Biomedical subjects

N Sagiv

Publications and source records attributed to N Sagiv.

6 recordsLinked to original sources

PubMed2003

Presynaptic and postsynaptic GABAA receptors in rat suprachiasmatic nucleus.

The mammalian suprachiasmatic nucleus (SCN), the brain's circadian clock, is composed mainly of GABAergic neurons, that are interconnected via synapses with GABA(A) receptors. Here we report on the subcellular localization of these receptors in the SCN, as revealed by an extensively characterized antibody to the alpha 3 subunit of GABA(A) receptors in conjunction with pre- and postembedding electron microscopic immunocytochemistry. GABA(A) receptor immunoreactivity was observed in neuronal perikarya, dendritic processes and axonal terminals. In perikarya and proximal dendrites, GABA(A) receptor immunoreactivity was expressed mainly in endoplasmic reticulum and Golgi complexes, while in the distal part of dendrites, immunoreaction product was associated with postsynaptic plasma membrane. Many GABAergic axonal terminals, as revealed by postembedding immunogold labeling, displayed GABA(A) receptor immunoreactivity, associated mainly with the extrasynaptic portion of their plasma membrane. The function of these receptors was studied in hypothalamic slices using whole-cell patch-clamp recording of the responses to minimal stimulation of an area dorsal to the SCN. Analysis of the evoked inhibitory postsynaptic currents showed that either bath or local application of 100 microM of GABA decreased GABAergic transmission, manifested as a two-fold increase in failure rate. This presynaptic effect, which was detected in the presence of the glutamate receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione and the selective GABA(B) receptor blocker CGP55845A, appears to be mediated via activation of GABA(A) receptors. Our results thus show that GABA(A) receptors are widely distributed in the SCN and may subserve both pre- and postsynaptic roles in controlling the mammalian circadian clock.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2001

GABA-induced current and circadian regulation of chloride in neurones of the rat suprachiasmatic nucleus.

1. We have shown previously that GABA, the main neurotransmitter in the suprachiasmatic nucleus (SCN), has dual effects on SCN neurones, excitatory during the day and inhibitory at night. This duality has been attributed to changes in [Cl(-)](i) during the circadian cycle. To unravel the processes underlying these changes we investigated the biophysical properties of the GABAergic receptors and the regulation of [Cl(-)](i) in SCN neurones. 2. We used voltage-clamp methodology in conjunction with local application of GABA to characterise the current induced by GABA in SCN neurones within acute brain slices. This current, mediated via GABA(A) receptors, shows moderate voltage dependence, does not desensitise and can significantly alter [Cl(-)](i). 3. Loading or depletion of intracellular Cl(-) was induced by a train of GABA pulses. The recovery of intracellular Cl(-) was deduced from the change in [Cl(-)](i) calculated from the response to a test GABA pulse presented at different intervals after the conditioning train of GABA application. The time course of recovery was described by an exponential curve. Recovery following Cl(-) depletion was slower than recovery from Cl(-) loading and was further delayed during the subjective night. 4. We concluded that: (a) SCN neurones express a large number of somatic GABA(A) receptors, which give rise to a modifiable, tonic Cl(-) conductance that modulates cell excitability; (b) two Cl(-) transport mechanisms operate in SCN neurones, one that replenishes the cell with Cl(-) following Cl(-) depletion and another that removes Cl(-) after Cl(-) loading; (c) the efficiency of the replenishing mechanism is reduced during the subjective night; and (d) this reduction explains a lower [Cl(-)](i) during the night phase of the circadian cycle.

Animals↗

PubMed2001

Structural encoding of human and schematic faces: holistic and part-based processes.

The range of specificity and the response properties of the extrastriate face area were investigated by comparing the N170 event-related potential (ERP) component elicited by photographs of natural faces, realistically painted portraits, sketches of faces, schematic faces, and by nonface meaningful and meaningless visual stimuli. Results showed that the N170 distinguished between faces and nonface stimuli when the concept of a face was clearly rendered by the visual stimulus, but it did not distinguish among different face types: Even a schematic face made from simple line fragments triggered the N170. However, in a second experiment, inversion seemed to have a different effect on natural faces in which face components were available and on the pure gestalt-based schematic faces: The N170 amplitude was enhanced when natural faces were presented upside down but reduced when schematic faces were inverted. Inversion delayed the N170 peak latency for both natural and schematic faces. Together, these results suggest that early face processing in the human brain is subserved by a multiple-component neural system in which both whole-face configurations and face parts are processed. The relative involvement of the two perceptual processes is probably determined by whether the physiognomic value of the stimuli depends upon holistic configuration, or whether the individual components can be associated with faces even when presented outside the face context.

Adult↗

PubMed2001

Neural fate of seen and unseen faces in visuospatial neglect: a combined event-related functional MRI and event-related potential study.

To compare neural activity produced by visual events that escape or reach conscious awareness, we used event-related MRI and evoked potentials in a patient who had neglect and extinction after focal right parietal damage, but intact visual fields. This neurological disorder entails a loss of awareness for stimuli in the field contralateral to a brain lesion when stimuli are simultaneously presented on the ipsilateral side, even though early visual areas may be intact, and single contralateral stimuli may still be perceived. Functional MRI and event-related potential study were performed during a task where faces or shapes appeared in the right, left, or both fields. Unilateral stimuli produced normal responses in V1 and extrastriate areas. In bilateral events, left faces that were not perceived still activated right V1 and inferior temporal cortex and evoked nonsignificantly reduced N1 potentials, with preserved face-specific negative potentials at 170 ms. When left faces were perceived, the same stimuli produced greater activity in a distributed network of areas including right V1 and cuneus, bilateral fusiform gyri, and left parietal cortex. Also, effective connectivity between visual, parietal, and frontal areas increased during perception of faces. These results suggest that activity can occur in V1 and ventral temporal cortex without awareness, whereas coupling with dorsal parietal and frontal areas may be critical for such activity to afford conscious perception.

Aged↗

PubMed2001

Two eyes make a pair: facial organization and perceptual learning reduce visual extinction.

We examined a patient with left spatial neglect and visual extinction due to right parietal damage in tasks where identical stimuli were presented before and after they were primed so as to be perceived as the eyes of schematic faces. In a first block, we presented alphanumeric stimuli (+, o, T, 6) on the right, left, or both sides of fixation on a blank background, and established that the patient could perceive unilateral stimuli on either side but extinguished most of the left-sided ones in the bilateral trials. In a second block, some of these stimuli (+, o) were presented again, but now in the position of eyes within the context of an oval frame which created the impression of a schematic face. Other stimuli (T, 6) were presented as previously on a blank background for an equal number of trials. In the third critical block, all stimuli were presented once again on a blank background, as in the first block. Now the patient extinguished very few of those left-sided stimuli primed to be seen as a pair of eyes in face configuration (+, o), but still extinguished most of the other stimuli (T, 6). A second control experiment showed no effect of repeatedly exposing stimuli in a common region of space defined by meaningless shape boundaries. These results suggest that facial organization can group eye features before the level where attentional selection or extinction occurs, and that such grouping may be influenced by rapid perceptual learning.

Facial Expression↗

PubMed1998

Research notes: Fresh and frozen pools of chicken red blood cells as substrates for direct polymerase chain reaction.

A method is presented for reliable use of pooled chicken blood samples for estimation of microsatellite frequencies by direct polymerase chain reaction (PCR) amplification of DNA. This method overcomes the variability of hematocrit values in individual chickens and eliminates the step of DNA preparation. The estimated frequencies of polymorphic alleles in fresh and frozen pooled blood samples were similar to those obtained by calculating these frequencies from the individual genotyping. When frozen pooled blood samples are used, pools should be prepared prior to their freezing.

Animals↗