Preservation of binocularity after monocular deprivation in the striate cortex of kittens treated with 6-hydroxydopamine

Suppressed visual adaptation of the vestibuloocular reflex in catecholamine-depleted cats

Intracisternal injections of 6-hydroxydopamine were used to deplete brain catecholamines in cats. These animals were then subjected to a prolonged period of lateral head rotations while viewing a visual scene that always rotated with them. In contrast to their predepletion behavior, these cats showed very little decrease in the gain of their vestibuloocular reflex. These results suggest that brain catecholamines may play a role in regulating neural plasticity in the vestibuloocular system just as they appear to do in the visual cortex.

Hormonal and humoral influences on brain development

This review discusses evidence for neurotransmitters as developmental signals in such ontogenic processes as neural tube formation (neurulation), germinal cell proliferation, and neuronal and glial differentiation during brain organogenesis, as well as evidence for other roles of these neurotransmitters in non-neuronal tissues of vertebrates and invertebrates. Evidence also is presented for hormonal regulation of brain development during postnatal neurogenesis and for interrelationships which may link neurotransmitters and hormones in a humoral milieu, providing a variety of control mechanisms for the central and peripheral nervous system during key phases of their development. Given the evidence for neurotransmitters and hormones as coordinating influence on neural ontogeny, it is possible that drugs, stress, and environmental influences may have the ability to perturb particular aspects of these developmental systems if present during those "critical periods" when such humoral influences are important for normal ontogeny.

Trophic effects of brain areas on the developing cerebral cortex: I. Growth and histological organization of intraocular grafts

Trophic interactions during development of brain regions were examined in rats using intraocular grafts of central nervous tissue. The increase in volume of transplanted fetal parietal cerebral cortex, as measured through the cornea, was markedly augmented by the presence of several different previously grafted CNS areas such as locus coeruleus, tectum, or cerebral cortex. DNA measurements and histological examinations suggested that this increased volume was due both to hyperplasia and hypertrophy. Previous grafts of iris, in contrast, did not significantly alter the final size of subsequently grafted cortex pieces. Contact between the two transplants was found to be critical in eliciting the trophic response. Growth-stimulated cortical grafts had a better organized cyto-architecture with larger neurons, including typical pyramidal cells, more neuropil, a lower cell density, and a more organotypic distribution of the cell bodies than non-stimulated controls. The experiments thus demonstrate a profound effect of adjacent neural tissue on development of neocortex. It is concluded that trophic interactions upon brain development can be revealed by sequential intraocular grafting.

Chronic intraventricular administration of lysergic acid diethylamide (LSD) affects the sensitivity of cortical cells to monocular deprivation

In kittens, but not in adult cats, depriving one eye of pattern vision by suturing the lids shut (monocular deprivation or MD) for one week reduces the proportion of binocular units in the visual cortex. A sensitivity of cortical units in adult cats to MD can be produced by infusing exogenous monoamines into the visual cortex. Since LSD interacts with monoamines, we have examined the effects of chronic administration of LSD on the sensitivity to MD for cortical cells in adult cats. Cats were assigned randomly to one of four conditions: MD/LSD, MD/No-LSD, No-MD/LSD, No-MD/No-LSD. An osmotic minipump delivered either LSD or the vehicle solution alone during a one-week period of MD. The animals showed no obvious anomalies during the administration of the drug. After one week the response properties of single units in area 17 of the visual cortex were studied without knowledge of the contents of the individual minipumps. With the exception of ocular dominance, the response properties of units recorded in all animals did not differ from normal. In the control animals (MD/No-LSD, No-MD/LSD, No-MD/No-LSD) the average proportion of binocular cells was 78%; similar to that observed for normal adult cats. However, in the experimental animals, which received LSD during the period of MD, only 52% of the cells were binocular. Our results suggest that chronic intraventricular administration of LSD affects either directly or indirectly the sensitivity of cortical neurons to MD.

Single cell responses in cat visual cortex to visual stimulation during iontophoresis of noradrenaline

We studied how iontophoresis of noradrenaline (NA) changes responsiveness of individual cells in the feline visual cortex when their visual receptive fields are stimulated with the appropriate visual stimulus. We found three populations of cortical cells which either increased, decreased or did not change their visual responsiveness during NA iontophoresis. About equal numbers of cells belonged to each of these three groups. In the majority of such cells that changed visual responsiveness during NA iontophoresis and that had measureable amounts of spontaneous activity, the ratio of visually evoked to spontaneous activity (signal-to-noise ratio) improved during NA iontophoresis. This improvement was independent of the direction of changes in the response magnitude to visual stimulation. There was a differential effect of NA on simple and complex visual cortical cells: Although most simple cells (86%) clearly changed their responsiveness during NA iontophoresis, the effects were seen in only one-third of complex cells. Furthermore, the effects on complex cells were usually weak compared to those typically seen in simple cells. In some cases the effects of NA were more complicated than an overall enhancement of suppression of the cortical cell's responses to visual stimulation. The possible dual role of NA in the visual cortex is briefly discussed.

Amphetamine, haloperidol, and experience interact to affect rate of recovery after motor cortex injury

Rats subjected to unilateral ablation of the motor cortex and placed on a narrow beam displayed transient contralateral paresis. An immediate and enduring acceleration of recovery was produced by a single dose of d-amphetamine given 24 hours after injury. This effect was blocked by haloperidol or by restraining the animals for 8 hours beginning immediately after amphetamine administration. A single dose of haloperidol given 24 hours after injury markedly slowed recovery. This effect was also blocked by restraining the animals.

Central core control of developmental plasticity in the kitten visual cortex: II. Electrical activation of mesencephalic and diencephalic projections

Fifteen dark-reared, 4- to 5-week-old kittens were stimulated monocularly with patterned light while they were anesthetized and paralyzed. Six of these kittens were exposed to the light stimuli only, in four kittens the light stimuli were paired with electric stimulation of the mesencephalic reticular formation and in five kittens with electric activation of the medial thalamic nuclei. Throughout the conditioning period, the ocular dominance of neurons in the visual cortex was determined from evoked potentials that were elicited either with electric stimulation of the optic nerves or with phase reversing gratings of variable spatial frequencies. In two kittens, ocular dominance changes were assessed after the end of the conditioning period by analyzing single unit receptive fields. Monocular stimulation with patterned light induced a marked shift of ocular dominance toward the stimulated eye, when the light stimulus was paired with electric activation of either the mesencephalic reticular formation or of the medial thalamus. Moreover, a substantial fraction of cells acquired mature receptive fields. No such changes occurred with light or electric stimulation alone. It is concluded that central core projections which modulate cortical excitability gate experience-dependent modifications of connections in the kitten visual cortex.

Central core control of developmental plasticity in the kitten visual cortex: I. Diencephalic lesions

In five, dark-reared, 4-week-old kittens the posterior two thirds of the corpus callosum were split, and a lesion comprising the intralaminar nuclei was made of the left medial thalamic complex. In addition, the right eye was closed by suture. Post-operatively, the kittens showed abnormal orienting responses, neglecting visual stimuli presented in the hemifield contralateral to the side of the lesion. Sudden changes in light, sound, or somatosensory stimulation elicited orienting responses that all tended toward the side of the lesion. These massive symptoms faded within a few weeks but the kittens continued to neglect visual stimuli in the hemifield contralateral to the lesion when a second stimulus was presented simultaneously in the other hemifield. Electrophysiologic analysis of the visual cortex, performed after the end of the critical period, revealed marked interhemispheric differences. In the visual cortex of the normal hemisphere most neurons were monocular and responded exclusively to stimulation of the open eye, but otherwise had normal receptive field properties. In the visual cortex of the hemisphere containing the thalamic lesion, the majority of the neurons remained binocular. In addition, the selectivity for stimulus orientation and the vigor of responses to optimally aligned stimuli were subnormal on this side. Thus, the same retinal signals, which in the control hemisphere suppressed the pathways from the deprived eye and supported the development of normal receptive fields, failed to do either in the hemisphere containing the thalamic lesion. Apparently, experience-dependent changes in the visual cortex require both retinal stimulation and the functioning of diencephalic structures which modulate cortical excitability and control selective attention.

The time of genesis, embryonic origin and differentiation of the brain stem monoamine neurons in the rhesus monkey

Neurogenesis of the locus coeruleus (LC), substantia nigra (SN) and raphe nuclei (RN) was analyzed in autoradiograms prepared from postnatal rhesus monkeys that had been exposed to a pulse of [3H]thymidine on selected embryonic (E) days of the 165-day gestational period. Heavily labeled monoamine (MA) neurons were present only in monkeys exposed to the isotope between E27 and E36 with the peak around E30-E33. The majority of neurons generated on E30 eventually become situated in the medial part of the LC, whereas most cells of the lateral portion are generated on E32 and E33, indicating the existence of a mediolateral spatiotemporal gradient. Proliferation of neurons destined for the compact portion of the LC peaks around E32, whereas production of subcoeruleus cells proceeds more evenly throughout the E30-E33 period. SN neurons are generated between E36 and E43, with peak labeling around E38-E40, and no appreciable spatiotemporal gradients. Neurons of the ventral tegmental area are also generated between E38 and E43. Neurogenesis of the RN occurs between E28 and E43 with only a moderate rostrocaudal spatiotemporal gradient. Neurons of raphe dorsalis and centralis superior undergo final mitosis between E28 and E35, with the peak on E30, whereas cells of raphe magnus, pontis, obscurus and pallidus are produced between E35 and E43, with the peak between E38 and E40. In general, MA neurons that project to different targets may be produced simultaneously within each nucleus irrespective of any spatiotemporal gradients. Examination of another series of fetuses sacrificed at various short intervals after exposure to [3H]TdR revealed that all MA neurons arise in the ventricular zone with each MA nucleus being generated at a specific level of the brain stem. Postmitotic MA cells migrate to their final location along specific pathways, and settle in patterns corresponding to the sequence of their genesis. Morphometric analysis indicated that after reaching their final destinations, the somas and nuclei of all MA neurons grow according to the same tempo and sequence irrespective of the developmental schedules of their synaptic targets.

Serotonin analog selectively ablates indentified neurons in the leech embryo

Exposure of embryonic leeches to 5,7-dihydroxytryptamine a cytotoxic analog of the monoamine neurotransmitter serotonin, results in the selective ablation of serotonin-containing neurons in the ventral nerve cord. Other neurons appear to be unaffected by this treatment, including those that contain another monoamine neurotransmitter, dopamine. Embryos with ablations continue to develop into juvenile leeches, but as juveniles they are unable to make normal swimming movements. However, normal swimming movements can be instated in such leeches by injecting them with serotonin.

The development of neocortical noradrenergic innervation in the mouse: a quantitative radioenzymatic analysis

The concentration of norepinephrine (NE) in the neocortex of the mouse has been determined radioenzymatically from the thirteenth embryonic (E13) through the sixtieth postnatal (P60) days. Detectable levels of the transmitter are present on E14, coincident with the emergence of the cortical plate. NE concentrations increase rapidly thereafter to reach a level 57% of adult values by P4. Subsequently, through P8 there is a decline followed again by a more gradual rise to maximum, or adult, concentrations by P30.

An immunohistochemical study of serotonin neuron development in the rat: ascending pathways and terminal fields

The ontogeny of the serotonergic axonal projections may be divided into three periods: one of initial axon elongation (E12-E16), the development of selective pathways (E15-E19) and terminal field development (E19-E21). All serotonergic axons that enter the prosencephalon ascend in the medial forebrain bundle From this bundle fascicles of immunoreactive axons enter several well-defined fiber tracts: specifically, the fasciculus retroflexus, stria medullaris, external capsule, fornix, and supracallosal stria. Axons from these pathways form terminal arborizations in the thalamus, hypothalamus, basal and limbic forebrain, and cerebral cortex. Serotonergic axons appear to be guided by pre-existing non-serotonergic tracts in reaching targets in the forebrain. Innervation of the cerebral cortex is a prolonged process extending from E19 through PND21. Axons enter directly into the marginal and intermediate zones of the immature cortex, at the medial, frontal and lateral edges of the hemisphere, and subsequently spread tangentially to cover the hemispheres. Terminal ramifications then arise from the bilaminar axons and fill in the middle cortical layers. This growth pattern gives rise to tangential and radial gradients in innervation density. While the growth of serotonin axons across the forebrain appears to be a continuous, sequential process, the development of terminal innervation is highly heterogeneous, occurring at different times and at different rates from region to region. Serotonergic axons do not innervate immature, primarily proliferative neuronal populations. The delay in serotonin innervation of the suprachiasmatic nucleus, striatum, and middle cortical layers long after the axons have reached these structures suggests that the formation of serotonin axon terminals is dependent on maturation of other elements in local neuronal circuitry.

Noradrenergic and serotonergic fibers innervate complementary layers in monkey primary visual cortex: an immunohistochemical study

Antisera directed against human dopamine beta-hydroxylase or serotonin were used to characterize the noradrenergic and serotonergic innervation patterns within the primary visual cortex of the squirrel monkey. The noradrenergic and serotonergic projections exhibit a high degree of laminar complementarity: layers V and VI receive a dense noradrenergic projection and a very sparse serotonergic projection, whereas layer IV receives a very dense serotonergic projection and is largely devoid of noradrenergic fibers. In addition, the noradrenergic fibers manifest a geometric order that is not so readily apparent in the distribution of serotonergic fibers. These patterns of innervation imply that the two transmitter systems affect different stages of cortical information processing--the raphe-cortical serotonergic projection preferentially innervates the spiny stellate cells of layers IVa and IVc, whereas the ceruleo-cortical noradrenergic projection innervates pyramidal cells.

Non-stationarity of ocular dominance in cat striate cortex

Interocular relationships, based on monocular directional tuning curves derived simultaneously for bar and for texture motion interleaved, are described for complex cells in the lightly-anaesthetised feline striate cortex. The results confirm earlier reports of stimulus-dependent differences in ocular dominance (Hammond 1979a, b) and demonstrate that "ocular-dominance" may be time-dependent and influenced by secondary stimulus characteristics including velocity of motion. Temporal and apparently spontaneous shifts in ocular dominance may take place other than in parallel for different classes of stimuli and may even occur simultaneously but in opposite directions. Thus absolute shifts in eye preference, as well as relative shifts between differing stimuli, must both occur with time, perhaps as the result of non-visual influences. The results present a challenge to strategies classically employed in defining cortical ocular dominance.

Increased concentration and accelerated synaptosomal uptake of noradrenaline in the central nervous tissues of neonatally hemispherectomized adult rats

Neonatally hemispherectomized adult rat brain was analyzed. The content of noradrenaline (NA) had increased significantly in the brain stem (+ 116%) and the cerebellum (+ 100%) by the 3rd month after operation. The Km for the high affinity uptake of NA by the brain stem synaptosomes stayed unchanged but Vmax was almost doubled by the operation. Fluorescence histochemical findings also suggest sprouting of noradrenergic axons in specific brain regions.

An ultrastructural and biochemical analysis of norepinephrine-containing varicosities in the cerebral cortex of the turtle Pseudemys

The fine structure and norepinephrine content of small granular vesicle-containing profiles were studied in normal and norepinephrine-depleted cerebral cortex of the turtle, Pseudemys. The cortex was fixed for electron microscopy with the KMnO4 procedure of Koda and Bloom ('77), while the norepinephrine content was assayed wit the radioenzymatic method of Coyle and Henry ('73). Green fluorescent fibers have been described by Parent and Poitras ('74) as located almost exclusively in the outer half of the molecular layer in turtle cortex. Small granular vesicle-containing profiles are found down to 100 microns below the pial surface, but over 50% lie within 20 microns of the surface. Within the outer 100 microns of cortex, the frequency of labeled varicosities is 1.39/1,000 microns2. The average area of the norepinephrine-containing varicosities is 0.61 microns2, and there is a mean of 18.4 vesicles per single section. The average number of large plus small vesicles in an entire varicosity was estimated to be 72. Synaptic membranes are not well-preserved with KMnO4 fixation, but good examples were found of small granular vesicle-containing profiles forming both symmetrical and asymmetrical membrane differentiations. Only a small percentage of the small granular vesicle profiles were associated with a synaptic membrane differentiation in single sections. When norepinephrine-fiber synapses are seen, they usually share a postsynaptic element with another unlabeled vesicle-containing profile. Normal turtle cortex contains an average norepinephrine concentration of 1.95 micrograms/gr, which is about eight times higher than in rat cortex. The ratio of norepinephrine to dopamine is about 18 to one, suggesting that dopamine is present predominantly in a precursor pool for norepinephrine. Small granular vesicle-containing profiles were eliminated after treatment with reserpine and 6-hydroxydopamine in concentrations that were shown to reduce norepinephrine concentration by 94% and 86%, respectively. The labeled varicosities were partially depleted by midbrain hemisection and by an inhibitor of dopamine-beta-hydroxylase (FLA-63). The norepinephrine-containing varicosities are remarkably coextensive with the distribution of thalamic fibers, both in the total extent of cortex where they are found and in the depth of cortex where they terminate. The results support the idea that there is a close structural and functional association between locus coeruleus and thalamic fibers in cerebral cortex, and the apparent difference in frequency of synapses suggests that each fiber system exerts its influence on cortical cells in a different way.

The effects of neonatal 6-hydroxydopamine treatment on morphological plasticity in the dentate gyrus of the rat following entorhinal lesions

Lesions of the entorhinal cortex, which is the major source of afferents to the outer two thirds of the molecular layer of the dentate gyrus, induce an expansion of the commissural projection, which is normally restricted to the inner third, and an intensification of acetylcholinesterase staining in the outer portion of the layer; these changes are thought to be due to the sprouting of the commissural fibers and certain cholinergic afferents to the dentate gyrus, respectively. We have studied these sequelae of entorhinal lesions in young adult rats which had been treated with 6-hydroxydopamine (6-OHDA) as neonates, in order to determine whether in the absence of its normal noradrenergic input, morphological plasticity in the dentate gyrus would be altered either in magnitude or extent. In animals treated with 6-OHDA, the levels of noradrenaline in the hippocampal formation were reduced by 93%. Despite this, there was clear evidence for an expansion of the commissural projection following entorhinal lesions, as judged both autoradiographically and in Timm-stained material. Similarly, the intensification of acteylcholinesterase staining in the outer part of the molecular layer appeared as marked as after comparable lesions in untreated animals. From these observations it would appear that in the dentate gyrus, at least, morphological plasticity does not require the presence of an intact noradenergic innervation.

The effects of LSD and some analogues on the responses of single cortical neurons of the cat to optical stimulation

The effects of lysergic acid diethylamide (LSD) and its analogues, 2-bromo-LSD (BOL) and methysergide, have been investigated on the responses to photic stimulation of single neurons in the striate cortex of the paralyzed, anesthetized cat. Systemic LSD (0.1--50 micrograms/kg, i.v.) produced: (a) enhancement or depression of evoked activity, the former being common with low, the latter with high doses; (b) changes in directional selectivity; and (c) changes in unstimulated background discharges. The effectiveness of the drug was reduced by repeated administration. Both BOL (10--75 micrograms/kg) and methysergide (100-700 micrograms/kg) produced effects qualitatively similar to LSD, but were considerably less potent. Microelectrophoretic administrations of LSD to single cortical neurons had actions similar to those caused by intravenous administration. BOL and methysergide required much larger currents to produce any effect and sometimes no effect could be induced by iontophoresis. It was concluded that these drugs influence visually evoked neuronal responses mainly by acting directly on cortical cells or synapses; and that the interference with visual cortical function could account for the distortion of visual perception caused by lysergic acid analogues; but that the hallucinogenic and psychotomimetic actions of LSD probably require additional subcortical effects.

Visual deficits and recovery following monocular lid closure in a prosimian primate

The behavioral consequences of long-term monocular lid closure in 6 deprived and 2 normal prosimian primates (Galago crassicaudatus) were examined using a visual field perimetry technique and a variety of visuomotor tests. Deprived galagos had their right lid sutured either within 8 (n = 4) or 38 (n = 2) days postpartum and were reared under these conditions for at least 8 months. Normal galagos responded briskly on all visuomotor tests including tracking, response to visual threat, ability to descend from a platform, and obstacle avoidance. Perimetry tests of these animals showed that the visual field with both eyes open was 180 degrees while each eye tested individually had a 135 degrees field. Deprived galagos tested with their experienced eye exhibited normal visuomotor behavior and normal visual fields. Following reverse suture all deprived animals behaved initially as if blind on all tests. Over the next 4-8 weeks these animals showed some limited recovery on the visuomotor tasks. Tests of perimetry indicated that by the end of four weeks of testing, vision was limited to the deprived monocular segment where it remained stable to the end of the testing period. With both eyes open, deprived galagos exhibited a normal 180 degrees visual field. Curiously, however, when stimuli were introduced simultaneously in corresponding points in both monocular segments of the field, the majority of orienting responses of deprived subjects favored the right deprived eye. Under similar testing conditions one normal subject favored its right eye and the other its left eye. The reversibility of monocular deprivation was explored further either by extending the period of forced use of the deprived eye in one animal or by removing the non-deprived eye in another animal. No further improvement in visual field perimetry was seen following a longer period of forced use of the deprived eye. In contrast, a substantial increase in the visual field could be demonstrated one month following removal of the originally experienced eye: the deprived eye field expanded to include the entire 90 degrees ipsilateral hemifield. The contralateral field of the deprived eye, however, continued to remain blind even after one year. These behavioral results suggest that although the original binocular competitive disadvantage suffered by the deprived eye during development is relatively permanent, it can partially be ameliorated by late removal of the non-deprived eye.