i Podrostkov APN SSSR, Moscow (1970). 8. V. M. Storozhuk, Synaptic potentials of the somatic cortex, Zh. Vyssh. Nervn. Deyat., 26, No. 4, (1976). 9. E. Fifkova and G. Marshall, Stereotaxic atlas of the brain of the cat, rabbit, and rat, in: Electrophysiologic Research Methods [Russsian translation], IL, Moscow (1962). 10. R. A. Chizhenkova, The Structural-Functional Organization of the Sensorimotor Cortex [in Russian], Nauka, Moscow (1986). 11. I. A. Shimko, The age-related dynamics of thalamocortical evoked potentials in the rabbit in the early period of life, Zh. Vyssh. Nervn. Deyat.,
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