Parvocellular Cells Research Articles

Ghrelin: a hypothalamic GH-releasing factor in domestic fowl (Gallus domesticus).

Ghrelin, a recently discovered peptide in the mammalian hypothalamus and gastrointestinal tract is thought to be the endogenous ligand for the GH secretagogue (GHS) receptor and it stimulates GH release in rats and humans. The possibility that ghrelin is present in birds was therefore assessed, since a GHS receptor is present in the chicken pituitary gland. Although immunoreactive ghrelin is readily detectable in the rat stomach and ileum, ghrelin immunoreactivity could not be detected in the chicken proventriculus, stomach, ileum or colon, whereas somatostatin immunoreactivity, in contrast and as expected, was readily detectable in the chicken gastrointestinal tract. Ghrelin immunoreactivity was, however, present in the chicken hypothalamus, although not in the arcuate (infundibular) nucleus, as in rats. Discrete parvocellular cells and neuronal fibers with ghrelin immunoreactivity were present in the anterior medial hypothalamus. This immunoreactivity was specific and completely abolished following the preabsorption of the antibody with an excess of human ghrelin. Ghrelin immunoreactivity was also present in clusters of large ovoid magnocellular cells in the nucleus magnocellularis preopticus pars medialis, nucleus magnocellularis preopticus supraopticus and in the chiasmaopticus. Immunoreactivity for ghrelin was restricted to the cytoplasm of the perikarya and their axonal sprouts. Immunoreactivity for ghrelin was not seen in any other hypothalamic nuclei. In a preliminary experiment, circulating GH concentrations in conscious immature chicks were promptly increased following bolus i.v. administration of human ghrelin. The increase in GH concentration (approximately three times that in the controls) was comparable with that induced by the same dose (10 microg/kg) of human GH-releasing hormone, although less than that (approximately sixfold) induced by thyrotropin-releasing hormone. These results demonstrate the presence of a ghrelin-like protein in the chicken hypothalamus and suggest that it participates in the regulation of GH secretion in birds.

Are primate lateral geniculate nucleus (LGN) cells really sensitive to orientation or direction?

There is considerable controversy over the existence of orientation and direction sensitivity in lateral geniculate nucleus (LGN) neurons. Claims for the existence of these properties often were based upon data from cells tested well beyond their peak spatial frequencies. The goals of the present study were to examine the degree of orientation and direction sensitivity of LGN cells when tested at their peak spatial and temporal frequencies and to compare the tuning properties of these subcortical neurons with those of visual cortex. For this investigation, we used conventional extracellular recording to study orientation and direction sensitivities of owl monkey LGN cells by stimulating cells with drifting sinusoidal gratings at peak temporal frequencies, peak or higher spatial frequencies, and moderate contrast. A total of 110 LGN cells (32 koniocellular cells, 34 magnocellular cells, and 44 parvocellular cells) with eccentricities ranging from 2.6 deg to 27.5 deg were examined. Using the peak spatial and temporal frequencies for each cell, 41.8% of the LGN cells were found to be sensitive to orientation and 19.1% were direction sensitive. The degree of bias for orientation and direction did not vary with eccentricity or with cell class. Orientation sensitivity did, however, increase, and in some cases orientation preferences changed, at higher spatial frequencies. Increasing spatial frequency had no consistent effect on direction sensitivity. Compared to cortical cell orientation tuning, the prevalence and strength of LGN cell orientation and direction sensitivity are weak. Nevertheless, the high percentage of LGN cells sensitive to orientation even at peak spatial and temporal frequencies reinforces the view that subcortical biases could, in combination with activity-dependent cortical mechanisms and/or cortical inhibitory mechanisms, account for the much narrower orientation and direction tuning seen in visual cortex.

Cyanamide-induced activation of the hypothalamo-pituitary-adrenal axis.

The present study investigated the effects of acute administration of cyanamide (a potent inhibitor of aldehyde dehydrogenase used to treat alcoholics), on the hypothalamo-pituitary adrenal (HPA)-axis. Cyanamide resulted in a significant increase in arginine vasopressin mRNA and corticotrophin releasing factor (CRF) mRNA in the parvocellular cells of the paraventricular nucleus and pro-opiomelanocortin (POMC) mRNA in the anterior pituitary. Plasma corticosterone concentrations were elevated by a range of doses of cyanamide which were maintained in the high dose group at 4 h following administration. These results suggest that cyanamide is able to activate the HPA axis at all levels of the axis. Arginine vasopressin mRNA, in the parvocellular cells of the paraventricular nucleus is an important component of the stress response. Silver grain counting of emulsion dipped slides is commonly used for its evaluation following in-situ hybridization. This method is however, not entirely satisfactory and very time-consuming. We compared this method with a film autoradiographic method, and show that the film autoradiographic method is valid for the determination of arginine vasopressin mRNA in the parvocellular cells of the paraventricular nucleus.

Stereological evaluation of neurons and glia in the monkey dorsal lateral geniculate nucleus following an early cerebral hemispherectomy.

The effects of an early, unilateral cerebral hemispherectomy on the cytoarchitecture of the dorsal lateral geniculate nucleus (dLGN) were quantitatively evaluated in the green monkey. The dLGN ipsilateral to the lesion showed a 73% reduction in size, more than 99% neuronal cell loss, 50% increase in glial cell density, but a 50% reduction in the total number of glial cells. The total number of neural and glial cells estimated for the dLGN contralateral to the ablation did not differ from control values. Despite evidence for substantial degeneration of the ipsilateral dLGN, cytochrome oxidase histochemistry revealed a small population of surviving cells that exhibited features of neuronal cells. More surviving cells were found in the parvocellular than in the magnocellular layers, and surviving parvocellular cells had the same size-frequency distribution as Nissl-stained neurons in an intact animal. These findings suggest that the intrinsic geniculate circuitry may be able to sustain the residual interneurons that can, in turn, contribute to maintaining retinal and brainstem afferents. The remaining neurons in the dLGN following hemispherectomy appear to be insufficient in number to be importantly implicated in the residual visual functions that have been reported in some hemispherectomized patients.

Size-selective neuronal changes in the anterior optic pathways suggest a differential susceptibility to injury in multiple sclerosis.

Axonal damage is found in both acute and chronic lesions of multiple sclerosis. Direct axon counting in post-mortem tissue has suggested that smaller axons might have a greater susceptibility to damage, but methodological limitations have precluded unequivocal interpretation. However, as neuronal and axonal sizes are linked and neuronal changes would be expected with retrograde or transsynaptic degeneration following axon injury, we hypothesized that an alternative strategy for studying this phenomenon would be to define multiple sclerosis-associated changes in neurones. To test this hypothesis, we measured both axonal loss and neuronal size changes in the anterior optic pathway [including the optic nerve (ON), optic tract (OT) and lateral geniculate nucleus] of the brains of eight patients who died with multiple sclerosis and in eight control brains. The ONs and OTs in brains from the multiple sclerosis patients showed a trend to smaller mean cross-sectional areas (ON, multiple sclerosis = 6.84 mm(2), controls = 9.25 mm(2); and OT, multiple sclerosis = 6.45 mm(2), controls = 7.94 mm(2), P = 0.08) and had reduced axonal densities (ON, multiple sclerosis = 1.1 x 10(5)/mm(2), controls = 1.7 x 10(5)/mm(2); and OT, multiple sclerosis = 1.4 x 10(5)/mm(2), controls = 1.8 x 10(5)/mm(2), P = 0.006). Estimated total axonal counts were reduced by 32 (OT)-45% (ON) in the patients relative to controls (ON, multiple sclerosis = 8.1 x 10(5) axons, controls = 14.8 x10(5), P = 0.05; and OT, multiple sclerosis = 9.1 x 10(5), controls = 13.3 x 10(5), P = 0.02). The size distributions of the magnocellular cells in the lateral geniculate nucleus were similar for the two groups, but in multiple sclerosis brains the parvocellular cells were significantly smaller (mean sizes: multiple sclerosis = 226 microm(2), controls = 230 microm(2), P < 0.001) and had a larger variation in size, suggesting a greater proportion of atrophic neurones. Axon loss in the optic nerves of multiple sclerosis patients correlated strongly with measures of increased dispersion of cell sizes in the parvocellular layer (r = 0.8, P < 0.04). These data demonstrate that both atrophy and decreased density contribute to the substantial axonal loss in the anterior visual pathway of these patients. This appears related to a relatively selective atrophy of the smaller neurones of the parvocellular layer in the lateral geniculate nucleus, supporting the hypothesis that smaller axons may be preferentially susceptible to injury in multiple sclerosis.

Spatial properties of koniocellular cells in the lateral geniculate nucleus of the marmoset Callithrix jacchus.

1. The receptive field dimensions, contrast sensitivity and linearity of spatial summation of koniocellular (KC), parvocellular (PC) and magnocellular (MC) cells in the lateral geniculate nucleus (LGN) of 11 adult marmosets were measured using achromatic sinusoidal gratings. 2. The receptive field centre diameter of cells in each (PC, KC and MC) class increases with distance from the fovea. There is substantial overlap in centre size between the three cell classes at any eccentricity, but the PC cells have, on average, the smallest centres and the KC cells have the largest. Some PC and KC cells did not respond at all to the grating stimulus. 3. The contrast sensitivity of the receptive field centre mechanism in KC cells decreases in proportion to the centre area. A similar trend was seen for the surround mechanism. These characteristics are common to PC and MC cells, suggesting that they originate at an early stage of visual processing in the retina. 4. The KC cells showed, in general, lower peak evoked discharge rates than PC or MC cells. The spontaneous discharge rate of KC cells was lower than that of PC cells and similar to that of MC cells. 5. The majority of cells in all divisions of the LGN show linear spatial summation. A few cells did show non-linear spatial summation; these cells were predominantly located in the MC and ventral KC layers. 6. The ventral KC layers below and between the MC layers contain cells with larger and more transiently responding receptive fields than cells in the more dorsal KC layers. 7. We conclude that many of the contrast-dependent spatial properties of cells in the marmoset LGN are common to PC, MC and KC cells. The main difference between KC cells and the other two classes is that there is more variability in their response properties, and they are less responsive to high spatial frequencies.

Central orexin-A activates hypothalamic-pituitary-adrenal axis and stimulates hypothalamic corticotropin releasing factor and arginine vasopressin neurones in conscious rats.

The effects of centrally injected orexin-A on plasma adrenocorticotropin (ACTH) and corticosterone levels and corticotropin releasing factor (CRF) and arginine vasopressin (AVP) mRNA in the parvocellular cells of the paraventricular nucleus (PVN) of the rat were investigated. In animals implanted previously with a lateral brain ventricle and femoral artery cannula, a single i.c.v. injection of orexin-A (10 microg/rat) resulted in a rapid, significant increase in plasma ACTH and corticosterone concentrations. Plasma ACTH reached a peak (12.5-fold greater than basal levels) at 30 min, which was maintained over 120 min before declining towards control levels by 240 min. Plasma corticosterone concentrations reached a peak (6.7-fold greater than basal levels) at 30 min. Orexin-A at a higher dose (30 microg/rat) also produced a rapid increase in plasma ACTH and corticosterone concentrations. The area under the curve for plasma levels of ACTH was similar for both doses of orexin-A. In a second study, orexin-A (10 microg/rat) was injected i.c.v. and brains and pituitaries were rapidly removed after 240 min. In situ hybridization histochemistry revealed that CRF and AVP mRNA levels were significantly increased in the parvocellular cells of the PVN. Pro-opiomelanocortin mRNA levels in the pituitary gland were not significantly elevated in response to orexin-A. These results suggest that orexin-A is able to act centrally to activate the hypothalamic-pituitary-adrenal axis involving stimulation of both CRF and AVP expression.

Chromatic sensitivity of ganglion cells in the peripheral primate retina.

Visual abilities change over the visual field. For example, our ability to detect movement is better in peripheral vision than in foveal vision, but colour discrimination is markedly worse. The deterioration of colour vision has been attributed to reduced colour specificity in cells of the midget, parvocellular (PC) visual pathway in the peripheral retina. We have measured the colour specificity (red-green chromatic modulation sensitivity) of PC cells at eccentricities between 20 and 50 degrees in the macaque retina. Here we show that most peripheral PC cells have red-green modulation sensitivity close to that of foveal PC cells. This result is incompatible with the view that PC pathway cells in peripheral retina make indiscriminate connections ('random wiring') with retinal circuits devoted to different spectral types of cone photoreceptors. We show that selective cone connections can be maintained by dendritic field anisotropy, consistent with the morphology of PC cell dendritic fields in peripheral retina. Our results also imply that postretinal mechanisms contribute to the psychophysically demonstrated deterioration of colour discrimination in the peripheral visual field.

Influence of Contrast on the Responses of Marmoset Lateral Geniculate Cells to Drifting Gratings

The responses of lateral geniculate nucleus (LGN) cells in the common marmoset (Callithrix jacchus) to drifting luminance or cone isolating gratings of different spatial frequencies and contrasts were measured. The response noise, defined as the variability of the responses to single sweeps in the complex plane, was independent of stimulus contrast and spatial frequency but increased with increasing overall responsiveness of the cell. The signal-to-noise ratio of parvocellular (PC) cells was smaller than of magnocellular (MC) cells. At each contrast, the response amplitude as a function of spatial frequency could be described with a difference of Gaussians model. With this model, the sizes and the peak sensitivities of the receptive field centers and surrounds were estimated. It was found that receptive field center and surround sizes of LGN cells decrease slightly with increasing contrast. Further, the peak sensitivity decreases with increasing contrast. The two factors are involved in a decrease in responsivity (the response per unit contrast) with increasing contrast which is compatible to response saturation for low spatial frequency stimuli. PC cells did not saturate as much to luminance stimuli although some saturation was found with cone isolating gratings. We found that the response phase lag of both PC and MC cells decreased with increasing contrast, which cannot be explained on the basis of linear response behavior. Apparently the phase of LGN cell responses to drifting gratings is altered in comparison with the retinal inputs by additional nonlinearities.

Neurophysiologic mechanisms of attention: a selective review of early information processing in schizophrenics.

Attention is an integral component of information processing. A pronounced attention deficit exists among people with schizophrenia and their first-degree relatives as compared to persons without this pathology. Schizophrenics demonstrate marked deficiencies on psychophysical tasks that require temporal and / or spatial integration, properties that are associated with the two primary visual pathways composed of magnocellular (M) and parvocellular (P) cells, respectively. The deficit expresses itself as a dysfunctional information processing system that affects higher order processes, for example, perceptual ability and memory. The focus of this review is to integrate results from several divergent areas of research to include those studies that identify the contributions of the M and P pathways associated with information processing and the attention deficit. The diverse approaches reviewed in this chapter converge to provide a neurophysiologic explanation of the attention deficit in schizophrenia.

Optimal nonlinear codes for the perception of natural colours

We discuss how visual nonlinearity can be optimized for the precise representation of environmental inputs. Such optimization leads to neural signals with a compressively nonlinear input-output function the gradient of which is matched to the cube root of the probability density function (PDF) of the environmental input values (and not to the PDF directly as in histogram equalization). Comparisons between theory and psychophysical and electro-physiological data are roughly consistent with the idea that parvocellular (P) cells are optimized for precision representation of colour: their contrast-response functions span a range appropriately matched to the environmental distribution of natural colours along each dimension of colour space. Thus P cell codes for colour may have been selected to minimize error in the perceptual estimation of stimulus parameters for natural colours. But magnocellular (M) cells have a much stronger than expected saturating nonlinearity; this supports the view that the function of M cells is mainly to detect boundaries rather than to specify contrast or lightness.

Fine Structure of Parvocellular Receptive Fields in the Primate Fovea Revealed by Laser Interferometry

Optical blurring in the eye prevents conventional physiological techniques from revealing the fine structure of the small parvocellular receptive fields in the primate fovea in vivo. We explored the organization of receptive fields in macaque parvocellular lateral geniculate nucleus cells by using sinusoidal interference fringes formed directly on the retina to measure spatial frequency tuning at different orientations. Most parvocellular cells in and near the fovea respond reliably to spatial frequencies up to and beyond 100 cycles/ degrees of visual angle, implying center input arising mainly from a single cone. Temporal frequency and contrast response characteristics were also measured at spatial frequencies up to 130 cycles/degrees. We compared our spatial frequency data with the frequency responses of model receptive fields that estimate the number, configuration, and weights of cones that feed the center and surround. On the basis of these comparisons, we infer possible underlying circuits. Most cells had irregular spatial frequency-response curves that imply center input from more than one cone. The measured responses are consistent with a single cone center together with weak input from nearby cones. By exposing a fine structure that cannot be discerned by conventional techniques, interferometry allows functional measurements of the early neural mechanisms in spatial vision.

Visual physiology of the lateral geniculate nucleus in two species of new world monkey: Saimiri sciureus and Aotus trivirgatis.

1. Visual responses were recorded from neurones in the magnocellular and parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus in two species of New World monkeys - the diurnal squirrel monkey (Saimiri sciureus) and the nocturnal owl monkey (Aotus trivirgatis). Recording sites were reconstructed in postmortem tissue and comparisons were made between the response properties of magnocellular and parvocellular neurones. 2. Receptive fields were characterized with both white noise and drifting gratings. We found that most of the differences between magnocellular and parvocellular neurones that have been described in the macaque monkey hold for the squirrel monkey and owl monkey. In squirrel monkey and owl monkey, receptive fields of magnocellular neurones were larger than those of parvocellular neurones at similar eccentricities. Although visual responses in the owl monkey were significantly slower than in the squirrel monkey, in both species magnocellular neurones differed from parvocellular neurones in that their responses (1) had higher contrast gains, (2) tended to peak at higher temporal frequencies (but with considerable overlap), (3) had shorter response latencies, and (4) were more transient. 3. The strength of a neurone's receptive-field surround was assessed by comparing neuronal responses to gratings of optimal spatial frequency with responses to gratings of low spatial frequency. Using this approach, receptive-field surrounds were found to be equally strong on average for magnocellular and parvocellular neurones. 4. Spatial summation, as measured by a null test, was linear for all magnocellular and parvocellular cells tested; that is, Y cells were not observed in either species. Finally, most magnocellular neurones showed a contrast gain control mechanism, although this was not seen for parvocellular neurones.

Differential regulation of corticotropin-releasing hormone and vasopressin transcription by glucocorticoids.

CRH and vasopressin (VP), the main regulators of pituitary ACTH secretion, co-exist in parvocellular cells of the PVN, but their levels of expression are regulated differentially during manipulations of the hypothalamic pituitary adrenal (HPA) axis. The effects of glucocorticoids on this system was studied using in situ hybridization with intronic and exonic probes to measure changes in CRH and VP messenger RNA (mRNA) and heteronuclear (hn) RNA in 48-h adrenalectomized (ADX) rats receiving injections of corticosterone (2.8 mg/100 g, ip) or vehicle. We also determined the time course of changes in VP expression following the first 72 h of ADX. Levels of VP heteronuclear (hn) RNA and the number of parvocellular cells containing VP hnRNA remained very low in sham operated rats, whereas biphasic changes were observed after ADX. Grain density levels increased 11.5-fold over sham-operated controls by 6 h, declined to 2-fold by 18 h, to increase again to 10- and 20-fold by 48 and 72 h, respectively. In 48-h ADX rats, vehicle injection increased CRH hnRNA levels transiently (11-fold the basal by 15 and 30 min), returning to basal at 60 min, whereas VP hnRNA levels increased progressively up to 28-fold the basal by 2 h. Corticosterone injection had no significant effect on vehicle-induced increases in CRH hnRNA, in spite of marked elevations in circulating corticosterone. In contrast to CRH, VP hnRNA levels increased only transiently by 15 min, and then decreased below basal (near sham-ADX levels) by 2 h. The data show that in normal conditions the responsiveness of parvocellular neurons to stress is under marked inhibition by the low resting levels of glucocorticoids, and that the sensitivity of CRH and VP transcription to glucocorticoid feedback is markedly different.

Vasopressin and corticotropin-releasing hormone gene responses to novel stress in rats adapted to repeated restraint.

Stress-responsive neurons of the hypothalamic paraventricular nucleus (PVN) show functional plasticity and adapt to repeated restraint (RR) stress. To investigate whether neuronal adaptation to the homotypic stress also affects their response to a heterotypic stressor, we used in situ hybridization with intronic and exonic probes to measure primary transcript (hnRNA) and messenger RNA (mRNA) levels for CRH and vasopressin (VP) in the PVN of control and RR rats after the heterotypic stress of i.p. hypertonic saline injection (ipHS). Two weeks of daily restraint blunted plasma corticosterone and parvocellular CRH, but not VP, transcript responses to a further restraint episode. IpHS increased circulating corticosterone in both groups, but levels were higher in RR rats. CRH hnRNA increased within 15 min and returned to baseline by 1 h in both naive and RR rats. CRH mRNA increased more slowly in both groups, peaking at 2 h, with RR rats showing greater responses at this time. Parvocellular VP hnRNA reached a peak 2 h after ipHS in naive rats, but more rapidly (1 h) and to higher levels in RR rats. The number of parvocellular neurons expressing VP hnRNA increased approximately 5-fold after ipHS in both groups. Basal VP mRNA levels and the number of parvocellular cells expressing VP mRNA were elevated in RR rats. Both ipHS and naive rats showed an increase in VP mRNA transcripts after ipHS, with RR rats showing greater levels at 2 and 4 h. Magnocellular cells in both PVN and supraoptic nuclei showed increases in VP hnRNA within 15 min. The data demonstrate VP responses in parvocellular neurons of both control and RR rats, whereas profound inhibition of CRH transcription is selective for the homotypic stressor with CRH responsiveness to the heterotypic stress preserved or increased.

Differential recruitment of hypothalamic neuroendocrine and ventrolateral medulla catecholamine cells by non-hypotensive and hypotensive hemorrhages

We performed c- fos expression experiments in conscious rats to quantify the threshold and extent of activation of hypothalamic neuroendocrine cells in response to non-hypotensive and hypotensive hemorrhages allowing us to assess whether their pattern of recruitment corresponded to known oxytocin, vasopressin and ACTH release patterns. Also, because previous studies have implicated ventrolateral medulla catecholamine cells in the generation of certain hypothalamic neuroendocrine cell responses, we examined the response of ventrolateral medulla catecholamine cells to non-hypotensive and hypotensive hemorrhages and directly tested their role in regulating neuroendocrine cell responses to hypotensive hemorrhage. Animals were subjected to hemorrhages of 0, 4, 8, 12 or 16 ml/kg BW, the latter two levels being hypotensive. We found that only supraoptic nucleus vasopressin cells were significantly activated by the smallest non-hypotensive hemorrhage (4 ml/kg), which corresponds to reports that only vasopressin is released into the plasma after a small hemorrhage. Hypotensive hemorrhages resulted in significant recruitment of paraventricular and supraoptic oxytocin and vasopressin cells and parvocellular cells of the medial division of the paraventricular nucleus. Vasopressin cells were recruited in much greater numbers than oxytocin cells, which is in agreement with previous findings that there is a greater release of vasopressin than oxytocin into the plasma after hypotensive hemorrhage. In addition, medial parvocellular cells of the paraventricular nucleus, most likely to be tuberoinfundibular-projecting corticotropin-releasing factor cells, were activated by hypotensive hemorrhage only when arterial pressure dropped below 60 mmHg which also corresponds well with the plasma release response of ACTH. Ventrolateral medulla catecholamine cells were only recruited by hypotensive hemorrhages. While caution must be exercised in interpreting an absence of response, this certainly suggests that catecholamine cells are unlikely to have a role in the activation of supraoptic neurosecretory cells in response to non-hypotensive hemorrhages. Unilateral lesions of the ventrolateral medulla catecholamine cell column, corresponding primarily to the location of A1 noradrenergic cells, significantly reduced the hypotensive hemorrhage-induced activation of hypothalamic vasopressin, oxytocin and medial parvocellular paraventricular nucleus cells. This suggests that A1 noradrenergic cells contribute to the activation of these neuroendocrine cell populations, including oxytocin cells, which is an unexpected finding. More significantly, however, because the reduction in responsiveness after A1 lesions was similar for all cell categories, it seems likely that other factors must determine the differential recruitment of hypothalamic neuroendocrine cells in response to a hypotensive hemorrhage.

Temporal contrast sensitivity in the lateral geniculate nucleus of a New World monkey, the marmoset Callithrix jacchus.

1. The temporal contrast sensitivity of koniocellular, parvocellular and magnocellular cells in the lateral geniculate nucleus (LGN) of nine adult marmosets was measured. The receptive fields of the cells were between 0.3 and 70 deg from the fovea. The stimulus was a large spatially uniform field which was modulated in luminance at temporal frequencies between 0.98 and 64 Hz. 2. For each cell group there was a gradual increase in modulation sensitivity, especially for temporal frequencies below 8 Hz, with increasing distance from the fovea. At any given eccentricity, magnocellular cells had the greatest sensitivity. In central visual field, the sensitivity of koniocellular cells lay between that of parvocellular and magnocellular cells. In peripheral visual field (above 10 deg eccentricity) koniocellular and parvocellular cells had similar sensitivity. 3. The contrast sensitivity of each cell class was dependent on the anaesthetic used. Cells from animals anaesthetized with isoflurane were less sensitive than cells from animals anaesthetized with sufentanil. This effect was more marked for temporal frequencies below 4 Hz. 4. These results are incompatible with the notion that the koniocellular pathway is functionally homologous to a sluggish, W-like pathway in other mammals. At least in terms of their temporal transfer properties, many koniocellular cells are more like parvocellular cells.

Segregation of receptive field properties in the lateral geniculate nucleus of a New-World monkey, the marmoset Callithrix jacchus.

The lateral geniculate nucleus (LGN) in humans and Old-World monkeys is dominated by the representation of the fovea in the parvocellular (PC) layers, and most PC cells in the foveal representation have red-green cone opponent receptive field properties. It is not known whether these features are both unique to trichromatic primates. Here we measured receptive field properties and the visuotopic organization of cells in the LGN of a New-World monkey, the marmoset Callithrix jacchus. The marmoset displays a polymorphism of cone opsins in the medium-long wavelength (ML) range, which allows the LGN of dichromatic ("red-green color blind") and trichromatic individuals to be compared. Furthermore, the koniocellular-interlaminar layers are segregated from the main PC layers in marmoset, allowing the functional role of this subdivision of the LGN to be assessed. We show that the representation of the visual field in the LGN is quantitatively similar in dichromatic and trichromatic marmosets and is similar to that reported for macaque; the vast majority of LGN volume is devoted to the central visual field. ON- and OFF-type responses are partially segregated in the PC layers so that responses are more commonly encountered near the external border of each layer. The red-green (ML) opponent cells in trichromatic animals were all located in the PC layers, and their receptive fields were within 16 degrees of the fovea. The koniocellular zone between the PC and magnocellular layers contained cells that receive excitatory input from short wavelength sensitive cones ("blue- cells") as well as other nonopponent cells. These results suggest that the basic organization of the LGN is common to dichromatic and trichromatic primates and provide further evidence that ML and SWS opponent signals are carried in distinct subdivisions of the retinogeniculocortical pathway.

Quantitative ultrastructural evidences suggest no age-related changes in biosynthesis and processing within parvocellular cells of the paraventricular nucleus in hamsters

The parvocellular neurons of the parvocellular division of the paraventricular nucleus (PVPA) were studied in hamsters at six point ages (from 3 to 30 months old). Standard manual morphometric techniques were used to obtain data of parvocellular activity including nuclear and nucleolar size, as well as the percentage of the cell occupied by Golgi apparatus (GA), mitochondria and rough endoplasmic reticulum (RER). Other directly age-related parameters like amounts of nuclear invagination and lipofuscin have also been studied. No significant differences in the measured subcellular components were detected among groups studied, except slight increases in lipofuscin. No age-related changes were found in the synthesizing apparatus, but a significant decrease in the cell area was observed in older groups. This finding could suggest a reduction in absolute terms in the protein synthesis of the parvocellular neurons during aging. Ultrastructural morphometric observations in parvocellular neurons are discussed in relation to synthesizing activity and hormone production during aging.

Evidence for a Possible Glycinergic Inhibitory Neurotransmission in the Midbrain and Rostral Pons of the Rat Studied by Gephyrin

The data on the glycinergic transmission in the rostral brainstem are both few and controversial. The present report provides evidence for a possible glycinergic transmission in Sprague-Dawley rats, based on observations of immunocytochemical labeling for gephyrin, a 93 kDa protein and a component of the functional glycine receptor. A monoclonal antibody against gephyrin was used, and the reaction product was visualized by means of avidin-biotin-peroxidase procedure. The reaction product in midbrain and rostral pons was found in neuronal perikarya and in proximal dendrites but in some cases the most distal dendritic branches were also labeled. The neuropil usually displayed a moderate staining with finely granulated reaction product. The most significant immunocytochemical signal was mainly encountered in large and medium-sized neuronal populations of the motor cranial nerve nuclei (III, IV, V), in the reticular formation (laterodorsal tegmental nucleus, pedunculopontine tegmental nucleus, deep mesencephalic nucleus), in the red nucleus, in the intermediate and deep gray strata of the superior colliculus. Only in the substantia nigra and the inferior colliculus the parvocellular cell populations were mainly labeled. The present data suggest a significant inhibitory glycinergic neurotransmission in the rostral brainstem, probably mediated by interneurons.