Effects of retinal deafferentation on serotonin receptor types in the superficial grey layer of the superior colliculus of the rat

Changes in the modulation of retinocollicular transmission through group III mGluRs long after an increase in intraocular pressure in a rat model of glaucoma

Metabotropic glutamate receptors (mGluRs) have been shown to be involved in the modulation of retinocollicular neurotransmission. In glaucoma, retinal ganglion cells (RGCs) degenerate, which may have an implication on this transmission as the superior colliculus is their major central target in the much-used rodent models of the disease. We have investigated this using an in vitro slice preparation of the superior colliculus by eliciting field excitatory postsynaptic potentials (fEPSPs) through optic tract stimulation in a rat ocular hypertension model of glaucoma. Application of the group III mGluR agonist L-AP4 reduced the peak amplitude of the fEPSP in superior colliculus slices through presynaptic mechanisms as previously shown in our lab. At 3 and 16 weeks after surgery, there were no significant differences in the effect of L-AP4 on fEPSP peak amplitude in the superior colliculus slices receiving input from the glaucomatous eyes [elevated intraocular pressure (IOP)] compared to those with input from the unoperated eyes (normal IOP). However, at 32 weeks, the fEPSP peak amplitude was reduced to a significantly greater degree during L-AP4 application in the elevated IOP slices compared to normal IOP slices. At all time points, there were no significant changes in the baseline amplitudes of fEPSPs or the stimulus intensities required to evoke fEPSPs. These results suggest that the modulation of synaptic transmission through group III mGluRs on RGC terminals to the superior colliculus is changed at later stages due to RGC degeneration through IOP elevation. These changes may be compensatory changes possibly through plasticity in the RGC terminals of surviving cells, which may be due to increases in the numbers of group III mGluRs. This result may have implications on further treatment studies carried out using these models of glaucoma as changes in the central visual system may need to be considered along with the retinal changes that occur.

Eph/ephrin expression in the adult rat visual system following localized retinal lesions: localized and transneuronal up-regulation in the retina and superior colliculus

Following unilateral optic nerve section in adult PVG hooded rat, the axon guidance cue ephrin-A2 is up-regulated in caudal but not rostral superior colliculus (SC) and the EphA5 receptor is down-regulated in axotomised retinal ganglion cells (RGCs). Changes occur bilaterally despite the retino-collicular projection being mostly crossed. Here we investigate the dynamics of Eph/ephrin expression using in situ hybridization and semi-quantitative immunohistochemistry after localized retinal lesions. Unilateral krypton laser lesions to dorso-nasal retina ablated contralaterally projecting RGCs (DN group); ventro-temporal lesions ablated contralaterally and ipsilaterally projecting RGCs (VT group). Lesions of the entire retina served as controls (Total group). Results are compared to normal animals in which tectal ephrin-A2 and retinal EphA5 are expressed, respectively, as shallow ascending rostro-caudal and naso-temporal gradients. In both SCs of DN and Total groups, tectal ephrin-A2 was up-regulated caudally; in the VT group, expression remained normal bilaterally. Unilateral collicular ablation indicated that bilateral changes in ephrin-A2 expression are mediated via intercollicular pathways. EphA5 expression in the VT group was elevated in the intact nasal region of experimental retinae. For each experimental group, EphA5 expression was also elevated in nasal retina of the opposite eye, resulting in uniform expression across the naso-temporal axis. Up-regulation of ephrin-A2 in caudal, but not rostral, SC suggests the enhancement of developmental positional information as a result of injury. Bilateral increases in retinal EphA5 expression demonstrate that signals for up-regulation operate interocularly. The study demonstrates that signals regulating guidance cue expression are both localized and relayed transneuronally.

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.

Sculpting the visual map: the distribution and function of serotonin-1A and serotonin-1B receptors in the optic tectum of the frog

Agonists of serotonin (5-HT)-1 receptors modulate the synaptic strength of the connection between retinal ganglion cells and neurons of the frog optic tectum in brain slices (Brain Res. 1998;781:167-181). We have now used autoradiographic receptor binding techniques to determine the location of 5-HT1A and 5-HT1B binding sites in the laminated optic tectum. 5-HT1A binding sites, as labeled with [3H]8-hydroxy-dipropylaminotetralin (8-OH-DPAT), were highest in the superficial, retinorecipient layers of the tectum, intermediate in layers 6 and 7 and low in the remaining layers. Binding densities in all of these layers were unaffected by optic nerve lesion. 5-HT1B binding sites were visualized using [125I]iodocyanopindolol (ICYP). Binding densities were highest in the plexiform layers 5 and 7 and intermediate in layers 6 and 8. Binding sites were present at low levels in layer 9; however, optic nerve lesion resulted in a strong upregulation of these sites in this layer. Pharmacological manipulation of receptor activation resulted in changes in the activity-dependent visual map that is created at the tectum by retinal ganglion cell terminals. Chronic treatment of the tectum with SB-224289, a selective antagonist of 5-HT1B receptors, disrupted the topographic map. In contrast, exposure to WAY-100635, a selective antagonist of 5-HT1A receptors, refined it. We conclude that both 5-HT1A and 5-HT1B receptors are present in the adult frog tectum and that changes in their activation levels can produce changes in retinotectal transmission levels that drive visual plasticity in opposite directions.

Organization of the corticotectal projection in hamsters with neonatally elevated levels of serotonin in the superior colliculus

A previous study from this laboratory showed that elevated serotonin (5-HT) levels in the hamsters superior colliculus (SC), induced by a single subcutaneous injection of 5,7-dihydroxytryptamine (5,7-DHT) at birth, resulted in an abnormally widespread distribution of the uncrossed retinotectal projection. The present study investigated whether the corticotectal projection in such animals was altered. Adult normal and 5,7-DHT-treated hamsters were injected with horseradish peroxidase (HRP) into occipital cortex and processed for anterograde tracing of corticotectal terminals in the SC. Quantitative analysis showed that normal and 5,7-DHT-treated hamsters were not significantly different in total labeling or in the gradient of labeling density within the SC. These data indicate that corticotectal axons achieve normal terminal fields after neonatal elevation of 5-HT in the SC.

Indirect evidence for an association of 5-HT(1B) binding sites with retinal and geniculate axon terminals in the rat suprachiasmatic nucleus

The purpose of the present study was to investigate the possible cellular location of 5-HT(1B) receptors on retinal and geniculate afferents in the rat suprachiasmatic nucleus (SCN). Biocular enucleation significantly decreased 5-HT(1B) binding site labeling (35%), specifically in the ventral part of the SCN, while monocular enucleation produced a decrease of smaller magnitude (12%), limited to the ventral part of the contralateral SCN, these results being consistent with the known distribution of retinal afferents in the nucleus. By contrast, bilateral geniculate lesion did not induce any significant variation of 5-HT(1B) binding site labeling in the SCN. Previously, we reported that serotonin (5-HT) synthesis inhibition by parachlorophenylalanine increases 5-HT(1B) binding site labeling in the SCN. Using saturation studies, we have now demonstrated that this upregulation reflected an increase in the total number of 5-HT(1B) binding sites (+41% in the dorsal and +67% in the ventral part of the SCN). Furthermore, we evaluated the effects of bilateral geniculate lesion after 5-HT stores depletion in order to overcome problems of technical resolution limits. The magnitude of upregulation was significantly decreased (27%) after bilateral geniculate lesion, suggesting that part of the 5-HT(1B) receptor population was located on geniculate axon terminals within the SCN. The possible involvement of 5-HT(1B) receptors, according to their cellular locations evidenced in the present study, in photic and nonphotic entrainment of the circadian clock is discussed.

5-HT1B receptor knock-out mice exhibit increased exploratory activity and enhanced spatial memory performance in the Morris water maze

In an attempt to characterize the contribution of the 5-HT1B receptor to behavior, 5-HT1B knock-out (KO) mice were subjected to a battery of behavioral paradigms aimed at differentiating various components of cognitive and emotional behaviors. In an object exploration task, wild-type (WT) and 5-HT1B KO mice did not differ in locomotor activity. 5-HT1B KO mice, however, displayed lower thigmotaxis (an index of anxiety) associated with a higher level of object exploratory activity, but no genotype differences were observed in the elevated plus maze. 5-HT1B KO mice also displayed a lack of exploratory habituation. In the spatial version of the Morris water maze, 5-HT1B KO mice showed higher performances in acquisition and transfer test, which was not observed in the visual version of the task. No genotype differences were found in contextual fear conditioning, because both WT and 5-HT1B KO mice were able to remember the context where they had received the aversive stimulus. The deletion of the 5-HT1B receptor, associated with appropriate behavioral paradigms, thus allowed us to dissociate anxiety from response to novelty, and perseverative behavior (lack of habituation) from adaptive behavioral inhibition underlying cognitive flexibility (transfer stage in the water maze). The deletion of the 5-HT1B receptor did not result in significant developmental plasticities for other major 5-HT receptor types but may have influenced other neurotransmission systems. The 5-HT1B receptor may be a key target for serotonin in the modulation of cognitive behavior, particularly in situations involving a high cognitive demand.

Cellular and subcellular localization of 5-hydroxytryptamine1B receptors in the rat central nervous system: immunocytochemical, autoradiographic and lesion studies

The localization of 5-hydroxytryptamine1B receptors in the rat central nervous system was investigated using anti-peptide antibodies that recognize a selective portion of the third intracytoplasmic loop of the receptor protein. At the light microscope level the densest 5-hydroxytryptamine1B receptor-like immunoreactivity was observed in ventral pallidum, globus pallidus, substantia nigra and dorsal subiculum. In addition, moderate immunoreactivity was found in the entopeduncular nucleus, the superficial gray layer of the superior colliculus, the caudate-putamen and the deep nuclei of the cerebellum. This distribution matched perfectly that previously described from radioligand binding studies. At the ultrastructural level, 5-hydroxytryptamine1B receptor-like immunoreactivity was associated with axons and axon terminals in the three areas examined: substantia nigra, globus pallidus and superficial gray layer of the superior colliculus. In all cases, immunostaining was located on the plasma membrane of unmyelinated axon terminals and in the cytoplasm close to the plasmalemma. Synaptic differentiations were never labelled but, in some cases, 5-hydroxytryptamine1B receptor-like immunoreactivity was found in their close vicinity. Injection of kainic acid into the neostriatum resulted in a marked decrease in receptor-like immunoreactivity in the globus pallidus and the substantia nigra, consistent with the location of 5-hydroxytryptamine1B receptors on terminals of striatopallidal and striatonigral fibres, respectively. A reduction in 5-hydroxytryptamine1B receptor-like immunoreactivity was also noted in the superficial gray layer of the superior colliculus after contralateral enucleation, as expected of the location of 5-hydroxytryptamine1B receptors on the terminals of retinocollicular fibres. In both lesion experiments, immunolabelled degenerating terminals were observed in the projection areas. Anterograde labelling experiments coupled with immunocytochemical detection further showed that 5-hydroxytryptamine1B receptors in the substantia nigra are located on axons of striatal neurons. These data provide anatomical support for the idea that 5-hydroxytryptamine1B receptors act as terminal receptors involved in presynaptic regulation of the release of various neurotransmitters, including 5-hydroxytryptamine itself.

Differential effects of serotonin (5-HT) lesions and synthesis blockade on neuropeptide-Y immunoreactivity and 5-HT1A, 5-HT1B/1D and 5-HT2A/2C receptor binding sites in the rat cerebral cortex

The present study was aimed at comparing the effects of serotonin (5-HT) synthesis blockade using chronic administration of p-chlorophenylalanine (PCPA) and 5,7-dihydroxytryptamine injections of variable volume (3 vs. 6 microl) on the density of NPY immunoreactive (Ir) neurons and binding of [3H]8-OH-DPAT, S-CM-G[125I]TNH2 and [125I]DOI to 5-HT1A, 5-HT1B/1D, and 5-HT2A/2C receptors in rat cortical regions. Three weeks after large but partial (89% depletion in 5-HT tissue concentration) lesions of 5-HT neurons no changes in neither NPY immunoreactivity nor 5-HT receptor binding were detected. The complete 5,7-DHT lesions produced increases in the number of NPY-Ir neurons in the upper regions of the cingular (134%), frontal (140%) and parietal cortex (48%) and corresponding decreases in 5-HT2A/2C binding (16-26%). No changes in 5-HT1A and 5-HT1B/1D binding were observed after lesions of this kind. After PCPA treatment, decreases in NPY-Ir neurons density (22-40%) and increases in 5-HT1A and 5-HT1B/1D receptor binding sites (20-50%) were distributed in both upper and deeper cortical regions. The lack of effect of the partial lesion suggests that spared 5-HT neurons may exert compensatory mechanisms up to a large extent. The changes in NPY immunoreactivity and 5-HT2A/2C binding detected in the upper regions of the cortex after complete 5-HT lesions probably result from local cellular rearrangements, whereas blocking 5-HT synthesis has more widespread influence on NPY neurons and on 5-HT1A and 5-HT1B/1D receptor subtypes. Moreover, decreases in DOPAC concentrations detected only after complete lesions suggest that the involvement of catecholaminergic transmission may also differentiate 5,7-DHT and PCPA treatments. Altogether, these data suggest that different receptor subtypes might be involved in 5-HT-NPY relationships.

Local administration of serotonin agonists blocks light-induced phase advances of the circadian activity rhythm in the hamster

Circadian rhythms in mammals are synchronized to environmental light-dark cycles through a direct retinal projection to the suprachiasmatic nucleus (SCN), a circadian clock. This process is thought to be modulated by other afferents to the SCN, including a dense serotonergic projection from the midbrain raphe. Previous work from this laboratory demonstrated that a systemically administered 5-hydroxytryptamine1A/7 (5-HT1A/7) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) dose dependently attenuates light-induced phase shifts of the circadian activity rhythm of the Syrian hamster. In this study, we demonstrate that local injections (1-100 microM) of the 5-HT1A/7 agonists 8-OH-DPAT or 5-carboxamidotryptamine into the region of the SCN inhibit light-induced phase advances of the circadian wheel-running rhythm. In addition, the inhibitory effects of systemically administered 8-OH-DPAT were unaffected by either radiofrequency-induced lesions of the intergeniculate leaflet or 5,7-dihydroxytryptamine-induced lesions of serotonergic projections to the SCN. These findings support a modulatory role of serotonin in photic regulation of circadian phase through an action at the level of the SCN.

Selective increases in serotonin 5-HT1B/1D and 5-HT2A/2C binding sites in adult rat basal ganglia following lesions of serotonergic neurons

Quantitative autoradiography was used to examine possible adaptive changes in serotonin 5-HT1B/1D and 5-HT2A/2C receptor binding sites in adult rat basal ganglia, after partial or severe lesions of serotonergic neurons produced by intraraphe injections of variable amounts of 5,7-dihydroxytryptamine. In controls, the 5-HT1B/1D sites labeled with S-CM-G[125I]TNH2 were evenly distributed in the core and the shell of the nucleus accumbens. The density of 5-HT1B/1D sites was higher in the ventral than dorsal part of the striatum and no regional differences were detected along the rostrocaudal axis of the structure. The 5-HT2A/2C sites labeled with [125I]DOI were preferentially distributed in the mediodorsal striatum and higher densities were detected in the shell than core of the nucleus accumbens. Following 5,7-dihydroxytryptamine injections, there were no changes in binding of either receptor subtype after partial lesions entailing 80-90% 5-HT depletions. After severe 5-HT depletions (over 95%), large increases in 5-HT1B/1D binding were observed in the substantia nigra (78%), but no changes took place in the globus pallidus. Increases in 5-HT1B/1D binding were also detected in the shell of the nucleus accumbens (27%). Similar sized increases in 5-HT2A/2C binding (22%) were restricted to the medial striatum. The present results suggest a preferential association between 5-HT1B/1D receptors and the striatonigral neurons containing substance P, as indicated by the striatal distribution of these receptors and their selective increases in the substantia nigra after severe 5-HT deprivation. We recently proposed a similar relationship between the 5-HT4 receptors and the striatopallidal neurons containing met-enkephalin. Moreover, the increases in 5-HT1B/1D binding in the substantia nigra and in the shell of the nucleus accumbens reinforce the view of an implication of this receptor subtype in motor functions. In contrast, the prominent increases in 5-HT2A/2C binding after severe 5-HT deprivation as restricted to the medial region of the striatum and suggest up-regulation of most probably 5-HT2C receptors in a region implicated in cognitive functions.

5-Hydroxytryptamine1B receptors modulate the effect of cocaine on c-fos expression: converging evidence using 5-hydroxytryptamine1B knockout mice and the 5-hydroxytryptamine1B/1D antagonist GR127935

Serotonergic transmission has been suggested to modulate the effects of cocaine. However, the specific receptors and brain structures underlying this phenomenon have not been identified. To test the possible contribution of the 5-hydroxytryptamine1B (5-HT1B) receptor, we studied the induction of the immediate-early gene c-fos elicited by cocaine in knockout mice lacking this receptor. 5-HT1B knockout mice display a markedly reduced effect of cocaine on c-fos induction in different brain structures, most notably in the striatum. In addition, the administration to wild-type mice of the 5-HT1B receptor agonist RU24969 results in a striatal induction of c-fos expression very similar to that induced by cocaine in its time course, cellular and anatomical distribution, and pharmacology. Here, we also report the ability of a 5-HT1D receptor antagonist, GR127935, to antagonize 5-HT1B receptors in vivo. Finally, when administered to wild-type mice, GR127935 reduces the increase in striatal c-fos expression elicited by cocaine. These converging lines of evidence obtained with the knockout mice and 5-HT(1B/1D) antagonist indicate that cocaine acts as an indirect agonist of 5-HT1B receptors in vivo and demonstrate that activation of 5-HT1B receptors contributes to the cellular responses elicited by cocaine.

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.

Neonatal administration of the neurotoxin 5,7-dihydroxytryptamine results in synaptic reorganization in the superficial gray layer of the hamster's superior colliculus

Neonatal subcutaneous administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to hamsters results in a marked depletion of serotonin (5-HT) in cortex and an increase in the concentration of this amine in the superior colliculus (SC). To determine whether this increase was associated with an alteration in the synaptic organization of 5-HT-containing axons in the superficial gray layer of the SC, immunocytochemistry was combined with electron microscopy. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles make synaptic contacts in the superficial gray layer of the hamster's SC. In 5,7-DHT-treated animals, examination of 400 individual profiles indicated that 25.5% of 5-HT-positive profiles made synaptic contacts (P < 0.05). Given the recently demonstrated effect of 5-HT on retinotectal transmission in this species, the present results suggest that the functional organization of the SC may also be markedly altered in animals that sustain neonatal 5,7-DHT administration.

Distractibility and locomotor activity in rat following intra-collicular injection of a serotonin 1B-1D agonist

The superior colliculus (SC) is thought to be the decision center for reactions to novel and/or moving stimuli in the peripheral visual field. Serotonin 1B (5-HT1B) receptors were previously demonstrated to be located on collicular axon terminals of retinal ganglion cells and their activation might depress afferent inputs from the retina. The effects of intra-collicular injections of 5-HT1 drugs on distractibility were studied in hooded rats trained to run toward illuminated targets for a food reward in a 2-choice runway. 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT), a 5-HT1A receptor agonist, RU 24969, a mixed 5-HT1A and 5-HT1B agonist, serotonin-O-carboxymethylglycyltyrosinamide (S-CM-GTNH2), a mixed 5-HT1B and 5-HT1D receptor agonist and saline (control) were alternately injected. Following the S-CM-GTNH2 treatment alone, animals exhibited an erratic running style, involving side-to-side movements of the head, without change in the overall accuracy of their locomotor trajectories, but with substantial decrease in their running speed. When distracting peripheral lights were introduced at the mid-points of the animals' run, in the weaker distracting condition (unilateral distractor) only, distraction indexes were found lower following the S-CM-GTNH2 treatment than following the other drug or saline treatments. It is concluded that serotonin, via 5-HT1B-1D receptors, may induce an elevation of the visual distractibility threshold by modulating directly the transmission of the primary visual signal.

Serotonin 1B receptor regulation after dorsal subiculum deafferentation

The subiculum may be the key structure in the transfer of relevant processed information from the hippocampal formation to cortical areas. We investigated the location of the serotonin 1B receptor (5-HT1B) in the hippocampus with the specific ligand serotonin-O-carboxymethyl-glycyl[125I]tyrosinamide in rat brain sections using in vitro autoradiography. A high density of 5-HT1B binding sites was found in the dorsal subiculum (DS), in the lacunosum moleculare, and in the most dorsal layer of the stratum oriens of the CA1 field. CA1 pyramidal neurons that contain 5-HT1B mRNA project primarily to the DS. We interrupted the pyramidal CA1 axons unilaterally by a stereotaxic knife cut. Histological analysis showed that the lesion was restricted to a trial of cells lost between CA1 and DS. Specific 5-HT1B binding site density was decreased in the DS on the ipsilateral side of the lesion compared to the contralateral side. We conclude that 5-HT1B receptors are located on CA1 pyramidal axon terminals in the DS. Serotonin, acting on these receptors, should inhibit CA1 neurotransmitter release and, in this way, modulate subicular functions.

Unilateral optic nerve transection decreases 2-[125I]-iodomelatonin binding in retinorecipient areas and visual pathways of chick brain

In chick brain, specific 2-[125I]-iodomelatonin-binding was localized primarily in the visual system, i.e., retinorecipient and relay nuclei and fiber tracts of the tectofugal, thalamofugal, circadian and accessory visual pathways. Unilateral transection of the optic nerve (ONX) significantly reduced the binding of 2-[125I]-iodomelatonin (75 pM) in many, but not all, primary retinal targets and visual pathways at 7 and 14 days, but not 1 day, postlesion. As measured using quantitative autoradiography, 2-[125I]-iodomelatonin binding was decreased by 90% in both the central portion of the lesioned optic tract and one of its targets, the nucleus of the basal optic root (nBOR). Other retinorecipient areas exhibiting substantial decreases (60%) in 2-[125I]-iodomelatonin-binding included the optic tectum, lateroventral and dorsolateral geniculate nuclei and tectal gray area contralateral to the lesion. These findings are consistent with the hypothesis that melatonin receptors are located presynaptically on incoming optic nerve terminals in many retinorecipient areas. This localization may account for most of the binding sites in nBOR. In other primary visual areas, however, melatonin receptors also appear to be located on postsynaptic cells and/or non-retinal afferents. ONX had no significant effect on 2-[125I] -iodomelatonin binding in two retinorecipient areas, the visual suprachiasmatic nucleus and the dorsolateral anterior thalamus, which are part of the circadian/oculomotor and thalamofugal pathways, respectively. An unexpected consequence of ONX was that 2-[125I]- iodomelatonin binding was decreased in certain secondary (nucleus rotundus, isthmi nuclei) and tertiary level (ectostriatum) nuclei along the prominent tectofugal visual pathway. Binding in the tectorecipient nucleus triangularis was not significantly altered, however. Analysis of secondary level relay nuclei in the oculomotor pathway revealed that binding after ONX was decreased in the ipsilateral Edinger-Westphal nucleus but not in the oculomotor nuclei. Selective transsynaptic changes in 2-[125I]-iodomelatonin binding after lesion of the visual input most likely reflect activity-dependent regulation and functional plasticity of central melatonin receptors.