Intrinsic control of electroresponsive properties of transplanted mammalian brain neurons

Transplanted embryonic entorhinal neurons make functional synapses in adult host hippocampus

Grafts of embryonic entorhinal cortex (EC) or non-entorhinal cortex (NEC) were placed into the hippocampus of adult rats with transection of the perforant paths. Graft-host connectivity was investigated at 4-6 months post-transplantation by recording extracellular evoked responses in hippocampal slice preparations. Electrical stimulation of the grafts evoked excitatory postsynaptic potentials (EPSPs) in the outer molecular layer of the dentate gyrus, and the stratum lacunosum moleculare of CA1, CA3, and elicited population spikes in the granule cell layer and the pyramidal cell layer of CA1, but not CA3. While the latencies and the forms of these evoked response were similar to those in matched control slices from the normal animals, the amplitudes were smaller than normal controls. However, in the slices with NEC grafts, no such responses were recorded when stimulus was applied in similar position in the grafts. The findings suggest that grafted entorhinal neurons make viable synaptic connections with the host hippocampus.

Reciprocal trophic interactions between climbing fibres and Purkinje cells in the rat cerebellum

In the adult cerebellum both the climbing fibre arbour and the Purkinje cell are very plastic and each element is able to exert a remarkable action on the other one. The adult phenotype of the Purkinje cell is strictly dependent on the presence of its climbing fibre arbour. When the climbing fibre is missing, the Purkinje cell undergoes a hyperspiny transformation and becomes hyperinnervated by the parallel fibres. However, this change is fully reversible. The climbing fibre-deprived Purkinje cell is able to elicit sprouting of nearby located intact climbing fibres and the new arbour is able to fully restore synaptic connections which appear normal both morphologically and functionally. Multiple climbing fibre innervation of a single Purkinje cell persists in the adult hypogranular cerebellum. The different fibres are distributed to separate dendritic regions, suggesting a local competition between the different arbours for their territory. It is postulated that in the intact rat, an activity dependent mechanism of the parallel fibre favours the predominance of one arbour with the elimination of its competitors. When the Purkinje cell is deleted, the climbing fibre arbour becomes heavily atrophic and reduced in size. The analysis of the pattern of this atrophy indicates that the climbing fibre arbour is made by two compartments: a proximal one, whose survival depends on the integrity of the inferior olive, and a distal one, which represents the true pre-synaptic site, which strictly depends on the target. The climbing fibre terminal arbour is able to extend its territory of innervation not only when adult intact climbing fibres are confronted with nearby denervated Purkinje cells, but also when an embryonic cerebellum is grafted onto the surface of an adult unlesioned cerebellum. In this case, collaterals of intact climbing fibre arbours elongate through the pial surface, enter the graft to innervate the Purkinje cells. This growth is likely under the influence of a tropic signal released by the embryonic Purkinje cells. This suggests that the sprouting observed in the adult rat following a subtotal inferior olive lesion is also triggered by a similar factor. The axonal elongation and the consequent synaptogenesis are likely guided by local cues. In this condition, the distribution of the new collateral reinnervation occurs within its projectional map. In addition, when the inferior cerebellar peduncle is sectioned at birth, the climbing fibres of the non-deafferented hemicerebellum emit collaterals which cross the midline and innervate cerebellar strips which are symmetrically positioned relative to the intact side. In the grafting experiments, both the migrated and non-migrated Purkinje cells show the typical electrophysiological properties of the mature cerebellum. These data show that the disappearance of neuronal elements is not a necessary prerequisite to allow new neurones to become fully morphologically and functionally integrated into an adult brain. The reciprocal trophic influence between the climbing fibres and the Purkinje cells shown in the present series of experiments are likely operative in the adult brain not only in pathological conditions and they could give a basic contribution to the synaptic plasticity underlying learned behaviour.

Postsynaptic currents and short-term synaptic plasticity in Purkinje cells grafted onto an uninjured adult cerebellar cortex

It has been shown recently that embryonic Purkinje cells grafted extraparenchymally into an intact cerebellum, in the absence of any sign of damage, are able to migrate into the host molecular layer where they receive a climbing fibre innervation. Using the same technique, we investigated the development of the electrophysiological properties of the synapses between the grafted cells and their main afferents. Purkinje cells either in the graft or having migrated into the molecular layer of the host were recorded using the whole-cell patch-clamp method in acutely prepared slices 17-112 days after grafting. Spontaneous postsynaptic currents with a single-exponential decay and mediated by GABAA receptors were very similar to those described in normal Purkinje cells. Excitatory postsynaptic currents (EPSCs) evoked by climbing fibre and by parallel fibre stimulation were blocked by an alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate antagonist, and displayed the linear current-voltage relation typical of postnatal Purkinje cells. The attainment of normal functional properties by the adult axons at the newly formed synaptic sites was shown by the expression of short-term facilitation of parallel fibre EPSCs and of short-term depression of climbing fibre EPSCs. The grafted Purkinje cells showed climbing fibre polyinnervation 17-20 days after grafting which evolved to monoinnervation at 23-45 days, confirming the completion of the developmental programme up to maturation. Our experiments support the view that the adult intact brain is able to accept and integrate an additional number of neurons which show fully mature electrophysiological properties which are electrophysiologically indistinguishable from those of the host neurons.

Development of fetal hippocampal grafts in intact and lesioned hippocampus

Functional recovery observed in Parkinson's disease patients following grafting of fetal substantia nigra has encouraged the development of similar grafting therapy for other neurological disorders. Fetal hippocampal grafting paradigms are of considerable significance because of their potential to treat neurological disorders affecting primarily hippocampus, including temporal lobe epilepsy, cerebral ischemia, stroke, and head injury. Since many recent studies of hippocampal transplants were carried out with an aim of laying the foundation for future clinical applications, an overview of the development of fetal hippocampal transplants, and their capability for inducing functional recovery under different host conditions is timely. In this review, we will summarize recent developments in hippocampal transplants, especially the anatomical and/or functional integration of grafts within the host brain under specific host conditions, including a comparison of intact hippocampus with various types of hippocampal lesions or injury. Improvements in grafting techniques, methods for analysis of graft integration and graft function will be summarized, in addition to critical factors which enhance the survival and integration of grafted cells and alternative sources of donor cells currently being tested or considered for hippocampal transplantation. Viewed collectively, hippocampal grafting studies show that fetal hippocampal tissue/cells survive grafting, establish both afferent and efferent connections with the host brain, and are also capable of ameliorating certain learning and memory deficits in some models. However, the efficacy of intracerebral fetal hippocampal grafts varies considerably in different animal models, depending on several factors: the mode of donor tissue preparation, the method of grafting, the state of host hippocampus at the time of grafting, and the placement of grafts within the hippocampus. Functional improvement in many models appeared to be caused partially by re-establishment of damaged circuitry and partially by a trophic action of grafts. However, exact mechanisms of graft-mediated behavioral recovery remain to be clarified due to the lack of correlative analysis in the same animal between the degree of graft integration and behavioral recovery. Issues of mechanisms of action, degree of restoration of host circuitry and amelioration of host pathological conditions will need to be sorted out clearly prior to clinical use of fetal hippocampal transplants for susceptible neurological conditions.

Low extracellular magnesium unmasks N-methyl-D-aspartate-mediated graft-host connections in rat neocortex slice preparation

The main purpose of this study was to investigate the role of N-methyl-D-aspartate receptors in host-graft synaptic transmission in the neocortex. The effects of low extracellular magnesium, the glutamate agonist N-methyl-D-aspartate and N-methyl-D-aspartate antagonists on the synaptic activation of connections between embryonic neocortical graft tissue and the surrounding host tissue were studied in 17 perfused slices of rat neocortex. In standard artificial cerebrospinal fluid, stimulation of the host white matter evoked field potentials in four of 17 grafts. However, in Mg(2+)-free medium, the same stimulation evoked field potentials in an additional six grafts, with significant increases in the mean duration of the evoked responses in the 10 responsive grafts. In five of these slices stimulation of the graft also evoked field potentials in the host tissue, suggesting reciprocal interaction between graft and host. Simultaneous extracellular recordings from graft and host tissues in Mg(2+)-free medium showed that spontaneous epileptiform discharges developed in the graft and host tissue synchronously. In Mg(2+)-free medium, application of N-methyl-D-aspartate induced a shift of the baseline with superimposed epileptiform discharges in both graft and host. Application of the non-competitive N-methyl-D-aspartate antagonist ketamine and the competitive antagonist D,L-2-amino-5-phosphonovaleric acid attenuated or reversibly blocked both the spontaneous epileptiform discharges and the evoked field potentials. Our data provides evidence that N-methyl-D-aspartate receptors are present at synapses created between fetal graft and host neocortex, and that the N-methyl-D-aspartate-activated receptor-channel complex plays an active role in mediating excitatory synaptic transmission in host-graft circuitry.

Anatomical and physiological localization of prelabeled grafts in rat hippocampus

Dissociated rat fetal hippocampal cells were grafted into normal adult rats. The fetal cells were incubated with one of a number of fluorescent compounds at the time of the dissociation to facilitate identification of the individual grafted cells. The fluorescent labels which were analyzed for this purpose included rhodamine latex microspheres, Cascade blue latex beads, rhodamine-dextran-amine, DiI, and carboxyfluorescein ester. The labeled cells were stereotaxically placed as a suspension into normal rat host hippocampi. The rats were sacrificed 2 to 6 weeks after the grafting for in vitro physiological recordings, and the prelabeled grafts were located using fluorescence optics. During intracellular recordings neurons within the prelabeled grafts were injected with Lucifer yellow to visualize the morphology and integration of neuronal processes within the host. Following the recordings the host slices with the grafts were fixed in 4% paraformaldehyde for anatomical analysis. The ability to prelabel cellular grafts allows subsequent anatomical and physiological analysis of the integration of grafted neurons at the resolution of a single neuron. Such an analysis will improve our understanding of the survival, differentiation, migration, and integration of the grafted neurons and their potential to replace lost function in the lesioned hippocampus.

Hippocampal grafts into the intact brain induce epileptic patterns

Spontaneous hippocampal EEG activity and evoked field potentials were investigated in intact rats and in animals with fetal hippocampal grafts. Pieces of hippocampal grafts, derived from 15- to 16-day-old fetuses, were used to prepare cell suspensions and grafted directly into the intact hippocampus. Control animals received suspension grafts of the cerebellum derived from fetuses of identical age. Host hippocampal electrical patterns were monitored with chronic single electrodes or with a 16-microelectrode probe from 7 to 10 months after grafting. In contrast to previously reported high survival rates of fetal grafts in studies with damage to the host brain prior to grafting, survival of both hippocampal (60%) and cerebellar grafts (20%) was very poor in the intact hippocampus. In animals with cerebellar transplants or without surviving grafted neurons the electrical activity of the host hippocampus was indistinguishable from normal controls. In rats with hippocampal grafts short duration, large amplitude EEG spikes (up to 10 mV) were recorded, predominantly during immobility. When the EEG spikes (putative interictal spikes) were of large amplitude and contained population spikes, test evoked responses delivered to the perforant path were suppressed after the spontaneous events. In contrast, evoked responses were facilitated by interictal spikes without population spikes. The threshold of electrically induced afterdischarges did not differ significantly between groups of intact rats and animals with or without hippocampal grafts. However, in three rats with hippocampal grafts the evoked afterdischarges were associated with behavioral seizures. In two of these rats spontaneously occurring seizures were also observed. Synaptophysin-immunoreactivity demonstrated growth of the host mossy fibers into the graft.(ABSTRACT TRUNCATED AT 250 WORDS)

An intracellular study of grafted and in situ preoptic area neurones in brain slices from normal and hypogonadal mice

1. Intracellular recordings have been obtained from forty-one preoptic area (POA) neurones at times up to 14 months after they were grafted into the third ventricle of the mouse. Thirty-one neurones were in grafts from hypogonadal (hpg) mice in which a reversal of the hypogonadism was seen (responders), six were in grafts from hpg mice in which no such reversal occurred (non-responders) and four were in grafts from normal mice. 2. The grafted neurones had a mean resting potential (Em) of -57 mV, a mean apparent input resistance (Rm) of 136 M omega and a mean membrane time constant (tau m) of 7.7 ms. The slopes of the current-voltage (I-V) relations were linear. Approximately a quarter of neurones in responders fired action potentials spontaneously either singly or in bursts. Such activity could underlie the release of gonadotrophin hormone-releasing hormone (GnRH) which is known to occur from such grafts. 3. Two types of response were seen when these neurones were depolarized to firing threshold from Em, in one group a single action potential was discharged; in the other group one or more action potentials arising from a transient, slowly rising and falling depolarization (low-threshold response, LTR) was recorded. Some cells in the former category exhibited a LTR when depolarized from a potential more negative than Em. 4. The commonest response to stimulation of the median eminence in responders was an EPSP either alone or in combination with an IPSP. Antidromic action potentials were seen in four neurones and in two of these cells excitatory synaptic inputs could be demonstrated when the host hypothalamus adjacent to the graft was stimulated. It is suggested that these responses may represent activation of an afferent input from the host to neurones in the graft. 5. The morphology of neurones in POA grafts was determined by intrasomatic injection of horseradish peroxidase (HRP). A variety of profiles were seen and although some neurones extended over distances of up to 635 microns and branched extensively only one appeared to enter the host tissue at the ventrolateral edge of the graft. 6. A comparison was made between grafted POA neurones and cells in the medial preoptic area (MPOA), a region which constituted a significant component of the grafted tissue. No significant difference was noted between neurones in the graft and neurones in the MPOA in terms of their passive membrane properties. With regard to the active properties MPOA neurones could also be classified according to whether or not a LTR was elicited when the neurone was depolarized from Em. The major difference between the grafted neurones and those in the MPOA lay in the proportion of cells which exhibited a LTR under such conditions, being significantly greater in the latter group.

The grafted hippocampus: an epileptic focus

Field potentials and unitary activity were investigated in the grafted and the host hippocampi in freely moving rats and in vitro. The subcortical afferents and efferents of the hippocampus (fimbria-fornix, FF) were removed by aspiration. Solid pieces of hippocampal grafts derived from 15- to 16-day-old fetuses were placed in the lesion cavity in rats with unilateral FF lesions, and cell suspensions prepared from fetal hippocampi were grafted directly into the host hippocampi in animals with bilateral FF lesions. Reciprocal communication between the grafted and the host hippocampi was monitored with a 16-microelectrode probe from 7 to 10 months after grafting. The fluorescent retrograde tracer, Fluorogold, was used to examine graft-host projections and acetylcholinesterase staining to reveal host-derived fibers in the graft. The most typical neuronal pattern of the hippocampal graft was a highly synchronous population burst with concurrent EEG spike. The speed of propagation of the EEG spike within the graft and across the graft-host interface was either fast (greater than 3 m/s) or slow (less than 0.5 m/s). Large amplitude, short duration EEG spikes usually propagated with a high speed, while smaller amplitude, wider spikes with broad population bursts spread at a lower velocity. The direction of propagation was usually uniform indicating that the population burst was triggered by a localized subgroup of highly excitable neurons ("focus"). Spontaneous seizures were also present in the solid graft which frequently invaded the host hippocampus. The incidence of EEG spikes was three times higher in rats with bilateral suspension grafts than in animals with FF lesion only. In about half of the grafted rats spontaneous behavioral seizures were also observed. Intracellular recordings from putative pyramidal cells in the graft and in the host revealed large amplitude (10-12 mV), spontaneously occurring EPSPs. IPSPs were difficult to detect even during depolarizations of up to 20 mV from rest. We suggest that the increased excitability of the hippocampal graft is due to the high incidence of recurrent excitatory collaterals terminating on or close to the somata of pyramidal neurons. Population bursts may spread fast via extensively arborizing axon collaterals or slowly by successively activating new sets of neighboring neurons. Spontaneous behavioral convulsions are explained by assuming that the grafted hippocampus serves as an epileptic focus which is capable of kindling the host brain by repeated seizure induction.(ABSTRACT TRUNCATED AT 400 WORDS)

Alterations in excitatory and GABAergic inhibitory connections in hippocampal transplants

Solid pieces of embryonic hippocampal tissue were implanted in a cavity formed by aspiration of the fimbria-fornix and the overlying cingulate cortex in adult rats. Six to 8 months after the transplantation, chronic recording electrodes were implanted into the graft and the host hippocampi for the recording of electroencephalogram and unit activity in the freely moving animal. Irregularly occurring sharp waves or electroencephalogram spikes and concurrent synchronous discharge of large groups of neurons dominated the electrical activity of the grafts, in contrast to the situation in normal animals. Light microscopy and GABA immunocytochemistry in the grafts revealed that the three major cell types of the hippocampal formation, i.e. pyramidal neurons, dentate granule cells and GABA-immunoreactive interneurons were present in the hippocampal grafts. At the ultrastructural level, however, significant alterations in connectivity were observed. The most striking finding was the absence or sparse occurrence of synapses on the axon initial segments of pyramidal neurons. The axon initial segments are normally densely covered by GABAergic synapses derived from a specialized type of interneuron, the chandelier or axo-axonic cell. On the other hand, numerous GABA-immunoreactive terminals were found in synaptic contact with somata of pyramidal neurons, suggesting that other types of GABAergic interneurons and their efferent connections may have developed in a normal manner. The cell bodies of pyramidal neurons received, in addition, several asymmetric synapses from GABA-negative terminals. These presumably excitatory synapses are not present on the somata of pyramidal cells in the normally developing hippocampus. We hypothesize that the somatic excitatory synapses originate, at least in part, from the axon collaterals of the neighbouring pyramidal cells in the graft. We suggest that the hyperexcitability of the neuronal circuitry within the graft is due to reduced inhibition (lack of axo-axonic synapses) coupled with increased collateral excitation of the pyramidal neurons.

Morphology and electrophysiology of ventral mesencephalon nerve cell cultures

In primary neuron cultures obtained from ventral mesencephalon of mouse fetuses, approximately 10-30% of the neurons were dopaminergic, as demonstrated by a rapid glyoxylic acid histofluorescence procedure, and another 10-30% were GABAergic as demonstrated by autoradiography. Resting membrane potentials averaged -58 mV and input resistances averaged 188 M omega. Action potential (AP) firing patterns were of 3 types: in 49% of cells, depolarizing current elicited bursts of APs of constant amplitude, duration, and interspike interval (Type 1); in 44% of cells, bursts consisted of APs of decreasing amplitude, increasing duration, and increasing interspike interval (Type 2); and in 7% of cells, bursts were initiated by a single high amplitude, short duration AP followed by a series of lower amplitude longer duration APs that progressively increased in amplitude and decreased in duration and interspike interval (Type 3). Calcium APs of two distinct types, differing in duration and rate of rise, were observed when cultures were exposed to tetrodotoxin. Abundant postsynaptic activity was recorded. Simultaneous intracellular recording between pairs of cells demonstrated reciprocal innervation. The neurotransmitter antagonists haloperidol, bicuculline, naloxone, atropine, hexamethonium and pirenzepine affected synaptic activity and/or resting membrane potential of some of the cultured neurons.

Ultrastructural organization of normal and transplanted rat fascia dentata: I. A qualitative analysis of intracerebral and intraocular grafts

Few studies have dealt with the general ultrastructure and synaptic organization of grafted brain tissue. This study was therefore performed to extend current light microscopic observations on intracerebral and intraocular grafts of hippocampal tissue to the ultrastructural level. Blocks of tissue containing the hippocampus and fascia dentata from day 21 embryonic rats were grafted into the brain of developing and adult rats and to the anterior eye chamber of adult rats. After 100 or 200 days of survival the recipient rat brains or eyes were processed for electron microscopy. Tissue containing the graft dentate molecular layer and adjacent granule cell layer was selected for ultrastructural analysis, together with a few samples of the hilus and CA3. Normal dentate tissue was analyzed as control. At the light microscopic level most intracerebral and intraocular grafts displayed an organotypic organization with clearly recognizable cell and neuropil layers. Under the electron microscope the grafted granule cells had normal-appearing dendrites bearing the normal types of spines and forming the normal types of synapses. This was the case even in the absence of the normal major extrinsic afferents like the perforant path. The graft dentate granule cells formed axons and terminals with characteristics of the normal mossy fiber system in the hilus and CA3, in addition to aberrant supragranular mossy fiber terminals known from light microscopic studies of dentate transplants. Abnormal structures included a few dendritic growth cones and an increased occurrence of polyribosomes in spines. Their occurrence indicates ongoing dendritic plasticity even 100 days after transplantation. There was also an increased density of glial elements, particularly in the intraocular grafts. In some of these grafts the granule cells displayed immature traits in terms of nuclear indentations. Dentate interneurons of the basket cell type were present in both the intracerebral and the intraocular grafts. We conclude that grafted dentate granule cells, in different surroundings and without the normal, major perforant path input, can develop a basically normal cellular morphology, which includes the normal ultrastructural characteristics of the dendrites with spines and synapses, and the mossy fibers and its terminals.

Ultrastructural organization of normal and transplanted rat fascia dentata: II. A quantitative analysis of the synaptic organization of intracerebral and intraocular grafts

As part of an ultrastructural analysis of the normal rat fascia dentata and intracerebral and intraocular dentate transplants the synapses in the dentate molecular layer were quantified. Hippocampal and dentate tissue from 21-day-old rat embryos were grafted into the brain of developing and adult rats and to the anterior eye chamber of adult rats. After 100 or 200 days of survival the recipient rat brains and the recipient eyes were processed for electron microscopy, and the graft dentate molecular layer with the adjacent granule cell layer selected for ultrastructural analysis. Tissue from the dentate molecular layer of normal adult rats served as controls. The dentate synapses were classified as asymmetric (Gray's type 1) or symmetric (Gray's type 2), and according to the postsynaptic element (cell body, dendritic shaft, dendritic spine). The spine synapses were further classified into simple and complex types according to the spine-terminal configuration. Also, the length of synaptic contacts of the individual synaptic types was measured in some grafts, just as the percentage of the cross sectional area of the neuropil covered by blood vessels. The results showed that the synaptic density, expressed as number per unit area of neuropil, to a large extent was the same within the different parts of the normal dentate molecular layer. Compared with this the synaptic density was reduced with 16.4% in dentate molecular layer of the intracerebral graft, primarily because of a 17.6% reduction of simple synapses on dendritic spines and almost halving of the symmetric synapses on dendritic shafts. The synaptic density was independent of the age of the recipient, the intracerebral location of the graft, and the survival time. Although the synaptic length of some of the individual synaptic types increased, this did not compensate for the loss of synapses. In the intraocular grafts the synaptic density was lower than in the intracerebral grafts. Despite the reduced synaptic density, which mainly involved two synaptic types, we conclude that grafted dentate granule cells can develop a remarkably normal, ultrastructural synaptic organization even in the absence of major afferent inputs. This outcome must accordingly be achieved by reorganization of the available intrinsic afferents.

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.

Cellular activity of intracerebrally transplanted fetal hippocampus during behavior

Hippocampal tissue derived from 12-, 20-, 25- and 34-mm rat fetuses was placed in a cavity formed by unilateral aspiration of the fimbria-fornix and the overlying neocortical tissue in adult rats. From 4 to 6 months after transplantation the rats were equipped with chronic recording and stimulating electrodes. Single cell activity of the transplant was monitored during running in a wheel, drinking, and sleeping. Both complex-spike cells (n = 151) and single-spike cells (n = 80) were recorded from the graft. A portion of the neurons changed their firing rates and discharge patterns as a function of ongoing behavior. About half of the single-spike cells increased their firing frequency during running. Fifteen per cent of the single-spike cells fired rhythmically at about 8 Hz during running, and the paradoxical phase of sleep and the discharge pattern correlated with rhythmic slow activity (theta) recorded concurrently from the contralateral (intact) hippocampus. These patterns were most frequently obtained from grafts of 20- and 25-mm (16 to 18 embryonic days) fetuses. Graft neurons could be activated by stimulating the ipsilateral hippocampus or the ipsilateral perforant path, with latencies of 8-30 ms. The most common electrical pattern in grafts of all groups was the synchronous bursts of several neighboring cells and concurrent electroencephalogram sharp-waves. Sharp-waves occurred during all behaviors. Large amplitude, high-frequency electroencephalogram spindles (14-18 Hz and 30-50 Hz) and associated neuronal bursts were recorded in grafts of 12-, 20-, and 25-mm fetuses. Based on these findings we suggest that both subcortical afferents and host hippocampal afferents send axons to hippocampal grafts and form viable synaptic connections with a portion of the neurons in the graft. The frequently encountered population bursts are explained by assuming that excitatory collaterals in the graft are more potent in the graft than in the normal hippocampus, and/or GABAergic inhibition is less efficient in the graft.

Interactions between grafted serotonin neurons and adult host rat hippocampus

We have followed the development of physiological and functional properties of serotonin-containing raphe neurons grafted into an adult host hippocampus as a model system for graft-host interactions. These raphe cells have no clear identifying properties on the day of grafting: they develop them while growing in the host. Raphe neurons, recorded 1 month after grafting, possess adult normal physiological properties. These include high input resistance, slow membrane time constant, lack of inward rectification, a transient outward rectification, broad spikes having a Ca2+ component, lack of accommodation, and a large afterhyperpolarization. The graft is first innervated by host fibers and later projects to the host tissue. When stimulated, postsynaptic hyperpolarized responses are recorded in hippocampal neurons. In the freely moving rat, raphe grafts can restore sleep-wakefulness variations in an evoked population response of the hippocampus to afferent stimulation, which is eliminated by depletion of serotonin. These studies illustrate that grafted serotonin neurons develop functional relations with a host brain.

Extracellular activation and membrane conductances of neurones in the guinea-pig deep cerebellar nuclei in vitro

The responses of cerebellar nuclear cells to extracellular stimulation in a slice preparation were studied and the ionic basis of their electroresponsiveness was investigated with blockers of membrane conductances and with ion substitutions in the extracellular medium. The cells could be activated antidromically from the cerebellar cortex and the white matter surrounding the nuclei. The dominating response to orthodromic stimulation was an inhibitory synaptic potential presumably produced by activation of Purkinje cell fibres. The action potentials and the subthreshold spikelets were shown to be Na+ dependent and are presumably generated by a voltage-dependent inactivating Na+ conductance. Plateau potentials with a low threshold were also Na+ dependent, but these long-lasting potentials are probably produced by activation of a voltage-dependent non-inactivating Na+ conductance. Plateau potentials with a high threshold and high-threshold spikelets were Ca2+ dependent and seem to be generated by non-inactivating and possibly inactivating Ca2+ conductances. The spike after-hyperpolarizations had an early voltage-dependent K+ component and a late Ca2+-dependent K+ component. They are therefore produced by voltage-sensitive and Ca2+-dependent K+ conductances. By analogy with the distribution of conductances in Purkinje cells it is proposed that the Na+ conductances are mainly located in the somatic and axonal membrane and that the Ca2+ conductances are located in the dendrites. The functional implications of the complex electroresponsive properties of cerebellar nuclear cells are discussed.