Ferret Primary Visual Cortex Research Articles

Relationships between Local Synaptic Connections and Orientation Domains in Primary Visual Cortex

Combined optical imaging of ferret primary visual cortex in vivo and scanning laser photostimulation in brain slices were used to determine the spatial relationships between synaptic inputs onto individual neurons and the pattern of orientation columns. In the upper cortical layers, both excitatory and inhibitory inputs originated primarily from regions with orientation tuning similar to that of the recorded neurons; the shapes of the input tuning curves were indistinguishable. The orientation distributions of both types of inputs centered around the orientation of the recorded neurons, and no evidence for preferential cross-orientation inputs, either excitatory or inhibitory, was observed. These patterns of synaptic connectivity are most consistent with feedforward models for generation of orientation selectivity and are inconsistent with the patterns required by models based on cross-orientation inhibition.

Development of Orientation Preference Maps in Ferret Primary Visual Cortex

The development of orientation preference maps was studied in ferret primary visual cortex using chronic optical imaging of intrinsic signals. The emergence and maturation of the maps were examined over time in single animals. The earliest age at which cortical domains selectively responsive to particular stimulus orientations were observed varied considerably between individuals, from postnatal day 31 to 36. In all cases, the earliest maps seen were low-contrast, with regions of orientation-specific activity that were difficult to distinguish from noise. These early maps matured over a period of several days into the high-contrast, patchy maps typical of adult animals. The structure of the orientation maps was remarkably constant over time. The indistinct features in the earliest maps were always patches of the same sizes and shapes and at the same locations as in the maps obtained in subsequent recording sessions. Details of the more mature maps, including the relative intensities of individual iso-orientation domains, were also constant from one recording session to another over periods of several weeks. The patterning of iso-orientation domains in ferret primary visual cortex thus is established early in development and remains stable over time, unaffected by either normal visual experience or the anatomical rearrangements of geniculocortical afferents into eye-specific domains.

Distinct laminar differences in the distribution of excitatory amino acid receptors in adult ferret primary visual cortex

In order to explore the relative contributions of the different ionotropic excitatory amino acid receptor subtypes to signalling in primary visual cortex, we have mapped their distributions in area 17 of adult ferret cerebral cortex by quantitative in vitro autoradiography. d,l-α-amino-3-hydroxy-5-methoxy-4-isoxazole proprionate (AMPA) and kainate receptors, gating fast, Na +-permeable channels, were localized with [ 3H]AMPA and [ 3H]kainate, respectively. Slower-acting N-methyl- d-aspartate receptors, which gate the influx of Ca 2+ as well as Na +, were localized with [ 3H]dizocilpine maleate ([ 3H]MK-801). All three radioligands bound to single sites, with K D s of 414 nM ([ 3H]AMPA), 78nM ([ 3H]kainate) and 16nM ([ 3H]MK-801), and each receptor subtype displayed a different laminar distribution pattern within area 17. AMPA receptors were concentrated in superficial layers, with intermediate densities in deep layers and lowest levels in layer IV. Kainate receptor levels were high in layers V and VI and low in all other layers. N-methyl- d-aspartate receptors were more homogeneously distributed than AMPA or kainate receptors, but were expressed at highest levels in layers I and IV and lowest levels in layers V and VI. The binding site densities found in the layers containing most receptors were B max= 2812fmol/mg for [ 3H]AMPA, B max= 626fmol/mg for [ 3H]MK-801 maleate and B max = 278fmol/mg for [ 3H]kainate. Thus, while AMPA receptors were predominant and kainate receptors least abundant in all cortical layers, a complementary relative distribution of excitatory amino acid receptors was apparent, with AMPA receptor density highest in superficial layers, kainate receptor density highest in inferior layers and N-methyl- d-aspartate receptor density highest in the middle granular layer, as well as in layer I. The results indicate that although AMPA receptors are principally involved in excitatory signalling in adult ferret primary visual cortex, kainate receptors in layers V and VI and N-methyl- d-aspartate receptors in layers I and IV may have particularly important roles in mediating synaptic transmission.

Development of orientation selectivity in ferret visual cortex and effects of deprivation

The orientation selectivity of cells in ferret primary visual cortex was studied during normal development and in animals deprived of vision or of visual cortical activity. In normal animals from the age when visual responses were first recorded (postnatal day 23) through postnatal week 5, only about 25% of cells showed orientation-selective responses. By postnatal week 7, cortical responses had matured to an adult-like state, with approximately 75% of cells clearly selective for orientation. This development of orientation selectivity was not merely a reflection of the development of cortical cell responsiveness: at all ages studied, there was no correlation between responsiveness and selectivity. Infusion of TTX into visual cortex to silence neuronal activity completely blocked the maturation of orientation selectivity. Visual deprivation by bilateral lid suture impaired but did not completely block the normal development of orientation selectivity. We conclude that the maturation of orientation-selective responses in ferret primary visual cortex requires cortical neuronal activity, and that normal development requires visually driven activity.

Genesis of GABA-immunoreactive neurons in the ferret visual cortex

The pattern of neurogenesis of GABA-immunoreactive neurons in the ferret primary visual cortex was determined using immunohistochemical and 3H-thymidine autoradiographic techniques. Neurons in the visual cortex of the ferret undergo their final cell division during a period extending from embryonic day 20 (E20) to postnatal day 14 (P14) and follow an inside-out pattern of neuronal production (Jackson et al., 1984) similar to that observed in other mammals. Earlier-generated neurons are found at deeper cortical positions in the adult than are those generated later. Layer I is an exception to this rule, since neurons destined for this layer are produced at both the beginning and end of neurogenesis. In this study, the pattern of neurogenesis of GABA-immunoreactive neurons is compared to the pattern observed for nonimmunoreactive neurons. The overall pattern of cortical neurogenesis (inside-out pattern) is similar for GABA-immunoreactive neurons and neurons that are not GABA-immunoreactive. However, the GABA-immunoreactive neurons born on a given day of development are more broadly distributed across the radial axis of the adult cortex than are nonimmunoreactive neurons generated on the same day. GABA-immunoreactive neurons generated later in neurogenesis are, on average, slightly smaller than those generated early. If GABA-immunoreactive neurons in the visual cortex are interneurons, then these findings suggest that interneurons follow the same pattern of neurogenesis as do projecting neurons in the visual cortex.