Noggin signaling from Xenopus animal blastomere lineages promotes a neural fate in neighboring vegetal blastomere lineages.

In Xenopus, localized factors begin to regionalize embryonic fates prior to the inductive interactions that occur during gastrulation. We previously reported that an animal-to-vegetal signal that occurs prior to gastrulation promotes primary spinal neuron fate in vegetal equatorial (C-tier) blastomere lineages. Herein we demonstrate that maternal mRNA encoding noggin is enriched in animal tiers and at low concentrations in the C-tier, suggesting that the neural fates of C-tier blastomeres may be responsive to early signaling from their neighboring cells. In support of this hypothesis, experimental alteration of the levels of Noggin from animal equatorial (B-tier) or BMP4 from vegetal (D-tier) blastomeres significantly affects the numbers of primary spinal neurons derived from their neighboring C-tier blastomeres. These effects are duplicated in blastomere explants isolated at cleavage stages and cultured in the absence of gastrulation interactions. Co-culture with animal blastomeres enhanced the expression of zygotic neural markers in C-tier blastomere explants, whereas co-culture with vegetal blastomeres repressed them. The expression of these markers in C-tier explants was promoted when Noggin was transiently added to the culture during cleavage/morula stages, and repressed with the transient addition of BMP4. Reduction of Noggin translation in B-tier blastomeres by antisense morpholino oligonucleotides significantly reduced the efficacy of neural marker induction in C-tier explants. These experiments indicate that early anti-BMP signaling from the animal hemisphere recruits vegetal equatorial cells into the neural precursor pool prior to interactions that occur during gastrulation.     

The dorsomedial frontal cortex of the macaca monkey: fixation and saccade-related activity

Summary The activity of 249 neurons in the dorsomedial frontal cortex was studied in two macaque monkeys. The animals were trained to release a bar when a visual stimulus changed color in order to receive reward. An acoustic cue signaled the start of a series of trials to the animal, which was then free to begin each trial at will. The monkeys tended to fixate the visual stimuli and to make saccades when the stimuli moved. The monkeys were neither rewarded for making proper eye movements nor punished for making extraneous ones. We found neurons whose discharge was related to various movements including those of the eye, neck, and arm. In this report, we describe the properties of neurons that showed activity related to visual fixation and saccadic eye movement. Fixation neurons discharged during active fixation with the eye in a given position in the orbit, but did not discharge when the eye occupied the same orbital positions during nonactive fixation. These neurons showed neither a classic nor a complex visual receptive field, nor a foveal receptive visual field. Electrical stimulation at the site of the fixation neurons often drove the eye to the orbital position associated with maximal activity of the cell. Several different kinds of neurons were found to discharge before saccades: 1) checking-saccade neurons, which discharged when the monkeys made self-generated saccades to extinguish LED's; 2) novelty-detection saccade neurons, which discharged before the first saccade made to a new visual target but whose activity waned with successive presentations of the same target. These results suggest that the dorsomedial frontal cortex is involved in attentive fixation. We hypothesize that the fixation neurons may be involved in codifying the saccade toward a target. We propose that their involvement in arm-eye-head motor-planning rests primarily in targeting the goal of the movement. The fact that saccaderelated neurons discharge when the saccades are self initiated, implies that this area of the cortex may share the control of voluntary saccades with the frontal eye fields and that the activation is involved in intentional motor processes.     

Hyperpolarization of hypothalamic parvocellular neurons by 17 β-estradiol and their identification through intracellular staining with procion yellow

Summary Intracellular recordings and injections of procion yellow (PY) were made in parvocellular neurons in hypothalamic slices of female guinea pigs. Eighty-five neurons, with an average resting membrane potential of -35 mV, were recorded in the arcuate (ARC) ventromedial (VM), and in the cellpoor zones between the ARC and VM. Eleven of the ARC neurons and four neurons from the cell-poor zone could be driven antidromically by median eminence (ME) stimulation, nine other neurons from the three areas could be driven orthodromically by stria terminalis (ST) stimulation.Twenty-eight parvocellular neurons were tested with 17 -estradiol (E₂), which was applied in the bathing medium as the free steroid. Eleven neurons (nine ARC and two cell-poor-zone neurons) were hyperpolarized 2 to 24 mV by 10^–10 M E₂ concentrations. 10^–8 M estrone concentration was without effect on three of these cells. Through the intracellular injection of PY, the estrogen-sensitive neurons (N = 11) were identified as small fusiform cells with few dendrites. Spine-like appendages were found on only one of these cells. None of the larger pyramidal-like neurons of these areas responded to the application of E₂.Postdoctoral research fellow of the National Institute of Neurological and Communicative Disorders and Stroke     

Intracerebral xenografts of dopamine neurons: the role of immunosuppression and the blood-brain barrier

Summary Fetal mesencephalic mouse tissue, rich in dopamine neurons, was xenografted as a dissociated cell suspension into the striatum of rats with unilateral 6-hydroxydopamine induced lesions of the mesostriatal pathway. The rats were either assigned to a 10-day, 21-day or 42-day Cyclosporin A (CyA) immunosuppression scheme, or given no immunosuppression. The functional effects of the grafts were followed over 6 months by monitoring changes in the recipient rats' amphetamine-induced turning behaviour. Without immunosuppression no grafts were functional at the end of the experiment. In the 10-, 21-and 42-day CyA treatment groups there was a significant reduction of rotational asymmetry at some timepoint following grafting in 26 of the 33 rats. However, by 6 months only 8 grafts remained functional suggesting that in several rats an immunological rejection took place following the termination of immunosuppression. This was supported by catecholamine histofluorescence analysis which revealed evidence of surviving grafts only in the few rats which had shown sustained functional graft effects at 6 months after grafting. In animals in which the grafts had undergone rejection, there was scarlike tissue in the striatum which appeared more extensive in rats that had lost their grafts after several weeks compared to rats in which the grafts were rejected at an early time-point. In a subgroup of the grafted animals the humoral antibody response against major transplantation antigens present on the grafted cells was investigated. All the studied rats were found to be immunized against the grafted mouse tissue following the intrastriatal implantation. This occurred irrespective of prior immunosuppressive treatment. In a parallel group of rats, the leakage of the blood-brain barrier was studied following intrastriatal implantation of a syngeneic fetal neural cell suspension. Evans Blue was infused into rats 3–12 days following transplantation surgery. At the early time-points there was a marked barrier leakage at the implantation site. This subsided with time such that there was minor leakage after 7–8 days and no leakage after 12 days. In summary, the results indicate the CyA is effective in promoting survival of intracerebral xenografts of fetal neural tissue, but that cessation of immunosuppressive treatment in most cases results in rejection of the grafted tissue. Temporary CyA treatment, even exceeding the time it takes for the blood-brain barrier to reform after transplantation surgery, is thus not sufficient to reliably support long term survival of xenografted dopamine neurons.     

Saccades and multisaccadic gaze shifts are gated by different pontine omnipause neurons in head-fixed cats

 Pontine omnipause neurons (OPNs) have so far been considered as forming a homogeneous group of neurons whose tonic firing stops during the duration of saccades, when the head is immobilized. In cats, they pause for the total duration of gaze shifts, when the head is free to move. In the present study, carried out on alert cats with fixed heads, we present observations made during self-initiated saccades and during tracking of a moving target which show that the OPN population is not homogeneous. Of the 76 OPNs we identified, 39 were found to have characteristics similar to those of previously described neurons, ”saccade” (S-) OPNs: (1) the durations of their pauses were significantly correlated with the durations of saccades; (2) the discharge ceased shortly before saccade onset and resumed before saccade end; (3) visual responses to target motion were excitatory; and (4) during tracking, S-OPNs interrupted the discharge for the duration of saccades and resumed firing during perisaccadic ”drifts”. However, the characteristics of 37 neurons (”complex” (C-) OPNs) were different: (1) the pause duration was not correlated with the duration of self-initiated saccades; (2) time lead of pause onsets relative to saccades was, on average, longer than in the group of S-OPNs, and firing resumed after the saccade end; (3) visual target motion suppressed tonic discharges; and (4) during tracking, firing was interrupted for the total duration of gaze shifts, including not only saccades but also perisaccadic ”drifts”. We conclude that cat OPNs can be subdivided into two main groups. The first comprises neurons whose firing patterns are compatible with gating individual saccades (”saccade” OPNs). The second group consists of ”complex” OPNs whose firing characteristics are appropriate to gate total gaze displacements rather than individual saccades. The function of these neurons may be to disinhibit pontobulbar circuits participating in the generation of saccade sequences and associated perisaccadic drifts. Received: 20 January 1998 / Accepted: 22 October 1998     

“Dark” (compacted) neurons may not die through the necrotic pathway

Dark neurons were produced in the cortex of the rat brain by hypoglycemic convulsions. In the somatodendritic domain of each affected neuron, the ultrastructural elements, except for disturbed mitochondria, were remarkably preserved during the acute stage, but the distances between them were reduced dramatically (ultrastructural compaction). Following a 1-min convulsion period, only a few neurons were involved and their environment appeared undamaged. In contrast, 1-h convulsions affected many neurons and caused swelling of astrocytic processes and neuronal dendrites (excitotoxic neuropil). A proportion of dark neurons recovered the normal structure in 2 days. The non-recovering dark neurons were removed from the brain cortex through two entirely different pathways. In the case of 1-h convulsions, their organelles swelled, then disintegrated and finally dispersed into the neuropil through large gaps in the plasma membrane (necrotic-like removal). Following a 1-min convulsion period, the non-recovering dark neurons fell apart into membrane-bound fragments that retained the compacted interior even after being engulfed by astrocytes or microglial cells (apoptotic-like removal). Consequently, in contrast to what is generally accepted, the dark neurons produced by 1-min hypoglycemic convulsions do not die as a consequence of necrosis. As regards the case of 1-h convulsions, it is assumed that a necrotic-like removal process is imposed, by an excitotoxic environment, on dark neurons that previously died through a non-necrotic pathway. Apoptotic neurons were produced in the hippocampal dentate gyrus by intraventricularly administered colchicine. After the biochemical processes had been completed and the chromatin condensation in the nucleus had reached an advanced phase, the ultrastructural elements in the somatodendritic cytoplasm of the affected cells became compacted. If present in an apparently undamaged environment such apoptotic neurons were removed from the dentate gyrus through the apoptotic sequence of morphological changes, whereas those present in an impaired environment were removed through a necrotic-like sequence of morphological changes. This suggests that the removal pathway may depend on the environment and not on the death pathway, as also assumed in the case of the dark neurons produced by hypoglycemic convulsions.     

Participation of Ia reciprocal inhibitory neurons in the spinal circuitry of the tonic neck reflex

Summary As part of our studies of the spinal circuitry of the tonic neck reflex, we have recorded extracellularly from Ia reciprocal inhibitory neurons of the decerebrate, labyrinthectomized cat. The activity of a majority of neurons driven by stimulation of the quadriceps nerve was modulated by sinusoidal rotation of the neck; such modulation was much less frequent in the case of neurons driven by stimulation of nerves to more distal muscles. The results suggest that some of the inhibition which is part of the tonic neck reflex is mediated by Ia reciprocal inhibitory neurons, but that other pathways must also play an important role.     

Effect of the gaba antagonist 4-ethylbicyclophosphate on global and synaptic neuronal evoked responses in hippocampal slices

Laboratory of Organic Synthesis, Reception of Physiologically Active Substances Group, Institute of Physiologically Active Substances, Academy of Sciences of the USSR, Chernogolovka, Moscow Region. Laboratory of Functional Synaptology, Brain Institute, All-Union Mental Health Research Center, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR O. S. Adrianov. Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 111, No. 4, pp. 339–341, April, 1991.     

Responsiveness of solitario-parabrachial relay neurons to taste and mechanical stimulation applied to the oral cavity in rats

Summary A total of 311 units, responsive to natural stimulation of the oral cavity, were isolated from the rostral part of the solitary tract nucleus (NTS) of rats. Of these, 169 taste neurons, activated by taste stimulation, and 142 mechanoreceptive units, exclusively sensitive to mechanical stimulation of the oral cavity, were found. Most taste units (62.3%) were also excited by mechanical stimulation. Forty-three (34.1%) of the 126 taste units examined were identified as solitario-parabrachial relay (SP) neurons by antidromic stimulation from the ipsilateral dorsal pons, while only eleven (12.6%) of the 87 mechanoreceptive units were SP neurons. Taste SP neurons could be divided into two subgroups according to their antidromic latency; the fast SP units with an antidromic latency shorter than 9 ms and slow SP units with a longer antidromic latency. These two subgroups were not differentiated in any physiological properties except that the fast SP neurons were frequently excited by sucrose. Taste neurons were classified according to the best stimulus of the four basic taste solutions to produce the largest number of discharges in each neuron. All types of taste neurons were found among the SP and non-SP neurons, but only a small number of quinine-best neurons (n = 2) were found in the SP neuron group compared to the number of quinine-best neurons in the non-SP neuron group (n = 10). A histological examination of the recording sites revealed that taste relay neurons were found at the central or dorsal part of the nucleus but mechanoreceptive relay neurons were found at the peripheral part, although relay and non-relay neurons of either class were intermingled in the nucleus.Supported by a Grant from the Ministry of Education, Science and Culture of Japan (No. 58106008)     

Developing neuronal populations of the cat retinal ganglion cell layer

An improved flat-mount procedure demonstrates that the developing ganglion cell layer of the cat retina contains two morphologically distinct populations of presumed neurons at all ages between embryonic day 36 (E36) and adulthood. One population resembles the adult "classical neurons" composing the ganglion cells and bar-cells of Hughes, while the remaining cells, which are smaller and possess much less Nissl substance, presumably correspond to precursors of the adult microneurons. Although the total neuron population of the retinal ganglion cell layer remains quite constant at all studied ages, its component subpopulations alter significantly during prenatal development; some 50% of classical neurons disappear before birth and the microneuron population doubles during the same period. An obvious centroperipheral gradient exists for classical neurons by stage E47, but the microneuron density gradient only becomes apparent at birth. A 2:1 centroperipheral ratio for the total neuron population is also apparent at E47. Centroperipheral neuronal density gradients continue to increase during postnatal growth. Loss of classical neurons during prenatal life as a result of cell death or transformation into microneurons, has been postulated as a mechanism for determining neuron density gradients. Cell death does occur in the ganglion cell population but it is not yet established whether microneurons of the ganglion cell layer originate from ganglion cell transformation or migrate as a differentiated class from the ventricular layer. However, it can be concluded that not all microneurons originate from ganglion cell transformation, because the total loss of classical neurons is less than the increase in microneuron numbers during development. The population magnitudes of both neuronal classes in the ganglion cell layer stabilise after birth. However, it is during the postnatal period that the adult cruciate density topography is achieved by both populations. It is concluded that differential areal growth is the prime mechanism for postnatal cell redistribution.