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, 7 (6), e00713
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Substance P NK1 Receptor in the Rat Corpus Callosum During Postnatal Development

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Substance P NK1 Receptor in the Rat Corpus Callosum During Postnatal Development

Paolo Barbaresi et al. Brain Behav.

Abstract

Introduction: The expression of substance P (SP) receptor (neurokinin 1, NK1) was studied in the rat corpus callosum (cc) from postnatal day 0 (the first 24 hr from birth, P0) to P30.

Methods: We used immunocytochemistry to study the presence of intracallosal NK1-immunopositive neurons (NK1IP-n) during cc development.

Results: NK1IP-n first appeared on P5. Their number increased significantly between P5 and P10, it remained almost constant between P10 and P15, then declined slightly until P30. The size of intracallosal NK1IP-n increased constantly from P5 (102.3 μm2) to P30 (262.07 μm2). From P5 onward, their distribution pattern was adult-like, that is, they were more numerous in the lateral and intermediate parts of the cc, and declined to few or none approaching the midline. At P5, intracallosal NK1IP-n had a predominantly round cell bodies with primary dendrites of different thickness from which originated thinner secondary branches. Between P10 and P15, dendrites were longer and more thickly branched, and displayed several varicosities as well as short, thin appendages. Between P20 and P30, NK1IP-n were qualitatively indistinguishable from those of adult animals and could be classified as bipolar (fusiform and rectangular), round-polygonal, and pyramidal (triangular-pyriform).

Conclusions: Number of NK1IP-n increase between P5 and P10, then declines, but unlike other intracallosal neurons, NK1IP-n make up a significant population in the adult cc. These findings suggest that NK1IP-n may be involved in the myelination of callosal axons, could play an important role in their pathfinding. Since they are also found in adult rat cc, it is likely that their role changes during lifetime.

Keywords: corpus callosum; immunocytochemistry; intracallosal neurons; neurokinin; tachykinin.

Figures

Figure 1
Figure 1
Photomicrographs showing NK1 immunoreactivity in different regions of the rat CNS at P0. (a) A dense plexus of NK1IP fibers in the medulla oblongata. The framed region is enlarged in b. (b) Labeled fibers, probably immature glia processes. (c) Numerous NK1IP ‐n in the CPu and cerebral cortex. The framed region, enlarged in d, shows NK1IP ‐n in the subcortical plate of the cerebral cortex. (d) Several NK1IP ‐n with different morphologies in the cerebral cortex. (e) NK1 labeling in the CPu and GP. The framed region, enlarged in f, shows numerous NK1IP processes and neurons. (g) Immunoreactivity in the rat hippocampus. Calibration bars: a, 500 μm; b, c, and g, 100 μm; d and f, 50 μm; e, 250 μm
Figure 2
Figure 2
Distribution of NK1IP ‐n present in the rat cc at different ages. a: P5; b: P10; c: P15; d: P30. Three different lateromedial levels/time point are shown. Numbers in the bottom left in D indicate lateromedial levels according to the atlas of Paxinos and Watson (1982). Numbers in parentheses in the bottom left corner in a, b, c indicate a lateromedial level comparable to that of the adult. Calibration bar: 1 mm
Figure 3
Figure 3
Age‐related change in the number of intracallosal NKIP ‐n. Counts were performed on pooled data from three rats per time point (45 sections/time point, 270 sections overall). Data were subjected to Student's t test. The difference between P5 and P10 was significant (**p ≤ .01)
Figure 4
Figure 4
Histograms showing the size increase of intracallosal NK1IP ‐n at different postnatal ages (a: P5; b: P7: c: P10; d: P15; e: P20; f: P30). Each histogram shows pooled data from three rats
Figure 5
Figure 5
Camera lucida reconstruction of intracallosal NK1IP ‐n at three different postnatal ages (P5‐P7‐P10). P5‐A: a morphologically immature NK1IP ‐n; P5‐B‐E: round neurons; P7‐A, ‐D: round neurons; P7‐B: bipolar neuron; P7‐C, P10‐A: triangular neurons; P10‐B: bipolar neuron P10‐C: round (polygonal) neuron. Arrowheads in P5‐A and P5‐B indicate preterminal growth buds at branching points. Calibration bar: 100 μm. Dors, dorsal; Ant, anterior; Ax, axon
Figure 6
Figure 6
Photomicrographs of intracallosal NK1IP ‐n at three postnatal ages (P5‐P7‐P10). a–e: P5; f: P7; g–j: P10. a–d: Four intracallosal NK1IP ‐n showing different morphologies. (a) An ovoid NK1IP ‐n with a thick principal dendrite directed toward the ependymal cc region. (b and d) Two round NK1IP ‐n close to the ependymal cc region, whose dendrites are directed toward the dorsal, posterior and anterior cc regions. (c) Two adjacent NK1IP ‐n in the middle of the cc. (e) Medial cc region, probably between lateral at 0.9 and 0.4 mm (comparable with those of the adult): no neurons are found at this cc level. Several NK1IP ‐n are visible over the cc, in the IG. (f) An ovoid NK1IP ‐n with dendrites directed in all directions, including the ependymal cc region. Arrowhead: a growth bud. (g) Polygonal NK1IP ‐n. (i) Several intracallosal NK1IP ‐n. A neuron (asterisk) sends its dendrites into the CPu. (j) A bipolar NK1IP ‐n close to the ependymal region of the cc. Arrowheads in d and f indicate growth buds at branching points. Calibration bars: 25 μm in a–d, f, g, i, j; 100 μm in e; 10 μm in h
Figure 7
Figure 7
Number of NK1IP ‐n detected on different stereotaxic planes from lateral (lat 3.9) to medial (lat 0.4). Five different lateromedial levels/time point are shown. Numbers in parentheses in a, b, c, d indicate lateromedial levels comparable with those of the adult. Stereotaxic levels in d (P30) indicate lateromedial levels according to the atlas of Paxinos and Watson (1982). Three rats per age (CC‐Nk1/1; CCNK1/2; CCNK1/3) are shown. Data for each rat come from pooling three adjacent sections
Figure 8
Figure 8
Camera lucida reconstruction of intracallosal NK1IP ‐n at three different postnatal ages (P15‐P20‐P30). P15‐A, ‐B, P20‐B: round neurons; P20‐A, P30‐B: bipolar neurons; P30‐A: a polygonal neuron. Calibration bar: 100 μm. Dors, dorsal; Ant, anterior; Ax, axon; wm, white matter; VI, sixth layer of the cerebral cortex
Figure 9
Figure 9
Photomicrographs of intracallosal NK1IP ‐n at different postnatal ages (P15, P20, P30). (a) A round intracallosal NK1IP ‐n whose dendrites are oriented in all directions (P15). (b) A triangular neuron; the apical dendrite crosses the white matter and reaches the cerebral cortex (P15). The framed area, enlarged in c, shows both dendritic appendages and dendritic spines (arrowheads). (d) A bipolar neuron bearing several dendritic appendages (arrowheads) and dendritic spines (P15). Coronal section. (e) Cluster of NK1IP ‐n in the splenium (P20). (f) Two NK1IP ‐n in the medial cc (P20). Some NK1IP ‐n are in the IG, over the cc. (g) A bipolar NK1IP ‐n close to the ependymal region of the cc (P30). (h) Several NK1IP ‐n in the middle cc region (P30). (i) A probable bipolar NK1IP ‐n in the splenium (P30). Calibration bars: 500 μm in a, b, and h; 250 μm in e, g, and i; 100 μm in f; 10 μm in c and d

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