Increased serotonin in the developing superior colliculus affects receptive-field size of retinotectal afferents but not that of postsynaptic neurons

Serotonergic Modulation of Sensory and Multisensory Processing in Superior Colliculus

The ability to integrate information across the senses is vital for coherent perception of and interaction with the world. While much is known regarding the organization and function of multisensory neurons within the mammalian superior colliculus (SC), very little is understood at a mechanistic level. One open question in this regard is the role of neuromodulatory networks in shaping multisensory responses. While the SC receives substantial serotonergic projections from the raphe nuclei, and serotonergic receptors are distributed throughout the SC, the potential role of serotonin (5-HT) signaling in multisensory function is poorly understood. To begin to fill this knowledge void, the current study provides physiological evidence for the influences of 5-HT signaling on auditory, visual and audiovisual responses of individual neurons in the intermediate and deep layers of the SC, with a focus on the 5HT2a receptor. Using single-unit extracellular recordings in combination with pharmacological methods, we demonstrate that alterations in 5HT2a receptor signaling change receptive field (RF) architecture as well as responsivity and integrative abilities of SC neurons when assessed at the level of the single neuron. In contrast, little changes were seen in the local field potential (LFP). These results are the first to implicate the serotonergic system in multisensory processing, and are an important step to understanding how modulatory networks mediate multisensory integration in the SC.

Activity dependent mechanisms of visual map formation--from retinal waves to molecular regulators

The refinement of neural connections requires activity-dependent mechanisms in addition to the genetic program initially establishing wiring diagrams. The well-understood organization of the visual system makes it an accessible model for analyzing the contribution of activity in the formation of connectivity. Prior to visual experience, patterned spontaneous activity in the form of retinal waves has an important role for the establishment of eye-specific and retinotopic maps by acting on the refinement of axon arborization. In the present review, which focuses on experimental data obtained in mice and ferrets, we highlight the features of retinal activity that are important for visual map formation and question whether synaptic release and Hebbian based competition rules apply to this system. Recent evidence using genetic tools that allowed the manipulation of different features of neural activity have clarified the controversy on whether activity is instructive or permissive for visual map formation. Furthermore, current evidence strongly suggests that different mechanisms are at play for different types of axons (ipsilateral vs. contralateral), maps (eye-specific vs. retinotopic) or targets. Many molecules that either modulate activity or are modulated by activity are important in the formation of the visual map, such as adenylate cyclase 1, serotonin, or molecules from the immune system. Finally, new players in the game include retrograde messengers signaling from the target cell to the retinal axons as well as microglia that could help to eliminate inappropriate synapses.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

Neonatally elevated serotonin levels alter terminal arbors of individual retinal ganglion cells in superior colliculus of hamsters

Previous studies from this laboratory showed that sprouting of serotoninergic (5-HT) axons in the hamster's superior colliculus (SC), induced by a single subcutaneous injection of 5,7-dihydroxytryptamine (5,7-DHT) at birth (postnatal day 0 [P-0]), resulted in an increased terminal distribution of the uncrossed retinocollicular projection that was not associated with any changes in the number or distribution of ipsilaterally projecting retinal ganglion cells. The present study was undertaken to determine what effect this manipulation had on the terminal arbors of such axons. Retinocollicular axons of normal and 5,7-DHT-treated animals were anterogradely labeled with small intraretinal injections of the lipophilic dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) on P-16. After tissue processing on P-19, single retinocollicular axon arbors were reconstructed by using confocal microscopy. Quantitative analysis indicated that arbors from 5,7-DHT-treated hamsters had significantly greater total fiber lengths, areas, and volumes than those from normal animals. There were no differences between axons from the two groups in number of branch points, distribution of relative branch lengths, and numbers of bouton-like swellings. These results support the hypothesis that increased SC concentrations of 5-HT alter development of the uncrossed retinocollicular pathway such that a greater territory is covered by individual terminal arbors but that the number of synaptic contacts per arbor remains constant. This may explain, at least in part, the abnormally widespread distribution of the aggregate ipsilateral projection.

Differences in somatosensory processing in S1 barrel cortex between normal and monoamine oxidase A knockout (Tg8) adult mice

Spatio-temporal processing of whisker information was analysed in vivo for single neurons in D2 barrel columns of S1 cortex in Tg8 mutant mice, which lack barrels. Findings were compared with normal C3H mice of the same genetic background. The topographical organization of functional columns was similar in Tg8 and normal mice. Response magnitudes (RMs) to D2 principal whisker deflections in D2 columns for Tg8 were similar to normals for layers I-III and layer IV cells but short latency responses (>10 ms post-stimulus) were twice the magnitude of normal mice. The surrounding whiskers D1 and D3 yielded smaller RMs in layer IV of mutants than normal mice whereas RMs in layers I-III were equipotent (P>0.5). Modal latencies were shorter in Tg8 mice in all layers. Latency distributions for whisker D2 responses in both laminae were bimodal in normal mice, peaking at 6-8 and 12 ms post-stimulus, but unimodal in Tg8 mice in both laminae, peaking at 6-8 ms. Hence, despite an absence of barrels, segregation of columns is enhanced in layer IV and sensory processing is faster in layers I-IV compared with normal mice. This contrasts with adenylyl cyclase knockout mice where both an absence of barrels and enhanced surrounding whisker responses have been observed. These findings suggest that factors other than barrels and clustering of thalamo-cortical terminals define receptive field geometry.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.