Involvement of neuropeptide mechanisms in the integration of heterotopic dentate fascia grafts and recipient brain

Embryonic dentate fascia was grafted into the somatosensory neocortex of adult rats. Nine months post-grafting, the ultrastructural and morphometric analysis of the giant synapses established between the grafted granular neurons and inappropriate targets in the recipient brain was performed. As compared to the intact synaptic endings in the control hippocampus, differences were found in both the number and distribution of large dense-core synaptic vesicles, which store the neuropeptide co-transmitters. The peptidergic vesicle proportion (of total vesicle pool) within the ectopic giant synapses was 5.8 +/- 0.6% (versus 3.3 +/- 0.6% in the control). Clusters of large dense-core vesicles near the active zones of aberrant connections were observed almost 7.9 times more frequently than that of normal contacts. These data provide evidence that neuropeptide transmitters are critical for the maintenance of synaptic connections between the heterotopic dentate grafts and host brain.     

A survey of the cytology and synaptic organization of the insular trigeminal-cuneatus lateralis nucleus in the rat, including an identification of spinal afferent inputs

The cytology and synaptic organization of the insular trigeminal-cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.     

Demonstration and localization of synapsin I in vestibular receptors in the cat: immunocytochemical study

The presence and localization of synapsin I, a neuron-specific phosphoprotein, was investigated in the cat vestibular epithelium, using a rabbit antisynapsin I anti-serum. The staining was performed by immunofluorescence or by a peroxidase-antiperoxidase (PAP) technique. A strong immunoreactivity was observed with both methods. This immunoreactivity appeared as spherical patches distributed in the lower part of the epithelium. This distribution pattern is very similar to that of the efferent synaptic endings which form axodendritic synapses with the afferent nerve chalice of type I hair cells, or axosomatic synapses with type II hair cells. Some of the nerve chalices were also labelled; in this case, the immunoreactivity was more evident with PAP staining. These results thus suggest the presence of large amounts of synapsin I in the vestibular efferent nerve endings. These endings are known to be filled with numerous synaptic vesicles. This localization of synapsin I is well correlated with previous work that report a close association between synapsin I and small synaptic vesicles. The presence of synapsin I in sensory endings such as the afferent nerve chalices was unexpected and is under investigation.     

Fine structural characteristics and synaptic connections of trigeminocerebellar projection neurons in rat trigeminal nucleus oralis

In order to classify the presynaptic terminals contacting trigeminocerebellar projection neurons (TCPNs) in rat trigeminal nucleus oralis (Vo), electron-microscopic examination of sequential thin sections made from TCPNs located in the border zone (BZ) of Vo, labeled by the retrograde transport of horseradish peroxidase, was undertaken. The use of BZ TCPNs, labeled in Golgi-like fashion so that many of their dendrites and axons were visible, allowed for the determination of the distribution of each bouton type along the soma and dendrites, as well as for the characterization of the morphology and synaptic relations of the labeled axon and its terminals. Three types of axon terminals contacting labeled BZ TCPNs have been recognized, depending upon whether they contain primarily spherical-shaped, agranular synaptic vesicles (S endings); predominantly flattened, agranular synaptic vesicles (F endings); or a population of pleomorphic-shaped, agranular synaptic vesicles (P endings). The S endings represent the majority of axon terminals contacting labeled BZ TCPNs and establish asymmetrical axosomatic and axodendritic synaptic contacts. Many S endings are situated in one of two types of synaptic glomeruli. One type of glomerulus has a large S ending at its core, whereas the other contains a small S ending. Large-S-ending glomeruli include only labeled distal dendrites of BZ TCPNs; small-S-ending glomeruli contain either a labeled soma, proximal dendrite, or distal dendritic shaft. The remaining S endings are extraglomerular, synapsing on distal dendrites. P endings are less frequently encountered and establish intermediate axosomatic and axodendritic synapses. These endings exhibit a generalized distribution along the entire somatodendritic tree. F endings make symmetrical axodendritic synapses with distal dendrites, are only found in glomeruli containing small S endings, and are the least frequently observed ending contacting labeled BZ TCPNs. The majority of axonal endings synapsing on labeled BZ TCPNs are located along distal dendrites, with only a relatively few synapsing terminals situated on proximal dendrites and somata. The axons of labeled BZ TCPNs arise from the cell body and generally give rise to a single short collateral near their points of origin. This collateral remains unbranched and generates several boutons within BZ, while the parent axon acquires a myelin sheath and, without branching further, travels dorsolaterally toward the inferior cerebellar peduncle. The collateral boutons resemble extraglomerular S endings. They contain agranular, spherical-shaped synaptic vesicles and make asymmetrical axodendritic synapses with small-diameter unlabeled dendritic shafts in the BZ neuropil.     

Fine structure of synapses in the hippocampus of the rabbit with special reference to dark presynaptic endings

Summary The stratum radiatum of h ₃ and h ₄ in the hippocampus of the rahbit, where the mossy fiber endings are distributed, was investigated under the electron microscope. These regions contain a certain number of electron dense presynaptic endings. These are characterized by highly dense synaptic vesicles and mitochondrial matrices. The dense endings are not considered as degenerated. Electron dense silver particles, substituted for zinc, occurred on the synaptic vesicles of these dense terminals as well as the mossy fiber endings after the application of Timm's histochemical method modified for electron microscopy. It is concluded that the dark synaptic endings observed might represent mossy fiber terminals in a special functional phase, or might be the result of structural alteration in the course of tissue preparation. The zinc localized in the synaptic vesicles is thought to be associated with the neurotransmitter present in these endings.     

Submicroscopic changes of the synapse after nerve section in the acoustic ganglion of the guinea pig; an electron microscope study

The degenerative changes of the synaptic regions after nerve section have been studied with the electron microscope in the interneuronal synapse of the ventral ganglion of the acoustic nerve of the guinea pig. Fixation with buffered osmic tetroxide was carried out 22, 44, and 48 hours after destruction of the cochlea on one side; the contralateral ganglion being used as control. The submicroscopic organization of normal axosomatic and axodendritic synapses is described. In the synaptic ending four morphological components are recognized: the membrane, the mitochondria, the synaptic vesicles (19, 20), and the cytoplasmic matrix. The intimate contact of glial processes with the endings and with the surface of the nerve cell is described. At the level of the synaptic junction there is a direct contact of the limiting membranes of the ending and of the cell body or dendrite. Both contacting membranes constitute the synaptic one with a total thickness of about 250 A. This membrane has regions of higher electron density where the synaptic vesicles come into intimate contact and fuse with it. Definite degenerative submicroscopic changes in the nerve endings were observed after 22 hours of destruction of the cochlea and were much more conspicuous after 44 and 48 hours. After 22 hours there is swelling of the ending and decreased electron density of the matrix. Most synaptic vesicles have disappeared or seem to undergo a process of clumping and dissolution. Some mitochondria also show signs of degeneration. After 44 hours the synaptic vesicles have practically disappeared; mitochondria are in different stages of lysis; the membrane of the ending becomes irregular in shape, and there is shrinkage and in some cases detachment of the ending. No changes in the postsynaptic cytoplasm were observed. These observations and particularly the rapid lysis of the synaptic vesicles are discussed in correlation with data from the literature indicating the early alteration of synaptic function and the biochemical changes occurring after section of the afferent nerve. The hypothesis that the synaptic vesicles may be carriers of acetylcholine or other active substances (19, 20) and that they may act as biochemical units in synaptic transmission is also discussed.(2)     

Axo-axonal synapses in the frog tectum opticum

A quantitative electron-microscopic investigation of synaptic endings in large sections showed that about 50% of all axo-axonal synapses are located in the outer zone of the neuropil (layer 9) of the tectum opticum ofRana temporaria L. These synapses are more numerous in the rostral part of the tectum than the caudal. Hardly any axo-axonal synapses lie deeper than 50–60 µ Most axo-axonal synapses are located on axon endings of retinal ganglionic cells, for after degeneration of the optic nerve the number of these synapses is reduced by two-thirds. During ontogenetic differentiation and regeneration of the optic nerve axo-axonal synapses develop before axo-dendritic and their presynaptic processes have the normal structure and differ sharply from the bulbs of growth of the optic fibers. On this basis the central origin of most presynaptic processes forming these synapses is postulated. The results point to the possibility of presynaptic control over the effectiveness of action of the efferent axons, primarily optic, terminating in the outer zone of the frog tectum opticum.     

A Survey of the Cytology and Synaptic Organization of the Insular Trigeminal—Cuneatus Lateralis Nucleus in the Rat,Including an Identification of Spinal Afferent Inputs

The cytology and synaptic organization of the insular trigeminal—cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.     

Morphofunctional types of synapses in the neurosecretory cells of the carp preoptic nucleus]

The ultrastructure of synaptic endings of the neurosecretory cells of the nucleus preopticus was examined in adult Cyprinus carpio L. Two of synpatic endings occur: type I--small agranular vesicles and large granular vesicles, type II--only agranular vesicles. The functioning of the nucleus preopticus neurosecretory cells in Cyprinus carpio L is presumably controlled by the synpatic endings of the adrenergic (synaptic endings of type I) as well as of the cholinergic (synaptic endings of type II) origin. By visual and morphometric methods different kinds of synpatic endings are distinguished among both the types of synapses according to their particular functional states. A quantitative analysis of the correlation of these kinds of synpatic endings allows a suggestion in respect to the state of the synaptic apparatus on the perikaria of neurosecretory cells.     

SUBMICROSCOPIC CHANGES OF THE SYNAPSE AFTER NERVE SECTION IN THE ACOUSTIC GANGLION OF THE GUINEA PIG. AN ELECTRON MICROSCOPE STUDY

The degenerative changes of the synaptic regions after nerve section have been studied with the electron microscope in the interneuronal synapse of the ventral ganglion of the acoustic nerve of the guinea pig. Fixation with buffered osmic tetroxide was carried out 22, 44, and 48 hours after destruction of the cochlea on one side; the contralateral ganglion being used as control. The submicroscopic organization of normal axosomatic and axodendritic synapses is described. In the synaptic ending four morphological components are recognized: the membrane, the mitochondria, the synaptic vesicles (19, 20), and the cytoplasmic matrix. The intimate contact of glial processes with the endings and with the surface of the nerve cell is described. At the level of the synaptic junction there is a direct contact of the limiting membranes of the ending and of the cell body or dendrite. Both contacting membranes constitute the synaptic one with a total thickness of about 250 A. This membrane has regions of higher electron density where the synaptic vesicles come into intimate contact and fuse with it. Definite degenerative submicroscopic changes in the nerve endings were observed after 22 hours of destruction of the cochlea and were much more conspicuous after 44 and 48 hours. After 22 hours there is swelling of the ending and decreased electron density of the matrix. Most synaptic vesicles have disappeared or seem to undergo a process of clumping and dissolution. Some mitochondria also show signs of degeneration. After 44 hours the synaptic vesicles have practically disappeared; mitochondria are in different stages of lysis; the membrane of the ending becomes irregular in shape, and there is shrinkage and in some cases detachment of the ending. No changes in the postsynaptic cytoplasm were observed. These observations and particularly the rapid lysis of the synaptic vesicles are discussed in correlation with data from the literature indicating the early alteration of synaptic function and the biochemical changes occurring after section of the afferent nerve. The hypothesis that the synaptic vesicles may be carriers of acetylcholine or other active substances (19, 20) and that they may act as biochemical units in synaptic transmission is also discussed.²     

AMPA receptor regulation during synaptic plasticity in hippocampus and neocortex

Discovery of long-term potentiation (LTP) in the dentate gyrus of the rabbit hippocampus by Bliss and L?mo opened up a whole new field to study activity-dependent long-term synaptic modifications in the brain. Since then hippocampal synapses have been a key model system to study the mechanisms of different forms of synaptic plasticity. At least for the postsynaptic forms of LTP and long-term depression (LTD), regulation of AMPA receptors (AMPARs) has emerged as a key mechanism. While many of the synaptic plasticity mechanisms uncovered in at the hippocampal synapses apply to synapses across diverse brain regions, there are differences in the mechanisms that often reveal the specific functional requirements of the brain area under study. Here we will review AMPAR regulation underlying synaptic plasticity in hippocampus and neocortex. The main focus of this review will be placed on postsynaptic forms of synaptic plasticity that impinge on the regulation of AMPARs using hippocampal CA1 and primary sensory cortices as examples. And through the comparison, we will highlight the key similarities and functional differences between the two synapses.     

Ultrastructure of nervous tissue developing in the anterior chamber of the eye. 1. Perikaryons and dendrites

Embryonic tissues of septum and hippocampus were transplanted into the anterior eye chamber (AEC) of adult rats. The morphology of initial embryonic tissues and of transplants within 3 to 4 months of cultivation in AEC was studied. The transplanted tissue consists of neuroblasts and immature neurones: no synaptic contacts are observed. Within 3 to 4 months, highly differentiated neurones establishing synaptic contacts can be seen in the transplants. At the same time the fine structure of perikaryons and dendrites undergoes some changes: increased vacuolization, transformation of ergastoplasm into lamellar bodies. These can be due to an elevated functional activity of some neurones. Another group of morphological abnormalities (increased number of dendrite processes and microphyllopodia, somatic spines, dendrite cones of growth, tight junctions between perikaryons) suggests incomplete tissue maturation. These might be due to the absence of normal afferent and trophic influences in AEC.     

Immuno-electron microscopical demonstration of vasopressin and oxytocin synapses in the limbic system of the rat

Summary The extensive distribution of exohypothalamic vasopressin or oxytocin containing nerve fibres is thought to be the anatomical basis for the involvement of these neuropeptides in central processes. Following light microscopic observations suggesting that these fibres terminate on other neurons, the present study was undertaken to demonstrate the existence of such endings in the limbic system, which is one of the main target areas for these peptides. For immunoelectron microscopy glutaraldehyde-paraformaldehyde perfused brains of male Wistar rats and Brattleboro rats, homozygous for diabetes insipidus, with and without postfixation in OsO₄, were used. Post-embedding staining revealed false positive reaction product on all dense core vesicles, e.g., in the lateral septum. With pre-embedding staining, however, intense and specific reactions were observed for both vasopressin and oxytocin at their sites of production, as well as the neurohypophysis and in the extrahypothalamic limbic brain regions.In the lateral septum and habenular nucleus only vasopressin-containing synapses could be demonstrated, while in the medial nucleus of the amygdala synapses containing either vasopressin or oxytocin were observed. These peptide containing synapses do not seem to differ in any fundamental way from the classical transmitter-containing synapses in the brain.Supported by the Foundation for Medical Research FUNGOThe authors wish to thank Prof. Dr. A.H.M. Lohman for having made the vibratome available, and Miss C. de Raay for her expert technical assistance     

Differential ultrastructure of synaptic terminals on ventral longitudinal abdominal muscles in Drosophila larvae

The innervation of ventral longitudinal abdominal muscles (muscles 6, 7, 12, and 13) of third-instar Drosophila larvae was investigated with Nomarski, confocal, and electron microscopy to define the ultrastructural features of synapse-bearing terminals. As shown by previous workers, muscles 6 and 7 receive in most abdominal segments “Type I” endings, which are restricted in distribution and possess relatively prominent periodic terminal enlargements (“boutons”); whereas muscles 12 and 13 have in addition “Type II” terminals, which are more widely distributed and have smaller “boutons.” Serial sectioning of the Type I innervation of muscles 6 and 7 showed that two axons with distinctive endings contribute to it. One axon (termed Axon 1) has somewhat larger boutons, containing numerous synapses and presynaptic dense bodies (putative active zones for transmitter release). This axon also has more numerous intraterminal mitochondria, and a profuse subsynaptic reticulum around or under the synaptic boutons. The second axon (Axon 2) provides somewhat smaller boutons, with fewer synapses and dense bodies per bouton, fewer intraterminal mitochondria, and less-developed subsynaptic reticulum. Both axons contain clear synaptic vesicles, with occasional large dense vesicles. Approximately 800 synapses are provided by Axon 1 to muscles 6 and 7, and approximately 250 synapses are provided by Axon 2. In muscles 12 and 13, endings with predominantly clear synaptic vesicles, generally similar to the Type I endings of muscles 6 and 7, were found, along with another type of ending containing predominantly dense-cored vesicles, with small clusters of clear synaptic vesicles. This second type of ending was found most frequently in muscle 12, and probably corresponds to a subset of the “Type II” endings seen in the light microscope. Type I endings are thought to generate the ?fast’? and ?slow’? junctional potentials seen in electrophysiological recordings, whereas the physiological actions of Type II endings are presently not known. © 1993 John Wiley & Sons, Inc.     

Synaptic input to the axon hillock and initial segment of inhibitory interneurons in the cerebellar cortex of the rat

Summary The axon hillock (AH) and initial segment (IS) of 10 Golgi neurons and 6 basket cells in the cerebellar cortex of the rat were investigated by electron microscopy using serial sections. An average of 10.4 and 11.3 synaptic terminals were observed to establish synaptic contact with the axon hillock region of Golgi and basket cells, respectively. Most of these terminals were identified as the varicosities of the ascending parallel fibers. It is suggested that the focal innervation of AH regions represents an excitatory input pattern which is basically different from the randomly distributed, huge, parallel-fiber input onto the dendritic trees of Golgi and basket cells. In contrast to Golgi and basket neurons, no accumulation of parallel-fiber synapses was observed around the AH of stellate cells. The IS proper of the three neuronal types were devoid of true axo-axonal synapses.     

Strength of synaptic transmission at neuromuscular junction of crustaceans and insects in relation to calcium entry

Crustacean and insect neuromuscular junctions typically include numerous small synapses, each of which usually contains one or more active zones, which possess voltage-sensitive calcium channels and are specialized for release of synaptic vesicles. Strength of transmission (the number of quantal units released per synapse by a nerve impulse) varies greatly among different endings of individual neurons, and from one neuron to another. Ultrastructural features of synapses account for some of the physiological differences at endings of individual neurons. The nerve terminals that release more neurotransmitter per impulse have a higher incidence of synapses with more than one active zone, and this is correlated with more calcium build-up during stimulation. However, comparison of synaptic structure in neurons with different physiological phenotypes indicates no major differences in structure that could account for their different levels of neurotransmitter release per impulse, and release per synapse differs among neurons despite similar calcium build-up in their terminals during stimulation. The evidence indicates differences in calcium sensitivity of the release process among neurons as an aspect of physiological specialization.     

Hippocampus morphology in tissue culture]

Explants of the hippocampus of newborn rats were studied neurohistologically and with electron microscope within 5--35 days of explantation. Two zones are found in the culture of the hippocampus: a zone of explant, and a zone of outgrowth. Neurons, glial cells and a network of their fibres are compactly arranged in the center of the former, whereas, the latter involves a layer of migrated glial cells. The explant is surrounded by glia limiting cells. Three types of neurons are identified in the long living culture of the hippocampus: pyramidal, polymorphic and granule cells. Numerous nerve endings observed in the hippocampic explant can be recognized as axodendritic, axosomatic and glomerular synapses. The availability of several types of neurons, a variety of synapses and their complication during outgrowth of the culture are suggestive of a formation in the hippocampic explant of a functional reflex activity.     

Neuronal organization of the medial part of the ventral horn of the cat spinal cord

An electron-microscopic study was made of the normal structure of the medial part of the ventral horn (Rexed's laminae VII and VIII) in the cervical portion of the cat's spinal cord, the region where fibers of reticulospinal and vestibulospinal tracts terminate. Neurons of this region can be divided on the basis of the density of their cytoplasmic matrix into "light" and "dark," the dark being much more numerous in this area (26% of the total number counted) than in other parts of the gray matter of the spinal cord. The mean diameter of the soma of the dark cells is smaller than that of the light cells, and it usually is 15–20 µ. Dendrites of the neurons can also be subdivided into "light" and "dark" respectively. The surface of the former is comparatively simple in shape with a small number of appendages and spine-like structures. On the surface of the dark dendrites there are many projections and irregularly shaped lacunae. The glial cells and their processes often completely cover the surface of the soma of the small neurons, and synaptic endings are found on it only where the dendrites leave the soma. Analysis of 1000 randomly chosen synaptic endings showed that 76.1% of them form axo-dendritic synapses, 14.2% axo-somatic, and 9.7% axo-axonal synapses. Of the total number of endings 50.9% contain spherical and 40.9% flattened synaptic vesicles. Some synaptic endings contain special structures under the postsynaptic membrane and have osmiophilic synaptic vesicles. The possible functional role of the pattern of neuronal organization revealed in this region is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 2, pp. 176–183, March–April, 1972.     

Functional integration of the nervous tissue of the rat following xenotransplantation into the brain of a rabbit

Surviving grafts of the nervous tissue taken from the septum and hippocampus of rat embryos and xenotransplanted into the rabbit's brain were observed in 4 out of 6 animals 2-3 months after surgery. The grafts contacted the neocortex or hippocampus of the recipients. Extracellular recording of neuronal activity in the grafts revealed spontaneous discharges with normal patterns and without any signs of pathology. Electrical stimulation of the recipients' brain (contralateral hippocampus, mesencephalic reticular formation, posterior cingulate cortex) induced changes of spontaneous discharges in 41% of the units in the grafts. Diffuse tonic shifts of the level of discharge were usually observed, though driving effects (in two units) were also encountered. Reactions to sensory stimulation of the recipients were observed in 61% of grafted units. Excitatory and inhibitory reactions, tonic, phasic and specific on-effects were evoked mainly by auditory and somatosensory stimuli. The data show the possibility of integration of xenografted tissue with the brain of the recipient and of its participation in processing of sensory information.