The cortical vibrissae representation is normal in transgenic mice lacking the 5-HT(1B) receptor

Maturation of thalamocortical synapses in the somatosensory cortex depends on neocortical AKAP5 expression

Sensory cortex topographic maps consist of organized arrays of thalamocortical afferents (TCAs) that project into distinct areas of the cortex. Formation of topographic maps in sensory cortices is a prerequisite for functional maturation of the neocortex. Studies have shown that the formation of topographic maps and the maturation of thalamocortical synapses in the somatosensory cortex depend on the cyclic adenosine 5'-monophosphate-(cAMP)-protein kinase A (PKA) signaling pathway. AKAP5 is a scaffold protein (also called AKAP79 in humans or AKAP150 in rodents; AKAP79/150) that serves as a signaling hub that links cAMP and PKA signaling. Whether AKAP5 plays a role in topographic map formation and the maturation of thalamocortical synapses during development of the somatosensory cortex is still unknown. Here, we generated cortex-specific AKAP5-knockout mice (CxAKAP5KO) to examine its roles in somatosensory cortex development. We found that CxAKAP5KO mice displayed impaired cortical barrel maps. Electrophysiological recordings showed that the AMPA/NMDA ratio was reduced, and silent synapses were increased in thalamocortical synapses of CxAKAP5KO mice during postnatal development. Morphological analysis of layer IV cortical neurons demonstrated that dendritic refinement of these neurons was abnormal. These results indicate that AKAP5 is necessary for both topographic map formation and maturation of thalamocortical synapses as well as morphological development of cortical neurons in the somatosensory cortex.

Multiple receptors mediate the trophic effects of serotonin on ventroposterior thalamic neurons in vitro

Serotonin (5-HT) exerts prominent morphogenetic roles during development. For example, somatosensory cortical barrel formation is altered in mouse models characterized by excessive extracellular 5-HT, suggesting that 5-HT affects development of thalamic afferents and/or neocortical target regions. The present study assessed 5-HT effects in primary cultures of fetal ventroposterior thalamic (VPT) neurons. 5-HT produces concentration-dependent trophic effects, with impressive 59% and 106% peak increases in total neurite length and number of branching points, respectively, at a dose of 30 microM 5-HT. The exposure of VPT neurons to specific 5-HT receptor agonists 8-OH-DPAT (5-HT(1A)), CGS-12066A (5-HT(1B)), DOI (5-HT(2A/2C)), and m-CPBG (5-HT(3)), enhances primary neurite length and number of branching points with rank-order potency 5-HT(1B) > 5-HT(2A/2C) = 5-HT(3) > 5-HT(1A) = vehicle. Trophic 5-HT effects on embryonic VPT neurons are thus much more prominent than previously reported, and can be mediated by multiple 5-HT receptor subtypes.

Lack of 5-HT(1B) receptor and of serotonin transporter have different effects on the segregation of retinal axons in the lateral geniculate nucleus compared to the superior colliculus

We have shown previously that raised levels of serotonin (5-hydroxytryptamine or 5-HT) during development prevent retinal ganglion cell axons from segregating into eye-specific regions in their principal targets: the superior colliculus and the dorsal lateral geniculate nucleus. Possible mediators of 5-HT in this system include its plasma membrane transporter, which is transiently expressed by a sub-population of retinal ganglion cells, and the presynaptic 5-HT(1B) receptor carried on retinal ganglion cell axons. We analysed the retinal projections of 5-HT(1B) knockout (n=15), serotonin transporter knockout (n=14), serotonin transporter/5-HT(1B) double knockout (n=4) and monoamine oxidase A/5-HT(1B) double knockout (n=3) mice. In all four different knockout mice, the ipsilateral retinal projection to the superior colliculus was more diffuse and lost its characteristic patchy distribution. The alterations were most severe in the serotonin transporter knockout mice, where the ipsilateral retinal fibres covered the entire rostrocaudal and mediolateral extent of the superior colliculus, whereas in the 5-HT(1B) and double knockout mice, fibres retracted from the caudal and lateral superior colliculus. Abnormalities in the 5-HT(1B) knockout mice appeared only after postnatal day (P) 4. Treatment with parachlorophenylalanine (at P1-P12) to decrease serotonin levels caused an exuberance of the ipsilateral retinal fibres throughout the superior colliculus (n=9). In the dorsal lateral geniculate nucleus in contrast, the distribution and size of the ipsilateral retinal projection was normal in all four knockout mice. In the serotonin transporter knockout mice however, the contralateral retinal fibres failed to retract from the mediodorsal dorsal lateral geniculate nucleus, an abnormality that was reversed by early treatment with parachlorophenylalanine and in the serotonin transporter/5-HT(1B) double knockout. OUR OBSERVATIONS INDICATE: (1) that the lack of 5-HT transporter and the associated changes in 5-HT levels impair the segregation of retinal axons in both the superior colliculus and the dorsal lateral geniculate nucleus; (2) that 5-HT and 5-HT(1B) receptors are necessary for the normal refinement of the ipsilateral retinal fibres in the superior colliculus, but are not essential for the establishment of eye-specific segregation in the thalamus. Thus, both an excess and a lack of 5-HT affect the refinement of the superior colliculus retinal projection, while the establishment of eye-specific patterns in the dorsal lateral geniculate nucleus appears not to be sensitive to the lack of 5-HT or 5-HT(1B) receptors.

Clorgyline treatment elevates cortical serotonin and temporarily disrupts the vibrissae-related pattern in rat somatosensory cortex

Manipulation of cortical serotonin (5-HT) levels in perinatal rodents produces significant alterations in the development of the layer IV cortical representation of the mystacial vibrissae. Monoamine oxidase A (MAO(A)) knockout mice have highly elevated cortical 5-HT and completely lack barrels in somatosensory cortex (S-I). The present study was undertaken to determine whether the effects on thalamocortical development seen in MAO(A) knockout mice can be replicated in perinatal rats treated with an MAO(A) inhibitor and, second, to determine whether these effects persist with continued treatment or after discontinuation of the drug. Littermates were injected with either clorgyline (5 mg/kg) or sterile saline five times daily. Clorgyline administration from birth to postnatal day (P) 6, 8, or 10 produced increases of 1,589.4 +/- 53.3%, 1660.2 +/- 43.1% and 1,700.5 +/- 84.5 %, respectively, in cortical 5-HT as compared with controls. Serotonin immunocytochemistry, 1,1;-dioctadecyl-3,3,3", 3;-tetramethylindocarbocyanine perchlorate (DiI) labeling of thalamocortical afferents and Nissl and cytochrome oxidase staining of layer IV cellular aggregates demonstrated that clorgyline treatment from P0 to P6 produced a complete absence of any segmentation of vibrissae-related patches in S-I. However, continued treatment until P8 or P10 did not prevent the appearance of these patches. Animals treated with clorgyline from birth to P6 and killed on P8 or P10 had increases of 546.8 +/- 33.2% and 268.8 +/- 6.3% in cortical 5-HT and they had qualitatively normal vibrissae-related patterns in S-I. These results indicate that clorgyline treatment produces a transient disruption of vibrissae-related patterns, despite the continued presence of elevated cortical 5-HT.

Selective facilitation of the serotonin(1B) receptor causes disorganization of thalamic afferents and barrels in somatosensory cortex of rat

Alteration of serotonin (5-HT) levels influences developing thalamocortical afferents (TCAs) in primary somatosensory cortex (SI) of rats and mice. The 5-HT(1B) receptor, present on TCAs during the first postnatal week, may be involved in these effects. The present study asked whether administration of 5-nonyloxytriptamine (NNT), a selective 5-HT(1B) receptor agonist, affects TCA organization in rat SI. Littermates were injected five times daily (5x/day), with either 0.1 mg/kg NNT or vehicle from birth to postnatal day 6 (P-6). Animals were killed on P-6, and their brains were processed for high-performance liquid chromatography (HPLC), cytochrome oxidase (CO) histochemistry, cresyl violet, or demonstration of TCAs by placement of 1,1'-dioctadecyl-3,3,3'' 3'-tetra-methylindocarbocyanine perchlorate (Di-I) on thalamocortical radiations. At P-6, NNT treatment decreased 5-HT levels slightly compared with controls, although this difference was not statistically significant. In NNT-treated rats, the Di-I-labeled vibrissae-related pattern showed a range of effects, from fusion of patches related to mystacial vibrissae in treated animals to a less distinct vibrissae-related pattern in SI barrelfield compared with controls. Staining for CO and Nissl stain in layer IV of SI showed a similar range of abnormalities. These results indicate that the agonist action of NNT at the 5-HT(1B) receptor causes TCA disorganization in rat barrel field cortex in the absence of elevated 5-HT.