Vasoactive intestinal polypeptide (VIP) containing cells in the developing rat occipital hemisphere

Development of NADPH-diaphorase activity in the rat neocortex

The activity of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), an enzyme related to the synthesis of nitric oxide (NO), was studied histochemically in rat neocortex from the day of birth (P0) to young adulthood. At birth, NADPH-d containing neurons were already identifiable, sparsely distributed in the deep half of the pallium of the cerebrum. In addition, weakly stained bands of NADPH-d positive neuropil were detectable in layers VI and deep V and the cortical plate (CP). During the first postnatal week, NADPH-d positive neurons increased markedly, especially in CP and the superficial layers. By P7 the cells were mainly in layers VIb and II/III. Differential NADPH-d activity in the neuropil during this period appeared as higher activity gradually moving upwards until the highest intensity localised in layers II and upper III. In the caudal part of the cortex, the higher activity covered the whole of the supragranular layers. By the end of the second postnatal week, both the number and laminar distribution of NADPH-d neurons were adult-like, still mainly in layers VIb and II/III. The staining intensity in the neuropil was generally reduced but the banding pattern seen at P7 was still present. No detectable changes in the patterns of NADPH-d positive cells and neuropil in the neocortex occurred after the second postnatal week. The rostral part of the cortex matured slightly earlier than the caudal part. These results show that the development of NADPH-d activity correlates with the laminar differentiation and suggest that NADPH-d or NO may play an important role in the maturation of cortical neurons including the establishment of functional connections.

Differentiation of transmitter phenotypes in rat cerebral cortex

Cortical neurons differ in their neurochemical properties. Projection neurons use excitatory amino acids as transmitters, most local interneurons contain the inhibitory transmitter GABA, and specific subtypes of local circuit neurons express distinct neuropeptides. How this cellular diversity is generated during development is not known. We have been studying the transmitter differentiation of cortical neurons in different in vitro systems using immunohistochemical techniques. Transmitter phenotypes of cortical neurons were examined in slice cultures, i.e. in the absence of extrinsic cortical connections, and in dissociated cortical cell cultures, i.e. in the absence of extrinsic and intrinsic cortical connections. The expression of vasoactive intestinal polypeptide in cortical interneurons occurred normally in slice cultures prepared from neonatal rats between birth and 2 days of age, but was strongly impaired in dissociated cell cultures prepared at the same time. These results suggest that the intact cortical environment present in the slice cultures exerts crucial influences for neuropeptide differentiation. In contrast, the transmitters glutamate and GABA were expressed normally in the appropriate cell types and similar in proportions in dissociated cell cultures prepared from cortices at embryonic day 19. Only cells dissociated during S-phase failed to express glutamate and GABA in vitro. When cells were kept for 24 h after mitosis in a cortical slice preparation in vitro, however, they later expressed their appropriate transmitter phenotypes. Thus, signals from the local cortical environment that act early in the cell cycle are required for the specification of transmitter phenotypes of cortical neurons.

Postnatal development of vasoactive intestinal polypeptide-containing neurons in the visual cortex of normal and dark-reared rats

The effects of dark rearing on the distribution and density of vasoactive intestinal polypeptide (VIP)-containing neurons in the visual cortical areas (17, 18 and 18a) of rats during postnatal development were examined immunohistochemically. Two groups of Wistar rats, one reared under normal lighting conditions and the other in complete darkness from birth, were used. VIP neurons showed a fairly similar distribution in the three visual areas, being predominantly present in layers II and III. Their pattern of development was found to be similar in the normal and dark-reared animals and was characterized by a marked increase from postnatal day (P) 7 to P21, followed by a gradual diminution to 24-31% of peak densities. Counts of labeled neurons at all ages examined showed that their density was similar in both groups at P7 and P14, but progressively greater in dark-reared animals from P21 and thereafter, so that they only fell to 38-43% of peak densities. Thus, by 60 days of age densities of VIP-labeled neurons in areas 17, 18 and 18a in dark-reared rats were 57%, 49% and 51% higher than in the corresponding areas of the age-matched control rats. These results indicate that the normal decline in the numbers of VIP neurons is not so marked under the conditions of dark rearing.

Developmental gradients of vasoactive intestinal polypeptide (VIP)-containing neurons in the rat visual cortex detected by image analysis

The postnatal development of vasoactive intestinal polypeptide-immunoreactive (VIP-IR) neurons was followed by computer-assisted image analysis in the rat visual cortex. The laminar distribution of all VIP-IR structures was measured. These structures were subdivided into cell somata, dendritic profiles and axonal boutons and measured separately. VIP-IR neurons were first seen on postnatal day 1, mostly in the upper half of the presumptive visual cortex. A localization of cell bodies similar to that in the adult is reached between days 12 and 16. VIP-IR dendrites have a protracted growth period as compared to perikarya, involving a developmental gradient from an even distribution to a concentration in the upper cortical layers. This is due to the formation of dendritic terminal arbors after the second postnatal week. Scattered VIP-IT axonal boutons appear on day 3 in the midportion of the presumptive visual cortex. Their typical laminar distribution in layers II, IV and lower VI was observed after day 12. Our results suggest that the biochemically detected sharp increase in VIP levels after the second postnatal week is due to the maturation of cell processes as a morphological basis of neuronal connectivity.