Synaptic adjustment after deafferentation of the superior colliculus of the rat.

Eyes were removed from rats shortly after birth, when there are few formed synapses in the colliculus. It was found that synaptogenesis continues to give a near-normal ratio of terminals containing either spheroidal or flattened vesicles. After eye removal in adult rats, however, reinvasion of synaptic sites vacated by degenerate optic terminals occurs, with an incomplete return toward a normal proportion of synaptic types.

Visual discrimination of movement: midbrain or forebrain?

Monkeys whose optic chiasm and forebrain commissures had been sectioned and control monkeys with only the optic chiasm cut were tested for interocular transfer of discriminations based on direction of movement. Only the control animals showed transfer to the untrained eye, which suggests that discrimination of movement, like pattern, is a function strongly dependent on the cortex.

Widespread periodic intrinsic connections in the tree shrew visual cortex.

Intrinsic connections within the tree shrew (Tupaia glis) visual cortex (area 17) are organized in periodic stripelike patterns within layers I, II, and III. This anatomical network resembles the regularly organized stripes of 2-deoxyglucose accumulation seen after stimulation of alert animals with uniformly oriented lines. Such connections imply that widespread lateral interactions are superimposed on the retinotopic organization of area 17 and suggest alternative interpretations of cortical columns.

Effect of dietary n-3 and n-6 fatty acids on fatty acid desaturation in rat liver.

To study the effect of high-fat diets with varying contents of n-3 and n-6 fatty acids on the metabolism of essential fatty acids, the rat liver microsomal fatty acid desaturases were measured. The rats were fed for 3 weeks with diets high in linseed oil (18:3(n-3)), sunflower seed oil (18:2(n-6)) or fish oil (20:5(n-3) and 22:6(n-3)) (20%, w/w) using pellet fed rats as a reference. The delta 6-desaturase using 18:2(n-6) or 18:3(n-3) as substrates was stimulated 1.5-2.5-fold by linseed or sunflower seed oil, compared to the pellet reference. The delta 5-desaturase was stimulated 3.5-fold with linseed oil and 2.5-fold with sunflower seed oil, while the delta 9-desaturase was inhibited by all the high-fat diets. The delta 6-, 5- and 9-desaturase activities were in all cases considerably reduced with fish oil as compared to linseed and sunflower seed oil diets. With pellet fed rats the rates were highest for delta 9-desaturation and in decreasing order lower for delta 5-desaturation, delta 6-desaturation with 18:3 (n-3) as substrate and finally delta 6-Desaturation with 18:2(n-6) as substrate. The content of 20:4(n-6) in liver phospholipids increased with the diets rich in 18:2(n-6), and was reduced for the fish oil diet enriched in 20:5 and 22:6(n-3) fatty acids. The amount of 20:5(n-3) in phospholipids was as high with linseed oil diet as with the fish oil diet, while the 22:6(n-3) content was only increased with the fish oil diet.

Local circuit neurons of macaque monkey striate cortex: I. Neurons of laminae 4C and 5A.

A study has been made, using Golgi preparations, of the organization of neurons with smooth or sparsely spined dendrites, here called local circuit neurons, of the macaque monkey primary visual cortex. Since these neurons include those responsible for inhibitory circuitry of the cortex, a better understanding of their anatomical organization is essential to concepts of functional organization of the region. This account describes those neurons found with cell body and major dendritic spread within the thalamic recipient zone of lamina 4C and its border zone with lamina 5A. The neurons are grouped firstly in terms of in which laminar division the soma occurred--4C beta, 4C alpha or the border zone of 5A-4C beta--and secondly, into varieties on the basis of the interlaminar projection patterns of their axons. Most, if not all, of the local circuit neurons of these divisions have interlaminar axon projections as well as an arbor local to their cell body and dendritic field. These interlaminar projections are highly specific, targeting from one to five laminar divisions depending on the variety of neuron; on this basis 17 varieties of local circuit neuron are described. While the number of varieties appears dauntingly large in terms of understanding the functional circuitry of the region, the clear-cut organization of the interlaminar links may provide clues as to the information processing that concerns each neuron. The local circuit neuron axon projections can be related to a wealth of information already available concerning the laminar organization of afferent axons and efferent cell groups, the organization of spiny neuron intrinsic relays (presumed to be excitatory), and physiological properties of different laminar divisions. It is hoped that the information derived from this study can serve as a guide for correlated physiological-anatomical studies on single cells of the region.

Plasticity in the developing visual system: the effects of retinal lesions made in young rats.

The central visual pathways of the rat have been used as a model for investigating the significance of axonal interactions in mammalian neural development. Attention is restricted largely to the aberrant distribution of optic axons to the ipsilateral side of the brain and their distribution in the superior colliculus after early unilateral eye damage. The normal ipsilateral retinotectal pathway in pigmented rats appears as a series of patches located anteriorly and laterally in the stratum opticum, whereas in albino animals it is a small area lying anteromedially. In both, a few axons are often found at the extreme posterior border of the superior colliculus. After unilateral eye enucleation at birth, an aberrant ipsilateral pathway from the remaining eye arises at the optic chiasm. It originates from all parts of the retina and terminates in the ipsilateral superior colliculus in a topographic fashion such that the upper retina projects laterally and the lower retina, medially. The pathway is heaviest anteromedially (from lower temporal retina) and lightest posterolaterally (from upper nasal retina). There is always a heavy projection to the extreme posterior border of the superior colliculus. In only two animals of a large series was direct intertectal sprouting found. After partial retinal lesions, there is again an ipsilateral pathway from the unlesioned eye which fills the projection area of the lesion. As after total enucleation, the pathway arises from most of the ipsilateral retina, not just that region homotypic to the lesion site, being heaviest from the lower temporal and lightest (or deficient) from the upper retina. There is suggestion of ordering of the projection into the deafferented region in that the ipsilateral degeneration after lesions in the intact eye is compact but does not fill the gap in the crossed projection completely. There is also indication that some intact parts of the retina lesioned at birth may also project in an inappropriate retinotopic fashion to the deafferented region. The corticotectal pathway shows a normal map. Study of the ipsilateral retinotectal pathway indicates that the axons terminating at the extreme posterior border of the superior colliculus arise from the lower temporal retina. The results are interpreted as indicating that the aberrant uncrossed pathways after complete or local retinal lesions, compare very closely in most features. Both distribute to the deafferented area of the superior colliculus -- in one, this is the whole surface, while in the other it is a small area. The fact that in the latter case axons are ending in quite inappropriate parts of the tectal map, may be explained more simply in terms of interactions between adjacent axons in the optic pathway rather than by an hypothesis involving a change in the cell labels across the tectal map.

Local circuit neurons of macaque monkey striate cortex: III. Neurons of laminae 4B, 4A, and 3B.

We continue an investigation of the organization of local circuit neurons (largely inhibitory, GABAergic neurons, with smooth or sparsely spined dendrites) in the primary visual cortex of macaque monkey (Lund, '87: J. Comp. Neurol. 257:60-92; Lund et al., '88: J. Comp. Neurol. 276:1-29). This account covers local circuit neurons of layers 4B, 4A, and 3B; these three layers each receive different intrinsic second-order relays of principal thalamic inputs as well as receiving primary thalamic inputs in the case of two of the three laminae (4A and 3B). The study shows the existence of a number of different local circuit neurons making interlaminar projections between 4B, 4A, and 3B; each provides specific cross links between different combinations of the three laminae. It is known that the functional properties recorded physiologically from layers 4B, 4A, and 3B differ from one another and so these anatomical cross links may allow for correlation between different attributes of visual stimuli, e.g., color or motion, while still enabling separate processing of these different attributes to proceed in each of the three layers and be passed on to extrastriate areas. Whereas no spine-bearing neurons of layers 4B, 4A, or 3B provide "feedback" circuits to layer 4C (the source of their major intrinsic excitatory afferents), some of the local circuit neurons provide precisely structured axon feedback projections to divisions of 4C. The local circuit neurons also project to either lamina 5 or lamina 6, but not both and to superficial layers 3A, 2, and 1. Some local circuit neuron axon projections are of a dimension that would be confined to single functional clusters, e.g., cytochrome-rich "blobs," others reach out far enough to contact nearest neighbor "unlike" functional clusters, and yet others spread far enough to link repeating clusters of single function.

Postnatal development of thalamic recipient neurons in the monkey striate cortex: I. Comparison of spine acquisition and dendritic growth of layer 4C alpha and beta spiny stellate neurons.

A quantitative study has been made from Golgi impregnations of the maturation of dendrites and their spines on spiny stellate neurons in the macaque monkey primary visual cortex. The neurons studied lay within either the alpha or the beta division of lamina 4C; previous workers have shown the alpha division neurons to be contacted by thalamic axon terminals arising from the magnocellular division of the lateral geniculate nucleus (LGN) of the thalamus and the beta division neurons to be contacted by parvocellular LGN inputs. Most thalamic terminals and perhaps the majority of other type 1 (Colonnier, '81), presumed excitatory, inputs to these cells make synaptic contacts on the tips of their dendritic spines. Measurement was made of relative changes in the total number of spines on these alpha and beta spiny neurons over age by measuring both spine density along the dendrites and dendritic arbor size in single 90-microns sections from Golgi rapid preparations. Our previous work (Lund et al., '77; Boothe et al., '79) showed a marked proliferation and attrition of spines and dendritic branches to occur in the early postnatal weeks; Rakic et al. ('86) have since proposed that there is a cortexwide synchrony of synapse acquisition and loss during this same period. However, different visual capacities channelled via the magnocellular and parvicellular geniculate relays show different maturational rates (Harwerth et al., '86). This study indicates that the anatomical maturation of spines on the alpha and beta neurons is not temporally coincident from birth to 30 weeks. During this period, phases of spine acquisition and loss on alpha neurons precedes similar phases on beta neurons. The alpha neurons carry a peak spine population at 5-8 weeks postnatal, whereas the beta neurons carry their peak spine populations between 8 and 24 weeks postnatal. At all ages prior to 30 weeks, the two sets of neurons carry quite different total spine populations. Close to 30 weeks of age, the total spine coverage has fallen on both sets of neurons and becomes identical between the alpha and beta neurons. In animals aged 30 weeks to adult, spine coverage per neuron is maintained at a common figure for the alpha and beta neurons despite further growth and disparate dendritic arbor sizes and different local spine densities in the two groups; this suggests that some common sampling paradigm between pre- and postsynaptic elements is adopted by the alpha and beta neurons and also suggests the development of a close functional correlation between the two sets of neurons.

Postnatal development of thalamic recipient neurons in the monkey striate cortex: III. Somatic inhibitory synapse acquisition by spiny stellate neurons of layer 4C.

The development of type 2 (Colonnier, '81) synapses on the cell bodies of thalamic recipient spiny stellate neurons in layers 4C alpha and 4C beta of primary visual cortex neurons was examined over the first 36 postnatal weeks and in the adult monkey. The type 2 synapses, known to be GABAergic (Ribak, '78) and therefore presumed to be inhibitory, developed faster on the alpha neurons than the beta neurons. Both neuron groups show a marked increase and then decline in the percentage of the somatic membrane covered by type 2 synaptic appositions during this 36-week time period. The time course of the type 2 synapses development is compared to that of the spine synapse development described in previous studies (Lund and Holbach, '91; Lund et al., '91), and it is clear that on both neuron groups this inhibitory synapse population is put in place and refined later than the spine synapses. These findings suggest that each cortical neural circuit has a unique time course for its early development within an overall time window (Rakic et al., '86), or sensitive period (Hubel and Wiesel, '70). Visual deprivation, although causing the alpha and beta neurons to adopt a more similar temporal and numerical developmental pattern than normal, did not prevent acquisition and loss phases of type 2 synapses or the assumption of a normal numerical loading by 36 weeks of age.

Local circuit neurons of macaque monkey striate cortex: IV. Neurons of laminae 1-3A.

We continue our Golgi studies (Lund [1987] J. Comp. Neurol. 257:60-92; Lund et al. [1988] J. Comp. Neurol. 276:1-29; Lund and Yoshioka [1991] J. Comp. Neurol. 331:234-258) of the organization of local circuit, largely gamma-aminobutyric acid (GABA)-containing neurons in macaque monkey visual cortex, area V1, with this account of the local circuit neurons lying in layers 1 and 2/3A. These layers receive intrinsic interlaminar excitatory and inhibitory relays from layers 3B, 4A, 4B, and 5. We describe seven varieties of local circuit neurons with somata within layers 1-2/3A, and we compare the lateral scale of spread of the axons and dendrites of these neurons with the size of the columnar connectional patch domains made by the laterally spreading axon collaterals of pyramidal neurons within the superficial layers (Lund et al. [1993] Cerebral Cortex 3:148-162). We conclude from this comparison that all of the neurons have dendritic fields that are limited to single patch domains. Furthermore, only two of the seven local circuit neuron varieties have sufficient axon spread to influence territory beyond single domains, reaching into neighboring territory likely to differ in function from that occupied by their dendrites. We have identified descending projections from particular varieties to layers 3B, 4A, 4B, and 5 and to the white matter. We discuss the contributions that these interneurons may make to function within the superficial cortical layers, and we summarize our overall conclusions, so far, from our set of studies on interneurons within area V1 of the macaque.

Heterogeneity of chandelier neurons in monkey neocortex: corticotropin-releasing factor- and parvalbumin-immunoreactive populations.

Chandelier neurons are a unique subclass of cortical nonpyramidal neurons. The axons of these neurons terminate in distinctive vertically arrayed cartridges that synapse on the axon initial segment of pyramidal neurons. In this study, the rapid Golgi method and immunohistochemical techniques were used to characterize the morphology, regional distribution, laminar location, and biochemical content of chandelier neurons in the prefrontal and occipital cortices of three monkey species. As in our previous studies of visual areas V1 and V2 (Lund: Journal of Comparative Neurology 257:60-92, 1987; Lund et al.: Journal of Comparative Neurology 202:19-45, 1981, 276:1-29, 1988), Golgi impregnations of areas 46 and 9 of macaque prefrontal cortex show chandelier neurons to be present in layers 2 through superficial 5. The vertical arrays of terminal boutons (axon cartridges) typical of this neuron class are also present in layers 2-6 of the prefrontal cortex, but are not found in layer 1 or the subcortical white matter. In immunohistochemical studies, a calcium-binding protein, parvalbumin, and a neuropeptide, corticotropin-releasing factor (CRF), identify rod-like structures that are morphologically similar to the axon cartridges of chandelier neurons seen in the Golgi material. In addition, both parvalbumin- and CRF-immunoreactive cartridges are located below the somata of unlabeled pyramidal neurons and appear to outline the axon initial segment of these neurons. However, we find that parvalbumin and CRF are present in only subpopulations of chandelier axon cartridges. For example, in adult primary visual cortex, parvalbumin-labeled cartridges are present in very low numbers only in layers 2-3, whereas in prefrontal and occipital association cortices these cartridges are a very prominent component of layers 2-superficial 3 and are present in much lower density in the deeper cortical layers. In contrast to these findings in adult macaque monkeys, prefrontal and occipital association cortices of infant macaque monkeys contain a very high density of parvalbumin-labeled cartridges in layer 4 and relatively few in the superficial cortical layers. Furthermore, in adult squirrel monkey prefrontal cortex, CRF-labeled cartridges are predominately present in layer 4, but these CRF-immunoreactive structures have not been observed in the homologous regions of infant or adult macaque monkeys. These findings indicate that even for neurons of such distinctive morphology and presumably constant functional role as chandelier neurons, factors such as regional and laminar location, age, and primate species are associated with differences in the biochemical content of subpopulations of these neurons.

Intrinsic laminar lattice connections in primate visual cortex.

Intracortical injections of horseradish peroxidase (HRP) reveal a system of periodically organized intrinsic connections in primate striate cortex. In layers 2 and 3 these connections form a reticular or latticelike pattern, extending for about 1.5-2.0 mm around an injection. This connectional lattice is composed of HRP-labeled walls (350-450 microns apart Saimiri and about 500-600 microns in macaque) surrounding unlabeled central lacunae. Within the lattice walls there are regularly arranged punctate loci of particularly dense HRP label, appearing as isolated patches as the lattice wall labeling thins further from the injection site. A periodic organization has also been demonstrated for the intrinsic connections in layer 4B, which are apparently in register with the supragranular periodicities, although separated from these by a thin unlabeled region. The 4B lattice is particularly prominent in squirrel monkey, extending for 2-3 mm from an injection. In both layers, these intrinsic connections are demonstrated by orthogradely and retrogradely transported HRP and seem to reflect a system of neurons with long horizontal axon collaterals, presumably with arborizations at regularly spaced intervals. The intrinsic connectional lattice in layers 2 and 3 resembles the repetitive array of cytochrome oxidase activity in these layers; but despite similarities of dimension and pattern, the two systems do not appear identical. In primate, as previously described in tree shrews (Rockland et al., '82), the HRP-labeled anatomical connections resemble the pattern of 2-deoxyglucose accumulation resulting from stimulation with oriented lines, although the functional importance of these connections remains obscure.

Neuronal composition and development in lamina 4C of monkey striate cortex.

Lamina 4C (Lund, '73) of the monkey, Macaca nemestrina, visual striate cortex occupies a key position as a principal recipient zone of axons from the lateral geniculate nucleus (LGN). Synaptic maturation in lamina 4C is of particular interest since it involves a competitive interaction between thalamic axons for postsynaptic territory: an interaction which is strongly influenced by afferent activity (Hubel et al., '77). As an initial step toward understanding the normal process of synapse maturation in 4C, this study examines Golgi material to define the adult neuron populations of subdivisions 4C alpha (receiving afferents from magnocellular LGN) and 4C beta (receiving afferents from parvocellular LGN). Three groups of spine-bearing neurons are described--one relatively confined to either alpha or beta subdivision, the other two bridging the depth of 4C; four groups of smooth dendritic neurons interact with the spine-bearing population. Electron microscopy of normal and Golgi-impregnated tissue is used to define key features of synapse populations on these neurons. From embryonic day 159 through adulthood the smooth and spiny neurons occur in the same constant proportions in the neuropil (5% smooth, 95% spiny). Changes in the distribution of synapses on the spiny neurons are analyzed qualitatively; type 1 axon terminals (asymmetric apposition--round vesicles) shift from dendritic shafts to spine tips during maturation. Each spine is found to bear a type 1 contact at all ages; these results allow us to conclude that the figures of Boothe et al. ('79) on changes in spine populations during maturation can now be interpreted as changes in type 1 synapse populations. It is shown that type 2 synapses (symmetric appositions--pleomorphic vesicles) arise from axons of the smooth dendritic neurons. These synapses are found to increase in number on the spiny cell somata in early postnatal development, and this is followed by a decrease in number to the adult level.

Spine formation and maturation of type 1 synapses on spiny stellate neurons in primate visual cortex.

This study continues an investigation (Mates and Lund, '83a) of neuronal development in lamina 4C of macaque monkey striate visual cortex. The maturational history of the type 1 synaptic contacts on spine-bearing stellate neurons, which comprise 95% of the neurons of the lamina, is described. It is shown that type 1 contacts are initially found on the dendritic shafts; these contacts appear to be carried out on spine outgrowths. This leads to the adult condition where type 1 contacts are found only on the spine tips. A similar phenomenon has been reported for pyramidal neurons of the rat (reported during the course of this study by Miller and Peters, '81). In later maturation the spine and its type 1 contact may be lost; profiles found in the neuropil illustrate a process of shrinkage and detachment of both the type 1 axon terminals and postsynaptic dendritic spines in normal maturation. These findings provide an explanation for the marked increase and decrease in spine populations observed to occur on these neurons during normal maturation in an earlier study by Boothe et al. ('79).

Developmental changes in the relationship between type 2 synapses and spiny neurons in the monkey visual cortex.

This study continues an exploration of synaptic development in the primary visual cortex of the monkey (Macaca nemestrina). In a prior study (Mates and Lund, '83a), we observed that type 2 synapses on the cell bodies of spiny stellate neurons of lamina 4C appeared not only to increase in number during early postnatal development but also subsequently decreased during maturation. Using quantitative, stereological electron microscopic methods, we examined the maturation of this synapse population from embryonic day 159 to adult, on spiny stellate neurons of 4C alpha and beta and, for comparison, on pyramidal neurons in upper and lower lamina 6. Tissue was also taken for comparison from two animals reared to 8 weeks of age with binocular eyelid closure from birth. We confirmed that a marked increase and subsequent decrease occurred in this somal type 2 synapse population on both neuron populations. However, due to the infrequency of the smooth dendritic neurons (approximately 5% of the neuron population) giving rise to the type 2 contacts, and due to expansion of the neuropil during maturation increasing intercell distances against constant volume of the type 2 axon arbors, it is concluded that the decrease in type 2 somal synapses may represent a redistribution to dendrites rather than loss from the neuropil. Cells of lamina 4C beta (receiving input from the parvocellular lateral geniculate nucleus-LGN) show a slower initial accumulation of type 2 contacts compared to neurons of lamina 4C alpha (receiving input from magnocellular LGN), or to pyramidal neurons of lamina 6.(ABSTRACT TRUNCATED AT 250 WORDS)

Local circuit neurons of developing and mature macaque prefrontal cortex: Golgi and immunocytochemical characteristics.

A study has been made of the nonpyramidal, local circuit neurons in developing and mature macaque monkey prefrontal cortex with Golgi and immunocytochemical techniques. The area chosen for study is located between the cingulate gyrus and the ventral bank of the principal sulcus, and contains areas 9 and 46 as described by Walker (J. Comp. Neurol. 73:59-86, '40). In Golgi studies, the unique axonal features of impregnated neurons made possible the identification of thirteen separate classes of local circuit neurons. Five of these cell types, in their general characteristics, resembled classes identified in human prefrontal cortex, as well as in other cortical areas of macaque monkeys and other species. Measurements of the scale of axon arbors and dendritic fields of the Golgi-stained local circuit neurons also suggested particular spatial relationships of certain classes to the scale of intrinsic lattice connections made by the axons of pyramidal neurons in the same region. Similarities in morphology between cells described in human prefrontal cortex and neuron varieties described in this study indicate that this region of monkey prefrontal cortex may serve as a useful model for neuron populations in human prefrontal cortex. Sufficient morphological detail was present in immunocytochemical studies to suggest one or more identifying biochemical characteristics for seven of the thirteen classes of local circuit neurons. The calcium binding proteins, parvalbumin, calbindin D-28K, and calretinin, were found in chandelier and wide arbor neurons, neurogliaform cells, and double bouquet neurons, respectively. In addition, cholecystokinin immunoreactivity was present in medium arbor neurons and in narrow arbor cells connecting layers 2 and 4. Somatostatin 28(1-12) immunoreactivity was detected in beaded axon neurons in layers 5 and 6. This biochemical characterization of local circuit neurons, although incomplete, confirms the separate identity of at least some of the varieties distinguished by Golgi morphology, and allows a start to be made on studies examining changes in their functional state. The general inhibitory nature of these interneurons suggests that they are likely to play a crucial role in determining patterns of neural activation in the prefrontal cortex.

Intrinsic cortical connections in macaque visual area V2: evidence for interaction between different functional streams.

Area V2 of macaque visual cortex represents an important but poorly understood stage in visual processing. To provide a better understanding of the region, we studied the organization of its intrinsic cortical connections by making focal (200-300 microns) iontophoretic microinjections of the tracer biocytin. Alternate tissue sections were tested for biocytin, cytochrome oxidase (CO), or Cat-301 immunoreactivity to localize biocytin label relative to the three stripelike compartments that characterize this area. Biocytin-labeled pyramidal neurons of layers 2/3, and, to a lesser extent, layer 5, provided laterally spreading axon projections that terminated in discrete patches (250-300 microns diameter), primarily in layers 1-3. Any injected locus in V2 projected to 10-15 similarly sized patches, up to 4 mm from the injection site, and distributed in an elongated field orthogonal to the stripe compartments. We noted prominent patchy connections within, as well as between, individual compartments, perhaps reflecting functional substructures within stripes. Each stripe compartment projected to all three compartments but with different relative frequencies; CO-rich compartments projected mainly to other CO-rich compartments (75%), whereas CO-poor compartments projected equally to CO-rich and CO-poor compartments. We therefore emphasize the existence of substantial interconnections among all three V2 compartments. As further evidence for crosstalk between visual channels, we also noted an input to the V2 "thick" CO stripes from V1 cells in layer 4A as a distinct population in addition to the neurons of layer 4B. Thus, the CO stripe architecture may not be a marker for strictly segregated parallel visual pathways through V2.

Local circuit neurons of macaque monkey striate cortex: II. Neurons of laminae 5B and 6.

This study investigates the intrinsic organization of axons and dendrites of aspinous, local circuit neurons of the macaque monkey visual striate cortex. These investigations use Golgi Rapid preparations of cortical tissue from monkey aged 3 weeks postnatal to adult. We have earlier (Lund, '87) described local circuit neurons found within laminae 5A and 4C; this present account is of neurons found in the infragranular laminae 5B and 6. Since the majority of such neurons are GABAergic and therefore believed to be inhibitory, their role in laminae 5B and 6, the principal sources of efferent projections to subcortical regions, is of considerable importance. We find laminae 5B and 6 to have in common at least one general class of local circuit neuron-the "basket" neuron. However, a major difference is seen in the axonal projections to the superficial layers made by these and other local circuit neurons in the two laminae; lamina 5B has local circuit neurons with principal rising axon projections to lamina 2/3A, areas whereas lamina 6 has local circuit neurons with principal rising axon projections to divisions of 4C, 4A, and 3B. These local circuit neuron axon projections mimic the different patterns of apical dendritic and recurrent axon projections of pyramidal neurons lying within laminae 5B and 6, which are linked together by both dendritic and axonal arbors of local circuit neurons in their neuropils extending between the two laminae. The border zone between 5B and 6 is a specialized region with its own variety of horizontally oriented local circuit neurons, and it also serves as a special focus for pericellular axon arrays from a particular variety of local circuit neuron lying within lamina 6. These pericellular axon "baskets" surround the somata and initial dendritic segments of the largest pyramidal neurons of layer 6, which are known to project both to cortical area MT (V5) and to the superior colliculus (Fries et al., '85). Many of the local circuit neurons of layer 5B send axon trunks into the white matter, and we therefore, suspect them of providing efferent projections. The axons of lamina 6 local circuit neurons have not been found to make such clear-cut contributions to the white matter.

Distribution of GABAergic neurons and axon terminals in the macaque striate cortex.

Antisera to glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) have been used to characterize the morphology and distribution of presumed GABAergic neurons and axon terminals within the macaque striate cortex. Despite some differences in the relative sensitivity of these antisera for detecting cell bodies and terminals, the overall patterns of labeling appear quite similar. GABAergic axon terminals are particularly prominent in zones known to receive the bulk of the projections from the lateral geniculate nucleus; laminae 4C, 4A, and the cytochrome-rich patches of lamina 3. In lamina 4A, GABAergic terminals are distributed in a honeycomb pattern which appears to match closely the spatial pattern of geniculate terminations in this region. Quantitative analysis of axon terminals that contain flat vesicles and form symmetric synaptic contacts (FS terminals) in lamina 4C beta and in lamina 5 suggest that the prominence of GAD and GABA axon terminal labeling in the geniculate recipient zones is due, at least in part, to the presence of larger GABAergic axon terminals in these regions. GABAergic cell bodies and their initial dendritic segments display morphological features characteristic of nonpyramidal neurons and are found in all layers of striate cortex. The density of GAD and GABA immunoreactive neurons is greatest in laminae 2-3A, 4A, and 4C beta. The distribution of GABAergic neurons within lamina 3 does not appear to be correlated with the patchy distribution of cytochrome oxidase in this region; i.e., there is no significant difference in the density of GAD and GABA immunoreactive neurons in cytochrome-rich and cytochrome-poor regions of lamina 3. Counts of labeled and unlabeled neurons indicate that GABA immunoreactive neurons make up at least 15% of the neurons in striate cortex. Layer 1 is distinct from the other cortical layers by virtue of its high percentage (77-81%) of GABAergic neurons. Among the other layers, the proportion of GABAergic neurons varies from roughly 20% in laminae 2-3A to 12% in laminae 5 and 6. Finally, there are conspicuous laminar differences in the size and dendritic arrangement of GAD and GABA immunoreactive neurons. Lamina 4C alpha and lamina 6 are distinguished from the other layers by the presence of populations of large GABAergic neurons, some of which have horizontally spreading dendritic processes. GABAergic neurons within the superficial layers are significantly smaller and the majority appear to have vertically oriented dendritic processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal development of thalamic recipient neurons in the monkey striate cortex: II. Influence of afferent driving on spine acquisition and dendritic growth of layer 4C spiny stellate neurons.

This study uses Golgi-impregnated material to examine the effects of altering the nature of afferent driving on the maturation of spines and dendrites on thalamic recipient spiny stellate neurons in layers 4C alpha and beta of the monkey striate cortex. These two laminae receive input from different sets of thalamic afferents with different functional properties. The development of dendritic spine and dendritic branch populations on these neurons in experimental animals is compared to the same features on similar groups of neurons in a series of normal animals described in the preceding study (Lund and Holbach, '91). Three conditions of rearing were used to alter afferent driving from normal: complete darkness (with in some cases return to normal diurnal light-dark cycle), bilateral eye lid suture, and monocular eye lid suture. Some of the normal and dark-reared infant monkeys were examined behaviorally for visual capacity in an earlier study (Regal et al., '76). All conditions of abnormal afferent driving caused changes from the normal developmental patterns of spine and dendritic arbor growth in these first-order neurons of the cortex and each condition differed in the nature of change produced. Major findings of this study are: 1. Vigorous spine acquisition and dendritic growth occurs under all conditions of visual deprivation on alpha and beta neurons. Eventual spine and dendritic attrition occurs under at least conditions of bilateral or monocular lid suture to produce a rather constant adult morphology. We assume, therefore, that visually driven activity is a modulator or shaper of the developmental process for thalamic recipient neurons of visual cortex, rather than being an initiator, terminator, or driving force for their maturation. 2. An innate "clock," whose nature is unknown but is apparently not driven by visual input, initiates and terminates a period of growth of the thalamic recipient neurons between birth and 30-32 weeks of age. 3. Factors controlling dendritic arbor growth and retraction are different from those controlling spine synapse addition or attrition. 4. Whereas the alpha and beta neurons normally show quite different early growth patterns between birth and 30 weeks of age, when both eyes are simultaneously deprived of vision, an early temporal and numerical convergence occurs in patterns of spine population development on the two groups of neurons. This convergent pattern assumes a different form in dark-reared and lid-sutured animals.(ABSTRACT TRUNCATED AT 400 WORDS)