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.

Excessive activation of serotonin (5-HT) 1B receptors disrupts the formation of sensory maps in monoamine oxidase a and 5-ht transporter knock-out mice.

Deficiency in the monoamine degradation enzyme monoamine oxidase A (MAOA) or prenatal exposure to the monoamine uptake inhibitor cocaine alters behavior in humans and rodents, but the mechanisms are unclear. In MAOA knock-out mice, inhibiting serotonin synthesis during development can prevent abnormal segregation of axons in the retinogeniculate and somatosensory thalamocortical systems. To investigate this effect, we crossed MAOA knock-outs with mice lacking the serotonin transporter 5-HTT or the 5-HT1B receptor, two molecules present in developing sensory projections. Segregation was abnormal in 5-HTT knock-outs and MAOA/5-HTT double knock-outs but was normalized in MAOA/5-HT1B double knock-outs and MAOA/5-HTT/5-HT1B triple knock-outs. This demonstrates that the 5-HT1B receptor is a key factor in abnormal segregation of sensory projections and suggests that serotonergic drugs represent a risk for the development of these projections. We also found that the 5-HT1B receptor has an adverse developmental impact on beam-walking behavior in MAOA knock-outs. Finally, because the 5-HT1B receptor inhibits glutamate release, our results suggest that visual and somatosensory projections must release glutamate for proper segregation.

Expression of laminin alpha2 chain during normal and pathological growth of myocardium in rat and human.

OBJECTIVES: Fibrosis is a classical feature of cardiac hypertrophy. To date changes within the basal lamina during normal and pathological cardiac growth have been poorly investigated. The goal of the present study was to determine if the expression of the muscle specific subunit of merosin (laminin alpha2 chain) together with that of fibronectin (FN) is modified in the diseased human heart. Laminin alpha2 chain expression was also investigated during physiological and pathological cardiac growth in the rat. METHODS: In ten normal human hearts and ten hearts with idiopathic dilated cardiomyopathy (IDCM), the laminin-alpha2 and FN mRNA levels were quantified by slot-blot using total RNA and the protein distribution was analysed using an immunofluorescence approach. In Wistar rats, laminin alpha2 and FN mRNA expression was analyzed using RNase protection assay (RPA) and slot-blot assays. RESULTS: The amount of laminin alpha2 mRNA did not vary in normal and pathological human hearts whereas it was significantly decreased in renovascular hypertensive rats (-20%) P<0.05 versus normal tissue). The amount of fibronectin mRNA increased in IDMC patients (x2, P<0.05 versus normal tissue), but was unchanged in hypertensive rats. A negative correlation was found between the cardiac laminin-alpha2 level and the age of the patients whatever the cardiac status. During postnatal development in the rat, a similar decrease in cardiac laminin-alpha2 level was observed between 3 and 30 weeks of age. Finally, the immunofluorescent approach failed to detect any alteration in laminin alpha2 distribution within the human myocardium. CONCLUSION: These data indicate that an imbalance between myocyte hypertrophy and the level of laminin-alpha2 might contribute to alterations in sarcolemmal properties, which occur during the development of cardiac hypertrophy and its transition to cardiac failure.

Excess of serotonin (5-HT) alters the segregation of ispilateral and contralateral retinal projections in monoamine oxidase A knock-out mice: possible role of 5-HT uptake in retinal ganglion cells during development.

Retinal ganglion cell (RGCs) project to the ipsilateral and contralateral sides of the brain in the dorsal lateral geniculate nucleus (dLGN) and the superior colliculus (SC). Projections from both eyes are initially intermingled until postnatal day 3 (P3) but segregate into eye-specific layers by P8. We report that this segregation does not occur in monoamine oxidase A knock-out mice (MAOA-KO) that have elevated brain levels of serotonin (5-HT) and noradrenaline. The abnormal development of retinal projections can be reversed by inhibiting 5-HT synthesis from P0 to P15. We found that in MAOA-KO mice, 5-HT accumulates in a subpopulation of RGCs and axons during embryonic and early postnatal development. The RGCs do not synthesize 5-HT but reuptake the amine from the extracellular space. In both MAOA-KO and normal mice, high-affinity uptake of 5-HT and serotonin transporter (SERT) immunoreactivity are observed in retinal axons from the optic cup to retinal terminal fields in the SC and dLGN. In the dLGN, transient SERT labeling corresponds predominantly to the ipsilateral retinal projection fields. We show that, in addition to SERT, developing RGCs also transiently express the vesicular monoamine transporter gene VMAT2: thus, retinal axons could store 5-HT in synaptic vesicles and possibly use it as a borrowed neurotransmitter. Finally we show that the 5-HT-1B receptor gene is expressed by RGCs throughout the retina from E15 until adult life. Activation of this receptor is known, from previous studies, to reduce retinotectal activity; thus 5-HT in excess could inhibit activity-dependent segregation mechanisms. A hypothesis is proposed whereby, during normal development, localized SERT expression could confer specific neurotransmission properties on a subset of RGCs and could be important in the fine-tuning of retinal projections.