A crossed parabigemino-lateral geniculate projection in rats blinded at birth

Summary Anatomical techniques are used to demonstrate a projection from the parabigeminal nucleus (PBG) to the contralateral dorsal lateral geniculate nucleus (dLGN) in adult pigmented rats which had been bilaterally enucleated at birth. No similar pathway is detected in normal animals. Ultrastructural examination of degeneration in dLGN of blinded rats after contralateral PBG lesion reveals a similarity of the PBG terminals to replacement terminals seen in dLGN of anophthalmic or neonatally enucleated mice (Cullen and Kaiserman-Abramof 1976)Supported by grants EY05381 and EY03414 from the National Institute of Health     

Anatomical origins of ocular dominance in mouse primary visual cortex

Ocular dominance (OD) plasticity is a classic paradigm for studying the effect of experience and deprivation on cortical development, and is manifested as shifts in the relative strength of binocular inputs to primary visual cortex (V1). The mouse has become an increasingly popular model for mechanistic studies of OD plasticity and, consequently, it is important that we understand how binocularity is constructed in this species. One puzzling feature of the mouse visual system is the gross disparity between the physiological strength of each eye in V1 and their anatomical representation in the projection from retina to the dorsal lateral geniculate nucleus (dLGN). While the contralateral-to-ipsilateral (C/I) ratio of visually evoked responses in binocular V1 is ∼2:1, the ipsilateral retinal projection is weakly represented in terms of retinal ganglion cell (RGC) density where the C/I ratio is ∼9:1. The structural basis for this relative amplification of ipsilateral eye responses between retina and V1 is not known. Here we employed neuroanatomical tracing and morphometric techniques to quantify the relative magnitude of each eye's input to and output from the binocular segment of dLGN. Our data are consistent with the previous suggestion that a point in space viewed by both eyes will activate 9 times as many RGCs in the contralateral retina as in the ipsilateral retina. Nonetheless, the volume of the dLGN binocular segment occupied by contralateral retinogeniculate inputs is only 2.4 times larger than the volume occupied by ipsilateral retinogeniculate inputs and recipient relay cells are evenly distributed among the input layers. The results from our morphometric analyses show that this reduction in input volume can be accounted for by a three-to-one convergence of contralateral eye RGC inputs to dLGN neurons. Together, our findings establish that the relative density of feed-forward dLGN inputs determines the C/I response ratio of mouse binocular V1.     

The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons

Summary The colour of an object is changed by surround colours so that the perceived colour is shifted in a direction complementary to the surround colour. To investigate the physiological mechanism underlying this phenomenon, we recorded from 260 neurons in the parvo-cellular lateral geniculate nucleus (P-LGN) of anaesthetized monkeys (Macaca fascicularis), and measured their responses to 1.0–2.0° diameter spots of equiluminant light of various spectral composition, centered over their receptive field (spectral response function, SRF). Five classes of colour opponent neurons and two groups of light inhibited cells were distinguished following the classification proposed by Creutzfeldt et al. (1979). In each cell we repeated the SRF measurement while an outer surround (inner diameter 5°, outer diameter 20°) was continuously illuminated with blue (452 nm) or red (664 nm) light of the same luminance as the center spots. The 1.0–1.5° gap between the center and the surround was illuminated with a dim white background light (0.5–1cd/m²). During blue surround illumination, neurons with an excitatory input from S-or M-cones (narrowand wide-band/short-wavelength sensitive cells, NSand WS-cells, respectively) showed a strong attenuation of responses to blue and green center spots, while their maintained discharge rate (MDR) increased. During red surround illumination the on-minus-off-responses of NS- and WS-cells showed a clear increment. L-cone excited WL-cells (wide-band/long-wavelength sensitive) showed a decrement of on-responses to red, yellow and green center spots during red surround illumination and, in the majority, also an increment of MDR. The response attenuation of narrow-band/long-wavelength sensitive (NL)-cellls was more variable, but their on-minus-off-responses were also clearly reduced in the average during red surrounds. Blue surround illumination affected WL-cell responses little and less consistently than those of NL-cells, but often broadened the SRF also in the WL-cells towards shorter wavelengths. The M-cone excited and S-cone suppressed WM-cells were strongly suppressed by blue but only little affected by red surround illumination. The changes of spectral responsiveness came out clearly in the group averages of the different cell classes, but snowed some variation between individual cells in each group. The zero-crossing wavelengths derived from on-minus-off-responses were also characteristically shifted towards wavelengths complementary to those of the surround. The direction of changes of spectral responsiveness of P-LGN-cells are thus consistent with psychophysical colour contrast and colour induction effects which imply that light of one spectral region in the surround reduces the contribution of light from that same spectral region in the (broad band or composite) object colour. Surrounds of any colour also decrease the brightness of a central coloured or achromatic light (darkness induction). We calculated the population response of P-LGN-units by summing the activity of all WS-, WM- and WL-cells and subtracting that of all NS- and NL-cells. The SRF of this population response closely resembled the spectral brightness function for equiluminous lights rather than the photopic luminosity function. With red or blue surrounds, this population SRF was lowered nearly parallel across the whole spectrum to about 0.7 of the amplitude of the control. In a psychophysical test on 4 observers we estimated the darkness induction of an equiluminous surround in a stimulus arrangement identical to the neurophysiological experiment, and found a brightness reduction for white, blue, green and red center stimuli to 0.5–0.7 of the brightness values without surround. This indicates that the neurophysiological results may be directly related to perception, and that P-LGN-cells not only signal for chroma but also for brightness, but in different combinations. The results indicate that both an additive (direct excitation or suppression of activity) and a multiplicative mechanism (change of gain control) must be involved in brightness and colour contrast perception. As mechanisms for the surround effects horizontal cell interactions appear not to be sufficient, and a direct adaptive effect on receptors feeding positive or negative (opponent) signals into the ganglion cells receptive fields by straylight from the surround must be seriously considered. This will be examined in the following companion paper. The results indicate that changes of spectral and brightness responses in a colour contrast situation sufficient to explain corresponding changes in perception are found already in geniculate neurons and their retinal afferents. This applies to mechanisms for colour constancy as well in as much as they are related to colour contrast.This paper is dedicated to the memory of Prof. Günter Baumgartner, Zürich. The material was first presented on the occasion of his 65^th birthday in 1989     

Alterations of retinal inputs following striate cortex removal in adult monkey

Summary The morphology of the retina and central retino-recipient nuclei was studied in two monkeys that had undergone total bilateral striate cortex removal as adults. These animals had been behaviorally tested for two years after lesioning and had demonstrated significant recovery of pattern vision. Light and electron microscopy and autoradiography were done on the central retino-recipient nuclei following a monocular intravitreal injection of ³H-proline.Light microscopic analysis of retinal ganglion cell number showed a 30% loss in the parafoveal retina due to retrograde trans-synaptic degeneration.The most striking central change in retinal axon distribution was in the dorsal lateral geniculate nucleus (dLGN) where the parvocellular but not the magnocellular region showed a marked reduction in retinal input. Despite the loss of almost all dLGN neurons through retrograde degeneration, at the EM level both parvocellular and magnocellular regions contained islands of neuropil made up of retinal and several other types of synaptic terminals as well as small dendrites and pale unidentified processes. Approximately equal numbers of retinal terminals were identified by EM autoradiography in both regions of dLGN, which did not explain the apparent differences in labeling between the two regions seen in the light microscope.A second change in central retinal pathways was found in the olivary pretectal nucleus where a significant loss of retinal input also occurred. A third change, but in the opposite direction, was found in the pregeniculate nucleus (PGN) where the area of retinal terminals appeared enlarged. The remaining central retino-recipient nuclei had the same distribution of retinal input as the control animals.Supported in part by NIH Research Grants EY-01208, EY-01730 and EY-07013, and in part by HD02274     

Evoked electrical responses of nucleus lateralis posterior of the rabbit thalamus and their dependence upon the functional state of the cortex and reticular formation

The electrical responses of the thalamic pulvinar (of its analogue, the nucleus lateralis posterior (nLP)) to light stimuli of various intensity were recorded in awake rabbits, and their interrelationships with the responses of the retina, superior colliculi, and visual cortex were analyzed. It is postulated that a major role in the afferent supply of the rabbit-nLP belongs to the optic track and the superior colliculi. The presence was demonstrated of a highly organized analyzer of the reticular control of nLP function that is of a facilitatory character. The most important role in reticular influence belongs to its adrenoresponsive mechanism. An inhibitory influence was demonstrated for the visual and sensorimotor regions of the cerebral cortex on the nLP neuronal apparatus producing responses to light stimuli. The influence of the visual cortex are the most pronounced and stable.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 69, No. 6, pp. 739–747, June, 1983.     

Anatomical and physiological investigation of auditory input to the superior colliculus of the echolocating megachiropteran bat Rousettus aegyptiacus

The objective of this study was to investigate whether a representation of auditory space in the superior colliculus (SC) of the echolocating megachiropteran bat (Rousettus aegyptiacus) exists. Additionally the subcortical auditory connectivity of the SC was investigated. A total of 207 units were recorded in five awake animals while presenting acoustic stimuli (white noise, clicks, and pure tones) at different positions in space. Six units responded to acoustic stimulation. Three of these located within the superficial layers and one located in the intermediate layers were classified as omnidirectional units. Two units were located within the deep layers. One was classified as a hemifield unit, and the other as a frontal unit. All units responded phasically to acoustic stimulation with a latency of 4–150 ms. None of them could be activated by visual stimuli. We further examined the interaction of paired auditory and visual stimulation in 116 visually responsive units. Responses to visual stimulation were markedly altered by acoustic stimulation in 5 units. The influence of the acoustic stimuli was temporally and spatially restricted, and resulted either in a reduction or an elevation of unit responsiveness. Horseradish peroxidase was injected into the SC of eight animals to investigate the auditory subcortical connectivity of the SC. Retrograde labeling in auditory structures was rare compared with labeling found in nonauditory structures (e.g., retina, substantia nigra, parabigeminal nucleus). In auditory structures retrograde labeling was found mainly in the external nucleus of the inferior colliculus and in the nucleus of the brachium of the inferior colliculus. To a lesser extent it was found in the nucleus sagulum and in the area medial to the lemniscal nuclei. In one case the dorsal nucleus of the lateral lemniscus and the anterolateral periolivary nucleus were labeled. Our results reveal only a sparse auditory input into the SC of the flying fox, R. aegyptiacus. On the basis of single-unit recordings, we did not find an elaborate representation of auditory space as it is described for several other species. The existence of auditory and bimodal neurones, in combination with their response properties, nonetheless indicate that there might be a representation of auditory space in the SC of R. aegyptiacus.     

The topographic relationship between shifting binocular maps in the developing dorsal lateral geniculate nucleus

Summary The major mammalian subcortical visual structures receive topographically ordered projections from both eyes. In the adult dorsal lateral geniculate nucleus (dLGN) each projection terminates in separate restricted regions of the nucleus. This pattern is different during development. Initially in ferrets the projections from each eye to the dLGN overlap throughout this structure. Although the projections do not occupy regions that are appropriate given the adult pattern, they are both retinotopically organised. Consequently, the formation of the adult pattern requires that the two retinotopic projections shift in relation to one another. The experiments undertaken here on the newborn ferret demonstrate the relationship between the two unsegregated projections in terms of their retinal origin and relative pattern of projection to the dLGN. By establishing the relationship between the projections at this stage of development it is possible to determine the relative changes that must be made between them in order to bring about the adult pattern of registration. By mapping the two unsegregated projections with a combination of retinal lesions and anterograde tracing methods it is demonstrated that at birth the ipsilateral projection arises from the temporal retina, and the contralateral projection arises from the entire retina. Because of the significant contralateral projection from the temporal retina the relatively sharp nasotemporal division found in the adult is not present at this stage. This element of the contralateral projection maps in continuity with the rest of this projection and terminates at the caudal pole of the nucleus. However, it is probably lost before the adult pattern has clearly started to develop. It is proposed that the representation of the naso-temporal division at the caudal pole of the dLGN is the starting point for the development of the adult pattern of registration. Once this point of registration has been established each map shifts in relation to the other and to the borders of the nucleus to bring about the adult pattern.     

Luminance and darkness detectors in the olivary and posterior pretectal nuclei and their relationship to the pupillary light reflex in the rat

Summary In order to identify the pretectal nucleus which contains pupillomotor cells in the rat, cells were sought which were sensitive to changes in luminance level at the eye. Two types were found: Luminance detectors which showed a graded increase in firing with increase in luminance, and darkness detectors which showed a graded increase in firing rate with graded dimming of luminance intensity. All luminance detectors were located in the olivary pretectal nucleus, whereas darkness detectors were located in the posterior pretectal nucleus. Consensual pupil responses were recorded in conscious normal and sympathectomised rats using an infra-red sensitive T.V. pupillometer. Pupil diameter varied 2mm in an approximately linear fashion over six log units range in luminance intensity. Sympathectomy produced a general constriction of the pupil, but the overall response to light was unaffected. The changes in pupil size occurred over the same range of luminance that the firing rates of both luminance and darkness detectors changed. The olivary pretectal nucleus may therefore be involved in pupilloconstruction in the light, and the posterior pretectal nucleus, with pupillodilation in the dark.This work was supported by a grant from the Special Trustees of St. Thomas' Hospital to H.I.; R.J.C. was a holder of a studentship of the Science Research Council of Great Britain during 1976–1979     

The central cervical nucleus in the cat. II

Summary After HRP injections in kittens and adult cats into parts of the cerebellum known to receive spinal afferents, retrogradely labelled neurones were found in the C1–C4 segments of the spinal cord, primarily in the central cervical nucleus (CCN). A few labelled neurones were also found in laminae IV and VI–IX of Rexed. The results obtained in kittens and adult cats were similar.Control HRP injections were made into the vestibular nuclei and the inferior colliculus. After injections mainly involving the vestibular nuclei a few faintly labelled neurones were found in the C1–C4 segments in, and occasionally also outside the CCN, in laminae VI–VIII. After injections into the inferior colliculus, however, no labelled neurones were observed in these segments.After spinal cord hemisections at C1 and bilateral HRP injections into the anterior lobe, nearly all labelled CCN neurones were found ipsilateral to the hemisections. Labelled axons of such CCN neurones could be traced across the midline to the lateral funiculus. A few neurones were found outside the CCN in laminae VII–IX. Contralaterally labelled neurones were found mainly in lamina VI.Abbreviations b.c. brachium conjunctivum - CN cuneate nucleus - Cr. I, II crus I and II - DV descending (inferior) vestibular nucleus - ECN external cuneate nucleus - i.c.p. inferior cerebellar peduncle (restiform body) - LV lateral vestibular nucleus - MV medial vestibular nucleus - N.d. dentate (lateral) nucleus - N.f. fastigal (medial) nucleus - N.i. interpositus nucleus - N.pr.V principal nucleus of trigeminal nerve - N.V motor nucleus of trigeminal nerve - Ol.s. superior olive - pfl. paraflocculus - pfl.d. dorsal paraflocculus - pmd. paramedian lobule - SC superior colliculus - s.c.p. superior cerebellar peduncle - SV superior vestibular nucleus - Tr.V spinal trigeminal tract - x group x of Brodal and Pompeiano (1957) - I-X cerebellar lobules of Larsell (1953) - I-IX spinal cord laminae (Rexed, 1952)     

Time-dependent decrease in the extent of visual deafferentation in the lateral geniculate nucleus of adult cats with small retinal lesions

Summary Small, round photocoagulator lesions of 3–6 ° (0.6–1.2 mm) diameter were placed nasally on the retina of adult cats. Histological controls proved the complete destruction of all retinal layers within the lesions. Changes in lesion size by shrinkage of the retinal scar did not exceed 0.1 mm or 0.5 °. At different times after photocoagulation, single cells were recorded in layers A and A₁ of the contralateral lateral geniculate nucleus (LGN) with tungsten microelectrodes. Acute lesions of this size completely deafferented single LGN cells in layer A whose receptive field (RF) area was within the lesion. Thirty days and more after coagulation, light-excitable cells were detected in the originally deafferented LGN region, with RFs in the immediate surround of the retinal lesion. The spontaneous activity and light excitability of these neurons were altered. The representation of 3–4 ° retinal lesions at 20 ° horizontal eccentricity was found to be completely filled in by excitation in the LGN. Excitation had spread from the unsevered parts of the retina into the region of deafferentation. Single cells with signs of multiple activation from more than one border region of the retinal lesion were occasionally detected.The investigations were supported by the Deutsche Forschungsgemeinschaft (Ey 8)     

Exposure to an open-field arena increases c-Fos expression in a subpopulation of neurons in the dorsal raphe nucleus, including neurons projecting to the basolateral amygdaloid complex

Serotonergic systems in the dorsal raphe nucleus are thought to play an important role in the regulation of anxiety states. To investigate responses of neurons in the dorsal raphe nucleus to a mild anxiety-related stimulus, we exposed rats to an open-field, under low-light or high-light conditions. Treatment effects on c-Fos expression in serotonergic and non-serotonergic cells in the midbrain raphe nuclei were determined 2 h following open-field exposure or home cage control (CO) conditions. Rats tested under both light conditions responded with increases in c-Fos expression in serotonergic neurons within subdivisions of the midbrain raphe nuclei compared with CO rats. However, the total numbers of serotonergic neurons involved were small suggesting that exposure to the open-field may affect a subpopulation of serotonergic neurons. To determine if exposure to the open-field activates a subset of neurons in the midbrain raphe complex that projects to forebrain circuits regulating anxiety states, we used cholera toxin B subunit (CTb) as a retrograde tracer to identify neurons projecting to the basolateral amygdaloid complex (BL) in combination with c-Fos immunostaining to identify cells that responded to open-field exposure. Rats received a unilateral injection of CTb into the BL. Seven to 11 days following CTb injection rats were either, 1) exposed to an open-field in low-light conditions, 2) briefly handled or 3) left undisturbed in home cages. Dual immunostaining for c-Fos and CTb revealed an increase in the percentage of c-Fos-immunoreactive BL-projecting neurons in open-field-exposed rats compared with handled and control rats. Dual immunostaining for tryptophan hydroxylase and CTb revealed that a majority (65%) of BL-projecting neurons were serotonergic, leaving open the possibility that activated neurons were serotonergic, non-serotonergic, or both. These data are consistent with the hypothesis that exposure to anxiogenic stimuli activates a subset of neurons in the midbrain raphe complex projecting to amygdala anxiety circuits.     

Circadian and photic regulation of immediate-early gene expression in the hamster suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus is the site of an endogenous circadian clock synchronized by daily light-dark cycles. At some daily phases, light exposure both shifts the clock and alters the expression of several immediate-early genes in cells of the suprachiasmatic nucleus. We have studied both spontaneous circadian and light-induced expression of several immediate-early gene messenger RNAs and proteins in hamsters in constant darkness or in response to brief light exposure. There was no detectable spontaneous expression of NGFI-A messenger RNA in suprachiasmatic nucleus cells at any circadian phase, but light pulses induced its expression selectively during the subjective night, with highest levels of expression 6 h into the night. We also found that there are two independent rhythms of expression of junB messenger RNA and JunB protein, as well as c-fos messenger RNA and c-Fos protein, in the suprachiasmatic nucleus of hamsters: a rhythm of photic sensitivity expressed throughout the night and a spontaneous rhythm of expression triggered around dawn. Induction of NGFI-A messenger RNA and c-fos messenger RNA and c-Fos protein in response to a light pulse were found throughout the suprachiasmatic nucleus, with the highest levels of expression in the ventrolateral subdivision; however, the spontaneous expression of JunB and c-Fos proteins was confined mainly to the dorsomedial suprachiasmatic nucleus. The temporal and anatomical differences in the expression of these immediate-early genes in the mammalian suprachiasmatic nucleus suggest that their protein products may be involved in different signaling mechanisms mediating either photic entrainment or endogenous oscillations within distinct subpopulations of suprachiasmatic nucleus cells.     

Morris水迷宫训练后即早基因c-Fos及c-Jun在大鼠丘脑前核的表达

目的为探讨Morris水迷宫训练后丘脑前核内c-Fos和c-Jun的表达。方法 30只成年大鼠均分为正常组,假训练组和Morris水迷宫训练组,通过免疫组化和WesternBlot检测c-Fos和c-Jun的表达。结果 Morris水迷宫训练组丘脑前核c-Fos表达水平与假训练组和正常组比较明显增强(P<0.05),丘脑前背侧核显著强于前腹侧核(P<0.05)。训练组c-Jun表达水平与假训练组和正常组比较明显增强(P<0.05)。结论 c-Fos和c-Jun可能参与空间学习记忆过程。     

Differential induction of c-Fos and c-Jun in the lateral geniculate nucleus of rats following unilateral optic nerve injury with contralateral retinal blockade

The post-injury responses of traumatic optic neuropathy elicit a number of neuronal reactions in the visual system which have not been fully elucidated. This study aimed to investigate the immediate early gene (IEG) expression in the lateral geniculate nucleus (LGN) following unilateral optic nerve (ON) crush in adult rats with or without contralateral blockade of retinal input, and its indication. Intravitreal injection of the B subunit of cholera toxin was used to label the retinogeniculate projection. In group 1 rats whose unilateral (right eye) ON was crushed, a large induction of c-Jun was observed in the bilateral LGN from 2 h to 3 days post crush. Expression of c-Fos was detected ipsilaterally in cells of the intermediate geniculate nucleus and the medial subdivision of ventral LGN mainly innervated by the intact ON. A majority of c-Fos positive cells were also calbindin D-28k immunoreactive neurons. In group 2 rats whose monocular (left eye) spiking was blocked by tetrodotoxin, a sodium channel blocker, prior to the unilateral (right eye) ON crush, the location of c-Fos expression was observed to be shifted to the side of LGN contralateral to the crush, while the expression pattern of c-Jun resembled the pattern of group 1 rats. Thus, the unilateral ON injury induced significant IEG expression in the LGN may involve different mechanisms, under which the emerging of c-Fos depends upon remaining visual input from the retina, while c-Jun appears independent of visual activity.     

Sources of subcortical afferents to the macaque's dorsal lateral geniculate nucleus

Background: The dorsal lateral geniculate nucleus (dLGN) is the thalamic region responsible for transmitting retina signals to cortex. Brainstem pathways to this nucleus have been described in several species and are believed to control the retinocortical pathway depending on the state of the animal (awake, asleep, drowsy, etc.). The purpose of this study was to determine all of the subcortical sources of afferents to the dLGN in a higher primate, the macaque monkey, whose visual system is similar to that of humans. Methods: Injections of horseradish peroxidase (HRP), with or without conjugation to wheat germ agglutinin, were made into the dLGNs of seven macaque monkeys, followed by perfusion, brain sectioning, and analyses of neurons in the brainstem, thalamus, and hypothalamus that contained the retrogradely transported marker. Results: The reticular nucleus of the thalamus, pedunculopontine nucleus, parabigeminal nucleus, pretectal nucleus of the optic tract, superior colliculus, dorsal raphe nucleus, and tuberomammillary region of the hypothalamus contained many retrogradely labeled neurons ipsilateral to the injections. In the contralateral brainstem, HRP-labeled cells were found only in the pedunculopontine nucleus, nucleus of the optic tract, and dorsal raphe nucleus. The number of labeled neurons on the contralateral side was about one-half of that in corresponding ipsilateral nuclei. The locus coeruleus contained no labeled neurons in four of the macaques that had injections limited to the dLGN. Conclusion: There are seven subcortical regions that send afferents to the dLGNs of macaque monkeys. Except for the locus coeruleus, these are the same as observed for other species, such as the cat and rat, and indicate the possible sources of subcortical control over the dLGNs of humans. © 1995 Wiley-Liss, Inc.     

A retrograde fluorescence double-labeling study of the cat's optic nerve cell which has a bifurcating axon.

Injection of the fluorescent tracers, Evans Blue (EB) and 4'-6-Diamidino-2-phenylindole dihydrochloride hydrate (DAPI) or Fluoro-Gold (FG) into the dorsal nucleus of the lateral geniculate body (dLGN) and the ipsilateral superior colliculus (SC) of adult cats demonstrated the existence of double-labeled optic nerve cells in the retina denoting that their axons bifurcate and project to both of these structures. These cells were seen in the temporal half and dorsal-dorsonasal and ventral-ventronasal octants of the ipsilateral retina and accounted for 11.5% of all the labeled cells. On the contralateral side, they were seen in the entire retina and accounted for 13.7% of all the labeled cells.     

Responses of cells in the cat lateral geniculate nucleus to moving stimuli at various levels of light and dark adaptation

Summary The responses of neurones in laminae A and A1 of the cat lateral geniculate nucleus to moving stimuli were investigated at different background luminances. Moving bright slits, dark bars and edges were employed; the contrast of stimuli against the background was held constant. Background intensities varied from 10^–3 to 10² td.Responses as stimuli passed across the centres of LGN receptive fields became stronger with increasing levels of light adaptation up to 10^–1–10¹ td and then remained constant. Responses as stimuli passed through surround regions altered qualitatively with adaptation level, generally increasing in strength and complexity with background luminance. As a bright slit for on-centre cells or dark bar for off-centre cells left the surround, in almost all units a strong secondary peak could be elicited by an appropriate selection of the adaptation conditions. Many features of the responses to moving stimuli could not be predicted from the responses to stationary stimuli under different adaptation conditions described in the previous paper.Trainee of the European Training Programme in Brain and Behaviour Research, 1975.     

Effects of previous noxious stimulus applied to remote areas on noxious stimulus-evoked c-Fos expression in the rat trigeminal nucleus caudalis

Noxious stimulus-evoked c-Fos expression in the spinal dorsal horn is modulated by noxious stimuli applied previously to remote areas of the body. To confirm the existence of such modulation in c-Fos expression in the trigeminal system, changes in c-Fos expression in the trigeminal nucleus caudalis induced by formalin injection into the rat whisker pad were examined by previously injecting formalin into different areas (contralateral whisker pad, ipsilateral or contralateral forepaw) of the body. Formalin injection-evoked c-Fos expression in this nucleus was significantly reduced by previous formalin injection into the contralateral whisker pad or ipsilateral forepaw but not into the contralateral forepaw. The interval between the two injections of formalin that produced a maximal reduction of formalin injection-evoked c-Fos expression was 1 h, and the reduction of c-Fos expression was less when the interval of the two noxious stimuli was longer or shorter than 1 h. These results suggested that noxious stimulus-evoked c-Fos expression in the trigeminal nucleus caudalis is reduced by noxious stimulus applied previously to remote areas, and the reduction is dependent on the area of previous noxious stimulation and interval between the two noxious stimuli.     

Lateral excitation in the cat lateral geniculate nucleus

Summary Visual responses were elicited by global phase reversal stimuli in cells of the cat dorsal lateral geniculate nucleus (dLGN) after small retinal lesions had been centered on each receptive field. After acute lesions of different sizes exclusively lateral inhibition was found. When GABAergic inhibition was blocked by continuous microiontophoretic application of bicuculline lateral excitation emerged in dLGN cells partially deafferented by small and medium size acute retinal lesions, but not in those affected by large lesions. This indicates the presence of excitatory retinal inputs at the periphery of the dLGN cell dendrites which are normally suppressed by strong, long-ranging lateral inhibitory processes. After chronic deafferentation, the remaining excitatory inputs increase in effectiveness and lateral excitation is seen without blockade of inhibition. The maximal lateral spread of excitation (300 m) in the dLGN is distinctly smaller than the extent of lateral inhibition (1000 m).     

Are locus coeruleus neurons involved in blinking?

To investigate the involvement of the noradrenergic locus coeruleus (LC) in the reflex blink circuit, c-Fos and neuronal tracer experiments were performed in the rat. LC neurons involved in reflex blink were localized by analyzing c-Fos protein expression after electrical stimulation of the supraorbital nerve.

Subsequently, neuronal tracers were injected in two different nuclei which are part of the reflex blink circuit. Anterograde tracer experiments in the sensory trigeminal complex (STC) explored the trigemino-coerulear connection; retrograde tracer experiments in the latero-caudal portion of the superior colliculus (SC) established coerulear–collicular connections. The combination of retrograde tracer injections into the latero-caudal SC portion combined with electrical stimulation of the supraorbital nerve identified c-Fos positive LC neurons that project to the latero-caudal SC. Our results revealed the existence of a STC–LC–SC loop.