Inhibition of dendritic L-type calcium current by memantine in frog tectum

The aim of the study was to explore the effects of memantine on responses elicited in the frog tectum by the bursts of spikes of moderate strength of a single retina ganglion cell and to gain an insight about the effect of memantine on the L-type Ca(2+) current.

MATERIAL AND METHODS

The experiments were performed in vivo on adult frogs (Rana temporaria). An individual retina ganglion cell (or its retinotectal fiber) was stimulated by current pulses delivered through a multichannel stimulating electrode positioned on the retina. Responses to the discharge of a single retinal ganglion cell were recorded in the tectum by an extracellular carbon-fiber microelectrode positioned in the terminal arborization of the retinotectal fiber in the tectum layer F. The solution of memantine (1-amino-3,5-dimethyladamantane) hydrochloride (30 or 45 μM) was applied onto the surface of the tectum by perfusion at a rate of 0.4 mL/min.

RESULTS

Memantine (30-45 μM) largely inhibited the L-type Ca(2+) channel-mediated slow negative wave and late discharges seen in the tectum responses without any effect on fast synaptic retinotectal transmission.

CONCLUSIONS

Our results suggest that the neuroprotective effect of memantine could arise not only through the inhibition of the NMDA receptor current but also through the suppression of the L-type Ca(2+) current.

EphA3 expressed in the chicken tectum stimulates nasal retinal ganglion cell axon growth and is required for retinotectal topographic map formation

Background

Retinotopic projection onto the tectum/colliculus constitutes the most studied model of topographic mapping and Eph receptors and their ligands, the ephrins, are the best characterized molecular system involved in this process. Ephrin-As, expressed in an increasing rostro-caudal gradient in the tectum/colliculus, repel temporal retinal ganglion cell (RGC) axons from the caudal tectum and inhibit their branching posterior to their termination zones. However, there are conflicting data regarding the nature of the second force that guides nasal axons to invade and branch only in the caudal tectum/colliculus. The predominant model postulates that this second force is produced by a decreasing rostro-caudal gradient of EphA7 which repels nasal optic fibers and prevents their branching in the rostral tectum/colliculus. However, as optic fibers invade the tectum/colliculus growing throughout this gradient, this model cannot explain how the axons grow throughout this repellent molecule.

Methodology/Principal Findings

By using chicken retinal cultures we showed that EphA3 ectodomain stimulates nasal RGC axon growth in a concentration dependent way. Moreover, we showed that nasal axons choose growing on EphA3-expressing cells and that EphA3 diminishes the density of interstitial filopodia in nasal RGC axons. Accordingly, in vivo EphA3 ectodomain misexpression directs nasal optic fibers toward the caudal tectum preventing their branching in the rostral tectum.

Conclusions

We demonstrated in vitro and in vivo that EphA3 ectodomain (which is expressed in a decreasing rostro-caudal gradient in the tectum) is necessary for topographic mapping by stimulating the nasal axon growth toward the caudal tectum and inhibiting their branching in the rostral tectum. Furthermore, the ability of EphA3 of stimulating axon growth allows understanding how optic fibers invade the tectum growing throughout this molecular gradient. Therefore, opposing tectal gradients of repellent ephrin-As and of axon growth stimulating EphA3 complement each other to map optic fibers along the rostro-caudal tectal axis.

Prion protein (PrP) deposits in the tectum of experimental Gerstmann-Sträussler-Scheinker disease following intraocular inoculation

The abnormal misfolded isoform of prion protein (PrPd; "d" for disease) is considered as a surrogate marker for infectivity in the transmissible spongiform encephalopathies (TSEs) or prion diseases, including Creutzfeldt-Jakob disease (CJD). In this experiment, we used intraocular inoculation to study PrPd deposition in the visual system of the brain of mice infected with the Fujisaki (K.Fu) strain of Gerstmann-Sträussler-Scheinker (GSS) disease. We report here that PrPd is deposited in the superior colliculus following contralateral intraocular inoculation and thus follows neuronal connections when it spreads into the brain. Until 26 weeks postinoculation, no PrPd-specific immunostaining was observed in the brain. At 27 weeks postinoculation, PrPd targeted to the contralateral superior colliculus as delicate granular synaptic deposits located in the superficial part of this structure. As already reported, a few spongiform vacuoles were visible in the same area by conventional H and E staining. In several other sections, vacuoles were visible but no PrPd staining could be detected.

Assembly of lamina-specific neuronal connections by slit bound to type IV collagen

The mechanisms that generate specific neuronal connections in the brain are under intense investigation. In zebrafish, retinal ganglion cells project their axons into at least six layers within the neuropil of the midbrain tectum. Each axon elaborates a single, planar arbor in one of the target layers and forms synapses onto the dendrites of tectal neurons. We show that the laminar specificity of retinotectal connections does not depend on self-sorting interactions among RGC axons. Rather, tectum-derived Slit1, signaling through axonal Robo2, guides neurites to their target layer. Genetic and biochemical studies indicate that Slit binds to Dragnet (Col4a5), a type IV Collagen, which forms the basement membrane on the surface of the tectum. We further show that radial glial endfeet are required for the basement-membrane anchoring of Slit. We propose that Slit1 signaling, perhaps in the form of a superficial-to-deep gradient, presents laminar positional cues to ingrowing retinal axons.

Early expression of axon guidance molecules in the embryonic chick mesencephalon and pretectum

Early axon tracts in the developing vertebrate brain are established along precise paths. Yet, little is known about axon guidance processes at early stages of rostral brain development. Using whole mount in situ hybridisation in combination with immunohistochemistry, we have analysed the expression patterns of Slits, Netrins, Semaphorins and the respective receptors during the formation of the early axon scaffold, particularly focusing on the pretectal-mesencephalic boundary. Many of these guidance molecules are expressed in close correlation with the growing tracts, and the nuclei of the corresponding neurons often express the respective receptors. The expression patterns of Slits and Netrins implicate them with the positioning of the longitudinal tracts along the dorsoventral axis, while Semaphorins could provide guidance at specific choice points. Our study provides a catalogue of gene expression for future studies on axon guidance mechanisms in the early brain.

Eph/ephrin gradients in the retinotectal system of Rana pipiens: developmental and adult expression patterns

Eph/ephrin-receptor/ligand A and B families play a variety of roles during CNS development, including patterning the retinotectal projection. However, the alignment of their expression gradients with developing retinotectal maps and gradients of cellular development is not well understood in species whose midbrain tecta undergo a protracted anterior to posterior development. By using anatomical tracing methods and (3)H-thymidine neuronography, we have mapped the retinotectal projection and the spatiotemporal progression of tectal cellular development onto Eph/ephrin expression patterns in the tectum of larval Rana pipiens, as studied by means of in situ affinity analysis with fusion proteins. EphA expression is maximal in anterior tectum (and temporal retina); ephrin-A expression is maximal at the posterior pole (and nasal retina). EphB expression is graded in the early larva, where it is maximal in the posterior tectum just anterior to the posterior pole (and in the ventral retina). Tectal EphB expression becomes uniform at later stages and remains so in the adult, although its retinal expression remains maximal ventrally. In the early larva, EphA, EphB, and ephrin-A protein gradients are parallel to each other and align with the temporonasal axis of the retinal projection. The early EphB expression maximum overlaps the boundary between the mantle layer of newly postmitotic cells and the posterior, epithelial region of cell proliferation, suggesting that the expression maximum is associated with the initial migrations of the postmitotic cells. Ephrin-B expression was detected in the olfactory bulb and dorsal retina at all ages, but not in the tectum.

The pretectal nuclei in two monotremes: the short-beaked echidna (Tachyglossus aculeatus) and the platypus (Ornithorhynchus anatinus)

We have examined the organization of the pretectal area in two monotremes (the short beaked echidna-Tachyglossus aculeatus, and the platypus-Ornithorhynchus anatinus) and compared it to that in the Wistar strain rat, using Nissl staining in conjunction with enzyme histochemistry (acetylcholinesterase and NADPH diaphorase) and immunohistochemistry for parvalbumin, calbindin, calretinin and non-phosphorylated neurofilament protein (SMI-32 antibody). We were able to identify distinct anterior, medial, posterior (now called tectal gray) and olivary pretectal nuclei as well as a nucleus of the optic tract, all with largely similar topographical and chemoarchitectonic features to the homologous regions in therian mammals. The positions of these pretectal nuclei correspond to the distributions of retinofugal terminals identified by other authors. The overall size of the pretectum in both monotremes was found to be at least comparable in size, if not larger than, the pretectum of representative therian mammals of similar brain and body size. Our findings suggest that the pretectum of these two monotreme species is comparable in both size and organization to that of eutherian mammals, and is more than just an undifferentiated area pretectalis. The presence of a differentiated pretectum with similar chemoarchitecture to therians in both living monotremes lends support to the idea that the stem mammal for both prototherian and therian lineages also had a differentiated pretectum. This in turn indicates that a differentiated pretectum appeared at least 125 million years ago in the mammalian lineage and that the stem mammal for proto- and eutherian lineages probably had similar pretectal nuclei to those identified in its descendants.

Brainstem areas activated by diazepam withdrawal as measured by Fos-protein immunoreactivity in rats

In the 1970s, chronic treatment with benzodiazepines was supposed not to cause dependence. However, by the end of the decade several reports showed that the interruption of a prolonged treatment with diazepam leads to a withdrawal syndrome characterized, among other symptoms, by an exaggerated level of anxiety. In laboratory animals, signs that oscillate from irritability to extreme fear-like behaviors and convulsions have also been reported. In recent years many studies have attempted to disclose the neural substrates responsible for the benzodiazepines withdrawal. However, they have focused on telencephalic structures such as the prefrontal cortex, nucleus accumbens and amygdala. In this study, we examined the Fos immunoreactivity in brain structures known to be implicated in the neural substrates of aversion in rats under spontaneous diazepam-withdrawal. We found that the same group of structures that originally modulate the defensive responses evoked by fear stimuli, including the dorso-medial hypothalamus, the superior and inferior colliculus and the dorsal periaqueductal gray, were most labeled following diazepam withdrawal. It is suggested that an enhanced neural activation of neural substrates of fear in the midbrain tectum may underlie the aversive state elicited in diazepam-withdrawn rats.

Lmx1bis essential forFgf8andWnt1expression in the isthmic organizer during tectum and cerebellum development in mice

Secreted factors FGF8 and WNT1 are essential either for the inductive activity of the isthmus organizer or for the regionalization of the midbrain-hindbrain boundary (MHB). However, transcriptional regulation of these secreted factors during development remains to be elucidated. Here we show that the LIM homeobox gene Lmx1b is expressed in the anterior embryo as early as E7.5 and its expression becomes progressively restricted to the isthmus at E9.0. Analysis of gene expression in the MHB of the mutant embryos showed that many genes were lost by E9.5. In the MHB of Lmx1b-/- embryos, the expression of Fgf8, which normally occurs at the 4-somite stage, was completely absent, whereas Wnt1 was downregulated before the 4-somite stage. Moreover,transcription factors En1 and Pax2 were also downregulated prior to the 4-somite stage, whereas Gbx2 downregulation occurred at the 4-somite stage. By contrast, Otx2 and Pax6 expression was not affected in Lmx1b-/- embryos. The requirement of specific Lmx1b expression in the MHB was further confirmed by Wnt1-Cre-mediated region-specific conditional knockout of Lmx1b. As a result of these molecular defects, the development of the tectum and cerebellum was severely impaired in Lmx1b-/-mice. Taken together, our results indicate that Lmx1b plays an essential role in the development of the tectum and cerebellum by regulating expression of Fgf8, Wnt1 and several isthmus-related transcription factors in the MHB, and is a crucial component of a cross-regulatory network required for the induction activity of the isthmic organizer in the MHB.

Regional brain cholecystokinin changes as a function of rough-and-tumble play behavior in adolescent rats

Brain cholecystokinin (CCK) levels have been shown to be elevated in animals defeated during adult social aggression. The present experiment evaluated whether similar effects are evident in prolonged bouts of juvenile social-play fighting, which tend to switch from largely positive to some negative affect after approximately 15 min into a half-hour play session, as indexed by a gradual shift from positively valenced 50 kHz ultrasonic vocalizations (USVs) to negatively valenced 20 kHz USVs. Given the role of CCK in both positive and negative emotional events, we examined levels of CCK-8 in tissue homogenates from 14 brain areas in animals 6h after a 30 min play bout compared to no-play control animals tested similarly in isolation for 30 min. As with patterns observed following adult defeat, significantly higher CCK levels were evident after play in the posterior neo-cortex compared to no-play control animals (+26%). Levels of CCK were also elevated in the midbrain (+35%). However, unlike in adult aggression, CCK levels were reduced in the hypothalamus (-40%) and basal forebrain (-24%) as compared to no-play animals. Posterior cortex CCK levels were positively correlated to the duration that each animal was pinned (r = +.50) which suggests that elevated CCK in the posterior cortex may be related to the negative aspects of play. Hypothalamic CCK levels were negatively related to dorsal contacts and pins (r's = -.57), and suggest that the lower CCK levels may reflect the more positive valenced aspects of play. The data indicate that CCK utilization in the brain is dynamically responsive to rough-and-tumble play.

Functional and ultrastructural neuroanatomy of interactive intratectal/tectonigral mesencephalic opioid inhibitory links and nigrotectal GABAergic pathways: Involvement of GABAA and μ1-opioid receptors in the modulation of panic-like reactions elicited by electrical stimulation of the dorsal midbrain

In the present study, the functional neuroanatomy of nigrotectal-tectonigral pathways as well as the effects of central administration of opioid antagonists on aversive stimuli-induced responses elicited by electrical stimulation of the midbrain tectum were determined. Central microinjections of naloxonazine, a selective mu(1)-opiod receptor antagonist, in the mesencephalic tectum (MT) caused a significant increase in the escape thresholds elicited by local electrical stimulation. Furthermore, either naltrexone or naloxonazine microinjected in the substantia nigra, pars reticulata (SNpr), caused a significant increase in the defensive thresholds elicited by electrical stimulation of the continuum comprised by dorsolateral aspects of the periaqueductal gray matter (dlPAG) and deep layers of the superior colliculus (dlSC), as compared with controls. These findings suggest an opioid modulation of GABAergic inhibitory inputs controlling the defensive behavior elicited by MT stimulation, in cranial aspects. In fact, iontophoretic microinjections of the neurotracer biodextran into the SNpr, a mesencephalic structure rich in GABA-containing neurons, show outputs to neural substrate of the dlSC/dlPAG involved with the generation and organization of fear- and panic-like reactions. Neurochemical lesion of the nigrotectal pathways increased the sensitivity of the MT to electrical (at alertness, freezing and escape thresholds) and chemical (blockade of GABA(A) receptors) stimulation, suggesting a tonic modulatory effect of the nigrotectal GABAergic outputs on the neural networks of the MT involved with the organization of the defensive behavior and panic-like reactions. Labeled neurons of the midbrain tectum send inputs with varicosities to ipsi and contralateral dlSC/dlPAG and ipsilateral substantia nigra, pars reticulata and compacta, in which the anterograde and retrograde tracing from a single injection indicates that the substantia nigra has reciprocal connections with the dlSC/dlPAG featuring close axo-somatic and axo-dendritic appositions in both locations. In addition, ultrastructural approaches show inhibitory axo-axonic synapses in MT and inhibitory axo-somatic/axo-axonic synapses in the SNpr. These findings, in addition to the psychopharmacological evidence for the interaction between opioid and GABAergic mechanisms in the cranial aspects of the MT as well as in the mesencephalic tegmentum, offer a neuroanatomical basis of a pre-synaptic opioid inhibition of GABAergic nigrotectal neurons modulating fear in defensive behavior-related structures of the cranial mesencephalon, in a short link, and through a major neural circuit, also in GABA-containing perikarya and axons of nigrotectal neurons.

Navigation of trochlear motor axons along the midbrain-hindbrain boundary by neuropilin 2

Trochlear motor axons project dorsally along the midbrain-hindbrain boundary (MHB) to decussate at the dorsal midline. We report on the roles of neuropilin 2 and its ligands in the molecular mechanisms controlling this trajectory. In chick embryos, neuropilin 2 was expressed in the neuroepithelium of the dorsal isthmus in addition to the trochlear neurons,and Sema3F transcripts were localized along the caudal margin of the midbrain. Misexpression of Sema3F demonstrated that Sema3F displays repulsive activity in vivo that guides the trochlear motor axons along the MHB. An unexpected result was that misexpression of neuropilin 2 canceled the midbrain-evoked repulsion, allowing trochlear motor axons to cross the MHB and invade the tectum. A binding assay with neuropilin 2 ectodomain revealed the existence of neuropilin 2 ligands in the midbrain, which were masked by ectopic neuropilin 2. We therefore propose that neuropilin 2 neutralizes the repulsive activity in order to steer trochlear motor axons towards the dorsal decussation point. Taken together, our results suggest that the interaction of neuropilin 2 with its ligands has crucial roles for establishing trochlear trajectory along the MHB.

The relevance of neuronal substrates of defense in the midbrain tectum to anxiety and stress: empirical and conceptual considerations

The medial hypothalamus, amygdala, and dorsal periaqueductal gray constitute the main neural substrates for the integration of aversive states in the brain. More recently, some regions of the mesencephalon, such as the superior and inferior colliculi have also been proposed as part of this system. In fact, fear-like behaviors often result when these sites are electrically or chemically stimulated. Both the behavioral and autonomic consequences of electrical stimulation of the mesencephalic tectum have been shown to be attenuated by minor tranquilizers, probably through enhancement of gamma-aminobutyric acid (GABA)-mediated neurotransmission, which exerts a tonic inhibitory control on the neural circuits responsible for the so-called defense behavior repertoire. Besides GABA, also 5-hydroxy tryptamine serotonin (5-HT), opioids, neuropeptides, histaminergic and excitatory amino acids have all been implicated in the regulation of anxiety-related behaviors induced by stimulation of midbrain tectum. Efforts have been made to characterize how these neurotransmitters interact with each other in the organization of these reactions to aversive stimulation. In this review, we summarize the evidence linking the brain's defense response systems to the concept of fear-anxiety. Furthermore, a case is made for the consideration of the relevance of this body of data to the search for the physiological underpinnings of depression and its consequences.

Topographic mapping in dorsoventral axis of the Xenopus retinotectal system depends on signaling through ephrin-B ligands

Ephrin-B and EphB are distributed in matching dorsoventral gradients in the embryonic Xenopus visual system with retinal axons bearing high levels of ligand (dorsal) projecting to tectal regions with high receptor expression (ventral). In vitro stripe assays show that dorsal retinal axons prefer to grow on EphB receptor stripes supporting an attractive guidance mechanism. In vivo disruption of EphB/ephrin-B function by application of exogenous EphB or expression of dominant-negative ephrin-B ligand in dorsal retinal axons causes these axons to shift dorsally in the tectum, while misexpression of wild-type ephrin-B in ventral axons causes them to shift ventrally. These dorsoventral targeting errors are consistent with the hypothesis that an attractive mechanism that requires ephrin-B cytoplasmic domain is critical for retinotectal mapping in this axis.

Age-associated changes in the serotonergic system in rat superior colliculus and pretectum

The purpose of this study was to investigate whether aging alters serotonergic innervation of the superior colliculus and pretectum in rats. The superior colliculus has one of the highest concentrations of serotonin in the rat central nervous system. Young and old male F344 rats (<6 months, and >18 months, albino and pigmented) were used in all experiments. Coronal sections through the superior colliculus and pretectum were incubated with antibodies to serotonin, the serotonin 2A receptor, and the serotonin transporter. Immunocytochemical staining was analyzed semi-quantitatively. The results indicate that with age there is an increase in serotonin immunoreactivity throughout the entire superior colliculus and pretectum, a decrease in levels of serotonin 2A receptor staining in select layers of superior colliculus, and no change in serotonin transporter immunoreactivity. Albino rats differ from pigmented rats in that they have enhanced serotonergic immunoreactivity in the superficial layers of superior colliculus, a region that receives direct retinal input. These data suggest that the age-related changes in the serotonergic system in the superior colliculus and pretectum may account for some of the alterations in light-mediated behaviors with aging.

The alteration of gamma-aminobutyric acid and glutamate contents in the discrete brain of female tilapia during certain stages of gonadal cycle

The alteration of gamma-aminobutyric acid (GABA) and glutamate (Glu) contents in the discrete brain of female tilapia during the certain stages of gonadal cycle was investigated. Both hypothalamic GABA and Glu showed a significantly higher content in the prespawning female than that in the recrudesced and regressed females, which there was no significant difference. In the different gonadal status of female, both telencephalic GABA and Glu showed a maximal content during the prespawning phase and a minimal content during the recrudesced phase. Neither GABA nor Glu content was altered with the gonadal phases in the optic lobe and cerebellum of female tilapia. These results indicate that the alteration of GABA and Glu contents in the hypothalamus and telencephalon, but neither in the optic lobe nor cerebellum, is consistent with the gonadal cycle.

[The prenatal development of the tectum mesencephali and substantia nigra in man]

The development of tectum of the midbrain was studied in situ in human 6-11 wks embryos. Using electron microscopy and immunocytochemistry processes that occur in the anlage of tectum of the midbrain at early stages of formation of dopaminergic populations of cells were followed up. Growth of pallium zone due to migration of cells from ventricular zone and their differentiation is proportional to growth of the terms of embryonal development. Proliferating cells were located not only in ventricular zone. Expression of tyrosine hydroxylase was found in 6.5 wks embryos in the fraction of cells that migrated from ventricular zone and was observed only during neuroblast migration. Cells migrate along the radial glia fibres in dorsoventral direction. Microgliocytes with short processes near to vessels located in the vicinity of ventricular zone were found in 7 wks embryos for the first time. Peculiarities of morphogenetic processes participating in anlage of tectum of the midbrain and possible role of microgliocytes in development of embryonal nerve tissue are discussed.

Chick optic lobe contains a developmentally regulated alpha2alpha5beta2 nicotinic receptor subtype

The most widely expressed neuronal nicotinic acetylcholine receptor subtype in chick brain is that containing the alpha4 and beta2 subunits. However, immunoprecipitation and localization studies have shown that some brain areas also contain the alpha2 and/or alpha5 subunits, whose role in the definition of receptor properties is still intriguing. Using subunit-specific polyclonal antibodies, we found that the optic lobe is the chick central nervous system region that expresses the highest level of alpha2-containing receptors. Immunoprecipitation studies of these immunopurified alpha2-containing receptors labeled with the nicotinic agonist [(3)H]epibatidine showed that almost all of them contained the beta2 subunit and that more than 66% contained the alpha5 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha2, alpha5, and beta2 subunits and the absence of the alpha3, alpha4, alpha6, alpha7, alpha8, beta3, and beta4 subunits. The alpha2-containing receptors are developmentally regulated: their expression increases 25 times from embryonic day 7 to posthatching day 1 in the optic lobe, compared with an increase of only 5-fold in the forebrain. The alpha2-containing optic lobe receptors bind [(3)H]epibatidine (K(d) = 29 pM) and a number of other nicotinic agonists with very high affinity and have a pharmacological profile very similar to that of the alpha4beta2 subtype. They form functional cationic channels when reconstituted in lipid bilayers, with pharmacological and biophysical properties different from those of the alpha4beta2 subtype. These channels are activated by nicotinic agonists in a dose-dependent manner and are blocked by the nicotinic antagonist d-tubocurarine.

Synaptic membrane freezing affects modulatory sites in avian central nervous system GABA(A) receptor

Studies were carried out to determine whether barbiturates and neurosteroids share common recognition sites at the GABA(A) receptor complex in avian CNS. To achieve this, differentially prepared fresh and frozen synaptic membranes were used. Both the barbiturate, pentobarbital, and the neurosteroid, 3alpha-hydroxy-5alpha-pregnan-20-one, were able to stimulate GABA binding in both types of membranes. Stimulation differed markedly when both drugs were added jointly to different treated tissue. In frozen membranes drugs acted synergistically and were differentially displaced by picrotoxinin, while in fresh ones, where both compounds were inhibited by the convulsant, this additivity was absent. Post-freezing wash supernatants were collected and used as a source of putative endogenous factors involved in the above mentioned membrane differences. Addition of a high molecular weight fraction from supernatants to frozen synaptic membranes led to an inhibition of barbiturate and neurosteroid potentiation, as well as a loss of their additive effect. Our results indicate that GABA(A) receptor modulation by barbiturates and neurosteroids is affected by synaptic membrane treatment, with a common modulatory site in fresh membranes and separate recognition sites after a freeze-thawing procedure. There may also be endogenous factors involved in overlapping of modulatory sites, which would thus regulate GABA(A) receptor functionality by direct interaction with the complex.

Functional alpha6-containing nicotinic receptors are present in chick retina

Despite the fact that the neuronal chick alpha6 subunit was first cloned several years ago and recently has been shown to form acetylcholine (ACh)-activated channels in heterologous systems, no information is yet available concerning the structure and function of the alpha6-containing nicotinic receptors in neuronal tissues. Using subunit-specific antibodies directed against two different epitopes of the chick alpha6 subunit, we performed immunoprecipitation experiments on immunopurified alpha6-containing receptors radiolabeled with the nicotinic agonist [3H]epibatidine (Epi): almost all of the alpha6 receptors contained the beta4 subunit, 51% the beta3 subunit, 42% the alpha3 subunit, and 7.5% the beta2 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha3, beta3, beta2, and beta4 subunits, and the absence of the alpha4, alpha5, and alpha7 subunits. The alpha6-containing receptors bind [3H]Epi (Kd = 35 pM) and a number of other nicotinic agonists with very high affinity, the rank order being Epi >> cytisine > nicotine > 1, 1-dimethyl-4-phenylpiperazinium > acetylcholine > carbamylcholine. The alpha6 receptors also have a distinct antagonist pharmacological profile with a rank order of potency of alpha-conotoxin MII > methyllycaconitine > dihydro-beta-erythroydine > MG624 > d-tubocurarine > decamethonium > hexamethonium. When reconstituted in lipid bilayers, the alpha6-containing receptors form functional cationic channels with a main conductance state of 48 pS. These channels are activated by nicotinic agonists in a dose-dependent manner, and blocked by the nicotinic antagonist d-tubocurarine.

Neuropeptide Y and enkephalin immunoreactivity in retinorecipient nuclei of the hamster pretectum and thalamus

This investigation was stimulated by the historical confusion concerning the identity of certain pretectal nuclei and by large differences reported between species with respect to which nuclei receive retinal innervation. Subcortical visual nuclei were studied using immunohistochemistry to identify retinal projections labeled following intraocular injection of cholera toxin, b fragment. In addition, neuropeptide Y (NPY) or enkephalin (ENK) immunoreactive cells and fibers were also evaluated in the retinorecipient pretectal and thalamic areas. The results confirm the established view that the retina directly innervates the nucleus of the optic tract (NOT), posterior (PPT), and olivary pretectal (OPT) nuclei. However, the retina also innervates the hamster medial (MPT) and anterior (APT; dorsal division) pretectal nuclei, results not previously reported in rodents. A commissural pretectal area (CPT) sparsely innervated by retina is also described. The data show for the first time that the posterior limitans nucleus (PLi) receives a moderately dense, direct retinal input. The PLi does not project to the cortex and appears to be a pretectal, rather than thalamic, nucleus. All retinal projections are bilateral, although predominantly contralateral. The PLi contains a moderately dense plexus of NPY- and ENK-IR fibers and terminals. However, peptidergic fibers also traverse the ATP and connect with the dorsomedial pretectium. The OPT contains ENK- and NPY-IR neurons and fibers, but is specifically identifiable by a moderately dense plexus of ENK-IR terminals. Numerous ENK-IR neurons are found in the NOT and PPT. The latter also has moderate numbers of ENK-IR fibers and terminals, but few NPY-IR neurons or fibers. The MPT contains modest numbers of ENK-IR fibers. The APT has no NPY-IR neurons or terminals, but an occasional ENK-IR neuron is seen and there is sparse ENK-IR innervation. Peptidergic innervation of the visual nuclei does not appear to be derived from the retina. The results show a set of retinally innervated, contiguous nuclei extending from the thalamic ventrolateral geniculate nucleus dorsomedially to the midbrain CPT. These nuclei plus the superior colliculus comprise a dorsal "visual shell" embracing a central core of caudal thalamus and rostral midbrain.

In situ hybridization localization of the GABAA receptor beta 2S- and beta 2L-subunit transcripts reveals cell-specific splicing of alternate cassette exons

We have recently described two variants of the chicken GABAA receptor beta 2 subunit which arise by alternative splicing of the corresponding primary gene transcript. The long form of the beta 2 subunit (beta 2L) differs from the short form (beta 2S) by the insertion of an additional 17 amino acids, in the large presumed intracellular loop, between the third and fourth membrane-spanning domains. In this study, we have utilized in situ hybridization with transcript-specific oligonucleotide probes to determine the regional and cellular localizations of the beta 2S- and beta 2L-subunit messenger RNAs in the one-day-old chick brain. We show that the beta 2-subunit gene is expressed in many brain areas that also transcribe the GABAA receptor alpha 1- and gamma 2-polypeptide genes. We also demonstrate that while the beta 2S- and beta 2L-subunit messenger RNAs frequently co-localize in many brain areas, certain structures (e.g., the ectostriatum, the hippocampus, the nucleus solitarius, the nucleus isthmi, pars parvocellularis, the nucleus isthmi, pars magnocellularis, the paleostriatum primitivum, the Purkinje cell layer, and the deep cerebellar nuclei) exclusively or predominantly contain either the beta 2S- or the beta 2L-subunit transcript. The distributions of the beta 2S- and beta 2L-polypeptide messenger RNAs resemble those previously described for the chicken GABAA receptor gamma 2S- and gamma 2L-subunit transcripts, respectively, which are also generated by alternative splicing. Our results indicate that a major GABAA receptor subtype in the avian brain is comprised of alpha 1, beta 2 and gamma 2 subunits. In addition, the data obtained reveal that many neurons in the chicken CNS are capable of producing more than one alternatively spliced form of a given primary gene transcript. However, the avian brain also appears to contain two small populations of neurons that possess mechanisms that result in either the incorporation of alternate cassette exons into mature transcripts, or the exclusion of such exons from processed messenger RNAs.

Neurosteroid modulation of the benzodiazepine receptor in developing chick optic lobe

The modulation of [3H]flunitrazepam binding by 3 alpha-hydroxy-5 alpha-dihydroprogesterone (3 alpha-OH-DHP) and 3 alpha, 5 alpha-tetrahydrodeoxicorticosterone (3 alpha-THDOC) was investigated in synaptic membranes isolated from chick optic lobe at three developmental stages, in order to evaluate the role of neurosteroids in central nervous system functional maturation. It was demonstrated that both steroids modulate [3H]flunitrazepam binding in a concentration-dependent manner at embryonic day 14, hatching and adult stage, producing maximal [3H]flunitrazepam binding enhancement at early stages of development and declining thereafter. EC50 values for 3 alpha-OH-DHP were lower than for 3 alpha-THDOC at all stages examined. On the other hand, Emax values were higher with 3 alpha-OH-DHP than with 3 alpha-THDOC. Scatchard analysis performed at embryonic day 14, hatching and adult stage showed that dissociation constants (Kd) were of 22.23 +/- 0.15, 18.38 +/- 1.15 and 19.86 +/- 0.62 nM, and maximal number of binding sites (Bmax) were 1.95 +/- 0.15, 3.13 +/- 0.21 and 2.25 +/- 0.13 pmol/mg protein, respectively. By adding 4 microM of either 3 alpha-OH-DHP or 3 alpha-THDOC, Kd values decreased significantly to 10.65 +/- 0.62, 9.71 +/- 0.85 and 13.25 +/- 0.74 nM or 9.54 +/- 0.65, 11.20 +/- 1.27 and 12.96 +/- 1.38 nM, at the above mentioned stages, respectively. Thus, either drug at the given concentration increased the affinity of benzodiazepine receptor sites for [3H]flunitrazepam, while the density of receptor sites remained unchanged. Our results suggest that these steroids display a positive allosteric modulation of benzodiazepine receptor sites which is inversely related to age.

Tissue distribution of the DNA binding oncoprotein Maf during chicken development

To assess the normal function of the c-maf protooncogene product, we examined its tissue distribution during fetal growth and organogenesis of the chicken using an immunohistochemical technique. Nuclei of neuronal cells in the optic lobe and in the outer granular layer of the cerebral cortex were efficiently stained by anti-Maf serum throughout all developmental stages examined. In the lung, mesenchymal cells were stained intensely by the antiserum, with the most intense staining around embryonic day 15. During the embryonic period, nuclei of mesenchymal cells in the perichondrial and periosteal tissues as well as intestinal submucosa were also stained specifically by the antiserum. In addition, we detected substantial Maf immunoreactivity in nuclei of renal glomerular and proximal-tubular epithelial cells. These results suggest that c-Maf protein plays some important roles in the development of the central nervous system and tissues of mesodermal origin such as connective and renal tissues.

Ontogeny of two different benzodiazepine binding sites in the chick optic lobe

The developmental time-course of type I and type II benzodiazepine receptors in the chick optic lobe was determined using a triazolopyridazine, CL 218872. At embryonic day 13 most of the binding sites corresponded to type II (98.23%), while type I represented only a minor proportion (1.77%). During development there was an increase in type I binding sites which reached 62.88% in adulthood, while type II binding sites decreased to 37.12%. These results demonstrate a differential ontogeny of two benzodiazepine receptor subtypes. Changes in the benzodiazepine binding population may account for the variability in the GABA-benzodiazepine receptor interaction during chick optic lobe development.

The extracellular matrix molecule tenascin: expression in the developing chick retinotectal system and substrate properties for retinal ganglion cell neurites in vitro

To investigate the molecular mechanisms involved in the outgrowth of retinal ganglion cell axons in the tectum, the expression of the extracellular matrix molecule tenascin was analysed in the tectum and retina of chickens by immunocytochemistry and in situ hybridization. Tissue was analysed between embryonic days 4 and 12, just before and during the period when retinal ganglion cell axons innervate their target region, the optic tectum. In the tectum, tenascin immunoreactivity becomes detectable at the anterior pole at embryonic day 4, 2 days before retinal ganglion cell axons arrive, and spreads caudally with increasing age. At early stages, tenascin is predominantly accumulated in the stratum opticum, the zone of ingrowing retinal ganglion cell axons, and along their prospective pathway. In the stratum opticum, the molecule is associated with radial glial fibres, glial endfeet and retinal ganglion cell axons located in the immediate neighbourhood of radial glial fibres. At all ages investigated, tenascin mRNA is mainly restricted to cells located in the periventricular region, suggesting that the molecule is synthesized by radial glial cells. In the retina, tenascin is expressed by amacrine, displaced amacrine and horizontal cells but not by retinal ganglion cells. To investigate whether the accumulation of tenascin in the developing and prospective pathway of retinal ganglion cell axons may affect their rate of growth we assayed the substrate properties of tenascin for retinal ganglion cell neurites in vitro. When retinal ganglion cell suspensions from 6-day-old chick embryos were maintained on homogeneous mouse or chick tenascin/polyornithine substrates, neurite length was significantly increased when compared to polyornithine substrates at coating concentrations of 10 or 20 micrograms/ml. Higher coating concentrations (35 or 70 micrograms/ml) resulted in neurite lengths comparable to control values. Together, these observations suggest that tenascin in the developing and prospective stratum opticum might serve as a performed pathway to support growth of retinal ganglion cell axons in the tectum.

Anti-peptide specific antibodies for the characterization of different alpha subunits of alpha-bungarotoxin binding acetylcholine receptors present in chick optic lobe

Chick optic lobe express alpha-Bungarotoxin receptors. We have recently purified these receptors which, when reconstituted in a lipid bilayer, behave as functional acetylcholine gated channels. In order to characterize this purified preparation, we raised polyclonal antibodies against peptides obtained from the putative cytoplasmic domain between the hydrophobic sequence M3 and M4 of two previously cloned alpha-Bungarotoxin receptor subunits, alpha 7 and alpha 8. Both antibodies recognized the receptors present in the membrane extract and in the purified preparation, although the amount of the alpha-Bungarotoxin receptors precipitated by the two antibodies was quantitatively different. In Western blots of both purified and membrane-bound receptors, these antibodies specifically reacted with an M(r) 57000-55000 band. A study was also undertaken to quantify the receptors containing these subunits in different chick brain areas; it was found that the number of these subunits, as well as their ratio, was similar in all the tested areas. Furthermore, the alpha-Bungarotoxin receptors were present in at least two subtypes, one containing only the alpha 7 subunit and the other both alpha 7 and alpha 8 subunits.

Characterization of homologous 5-hydroxytryptamine4 receptor desensitization in colliculi neurons

Exposure of mouse colliculi neurons to selective 5-hydroxytryptamine (5-HT)4 agonists was accompanied by a rapid desensitization of the receptor-stimulated adenylyl cyclase response. Half-maximal desensitization occurred after 2 min. Only exposure of neurons to selective 5-HT4 agonists led to a potent desensitization of the 5-HT4-mediated response. Neurons exposed to other agents, like isoproterenol, vasoactive intestinal peptide, or forskolin, that increase cAMP levels did not undergo any desensitization of 5-HT4 receptors. Activation of protein kinase A with either 8-bromo-cAMP or dibutyryl-cAMP or application of inhibitors of protein kinase A-dependent phosphorylation did not change the rate of 5-HT4-induced desensitization. No shift to lower potency of 5-HT4 agonists in the concentration-response curve was observed. These results suggest that 5-HT4 receptor agonists induced homologous but not cAMP-mediated heterologous desensitization. A good correlation was found between the affinities of nine 5-HT4 agonists and their abilities to desensitize the adenylyl cyclase response. This may indicate that homologous desensitization is a function of the mean occupancy time of the receptors by agonists. When permeabilized neurons were loaded with heparin, an inhibitor of the beta-adrenergic receptor kinase (beta ARK), 5-HT4 receptor desensitization was reduced by 30-40%. Interestingly, Zn2+, an other inhibitor of beta ARK, totally prevented 5-HT4-induced desensitization. Pretreatment of neurons with concanavalin A, reported to inhibit sequestration of beta-adrenergic receptors from the cell surface, reduced the desensitization process by 70%. These data suggest that both sequestration and phosphorylation by beta ARK, or another specific agonist-dependent receptor kinase, are involved in homologous desensitization of 5-HT4 receptors coupled to adenylyl cyclase.

Electrophysiological evidence for excitatory 5-HT2 and depressant 5-HT1A receptors on neurones of the rat midbrain tectum

It has been claimed that the aversive behaviour induced by electrical stimulation of the midbrain tectum (MT) has validity as an animal model of panic attack. A great deal of evidence obtained from behavioural studies suggests that 5-HT2 mechanisms phasically inhibit the substrates of aversion in the MT. In order to test this hypothesis we employed the technique of microiontophoresis of drugs onto neurons of the MT to assess the identity of the receptors mediating the effects of 5-hydroxytryptamine (5-HT). The results obtained show that the majority of 5-HT responsive cells in MT are cells excited by 5-HT (72%). These cells were silent or showed very low spontaneous firing activity, whereas cells depressed by 5-HT showed high spontaneous firing activity at baseline. The 5-HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), buspirone and gepirone caused consistent reduction in the firing rate of cells depressed by 5-HT while they did not change the firing activity of cells excited by 5-HT. The excitatory effects induced by 5-HT on MT neurones were clearly attenuated by concomitant application of ketanserin, a highly specific 5-HT2 antagonist. Excitatory responses to DL-homocysteic acid were not affected by ketanserin. Previous administration of zimelidine, a selective 5-HT uptake inhibitor, caused a significant enhancement of the excitatory effects of 5-HT while similar application of gepirone did not affect the size of the excitatory responses to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

A unique neuronal organization in the cat pretectum revealed by antibodies to the calcium-binding protein calbindin-D 28K

The pretectum is an important center for visual reflexes. However, the location, boundaries, and connections of individual nuclei of the pretectum are incompletely understood. In cat, the traditionally defined nuclear boundaries have been placed in doubt by recent evidence showing that the retinal input to the pretectum forms four continuous projection zones that do not match the cytoarchitectural boundaries of individual pretectal nuclei defined by previous studies. We now show that antibodies to the calcium-binding protein calbindin-D 28K (CaBP) label clusters of neurons within the pretectum that match the zones of retinal termination. Four obvious cell clusters within the pretectum were labeled by CaBP antisera. Computer three-dimensional reconstruction of these cell clusters revealed that they form four distinct but continuous zones that run the rostrocaudal length of the pretectum in a medial-to-lateral direction. By combining anterograde HRP labeling of retinal terminals with CaBP immunocytochemistry, these CaBP-labeled cell clusters were found virtually to overlap the retinal projection zones. The CaBP-labeled neurons included both multipolar and fusiform morphologies, and most were medium- to large-sized cells. HRP retrograde transport studies showed that many CaBP-labeled neurons in the clusters projected to the LGN, while none projected to the inferior olive (IO). GABA-immunoreactive neurons were also found within the CaBP cell clusters, but these neurons were smaller than most CaBP-labeled neurons, and none were retrogradely labeled following HRP injections into the LGN or IO. Two-color antibody double-labeling experiments did not reveal any GABA neurons within the clusters that colocalized CaBP. In summary, calbindin is a precise marker of neuron clusters that overlap the retinal projection zones in the cat pretectum. Many of these CaBP neurons project to the LGN, and none contain GABA.

Benzodiazepine receptor sites in the chick optic lobe: development and pharmacological characterization

To investigate the interaction between gamma-aminobutyric acid (GABA) and benzodiazepine (BZD) receptor sites during development, the time-course of appearance of flunitrazepam (FNZ) binding sites and their pharmacological characterization were studied in developing chick optic lobe. At the earliest stage examined, embryonic day (Ed) 12, the receptor density was 30.9% (0.05 +/- 0.01 pmol/mg protein) of that found in the chick optic lobes of adult chicks. The adult value was achieved on Ed 16 (0.16 +/- 0.01 pmol/mg protein). After this stage there was a sharp and transient increase in specific [3H]FNZ binding of about two-fold reaching a maximal value between hatching and the postnatal day (pnd) 2 (0.33 +/- 0.01 pmol/mg protein). Scatchard analysis at different stages of development revealed the presence of a single population of specific FNZ binding sites. The increase in [3H]FNZ binding during development was due to a large number of binding sites while their affinity remained unchanged. Competition experiments in the chick optic lobe revealed that the order of potency for displacement of specific [3H]FNZ binding paralleled the pharmacological potency of the BZDs tested. The IC50s for clonazepam, flunitrazepam, Ro 15-1788 and chlordiazepoxide were 3.02, 4.30, 0.32, and 4778.64 nM respectively. Ro 5-4864, a potent inhibitor of BZD binding to peripheral tissues, had no effect on specific [3H]FNZ binding indicating that only central BZD binding sites are present in the chick optic lobe. The peak of maximal expression of BZD receptor sites precedes in 5-6 days the peak of GABA receptor sites indicating a precocious development of BZD receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a simple visual pattern on the early postnatal development of GABA receptor sites in the chick optic lobe

It is known that manipulation of the visual environment results in changes in the developmental pattern of several neurotransmitter receptors and that the GABA receptor shows a high degree of plasticity in differential illumination experiments. In the present paper we investigated whether exposure to a visual pattern has a developmental effect on GABA receptor expression during early postnatal life. Two groups of newly hatched chicks were used: one was exposed to a simple and specific visual pattern and the other was deprived of any visual pattern. GABA receptors at each developmental stage were determined by binding experiments performed in a crude membrane fraction. Saturation studies were carried out in a fraction enriched in synaptic membranes. The developmental pattern of both high and low affinity GABA binding sites was affected by the visual pattern. This effect displays its maximal expression by the end of the first postnatal week. The modification in receptor expression was due to a change in the receptor density while the affinity was not affected. The change in receptor density induced by the presence of a visual pattern was highest at the end of the first postnatal week suggesting that at that time there is a brief period of higher plasticity for GABA receptor expression in the visual system than at other times. Our results also suggest that variations in GABA receptor density could be instrumental in adaptative changes in the visual system in response to variations in the environmental stimulation.

Metabolic activity in rat tectal grafts is influenced by host sensory innervation

It has been shown previously that fetal tectal tissue grafted to the midbrain of newborn host rats grows, differentiates, and receives input from the host brain. In the present study, 4 neuroanatomical techniques have been combined to examine how metabolic activity in tectal transplants is influenced by an identified host sensory pathway. Tectal tissue from E15 pigmented rat embryos was transplanted to the midbrain region of anesthetized newborn rats of the same strain. Six to 22 weeks later, the functional relationship between tectal transplants and the visual system of the host animal was examined by mapping metabolic activity in the grafts and relating this activity to the presence or absence of host retinal innervation. Metabolic activity in tectal grafts was assessed using the radioactive 2-deoxy-glucose (2-DG) method and cytochrome oxidase (CO) histochemistry. Graft regions receiving input from host retinal axons were demonstrated by anterograde labeling after bilateral intraocular injections of HRP or WGA-HRP; all areas in grafts that were homologous to the superficial layers of normal superior colliculus (SC) were identified using AChE histochemistry. The levels of metabolic activity demonstrated with 2-DG and CO varied between animals and within individual grafts. Grafts that did not connect with the host showed only low metabolic activity. In grafts that received host input, localized areas of high metabolic activity were seen with both 2-DG and CO. Highest levels of activity were consistently found in area containing both intense AChE activity and a high density of host retinal innervation.

Evidence for a nigro-pulvinar-lateralis posterior complex projection in the cat using horseradish peroxidase neuronal retrograde technique

The possible existence of a direct projection from the substantia nigra to the pulvinar-lateral posterior complex (Pul-LP) was investigated in the cat by using the horseradish peroxidase technique. In particular horseradish peroxidase was injected in the Pul-LP of 8 animals, either unilaterally or bilaterally. Tissue sections obtained from the cat's brain 24-48 hrs. after injection were prepared according to Mesulam's method as slightly modified by the authors. Retrogradelly labelled neurons were observed in substantia nigra pars lateralis and reliculata ipsilaterally to the injected pulvinar-lateral posterior complex. A small number of labelled cells were also found in the contralateral substantia nigra. These findings demonstrate the existence of a close connection between two system which are involved in turning behavior: the nigrostriatal and the pulvinar-lateral posterior complex-superior colliculus.

Effects of light- and dark-rearing on the postnatal development of GABA receptor sites in the chick optic lobe

To investigate the ability of GABA receptor sites to undergo environmental-dependent plastic changes, the postnatal developmental pattern of GABA receptors was studied under different levels of light stimulation, i.e. normal-, light- and dark-rearing. At hatching the specific binding of [3H]GABA was 1.74 +/- 0.36 pmol/optic lobe. In normally reared chicks the number of GABA binding sites showed a transient increase with the highest value at the 6th day (7.0 +/- 1.32 pmol/optic lobe). This value is higher than the one reached at the adult stage. Between the 3rd and 6th day, there was a 33.7% increase in specific [3H]GABA binding in light-reared compared with normally reared animals (P less than 0.05). In the dark-reared chicks, the specific binding was 36.4% lower than that found in normally reared (P less than 0.02). However, the changes in receptor density were transient since at the 17th day the number of GABA binding sites returned to adult levels. Scatchard analysis revealed that the differences observed in the high affinity GABA binding sites between the three groups were due to modifications in the total number of binding sites while the affinity remained unchanged. The maximal number of binding sites were: 2.71, 7.01 and 1.79 pmol/mg protein in normally, light- and dark-reared chicks, respectively; while the apparent dissociation constants were unaffected: 3.2, 3.4 and 3.6 nM, respectively. These results show that, during postnatal development, different conditions of visual experience produce synaptic changes at the molecular level. These changes probably occur within a period of high plasticity, prior to the end of a critical period.

Effects of hypothermia on the in vivo measurement of rapid axonal transport in the rat: a cautionary note

Rapid axonal transport of glycoproteins was examined in the retinofugal projections of hypothermic and normothermic adult male Long-Evans hooded rats previously receiving intraocular injections of [3H]fucose. The amount of retinal fucosylation appeared normal in the hypothermic animals 3.5 h after isotope injection, but glycoprotein transport was reduced relative to normothermic controls. This reduction was especially pronounced in the most distal structure of the retinofugal tract (superior colliculus). We conclude that rapid axonal transport decreases with reductions in mammalian body temperature. This finding emphasizes the importance of controlling body temperature in in vivo studies of mammalian axonal transport.

Altered colchicine-binding capacity in chick brain regions: relationship to intensity or information content of visual input

The binding of [3H]colchicine to postmitochondrial supernatant fractions of chick brain has been studied, absorbing colchicine-bearing proteins on DEAE-cellulose filter discs. This was assayed at several times after unilateral enucleation of day-old chicks. Binding was unaltered in optic lobes or anterior dorsal forebrain regions contralateral to the removed eye, relative to the corresponding regions contralateral to and thus directly or secondarily innervated by the intact eye. Colchicine binding was also assayed after training chicks to suppress pecking at a metallic bead coated with aversive-tasting methylanthranilate. At 4 but not 24 h after the one-trial training, binding was selectively elevated in the anterior dorsal forebrain roof. Results are taken to imply that alterations in tubulin content of chick brain may reflect stress-related changes, possibly mediated by systemic endocrine flux, rather than a more localized learning experience.

Biochemical characterization of two nicotinic receptors from the optic lobe of the chick

We have studied putative nicotinic acetylcholine receptors in the optic lobe of the newborn chick, using 125I-labeled alpha-bungarotoxin, a specific blocker of acetylcholine receptors in the neuromuscular junction, and [3H]acetylcholine, a ligand which in the presence of atropine selectively labels binding sites of nicotinic character in rat brain cortex (Schwartz et al., 1982). [3H]Acetylcholine binds reversibly to a single class of high affinity binding sites (KD = 2.2 X 10(-8) M) which occur at a tissue concentration of 5.7 pmol/g. A large fraction (approximately 60%) of these binding sites is solubilized by Triton X-100, sodium cholate, or the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. Solubilization increases the affinity for acetylcholine and several nicotinic drugs from 1.5- to 7-fold. The acetylcholine-binding macromolecule resembles the receptor for alpha-bungarotoxin present in the same tissue with respect to subcellular distribution, hydrodynamic properties, lectin binding, and agonist affinity rank order. It differs from the toxin receptor in affinity for nicotinic antagonists, sensitivity to thermal inactivation, and regional distribution. The solubilized [3H]acetylcholine binding activity is separated from the toxin receptor by incubation with agarose-linked acetylcholine, by affinity chromatography on immobilized Naja naja siamensis alpha-toxin, and by precipitation with a monoclonal antibody to chick optic lobe toxin receptor.

Age-dependent changes in brain GABA turnover rates in two inbred strains of mice

gamma-Aminobutyric acid (GABA) steady-state levels and turnover rates have been determined in 15 brain areas of 21-day- and 3-month-old DBA/2J (DBA) and C57B1/6J (C57) mice. These two inbred strains differ by their susceptibility to audiogenic seizures; moreover, the involvement of GABAergic neurotransmission has been suggested in the control of this behavior. Turnover rates are generally higher at 21 days than at 3 months of age. There are few significant differences in the GABA steady-state levels between 21-day-old seizure-prone DBA mice when compared with seizure-resistant C57 mice. In the DBA mice, the steady-state level is higher in the olfactory bulbs and lower in the posterior colliculus and the olfactory tubercles than in the C57 mice. Although there are some significant differences in GABA turnover rates and steady-state levels, intra or inter strains, it is difficult to correlate directly these differences with seizure susceptibility.

GABA-containing neurons in the thalamus and pretectum of the rodent. An immunocytochemical study

Antisera produced by immunizing rabbits with GABA conjugated to bovine serum albumin reacted, after purification, strongly with GABA fixed with glutaraldehyde to rat brain macromolecules, but insignificantly with other fixed amino acids (Storm-Mathisen et al. 1983). Sections through the diencephalon of perfusion-fixed mouse and rat brains showed a highly selective labeling pattern after incubation with these antisera. All cells of the reticular nucleus appeared to be stained. Smaller proportions of stained perikarya occurred in the dorsal and ventral subdivisions of the lateral geniculate body, in the medial geniculate body, in the lateroposterior nucleus, and in all nuclei of the pretectum. Labeled cell bodies were only rarely encountered in the ventrobasal complex, and were not found in the anterior and medial groups of thalamic nuclei. Stained axons were particularly concentrated in the ventrobasal complex, and in the stria medullaris, stria terminalis and inferior thalamic peduncle. The arrangement and density of labeled bouton-like dots varied markedly among nuclei, the highest densities occurring in the paraventricular and parataenial nuclei, and in the ventral subdivision of the lateral geniculate body. The mean staining intensity of the thalamic neuropil was lower than that of nearby structures, such as the hypothalamus and zona incerta. The present results on direct immunocytochemical detection of GABA are consistent with, and extend, data from immunocytochemical studies of the GABA-synthetizing enzyme, glutamic acid decarboxylase.

Monoclonal antibodies against the alpha-bungarotoxin-binding protein of chick optic lobe

The nicotinic acetylcholine (ACh) receptor probe alpha-bungarotoxin (alpha-Butx) binds with high affinity to a membrane protein of the vertebrate central nervous system. To characterize further this putative neuronal ACh receptor, we have prepared monoclonal antibodies (mAbs) against the alpha-Butx-binding protein of chick optic lobe. Mice were immunized with affinity-purified protein preparations which were estimated to be 10 to 20% pure. Spleen cells from an immunized mouse were fused with the mouse myeloma cell line X63-Ag 8.653. From this fusion, 14 stable hybridoma lines were isolated which produce mAbs against the chick neuronal alpha-Butx-binding protein. Most of the antibodies inhibited alpha-Butx-binding to membrane fractions and/or detergent extracts of chick optic lobe. Some of the mAbs cross-reacted with the alpha-Butx-binding protein of the rat pheochromocytoma cell line PC12. However, none of the mAbs bound to a significant extent to the nicotinic ACh receptor of chick skeletal muscle or of Torpedo californica electric organ. All antibodies specifically isolated a polypeptide of Mr = 57,000 (+/- 2,000) from radiolabeled neuronal protein preparations. The present data show that these mAbs constitute useful tools for the further molecular and functional characterization of the neuronal alpha-Butx-binding protein.

Evidence that nigral substance P controls the activity of the nigrotectal GABAergic pathway

The unilateral intranigral application of either of two substance P (SP) analogs with antagonist properties ( [DPro2,D-Phe7,D-Trp9]-SP and [D-Pro2,D-Trp7,9]-SP) induced a significant (37%) decrease in GABA turnover in the target region of the nigrotectal projections, namely the deep layers of the superior colliculus in the homolateral hemisphere, and was without any effect in the superficial layers. This action was not prevented by coinjection of the GABA antagonist bicuculline-methiodide. There was no effect of these SP analogs on GABA turnover in the deep layers of superior colliculus when they were microinjected in the ipsilateral median forebrain bundle or in the contralateral substantia nigra. These results are consistent with the proposal that nigral SP exerts a tonic excitatory action on efferent GABAergic projections from pars reticulata of substantia nigra.

Muscarinic cholinergic binding in chick embryo retino-tectal system: effects of corticosterone

Muscarinic binding sites were measured using the radioligand [3H]quinuclidinyl benzilate (QNB) in the retina and tectum of 11-day-old chick embryos, after intracerebral administration of 0.02 microgram of corticosterone at 8 days of incubation. This age was chosen because the hormone preferentially accumulates in retinas at 8 days of development. Hormone treatment significantly affected the affinity of 3H-QNB-binding sites in retinas and slightly affected the affinity in treated tecta, whereas the number of binding sites remained unchanged. The specific binding was determined with either atropine or unlabeled QNB. Scatchard plot analysis of specific 3H-QNB binding revealed the presence of nonsaturable binding at high 3H-QNB concentrations (6-11 nM) in the treated retinas, but not in controls. It can be concluded from these data that the hormone has a primary effect on retinal cells during early growth in the chick embryo. The possibility that the hormone delays maturation of specific populations of retinal cells is considered in the discussion.

A comparative analysis of [3H]thymidine labeling in the embryonic tectum of the rhesus monkey (Macaca mulatta) and C57BL mouse

The pattern of [3H]thymidine incorporation in the dorsolateral wall of the embryonic tectum was studied and compared one hour after injection of the label in the rhesus monkey (Macaca mulatta) at stages 11-20 (25-37 days of gestation) and in the C57BL mouse at stages 14-22 (9-14 days of gestation). During the early stages of development, the labeled nuclei were located peripherally in the ventricular zone in both the rhesus monkey and mouse embryo, although a number of labeled nuclei tended to occur closer to the ventricular border in the mouse, whereas there was little or no encroachment at the ventricular border in the rhesus monkey. The ventricular zone of the rhesus monkey and mouse embryos initially showed a high labeling index (LI) of about 59% which subsequently declined with increasing age. However, the decline occurred earlier and more precipitously in the rhesus monkey. At stage 17 of the rhesus monkey the LI had dropped to about 42%, whereas it still remained at 59% in the 12-day mouse, and by stage 20 of the monkey the LI was approximately 26%, in contrast to 41% in the stage 22 (14-day) mouse. At stage 20 of the mouse (12 days of gestation) the intermediate zone became much thicker than in the comparable stage (17) of the rhesus monkey, and this discrepancy continued at each successive stage observed in the current study. Also, whereas lamination became apparent in the intermediate zone of the mouse at stage 22, the monkey tectum at a comparable stage (20) was poorly differentiated.

[Analysis of glioneuronal interrelationships after ultrasound exposure]

By means of quantitative cytochemical methods, changes in contents of protein amino groups on neurons and their surrounding glia have been studied in the rabbit midbrain tectum colliculi and in the cerebral visual cortex after a single and successive stimulation of the animal's eyes with impulsive ultrasound (880 kHz) at various intensity (0.2, 0.4-0.6, 1.0 and 2.0 W/cm2) and exposition for 5 min. The number of mantle gliocytes has been counted. It has been stated that certain metabolic manifestations in the glia overtake considerably (on an average 4-12 h) those in the neurons, and at the ultrasound intensity as great as 0.2 W/cm2 they are registered exclusively in the mantle gliocytes. It is probable that cells of the perineuronal glia, being an intermediate link between the neurons and capillaries, react selectively by changing their protein contents and, thus, perform a leading role in the reaction of the visual analyser to ultrasound stimulations. The ultrasound effect of therapeutic intensity or nearly so, while affecting metabolism of the neurons and neuroglia, does not change their interactions. It is possible that a prolonged active functional state of the neurons is not always ensured by increasing number of their satellite-cells but is realized by some other mechanisms.