Electrophysiologic research on the afferent connections of embryonic neocortex allografts inserted into the projection zones of the cortex in adult rats

Grafts of the rat fetal neocortex (the 17th-18th day of gestation) were placed into the cavity made by aspiration in the primary visual or somatosensory cortex of adult rats. Electrophysiological studies performed 3-3.5 months later showed that approximately in 50% of animals the neurons of the transplants responded to sensory stimuli of modality specific for cortical regions replaced by the transplant. These responses were elicited from local receptive fields that in some animals revealed topographic organization. Neuronal responses of transplants were elicited by local electrical stimulation of thalamic nucleus projecting to the cortical site of grafting, as by stimulation of contralateral homotopic cortical areas. Latency and temporal patterns of neuronal responses were similar to normal ones. Thus it may be concluded that afferent inputs to the cortical transplants retrace normal cortical inputs. The possible mechanisms of re-innervation of the grafts are discussed.     

Heterotopic neocortical transplants. An anatomical and electrophysiological analysis of host projections to occipital cortical grafts placed into sensorimotor cortical lesions made in newborn rats.

Plates of presumptive occipital neocortex obtained from fetal rats at 14-16 days gestation were grafted into the cerebral hemisphere of newborn rats. The transplants were placed heterotopically into sensorimotor cortical lesion cavities made immediately prior to grafting. At maturity, some of the transplants were injected with the retrograde fluorescent tracers Fast Blue and Diamidino yellow. In other animals, single-unit activity in the transplants or in normal cortex was recorded using standard electrophysiological techniques. Histologically, host projections to the transplants were demonstrated by the presence of retrogradely labeled neurons in the host primary and secondary somatosensory cortices as well as several thalamic areas including the anteroventral, anteromedial, ventrobasal, mediodorsal and central medial nuclei. Additional labeling was found in the claustrum, lateral hypothalamus, zona incerta and basal forebrain. Electrophysiologically, transplant single-unit activity was evoked in 43/69 (62%) neurons by thalamic stimulation, but only 1/69 transplant neurons responded to electrical stimulation of the contralateral forepaw. In further work, volumetric measurements showed that the transplants did not ameliorate the thalamic atrophy found after neocortical lesions. These results are compared to previous studies involving the homotopic placement of sensorimotor cortical grafts.     

Changes in the functional characteristics of cells of the Clare-Bishop zone after unilateral partial isolation of the cat neocortex

Evoked single cell activity in the Clare-Bishop area of the cat neocortex was recorded after partial transection of subcortico-cortical connections. Both functional properties of visual, auditory and somatosensory responsive cells and their responses to electrical stimulation of the primary projection area were studied. A week after transection some neurons in the Clare-Bishop area on the operated site recovered their reactions to applied stimuli. Within the first week the sensitivity of the neurons in the intact hemisphere changed; all the recorded cells reacted to light; 80% of the cells had receptive fields that were situated in the contralateral part of the visual area; 62% of the cells reacted to somatosensory stimuli. A week after the operation the number of responsive cells decreased. The peculiar features of the neuronal compensatory reorganization in the Clare-Bishop area and its role in the recovery of visual function is discussed.     

Retinal afferents innervate functionally tectal but not neocortical grafts placed in lesioned superior colliculus of adult rats

Solid pieces of tectum or occipital neocortex derived from 17-day rat fetuses were placed over the lesioned right superior colliculus (SC) in adult rats as sheets retaining the internal structure of the embryonal tissue. The upper laminae of the recipient's SC (approximately up to stratum opticum) were removed by aspiration after the neocortex overlying the SC was aspirated out. Two to 5 months after the operation a microelectrode study of the neuronal electrical activity in the grafts was performed. Recordings from the tectal transplants revealed normal patterns of the spontaneous neuronal activity in all grafts wad clear neuronal reactions to visual stimuli in a large portion of them (6 out of 11). Visual reactions in these grafts were recorded from the majority of studied neurons(185/226). The properties of the receptive fields as well as the range of latencies of the reactions corresponded to those characteristic of the normal SC. Topographic representation of the visual field upon the transplants was found. Recordings from the cortical grafts showed an abnormal character of the spontaneous neuronal activity and the absence of reactions to any sensory stimulation of the recipients. The data obtained suggest that regenerating optic axons in adult hosts retain specificity in functional innervation of only appropriate target neurons and can re-establish the topographic representation of the retina upon tectal grafts. Retinal afferents innervate functionally tectal but not neocortical grafts placed in lesioned superior colliculus of adult rats.     

The comparative characteristics of the neuronal activity in surviving slices of transplanted and intact neocortex

The background single unit activity and single or multiunit responses to electrical stimulation of the rat's embryonic cortex, grafted into the somatosensory cortex of adult rats were investigated in a slice preparation. The same characteristics of the intact cortex neurons were observed as a control. The percentage of neurons with background activity within grafts was higher, than that within intact neocortex slices (23% and 6%, respectively). However the percentage of neurons responding to electrical stimulation of the recipient neocortex was lower in grafts. The mean latency of multiunit responses of graft neurons was longer, than that of neurons of the intact neocortex recorded at the same distance from the stimulating electrode (19.4 +/- 5.0 and 5.8 +/- 1.1 ms, respectively). Duration of the evoked population discharges was approximately ten times longer in grafts. It is concluded that there are local functional connections between a graft and the host brain and that inhibitory processes within grafts are weak. Functional integration of grafts with the host brain of the recipients after cranial trauma is shown.     

Long-term depression is induced in Ca2+/calmodulin kinase-inhibited visual cortex neurons.

To elucidate a role of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in induction of long-term potentiation (LTP), KN-62, a selective inhibitor for CaMKII, was injected into layer 2/3 neurons of sliced visual cortex obtained from young rats. Tetanic stimulation (5 Hz, 1 min) applied to the white matter after the KN-62 injection induced long-term depression (LTD) of excitatory postsynaptic potentials (EPSPs) evoked by test stimulation of the white matter in 9 of the 14 cells tested. However, EPSPs evoked by test stimulation of the non-tetanized site were not changed, indicating that the induction of LTD was input-specific. Simultaneously, recorded field potentials which were derived from neurons with intact CaMKII showed LTP. These results suggest that postsynaptic CaMKII plays a role in the induction of LTP/LTD in visual cortex.     

Fetal frontal cortex transplanted to injured motor/sensory cortex of adult rats. II. VIP-, somatostatin-, and NPY-immunoreactive neurons

Fetal frontal cortex transplants that survived 2-9 months in cavities in adult rat motor/sensory cortex were processed for vasoactive intestinal polypeptide (VIP), somatostatin 14 (SS), and neuropeptide Y (NPY) immunocytochemistry, and NADPH-diaphorase (NADPH-d) histochemistry. All transplants had surviving VIP, SS, NPY, and NADPH-d neuronal perikarya and fibers with normal adult morphology. The number of peptidergic neurons within transplants, however, often appeared to be less than that in equivalent areas of host cortex. Most transplanted SS and VIP neuronal perikarya did not migrate to form the laminae characteristic of normal cortex. A few transplants had SS and VIP cells arranged in laminae in which the VIP processes were parallel to one another and perpendicular to one transplant surface, approximating normal host neocortex. VIP, NPY, and SS fibers crossed between host brains and transplants, suggesting that peptide host-transplant interactions are possible. All adult host cortical and most transplanted NPY neurons colocalized with NADPH-d. The failure of some transplanted NPY neurons to express NADPH-d suggests these transplanted cells may be functionally impaired, but that they can survive without the NADPH-d enzyme.     

Functional influences of the visuocortical projection on superior colliculus neurons in the rabbit

The effects of electrical stimulation of the visual cortex on superior colliculus neurons were investigated in adult Dutch-belted rabbits. Single units were recorded in the superior colliculus and classified as to receptive field type. Stimulation of the ipsilateral visual cortex activated 29% of the recorded superior colliculus units. No units were driven by stimulation of the contralateral visual cortex. Comparison of the relative proportional distributions of cortically driven and not driven cells having various receptive field types revealed an over-representation of driven motion type cells. The excitatory influence of the visuocortical projection to the superior colliculus in the rabbit shows a preference for neurons responsive to moving visual stimuli.     

Fetal frontal cortex transplant (14C) 2-deoxyglucose uptake and histology: survival in cavities of host rat brain motor cortex

Fetal frontal neocortex from 18-day-old rat embryonic brain was transplanted into cavities in 30-day-old host motor cortex. Sixty days after transplantation, 5 of 15 transplanted rats had surviving fetal transplants. The fetal cortex transplants were physically attached to the host brain, completely filled the original cavity, and had numerous surviving cells including pyramidal neurons. Cell lamination within the fetal transplant was abnormal. The (14C) 2-deoxyglucose uptake of all five of the fetal neocortex transplants was less than adjacent cortex and contralateral host motor-sensory cortex, but more than adjacent corpus callosum white matter. The results indicate that fetal frontal neocortex can be transplanted into damaged rat motor cortex. The metabolic rate of the transplants suggests they could be partially functional.     

Integration of neocortical embryonal grafts with the neocortex of host rats examined by Leao's spreading cortical depression

Cortical spreading depression (SD) was used to assess the density and organization of neural elements in neocortical transplants and their connectivity with the host brain. Embryonal neocortex (E14) was transplanted into cavities in the frontoparietal cortex of 3-month-old rats. SD elicited in the cortex of anesthetized host rats (n = 12) 3 to 8 months after transplantation did not penetrate into the grafts. SD could be elicited in large transplants but did not propagate to the surrounding host neocortex. Spontaneous unit activity in the transplants was affected by SD elicited in the neocortex of the host rats anesthetized with urethane. Most units (n = 49) displayed excitatory-inhibitory (52%) or inhibitory (29%) reactions, whereas purely excitatory reactions were less frequent (8%). The results suggest that the packing density of neurons in the transplant can support SD but that the conditions at the graft-host boundary (glial scar, scarcity of neurons) stop SD propagation. High reactivity of the graft neurons to SD in the host neocortex indicates that afferentation from the host brain represents an important, predominantly excitatory contribution to the spontaneous activity of the transplant.     

Expansion of the ipsilateral visual corticotectal projection in hamsters subjected to partial lesions of the visual cortex during infancy: Electrophysiological experiments

Single unit recording from cells in the superior colliculus ipsilateral to the damaged hemisphere in hamsters subjected to unilateral removal of a part of the posterior neocortex during infancy was combined with electrical stimulation of the cortical remnant and the visual cortex in the undamaged hemisphere. Cells activated by stimulation of the cortical remnant were recorded in all portions of the colliculus. No differences in percentages of driven cells or threshold current intensities were noted between electrode penetrations in which collicular neurons having receptive fields within the remaining visual cortical representation were recorded and tracks where units with receptive fields outside this region were isolated. In the medial part of the tectum ipsilateral to the damaged hemisphere cells driven by stimulation of either cortex were encountered. It was also demonstrated that stimulation of the ipsilateral cortical remnant and/or the contralateral cortex was capable of suppressing discharges normally elicited by optic chiasm or visual stimulation in a manner qualitatively similar to that observed for collicular cells in normal hamsters. The response properties of cells functionally influenced by the ipsilateral and/or contralateral corticles were not different from those of neurons which received no demonstrable cortical input. The receptive field characteristics of the sample of neurons recorded were, on the whole, quite similar to those of collicular neurons in hamsters subjected to lesions of the visual cortex as adults.     

Morphological damage to the premature fetal rat brain after acute carbon monoxide exposure

For most brain regions the responses of neurons to stimulation of the primary visual cortex (CTX) are unknown. The objective of this study was to determine how neurons (single-unit activity) in major limbic, thalamic, hypothalamic, and visual regions respond to electrical stimulation of the CTX. Cats underwent low cerveau isolé surgery (isolate forebrain) and electrode implantation. Unit activity was analyzed as action potentials recorded extracellularly under control (spontaneous firing rate) and experimental (poststimulus rate) conditions. A total sample of 2055 units were localized anatomically. The effect of CTX stimulation was primarily inhibitory in the initial 500-ms poststimulus responses. Only 20 cells (<1%) showed initial excitatory responses. Highly responsive regions included areas 17, 18, 19 and the hippocampal formation. Subcortical regions which showed marked (P < 0.01) inhibitory responses form part of the ascending reticular system (anterior, intralaminar, and reticular thalamic nuclei, and tegmental nucleus). Similar effects were also observed in the anteromedial hypothalamus. Only two regions showed significant (P < 0.05) prolonged responses (1 to 5 s) to CTX stimuli. One of them, the prostriatal cortex, exhibited increases in firing rates. The other, the cingulate cortex, showed inhibitory initial and prolonged responses. Therefore, information provided by CTX stimulation is distributed to different limbic and subcortical targets. We suggest that the mechanisms responsible for the effects of visual stimuli on arousal and emotion may be mediated by inhibition of ascending reticular pathways.     

Synaptic Evoked Potentials from Regenerating Dorsal Root Axons within Fetal Spinal Cord Tissue Transplants

Previously injured dorsal roots were electrically stimulated to determine if regenerating sensory axons can form physiologically active synaptic contacts with neurons within fetal spinal cord tissue transplants. Dorsal rootlets, sectioned at their spinal cord entry zone, were apposed to intraspinal transplants of fetal spinal cord tissue grafted along each side of a nerve growth factor-treated nitrocellulose implant. Two to six months later, the rootlets were transected between the spinal cord and their respective ganglia and electrically stimulated. Evoked potentials were recorded from the dorsal surface of the transplant, but were absent from adjacent ipsilateral and contralateral spinal cord regions. A glass micropipette was advanced through the transplant and used to record intramedullary field potentials evoked by dorsal root stimulation. Maximal negative potentials occurred 400–700 μm below the dorsal surface of the transplant, shifting to positive potentials deeper into the transplant. Additionally, both spontaneous and electrically evoked single neuronal action potentials were observed along the microelectrode track. Evoked potentials were abolished following transection of the rootlets between the stimulation site and the transplant. Immunocytochemical evidence of the production of fos protein following electrical stimulation of the regenerated dorsal rootlets was demonstrated within transplant neurons and some ventrally located host neurons, providing an anatomical correlate to the electrophysiological recordings of synaptic activation. These results provide evidence of the structural and functional integration of regenerated sensory axons with both transplant and host neurons.     

Electrophysiological study of synaptic connections between a transplanted lateral geniculate nucleus and the visual cortex of the host rat

The lateral geniculate nucleus (LGN) of a fetal rat was transplanted to the visual cortex (VC) of a neonatal rat. A current source-density analysis of field potentials and an intracellular study of neuronal responses were conducted in slice preparations by electrical stimulation of transplanted LGN and host VC. The results indicated that synaptic connections were established reciprocally between the transplanted LGN and the host VC.     

Neuronal activity of the septum transplanted into the neocortical barrel field of the rat

Embryonic (E16-17) septal solid grafts were transplanted into acute cavities in the barrel field of somatosensory neocortex of adult rats. Extracellular recording of the graft's neuronal activity was performed in lightly anesthetized rats, or in brain slices in vitro 8-10 months after grafting. Analysis of Nissl and Golgi-Cox stained preparations showed that 81% of the grafts survived. Judging by combined histological and electrophysiological criteria, 69% of surviving grafts were integrated with the host brain. All septal grafts contained neurons with high spontaneous activity (mean 14.9 ± 8.3 spikes · s -1). Irregular bursts or rhythmic theta-bursts were present in background activity. The frequency of theta-bursts varied in parallel with the state of the animal from 3 Hz (deep barbiturate sleep) up to 7-8 Hz (arousal). Somatosensory stimulation evoked initial bursts or suppression of activity, often followed by a period of rhythmic theta-bursts. Though a high level of convergence for stimulation of vibrissae and body surface was typical of the grafted neurons, a certain spatial gradient of body representation was present. Many neurons responded also by initial burst or prolonged suppression of activity to electrical stimulation of thalamus and homolateral motor cortex. In the frontal slices of neocortex the neurons of septal grafts were highly reactive to stimulation of adjacent neocortex, but usually when the distance between the stimulating electrodes and the graft border did not exceed 1-1.5 mm. The data obtained in the septal slices are compared to the results described earlier in homotopic neocortical and heterotopic hippocampal grafts to the barrel field. It is concluded that heterotopic embryonic septal grafts can successfully develop and establish functional afferent connections with the host brain. Their neurons can participate in processing of sensory information, appropriate to the substituted cortical area, though the characteristics of responses are to a great extent determined by intrinsic properties of the septal neurons.     

Stimulus-specific oscillatory responses of the brain: a time/frequency-related coding process.

OBJECTIVES: To review the coherent, rhythmic oscillations above approximately 20 Hz that occur in response to sensory inputs in the firing rate and membrane or local field potentials of distributed neuron aggregates of CNS layered structures.RESULTS: Oscillatory activity at approximately 20-80 Hz occurs in response to either olfactory, auditory and visual (contrast) stimuli; oscillations at frequencies centered on 100-120 Hz or 600 Hz are recorded, respectively, from the visual system (luminance stimulation) and from the somatosensory cortex. Experimental evidence suggests sources/mechanisms of generation that depend on inhibitory interneurons and pyramidal cells and are partially independent from those of conventional (broadband) evoked responses. In the olfactory and visual systems, the oscillatory responses reflect the global stimulus properties. A time/phase correlation between firing rate, spiking coincidence and oscillatory field responses has been documented. The oscillatory responses are postsynaptic both in cortex and in precortical structures (e.g. retina; LGN). Evidence indicates intracortical and thalamocortical interacting mechanisms of regulation as well as GABAergic and cholinergic modulation. In the visual cortex the oscillatory responses are driven by oscillations in the synaptic input. Oscillatory potentials are dependent on resonance phenomena and produce narrow-band synchronization of activated neurons. They may have a role in the 'binding' of separate neuronal aggregates into sensory units. CONCLUSIONS: Oscillatory responses contribute as a time/frequency coding mechanism to pacing neurons selectively for the physical properties of stimulus and are involved in sensory information processing.     

Local disinhibition of neocortical neuronal circuits causes augmentation of glutamatergic and GABAergic synaptic transmission in the rat neostriatum in vitro

Intra- and extracellular recordings were performed to investigate the influence of local disinhibition of neocortical circuits on corticostriatal synaptic transmission. In rat brain slices with preserved corticostriatal connections, electrical stimulation of the neocortex elicited composed postsynaptic responses in neostriatal neurons consisting of glutamatergic excitatory postsynaptic potentials (EPSPs) and weakly expressed GABAA receptor-mediated inhibitory postsynaptic potentials (IPSPs). Following local application of the GABAA receptor antagonist bicuculline to the neocortex, neocortical neurons responded to intracortical stimulation with transient paroxysmal depolarizations. Simultaneously, the amplitude of neocortically evoked EPSPs recorded from neostriatal neurons was found to be enhanced without changes in duration. Similarly, the amplitude of IPSPs increased following disinhibition of neocortical circuits. In addition and in contrast to EPSPs, the duration of the IPSPs was found to be markedly prolonged. The results demonstrate that local disinhibition of neocortical neuronal circuits potentiates both excitatory and inhibitory synaptic transmission in striatal neurons. However, compared to AMPA receptor-mediated excitation, GABAA receptor-mediated inhibition becomes more efficient due to a marked prolongation of IPSPs. The pronounced augmentation of inhibition can be attributed to a strong activation of inhibitory interneurons within the striatum.     

Cortical deafferentation in cat focal ischemia: disturbance and recovery of sensory functions in cortical areas with different degrees of cerebral blood flow reduction

During and after 15-min occlusion of the middle cerebral artery (MCA) in cats, local CBF and neuronal activity were measured in cortical areas varying in the degree of CBF reduction. In an area within the ischemic center (primary auditory cortex, middle ectosylvian gyrus), CBF was severely suppressed. Click-induced auditory evoked potentials and evoked as well as spontaneous single-unit activity ceased within 1 min after occlusion. Recirculation resulted in a recovery of the different neurophysiological parameters with a time delay ranging from several minutes to 2 h. In two areas surrounding the ischemic focus (a visual area in the marginal gyrus and the forelimb representation area in the primary somatosensory cortex), CBF was reduced but remained above 30 ml/100 g/min during MCA occlusion. Visual flash-induced evoked potentials and somatosensory evoked potentials induced by median nerve electrical stimulation ceased in the corresponding areas with a somewhat slower time course as compared to the auditory responses and they recovered faster after recirculation. In another somatosensory area (hindlimb projection area in the primary somatosensory cortex), CBF stayed nearly at control levels during occlusion. Evoked potentials and single-unit activity induced by tibial nerve electrical stimulation decreased approximately 5 min after occlusion and were abolished approximately 5 min later. At that time, single-unit activity had changed to a nonresponsive pattern but persisted. However, potentials evoked transcallosally by electrical stimulation of the contralateral hemisphere were still recorded. After reopening the MCA, the recovery of neuronal functions was usually complete and occurred within approximately 5 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabotropic glutamate receptors mediate expression of LTP in slices of rat visual cortex

Long-term-potentiation (LTP) can be induced by application of a standard theta-burst stimulation protocol in slice preparations of the neocortex. This type of LTP is known to be dependent on the activation of NMDA receptors. The present study used specific experimental conditions to evoke a non-NMDA receptor mediated type of LTP. By use of weak theta-burst stimulation (wTBS) we describe a non-NMDA receptor dependent LTP in rat visual cortex in vitro, which is sensitive to an antagonist of metabotropic glutamate receptors (mGluR). In slices of the visual cortex we stimulated ascending inputs in cortical layer IV and recorded extracellular field potentials (FPs) from cortical layers II/III. In disinhibited slices (with 1 microm picrotoxin), a wTBS induced LTP to 138% of control. The expression of this potentiation was insensitive to the NMDA-receptor antagonist, D-AP5, but could be abolished by application of the mGluR antagonist MCPG. These data suggest an NMDA-independent mechanism for LTP induction in the visual cortex which can be observed in layer II/III neurons.     

Mechanical vibratory stimulation of feline forepaw skin induces long-lasting potentiation in the secondary somatosensory cortex

We investigated the long-lasting effects of mechanical vibratory stimulation of the skin on the excitability of feline cortical neurons in the forelimb areas of the primary (SI) and secondary (SII) somatosensory cortices. Conditioning mechanical stimuli were 300 bursts of 10 pulses at 200 Hz delivered with a 10-s interburst interval from a mechanical stimulator. Test field potentials and unit discharges were evoked by electrical stimulation to the ventral posterolateral thalamic nucleus (VPL) or by single mechanical stimuli applied to the skin. In SII, the mechanical burst stimulation to the skin increased the amplitudes of field potentials and the frequency of unit discharges elicited by single mechanical stimuli applied to the skin. The vibratory conditioning stimulus also produced a similar potentiation of the VPL-evoked field potentials (126-139% increase in amplitude, P < 0.05) with an associated increase in firing rates of extracellularly recorded neuronal activity (117%, P < 0.001). These potentiations persisted through the entire experimental period of 120 min. The translaminar current source density analysis calculated from the VPL-evoked field potentials increased to 127% of the control value (P < 0.01). In contrast, in SI we observed no significant changes in the field potential amplitudes or in the currents generated in superficial layers (91-117%). Taken together with the previous finding that tetanic electrical stimulation of VPL induces long-lasting potentiation of the VPL-evoked cortical responses in SII but not of those in SI, the present results suggest that SII has a large capacity for the rapid functional plasticity involved in the learning that occurs during repeated tactile experiences.