Effects of 5-hydroxytryptamine, LSD and related compounds on electrical activities evoked in vitro in thin sections from the superior colliculus

Effect of methylphenidate on visual responses in the superior colliculus in the anaesthetised rat: Role of cortical activation

The mechanism of action of psychostimulant drugs in the treatment of Attention Deficit Hyperactivity Disorder is still largely unknown, although recent evidence suggests one possibility is that the drugs affect the superior colliculus (SC). We have previously demonstrated that systemically administered d-amphetamine attenuates/abolishes visual responses to wholefield light flashes in the superficial layers of the SC in anaesthetised rats, and the present study sought to extend this work to methylphenidate (MPH). Anaesthetised rats were administered MPH at a range of doses (or saline) and subjected to monocular wholefield light flashes at two intensities, juxta-threshold and super-threshold. In contrast to d-amphetamine, systemic MPH produced an enhancement of visual activity at both intensities. Methylphenidate was also found to produce activation of the cortical EEG in anaesthetised rats. Furthermore, cortical activation induced by electrical stimulation of the pons was found to enhance visual responses in superficial layers of the SC, and when MPH was paired with pontine-induced cortical activation, the response-enhancing effects of MPH were substantially attenuated. Taken together, the results suggest that the enhancement of visual responses in the superficial layers of the SC by MPH in the anaesthetised rat is an artefact of the drug's interaction with cortical arousal.

Enhanced visual responses in the superior colliculus in an animal model of attention-deficit hyperactivity disorder and their suppression by D-amphetamine

Attention-deficit hyperactivity disorder (ADHD) is a prevalent neurodevelopmental disorder characterized by overactivity, impulsiveness and attentional problems, including an increase in distractibility. A structure that is intimately linked with distractibility is the superior colliculus (SC), a midbrain sensory structure which plays a particular role in the production of eye and head movements. Although others have proposed the involvement of such diverse elements as the frontal cortex and forebrain noradrenaline in ADHD, given the role of the colliculus in distractibility and the increased distractibility in ADHD, we have proposed that distractibility in ADHD arises due to collicular sensory hyper-responsiveness. To further investigate this possibility, we recorded the extracellular activity (multi-unit (MUA) and local field potential (LFP)) in the superficial visual layers of the SC in an animal model of ADHD, the New Zealand genetically hypertensive (GH) rat, in response to wholefield light flashes. The MUA and LFP peak amplitude and summed activity within a one-second time window post-stimulus were both significantly greater in GH rats than in Wistar controls, across the full range of stimulus intensities. Given that baseline firing rate did not differ between the strains, this suggests that the signal-to-noise ratio is elevated in GH animals. D-Amphetamine reduced the peak amplitude and summed activity of the multi-unit response in Wistar animals. It also reduced the peak amplitude and summed activity of the multi-unit response in GH animals, at higher doses bringing it down to levels that were equivalent to those of Wistar animals at baseline. The present results provide convergent evidence that a collicular dysfunction (sensory hyper-responsiveness) is present in ADHD, and that it may underlie the enhanced distractibility. In addition, D-amphetamine - a widely used treatment in ADHD - may have one of its loci of therapeutic action at the level of the colliculus.

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.

The development of the synaptic organization of the serotonergic system differs in brain areas with different functions

The serotonergic innervation of the developing superior colliculus and ventrolateral nucleus of the thalamus of the rat were studied with light and electron microscope immunocytochemistry. We compared the pattern of innervation and synaptic organization of the serotonin (5-HT) system in the superficial and deep layers of the superior colliculus. We also compared the developmental pattern of synaptic incidence of 5-HT varicosities in the superior colliculus with that in the ventrolateral nucleus. Serotonin fibers were present in the superior colliculus at birth, concentrated mainly in the deep layers, whereas the superficial layers were only sparsely innervated. By the end of the first postnatal week the overall density of 5-HT fibers increased, but was still higher in the deep than in the superficial layers. The distribution pattern, density, and morphology of serotonergic axons acquired mature features by the end of the third postnatal week. In the adult, these axons were thin, varicose, forming a complex network which was denser in the lower part of the superficial layers and the upper part of the deep layers. Electron microscopical analysis revealed that the vast majority of 5-HT varicosities established symmetrical synapses with dendritic shafts in all layers of the superior colliculus throughout development. In the superficial layers, known to be involved in visual functions, the proportion of varicosities forming synapses increased gradually from birth to reach a peak at the end of the first postnatal week, then declined markedly in the subsequent 2 weeks before rising again at later stages. In contrast, in the deep layers and in the ventrolateral nucleus of the thalamus, areas involved in motor functions, the proportion of 5-HT varicosities engaged in synaptic contacts showed a continuous increase from birth until adulthood. Considering these results together with data from our previous studies, we speculate that the regional heterogeneity in the synaptic organization of the serotonergic system may reflect a differential role of 5-HT in the development of brain areas with different functions.

Serotonin modulates induced synaptic activity in the optic tectum of the frog

The extent to which retinal signals are modulated at central sites is unknown. We sought to determine the effects of serotonin, a neurotransmitter present in the retinorecipient layers of the frog tectum, on retinotectal transmission. Acute electrical stimulation delivered to the retinorecipient layer of optic tectum brain slices was used to model the activation of tectal neurons by visual inputs. This stimulation evoked either a monosynaptic or a polysynaptic current response in patch-clamped tectal neurons. External application of serotonin blocked both of these induced currents as did 5-carbotryptamine (5-CT), a nonselective agonist of 5-HT1 receptors. Alpha-methylserotonin, a nonselective agonist of 5-HT2 receptors, also blocked polysynaptic responses but was less effective than either serotonin or 5-CT in blocking monosynaptic ones. Lateral synaptic interactions between tectal cells, modeled by acute electrical stimulation in the main cellular layer of the tectum, were also blocked by serotonin, 5-CT or alpha-methylserotonin. The presented data suggest that endogenous serotonin may strongly affect visual signal processing by modulating synaptic transmission between both the retina and the tectum as well as between tectal neurons. This modulation is likely to be due, at least in part, to a demonstrated outward current induced by serotonin in a subpopulation of tectal cells.

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.

Increased serotoninergic innervation of the hamster's superior colliculus alters retinotectal projections

Anterograde tracing with horseradish peroxidase (HRP) was used to compare the organization of retinotectal projections in normal adult hamsters and in animals that sustained subcutaneous injections of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) on the day of birth. Neonatal injection of this neurotoxin decreases the density of the serotoninergic (5-HT) innervation of the cerebral and cerebellar cortices, but increases the density of these fibers in the brainstem including the superior colliculus (SC). Analysis of tissue from the retinorecipient laminae of the SC by high-pressure liquid chromatography indicated that these lesions increased the amount of 5-HT in the adult SC by 47%. The increased serotoninergic innervation of SC was associated with a marked change in the distribution of the uncrossed retinotectal projection. In normal adult hamsters, fibers from the ipsilateral eye form dense clusters in the lowermost stratum griseum superficiale (SGS) and stratum opticum (SO). A small number of uncrossed fibers are also visible in the more caudal portions of these layers. In the animals that sustained neonatal 5,7-DHT injections, uncrossed retinotectal fibers formed a nearly continuous band in rostral SO and lower SGS, and numerous labeled fibers were present in the caudal SC, primarily in the SO. Neonatal treatment with 5,7-DHT also produced alterations in the crossed retinotectal pathway and in the crossed and uncrossed retinogeniculate projections. These results indicate that the 5-HT input to the developing brainstem may strongly influence the development of retinofugal projections.

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

The effects of retinal axon terminal degeneration on the serotonin-1A, -1B, -2, nuerokinin-1 and gamma-amionobutyric acid-A high affinity binding sites in the superficial grey layer of the superior colliculus were tested with quantitative autoradiography on rat brain sections. The binding to serotonin-2, neurokinin-1 and gamma-aminobutyric acid-A high affinity receptors was not changed in the deafferented superficial grey layer of the superior colliculus after unilateral enucleation. By contrast, we demonstrate that the previously described 21% decrease in the binding of [3H]serotonin to serotonin-1 receptors observed in the deafferented superficial grey layer of the superior colliculus after enucleation, was not due to a decrease in the affinity of the serotonin-1 receptors for the radioligand, but to a decrease in the number of binding sites. Of the different serotonin-1 receptor subtypes, only the serotonin-1B was lost. This signifies that these receptors are probably located on the optic fibre terminals. Visual cortex lesion caused no apparent regulation of the serotonin-1 binding sites in the superficial grey layer of the superior colliculus. A bilateral enucleation produced a smaller decrease in serotonin-1 receptor density than that observed after unilateral enucleation, suggesting the existence of a compensatory mechanism.

The properties of the long-term potentiation (LTP) in the superior colliculus

(1) Postsynaptic potential (PSP) was recorded in the SGL of guinea pig SC slices after stimulation to the OL. (2) Tetanic stimulation (optimum parameter: 50 Hz in frequency, 20 sec in duration) to the OL induced LTP in the PSP of SGL. (3) NMDA-receptor antagonist MK801 inhibited the LTP occurrence but D-APV and gamma-DGG masked the appearance of LTP, suggesting that the mode of involvement of the NMDA receptor for LTP formation in the SC may be different from that reported in the hippocampus. (4) Protein kinase C inhibitors such as H-7, polymixin B and K-252a inhibited the maintenance of LTP. (5) Application of GABA prevented the occurrence of LTP and bicuculline facilitated the formation of LTP. (6) In in vivo preparations of the rat, the LTP in the SC was only elicited by tetanic stimulation to the optic nerve either when the visual cortical are ipsilateral to the SC tested was ablated or when picrotoxin was administered to the animal before tetanic stimulation.

Synaptic organization of the serotoninergic input to the superficial gray layer of the hamster's superior colliculus

Immunocytochemistry with an antibody directed against a serotonin (5-HT)-bovine serum albumin (BSA) conjugate was combined with electron microscopy and serial section analysis to examine the synaptic organization of the serotoninergic projection to the stratum griseum superficiale (SGS) of the hamster's superior colliculus (SC). Most of the 5-HT-immunoreactive swellings in the hamster's SC did not make conventional synaptic contacts. Examination of 250 such swellings in single thin sections revealed only 11 synapses. Assessment of an additional 50 swellings followed through serial sections showed that only 6% (N = 3) of these varicosities made conventional synaptic contacts. All the synapses made by serotoninergic axons were with the dendrites of SC neurons. These results demonstrate that the inhibitory effects of 5-HT on SC neurons in hamster are mainly mediated by the nonsynaptic release of this indoleamine.

The distribution and function of gamma-aminobutyric acid (GABA) in the superior colliculus

Laminer analysis of the distribution of GABA and GAD in the superior colliculus has shown that the distribution pattern of GABA within the SC is similar in rabbit, cat, and guinea pig. The highest levels of GABA were found in the superficial gray layer (SGL), averaging 37-40 mmol/kg dry weight. The GABA concentrations in the deep layers were each only half that of the levels in the SGL. The concentrations of both GABA and GAD in the upper half of SGL are the same as those in the substantia nigra and medial forebrain bundle which have the highest amounts of GABA in the CNS. Denervation studies of the fibers projecting to SGL suggest that the GABA concentrated in the SGL is intrinsic to the layer. The results obtained from immunohistochemical and electron microscopic studies on the localization of GABA neurons corresponds well with the regional distribution pattern of GABA and GAD reported here. However, pharmacological and electrophysiological studies do not necessarily accord well with the GABA distribution studies because they indicate that there are many GABA sensitive neurons in both the SGL and DGL. To investigate the role of GABA in the SGL, the effect of GABA and its agonists and antagonists on neurotransmission in SGL has been studied in SC slices in a perfusion system. Bath applied GABA (100 microM to 1 mM) enhanced the amplitude of postsynaptic field potentials (PSP) in SGL in a dose-dependent fashion and at concentrations above 1 mM it depressed the PSP in a dose-dependent fashion. A similar response pattern was obtained with muscimol (0.1-10 microM excitation; greater than 10 microM inhibition). However (-)-baclofen only inhibited the PSP. Bicuculline (1 microM) shifted the dose-response inhibitory curve of GABA to the right, while the excitatory effect was enhanced. These results indicate that GABA has an excitatory and inhibitory action on neurotransmission in the SGL. The nigro-tectal GABAergic fibers terminate in the intermediate and deep layers of SC. Inhibition of GABAergic activity in the SC causes irrepressible saccades made toward the center of the movement field while GABA activation delays and slows saccadic eye movements. Thus, GABA in the SC plays an important role in the control of eye movements. The same GABAergic projection is also related to the propagation of generalized seizures. There exist collicular neurons which suppress the propagation of seizures.(ABSTRACT TRUNCATED AT 400 WORDS)

Fine structure of synapses and retinal innervation of substance P and adenosin deaminase containing neurons in the superior colliculus of the rat

The fine structure of substance P (SP) and adenosine deaminase (ADA) immunoreactive structures in synaptic contacts localized to the superficial layers of the superior colliculus of the rat was investigated by means of immunoelectron microscopy. We also examined the possibility of retinal innervation of SP- and ADA- containing neurons by immunohistochemistry after degeneration of retinal terminals caused by enucleation. SP-like immunoreactive presynaptic terminals of the stratum griseum superficiale (SGS) formed both asymmetric and symmetric synaptic contacts. Presynaptic dendritelike structures were also observed. SP immunoreactive postsynaptic elements made contacts with terminals showing diverse features. ADA-like immunoreactive structures were seen only as postsynaptic elements to different kinds of nonimmunoreactive terminals and were mostly localized in the ventral third of the SGS and the dorsalmost stratum opticum (SO). After enucleation, degenerating retinal terminals were found to form synaptic contacts with SP and ADA immunoreactive structures. The highest number of such degenerating terminals on ADA immunoreactive structures was observed 2 days after retinal denervation, very few being seen after 5 days. These degenerating terminals were restricted to the ventral SGS and dorsal SO. SP immunoreactive structures postsynaptic to degenerating retinal terminals were most numerous 5 days after enucleation and mainly localized in the dorsal SGS. Occasionally, SP immunoreactive dendritelike processes forming synapses with degenerating retinal terminals were simultaneously presynaptic to other nonimmunoreactive profiles, defining, therefore, serial synapses. The present results suggest that SP-I and ADA-I collicular neurons may be part of distinct channels carrying visual information to the lateral posterior and lateral geniculate nuclei of the thalamus, respectively.

5-HT1 receptor binding sites in the guinea pig superior colliculus are predominantly of the 5-HT1D class and are presynaptically located on primary retinal afferents

The superficial layers of the guinea pig superior colliculus are characterized by high densities of [3H]5-HT binding sites. We have chosen receptor autoradiography to establish the drug binding profile and the localization of these sites. The binding of [3H]5-HT to guinea pig superior colliculus was nearly completely blocked by drugs such as 5-carboxamido-tryptamine and yohimbine, but only slightly sensitive to the 5-HT1 receptor agonist 8-hydroxy-dipropylamino-tetralin. 5-HT1C antagonists such as mesulergine or the beta-adrenergic receptor blocking compound SDZ 21-009 did not show any effect. The profile of the majority of these sites corresponds to that of 5-HT1D sites. Unilateral enucleation resulted in a nearly complete depletion of [3H]5-HT binding in the contralateral superior colliculus, whereas [125I]Bolton-Hunter-8-methoxy-N-propylaminotetralin binding sites, corresponding to 5-HT1A receptors, were preserved. These results indicate that 5-HT1D (and not 5-HT1A) receptors might be presynaptically localized on non-serotoninergic neuronal pathways. The guinea pig visual system may be a useful model for the study of the properties of these presynaptic 5-HT heteroreceptors.

Formation of long-term potentiation in superior colliculus slices from the guinea pig

Superior colliculus slices of sagittal section were prepared from the guinea pig. Postsynaptic potential (PSP) was evoked in the superficial grey layer (SGL) after the electrical stimulation to optic layer. Tetanic stimulation to the optic layer elicited long-term potentiation (LTP) in the PSP of the SGL. Tetanic stimulation of 20 s in duration and 50 Hz in frequency was most effective for the formation of LTP. The LTP formation was masked during application of 2-amino-5-phosphonovalerate (APV), a specific antagonist for N-methyl-D-aspartate (NMDA) receptor, but LTP was observed when APV was removed from the perfusion medium.

Excitatory and inhibitory action of GABA on synaptic transmission in slices of guinea pig superior colliculus

The laminar distribution of gamma-aminobutyric acid (GABA) in the superior colliculus of the guinea pig was analyzed. The superficial gray layer, especially the upper half, was found to contain the highest amount, 37.4 +/- 1.1 mmol/kg dry. To investigate the role of GABA in the superficial gray layer, the effect of GABA on neurotransmission in the superficial gray layer was studied in superior colliculus slices in a perfusion system. Bath-applied GABA, 100 microM-1 mM, enhanced the amplitude of the postsynaptic field potential (PSP) in the superficial gray layer dose dependently and at concentrations above 1 mM it depressed the PSP dose dependently. A similar response pattern was obtained with muscimol (0.1-10 microM, enhancement; greater than 10 microM, depression). However, (-)-baclofen only inhibited the PSP dose dependently (0.1-1 microM). The excitatory effect of GABA was enhanced at concentrations of nipecotic acid less than 0.5 mM. Bicuculline 1 microM shifted the dose-response curve of GABA to the right and the excitatory effect was also enhanced. These results indicate that GABA has a dual action on neurotransmission in the superficial gray layer: an excitatory effect, possibly mediated by GABAA receptors and an inhibitory effect mediated by both GABAA and GABAB receptors.

Dorsal raphe serotoninergic branching neurons projecting both to the lateral geniculate body and superior colliculus: a combined retrograde tracing-immunohistochemical study in the rat

Injections of HRP into the superior colliculus labelled cells in the lateral cell groups of the dorsal raphe nucleus. The cytoarchitectural features and location of these cells showed remarkable similarities with those known to project to the lateral geniculate body, and, therefore, the possible existence of branching neurons in the dorsal raphe nucleus projecting to these two visual structures was tested. Injections into the lateral geniculate body and the superior colliculus of several fluorescent tracers--namely, Fast Blue, Fluoro-Gold, propidium iodide, rhodamine-B-isothiocyanate, and Diamidino Yellow, used in different combinations, showed single- and double-labelled neurons in the lateral wings of the dorsal raphe nucleus. In order to verify the chemical nature of these cells, the tissue was processed for immunofluorescence with serotonin antibodies. The results obtained showed several triple-labelled cells exhibiting two fluorescent tracers as well as 5-hydroxytryptamine-like immunoreactivity. Some immunonegative tracer-positive cells were also observed, suggesting their nonserotoninergic nature. Finally, electrolytic lesions of the lateral wings of the dorsal raphe nucleus caused a gradual disappearance of serotonin-immunoreactive fibers in these visual areas following different survival times. This correlated well with a decrease in the serotonin content studied by high-pressure liquid chromatography. These results support a role of the serotoninergic dorsal raphe projection to the lateral geniculate body and to the superior colliculus in the processing of visual information, and they suggest that serotonin may have a coordinating influence on primary visual centers.

The organization of serotonin fibers in the mammalian superior colliculus. An immunohistochemical study

The distribution of serotonin immunoreactivity in the superior colliculus (SC) of the rat, hamster, chipmunk, cat, and monkey was studied using a sensitive immunohistochemical method. In all of these animals, serotonin immunoreactivity formed a dense network of varicose fibers throughout the SC. These fibers had a characteristic arrangement corresponding to the laminar structures of the SC. Except in the chipmunk, serotonergic fibers were more dense in the stratum griseum superficiale than in the other layers. In the SC of the chipmunk, these fibers appeared evenly distributed. To explore the degree of serotonergic innervation in each layer, a semi-quantitative assay of serotonin immunoreactive varicosities was conducted in the rat, chipmunk, cat, and monkey. Peaks in varicose density were seen in the stratum griseum superficiale, the stratum griseum intermedium and the stratum griseum profundum. In the rat, cat, and monkey, the highest density of these varicosities was in the stratum griseum superficiale. On the other hand, the stratum griseum intermedium of the chipmunk SC received the greatest innervation of serotonergic varicose fibers.

Presynaptic uptake blockade hypothesis for LSD action at the lateral inhibitory synapse in Limulus

We investigated the action of LSD at the putative indoleaminergic lateral inhibitory synapse in the lateral eye of Limulus polyphemus. We recorded extracellular and intracellular voltage responses from eccentric cells while producing inhibition either by light or by antidromic stimulation of the optic nerve in the presence of LSD, serotonin (5-HT), chlorimipramine, or a bathing medium whose high Mg++ and low Ca++ concentrations partially or completely blocked synaptic transmission. We found (a) light-evoked and antidromically stimulated lateral inhibition is enhanced during superfusion of low (1-5 microM) concentrations of LSD and suppressed by higher (5-20 microM) concentrations; (b) these actions of LSD are markedly reduced by bathing the retina in a medium high in Mg++ and low in Ca++; (c) very low concentrations of chlorimipramine, a putative uptake blocker of serotonin, appear to mimic actions of LSD both on eccentric cell firing rate and on lateral inhibition; (d) superfused 5-HT depresses lateral inhibition at all superthreshold concentrations (0.1-25 microM). These results suggest that LSD's action may require an intact inhibitory transmitter release and postsynaptic response mechanism, whereas serotonin exerts a direct postsynaptic effect. We propose that LSD blocks presynaptic uptake of transmitter at the lateral inhibitory synapse. The concentration dependence of LSD's action can be accounted for as follows: low concentrations partially restrict transmitter reuptake, thereby prolonging the lifetime of the transmitter in the synaptic cleft and thus increasing the magnitude and duration of postsynaptic inhibition. Higher concentrations cause more presynaptic uptake sites to be blocked; this causes accumulation of transmitter in the synaptic cleft, which causes a functional blockade of the synapse because of postsynaptic desensitization. As an alternative, we propose a hypothesis based on LSD action at presynaptic autoreceptors. Similar hypotheses can account for many aspects of LSD's action in mammalian brain.

Slice preparation of rat medulla and pons maintained for five hours in vitro

Slices of rat medulla and pons were incubated in bicarbonate-buffered medium and their electrical activity was monitored for five hours with microelectrodes. The morphology of these slices was compared with that of the same region of rats of the same age using prior perfusion or immersion in fixatives before incubation. Many neurons in incubated slices show shrinkage necrosis (apoptosis) but not dilatation of the endoplasmic reticulum seen in most neurons fixed immediately after slicing. In incubated slices, some processes but not somata of glia appeared swollen: to a lesser extent some dendritic and axonal processes were swollen. Glia showed no cytoplasmic reaction after five hours to indicate that they might phagocytose damaged tissue components. Synapses appeared morphologically normal after the period of incubation and there was an apparent increase in numbers of profiles resembling growth cones.

Inhibitory action of adenosine and adenosine analogs on neurotransmission in the olfactory cortex slice of guinea pig - structure-activity relationships

The postsynaptic potential (PSP) was recorded from thin slices of the olfactory cortex of the guinea pig. Application of adenosine and adenine nucleotides such as 5'-ATP, 5'-ADP and 5'-AMP in the incubation medium, depressed the amplitude of the PSP without altering the presynaptic fiber potential. The other purine and pyrimidine derivatives had no inhibitory effect. The inhibitory action of adenosine and adenine nucleotides on the PSP were manifest at concentrations of 5 microM-1 mM. Adenosine, 5'-ATP, 5'-ADP and 5'-AMP were equipotent in evoking depression of PSPs. Inhibition occurred within 10-20 sec after administration of the agents and the depressant effect disappeared rapidly after the removal of the compounds from the medium. Theophylline reversed and prevented the inhibition produced by adenosine and adenine nucleotides. To test the structure-activity relationships of these compounds, adenosine analogs and adenine nucleotide derivatives were applied to the medium. The 6-aminopurine riboside (adenosine radical) was found to be essential for inhibitory action on the PSP. Among adenosine analogs, the presence of at least one hydrogen atom in the amino group at the 6-position of the purine, and the OH group at the 2'-position of the ribose was essential for inhibitory activity.

Increase by visual stimuli in turnover of 5-hydroxytryptamine in the superior colliculi of the rabbit

1. Irregular light falshes were played on to one eye of dark adapted rabbits for periods of 20-80 min. The concentration of 5-hydroxyindol-3-ylacetic acid (5-HIAA) and of 5-hydroxytryptamine (5-HT) were estimated in left and right superior colliculi, thalami and hippocampi. 2. In rabbits exposed to such visual stimuli for 30-60 min, there was an increase in the 5-HIAA content of the colliculus contralateral to the stimulated retina which aberaged 17% (P = 0-02), but no rise was seen if the exposure was shortened to 20 or prolonged to 80 min. At no time was there a difference in 5-HIAA content between right and left thalamus or right and left hippocampus. 3. Stationary or strictly repetitive visual stimuli produced no difference between the 5-HIAA content of left and right superior colliculus. 4. No difference in 5-HT concentration between the two colliculi was found after any form of visual stimulation, nor did any changes occur in the other parts of the brain which were examined. 5. Irregular, prolonged visual stimualtion thus appears to activate tryptaminergic neurones terminating in the colliculi. The possibility is discussed that the 5-HT released at this site might act as a brake to neuronal activity under conditions when habituation to the stimuli is not yet complete.

Antagonism of 5-hydroxytryptamine by LSD 25 in the central nervous system: a possible neuronal basis for the actions of LSD 25

1. 5-Hydroxytryptamine (5-HT), acetylcholine (ACh), noradrenaline (NA), glutamate, D,L-homocysteic acid (DLH), glycine and gamma-aminobutyric acid (GABA) were applied to single neurones in the brain stem of decerebrate cats by microiontophoresis. The abilities of D-lysergic acid diethylamide tartrate (LSD 25), methysergide maleate (UML 491) and 2-bromo-lysergic acid diethylamide (BOL 148) to antagonize the actions of these compounds were studied.2. LSD 25 antagonized 5-HT excitation of single neurones when applied iontophoretically or administered intravenously. LSD 25 also antagonized glutamate excitation of neurones which could be excited by 5-HT. Inhibitory effects of 5-HT, the action of glutamate on neurones which could be inhibited by 5-HT and the actions of all the other compounds tested were unaffected by LSD 25.3. Iontophoretically applied UML 491 was also a specific antagonist to 5-HT and glutamate excitation but was less potent than LSD 25, and BOL 148 rarely exhibited antagonism.4. It is suggested that antagonism to 5-HT and glutamate excitation of brain stem neurones may be the basis of the psychotomimetic action of LSD 25. It is also suggested that there may be similarities in the mechanisms by which 5-HT and glutamate produce excitation where they act on the same neurone.

Lysergic acid diethylamide: role in conversion of plasma tryptophan to brain serotonin (5-hydroxytryptamine)

Injections of D-lysergic acid diethylamide decrease the turnover rate of 5-hydroxytryptamine of rat brain, as measured from the conversion of (14)C-tryptophan into (14)C-5-hydroxytryptamine. The 2-bromolysergic acid diethylamide given in doses fivefold greater than those of lysergic acid diethylamide fails to change the rate of (14)C-tryptophan conversion into (14)C-5-hydroxytryptamine. The effect of D-lysergic acid diethylamide is discussed with regard to its action on brain serotonergic neurons and its psychotomimetic effects.