Richard D. Mooney

Chapter 4 Serotonin modulates retinotectal and corticotectal convergence in the superior colliculus

A dense serotonin (5-HT)-containing projection to the superficial layers of the superior colliculus (SC) has been demonstrated in diverse mammalian species, but how 5-HT may affect visual signals within these laminae is largely unknown. This study undertook to investigate the distribution of 2 types of 5-HT receptors in the SC and to ascertain their physiological effects on transmission of visual signals to the SC from the retinotectal and corticotectual pathways. Autoradiography of tissue sections exposed to [3H]-8-OH-DPAT (8-hydroxy-dipropylaminotetraline) or to [125I]cyanopindolol plus isoproterenol showed that 5-HT1A and 5-HT1B receptors, respectively, were present in the superficial SC layers. In unilaterally enucleated animals, binding of ligand to 5-HT1B receptors was greatly reduced on the deafferented (contralateral) side, which is consistent with the possibility that these receptors are located on preterminal axons. Binding to 5-HT1A receptors was unaltered by enucleation. In recordings of superficial layer neurons from SC slices, application of 5-HT during blockade of 5-HT1A receptors with spiperone reduced the amplitude of EPSPs evoked by stimulation of the optic tract. The 5-HT concentration for a 50% reduction in EPSP amplitude was 6 microM. Under these conditions, there were no significant alterations in either membrane potential or input resistance concurrent with 5-HT mediated reduction in EPSPs. During extracellular in vivo recordings, 5-HT, applied by iontophoresis or micropressure or by endogenous release produced by electrical stimulation of the dorsal raphé nucleus, strongly suppressed visual activity in SC neurons. The effectiveness of 5-HT application was significantly stronger on responses evoked by electrical stimulation of the optic chiasm (an average response decrement of 92.2%) than on these evoked in the same neurons by stimulation of visual cortex (an average response reduction of 32.3%). These results support the following conclusions. The 5-HT1B receptors are located preferentially on optic axon terminals and exert presynaptic inhibition of retinotectal inputs. Secondly, 5-HT1A receptors probably have a postsynaptic localization and may affect activity of SC neurons irrespective of the source of input. The combined effect of 5-HT at both subtypes would bias SC visual activity toward information received from the corticotectal pathway.

Organization, Development and Enucleation‐induced Alterations in the Visual Callosal Projection of the Hamster: Single Axon Tracing with Phaseolus vulgaris leucoagglutinin and Di‐l

The distribution of callosal axons interconnecting lateral area 17 and medial area 18 of the rodents occipital cortex is dramatically altered by neonatal enucleation, but it is not known how this manipulation affects the morphology of individual callosal axons or whether the enucleation‐induced changes in this pathway reflect maintenance of a transient developmental state by these fibres. In the present study, these questions were addressed by tracing the individual callosal axons in normal adult and neonatally enucleated adult hamsters with Phaseolus vulgaris leucoagglutinin (PHAL) and by anterograde labelling of developing callosal axons with the carbocyanine dye, Di‐l. In normal adults, injections of PHAL into the region of the 17–18a border produced dense labelling in all layers in the region of the contralateral 17–18a border. Larger injections resulted in callosal labelling that extended across the lateral one‐half of area 17, primarily in layers l and V. Thirty‐four callosal axons from normal adult hamsters were reconstructed through all the cortical laminae. Most of these had very simple terminal arbors. They gave off short collaterals in the infragranular layers and branched more extensively in the uppermost part of layer II‐III and in lamina l. Small injections of PHAL into the occipital cortex of neonatally enucleated adult hamsters resulted in labelled axons throughout most of areas 17 and 18a in the contralateral hemisphere. The terminal arbors of most individual callosal axons in eyeless hamsters were not appreciably different from those in sighted animals. However, 26.8% of 28 fibres reconstructed through all cortical laminae in the neonatally enucleated hamsters had much more widespread branches than any of the axons recovered from normal hamsters. As a result, the average total length of the callosal axons from the blinded hamsters was significantly greater than that for such fibres from the sighted animals. Anterograde labelling with Di‐l demonstrated axons in the anterior commissure and anterior part of the corpus callosum on P‐O. Labelled fibres extended into the white matter underlying the occipital cortex on P‐1 and entered the cortical plate on P‐2. Some of these axons reached into the marginal layer. Many developing callosal axons had short branches in the white matter, but generally extended only a single collateral into the cortical grey matter. Callosal axons in perinatal animals branched very little within the cortex and, in this respect, resembled fibres labelled with PHAL in adult hamsters. These results support the conclusion that the expanded tangential distribution of the occipital callosal projection in neonatally enucleated adult hamsters results, at least in part, from individual axons with abnormally widespread terminal arbors which are not present in large numbers at any time during normal development.

A substance P projection from the superior colliculus to the parabigeminal nucleus in the rat and hamster

Immunocytochemical staining with antisera directed against substance P (SP) demonstrated the existence of numerous immunoreactive neurons throughout the mediolateral and rostrocaudal extents of the stratum griseum superficiale (SGS) of the superior colliculus (SC) of both rat and hamster. In both of these species, very dense SP-like immunoreactivity (SPLI) was also visible in the parabigeminal nucleus. Combination of retrograde tracing with True blue or Fluorogold and immunocytochemistry demonstrated that SP-positive SC neurons projected to the parabigeminal nucleus in both hamster and rat. Retrogradely labelled and double-labelled cells were most numerous in the rostromedial portion of the SC and rare in the caudal portion of the colliculus. Destruction of the superficial layers of the SC resulted in a virtually complete loss of SPLI in the ipsilateral parabigeminal nucleus in both species. SPLI was also visible in two other targets of the superficial SC laminae: the intergeniculate leaflet and the ventral lateral geniculate nucleus. Ablation of the dorsal SC laminae did not reduce SPLI in either of those nuclei. Our results thus indicate that at least some tectoparabigeminal neurons in hamster and rat contain SPLI and further that the SC appears to be the sole source of SP-positive input to this nucleus.

Augmentation of serotonin in the developing superior colliculus alters the normal development of the uncrossed retinotectal projection

A previous study from this laboratory showed that sprouting of serotoninergic axons in the hamsters 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 abnormal terminal distribution of the uncrossed retinotectal projection. The present study provided further evidence to support the role of increased 5‐HT levels within the SC in this phenomenon. Slow‐release polymer (ELVAX) chips impregnated with serotonin (5‐HT) were placed over the SC on either P‐1 or P‐3, and retinotectal projections were assessed via anterograde transport of horseradish peroxidase when animals reached P > 18. Analysis of ELVAX chips indicated that they released 5‐HT in amounts of ≥1 pmole/hour for at least 12 days. Assessment of the SC of treated hamsters indicated significantly elevated 5‐HT concentrations as late as P‐12, but not on P‐16. Implantation of 5‐HT chips, but not control chips, resulted in abnormalities in the uncrossed retinotectal projection similar to those observed in the 5,7‐DHT‐treated animals. The patches that normally develop in the rostral part of the stratum opticum were not present, and uncrossed axons were distributed densely in this layer and in the lower portion of the stratum griseum superficiale throughout the rostrocaudal and mediolateral extents of the SC. Quantitative analysis of these changes indicated significant differences between the organization of the uncrossed retinotectal projections of 5‐HT‐treated animals vs. either blank‐implant treated or completely untreated animals but not between 5‐HT‐treated hamsters and animals that received neonatal 5,7‐DHT injections. All of these results support the conclusion that increased SC concentrations of 5‐HT altered retinotectal development. J. Comp. Neurol. 393:84–92, 1998.

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 (MAOA) 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 MAOA knockout mice can be replicated in perinatal rats treated with an MAOA 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. J. Comp. Neurol. 427:139–149, 2000.

Development of the occipital corticotectal projection in the hamster

SummaryAnterograde and retrograde labelling with the carbocyanine dye, Di-I, was used to assess the development of the visual cortical projection to the superior colliculus (SC) in pre- and postnatal hamsters. Posterior cortical axons arrive in the SC on postnatal (P-) day one (the first 24 hours after birth = P-0) and begin to arborize in the superficial laminae (the stratum griseum superficiale [SGS] and stratum opticum [SO]) within one day after they enter the tectum. Over succeeding days, the density of the projection increases and numerous labelled fibers are visible throughout the depth of the SGS and SO. Beginning on P-6, there is a decrease in the density of labelled fibers in the upper SGS and by P-10, the laminal distribution of the occipital corticotectal pathway appears adult-like. Anterograde tracing with Di-I also revealed the presence of a few corticotectal fibers that crossed the midline in both the SC and posterior commissures to terminate mainly in the superficial tectal laminae contralateral to the injection site. Crossed corticotectal fibers were visible in hamsters aged between P-3 and P-12. Retrograde tracing with Di-I in hamsters killed between P-3 and P-12 demonstrated that both the ipsilateral and crossed corticotectal projections arose exclusively from pyramidal cells in developing lamina V.

Effects of norepinephrine upon superficial layer neurons in the superior colliculus of the hamster: in vitro studies.

Intracellular recording techniques were used to evaluate the effects of norepinephrine (NE) on the membrane properties of superficial layer (stratum griseum superficiale and stratum opticum) superior colliculus (SC) cells. Of the 207 cells tested, 44.4% (N = 92) were hyperpolarized by > or = 3 mV and 8.7% (N = 18) were depolarized by > or = 3 mV by application of NE. Hyperpolarization induced by NE was dose dependent (EC50 = 8.1 microM) and was associated with decreased input resistance and outward current which had a reversal potential of -94.0 mV. Depolarization was associated with a very slight rise in input resistance and had a reversal potential of -93.1 mV for the single cell tested. Pharmacologic experiments demonstrated that isoproterenol, dobutamine, and p-aminoclonidine all hyperpolarized SC cells. These results are consistent with the conclusion that NE-induced hyperpolarization of SC cells is mediated by both alpha2 and beta1 adrenoceptors. The alpha1 adrenoceptor agonists, methoxamine and phenylephrine, depolarized 35% (6 of 17) of the SC cells tested by > or = 3 mV. Most of the SC cells tested exhibited responses indicative of expression of more than one adrenoceptor. Application of p-aminoclonidine or dobutamine inhibited transsynaptic responses in SC cells evoked by electrical stimulation of optic tract axons. Inhibition of evoked responses by these agents was usually, but not invariably, associated with a hyperpolarization of the cell membrane and a reduction in depolarizing potentials evoked by application of glutamate. The present in vitro results are consistent with those of the companion in vivo study which suggested that NE-induced response suppression in superficial layer SC neurons was primarily postsynaptic and chiefly mediated by both beta2 and beta1 adrenoceptors.

Longitudinal MRI evaluations of human global cortical thickness over minutes to weeks

Magnetic resonance imaging (MRI) was used to evaluate within-subject variability in global mean cortical thickness over test-retest intervals of minutes-weeks in five healthy adults. Within-subject measures of global mean thickness were consistent over these intervals. Test-retest assessments of absolute thickness differences and percent thickness differences indicated variations of, respectively, < or =0.05-0.06 mm and < or =+/-1.9-2.3%. There have been few evaluations of normal within-subject variations in cortical thickness. The present results suggest that within-subject variability in global mean cortical thickness can be low over test-retest intervals of minutes-weeks, and that longitudinal scans can establish useful baseline estimates of variability from which to assess changes due to injury, disease, or other experiences.

Tectospinal neurons in hamster contain glutamate-like immunoreactivity

Immunocytochemistry with a monoclonal antibody directed against glutamate revealed numerous immunoreactive cells in the hamsters superior colliculus (SC). A large number of these neurons were located in the deep layers and many were in the stratum album intermedium (SAI). These neurons appeared similar to the large multipolar cells that have been shown to project to the cervical spinal cord in this species. The combination of retrograde tracing using either Fluoro-Gold- or FITC-labelled latex beads with immunocytochemistry for glutamate revealed that many of the immunoreactive cells did, in fact, project into the predorsal bundle and extend axons as far as the cervical spinal cord.

Serotonin immunoreactive neurons are present in the superficial layers of the hamster's, but not the rat's, superior colliculus.

SummaryImmunocytochemistry for serotonin (5-HT) was carried out in both hamsters and rats in order to determine whether or not 5-HT-positive cells existed in the superior colliculus (SC) of either species. In both hamster and rat, the superficial and deep SC laminae contained dense networks of 5-HT-positive fibers. The rats SC contained no 5-HT-positive neurons. In hamster, numerous 5-HT-immunoreactive cells were visible throughout the depth of the stratum griseum superficiale (SGS). These neurons had a variety of morphological characteristics and included marginal cells, horizontal cells, and neurons with vertically oriented dendritic trees. No 5-HT-positive neurons were found in any other portion of the hamsters SC. 5-HT-positive SC cells were observed with antisera from two different sources and they were not seen in animals that were pretreated with reserpine. Pretreatment with fluoxetine (an inhibitor of 5-HT uptake) also resulted in a disappearance of 5-HT-positive neurons in the hamsters SC. This result indicated that “serotonergic” cells in the colliculus of this species are capable of taking up, but probably not synthesizing, this indoleamine. The dorsal and ventral lateral geniculate nuclei (LGNd and LGNv, respectively) both contain numerous 5-HT-positive fibers and both of these structures receive input from the SGS. Combination of retrograde tracing with fluorogold and immunocytochemistry indicated that 5-HT-accumulating SC neurons were not the source of these fibers. Unilateral ablation of the superficial SC laminae also failed to reduce 5-HT immunoreactivity in either the LGNd or LGNv. These results are consistent with the possibility that 5-HT-accumulating cells in the hamsters SC may be interneurons that take up this transmitter after it is released by afferents to this nucleus.