Effect of alpha-bungarotoxin on retinotectal synaptic transmission in the goldfish and the toad

Intracellular and current source density analysis of pretectal input to the optic tectum of the frog

OBJECTIVE

Electrophysiological examination of the ipsilateral pretectotectal projection has proved that pretectal cells elicit strong suppressive responses to the ipsilateral tectum. However, the neural mechanisms underlying the contralateral pretectotectal prejection are still obscure. The present study aimed to examine the synaptic nature of pretectal nuclei and contralateral tectal cells, and to demonstrate the spatiotemporal pattern of neuronal activity in the 2 main brain structures.

METHODS

Intracellular recording and current source density (CSD) analysis were used to test the complexity of neuronal mechanism of pretectotectal information transfer.

RESULTS

The pretectal stimulation elicited only one type of response on the contralateral tectum, the inhibitory postsynaptic potential (IPSP). The majority of contra-induced IPSPs were assumed to be polysynaptically driven. In the CSD analysis, only one sink with short latency was observed in each profile. The ipsilateral projection produced a prominent monosynaptic sink in layer 8 of tectum. Recipient neurons were located in layers 6 and 7 of tectum. The result confirmed former findings from ipsilateral intracellular recordings.

CONCLUSION

These results suggest the following neuronal circuit: afferents from the pretectal nuclei broadly inhibit both tectal neuron, and since no second sink occurs in tectal layers, the pretectotectal excitatory afferents probably do not extend over the whole tectum, but are within limited state. The results of intracellular recording and CSD analysis further provide evidence of how pretectal afferent activity flows within the tectal laminae.

Current source density analysis of contra- and ipsilateral isthmotectal connections of the frog

The nucleus isthmi (NI) of the frog receives input from the ipsilateral optic tectum and projects back to both optic tecta. After ablation of NI, frogs display no visually elicited prey-catching or threat avoidance behavior. Neural mechanisms that underlie the loss of such important behavior have not been solved. Electrophysiological examination of the contralateral isthmotectal projection has proved that it contributes to binocular vision. On the other hand, there are very few physiological investigations of the ipsilateral isthmotectal projection. In this study, current source density (CSD) analysis was applied to contra- and ipsilateral isthmotectal projections. The contralateral projection produced monosynaptic sinks in superficial layers and in layer 8. The results confirmed former findings obtained by single unit recordings. The ipsilateral projection elicited a prominent monosynaptic sink in layer 8. Recipient neurons were located in layers 6–7. These results, combined with those from the former intracellular study, led to the following neuronal circuit. Afferents from the ipsilateral NI inhibit non-efferent pear shaped neurons in the superficial layers, and strongly excite large ganglionic neurons projecting to the descending motor regions. Thus feedback to the output neurons strengthens the visually elicited responses.

Intracellular and current source density analyses of somatosensory input to the optic tectum of the frog

This is the first report of current source density (CSD) and intracellular analyses of non-optic processing in the frog optic tectum. Sciatic nerve stimulation was used to test for somatosensory input to the optic tectum. To demonstrate the distribution of somatosensory input, field potentials were recorded from the whole surface of both tecta. Two components were observed. An early component was found in the whole area, but a late component was detected only in medial and caudal regions of the contralateral tectum. The effect of different stimulus intensity suggested that the optic tectum receives mainly the tactile sensation with fast conducting, low threshold level afferents from the sciatic nerve. The result of CSD analysis suggests that somatosensory afferents terminate on the tectal neurons with vertically expanding dendrites at the medial site of the contralateral optic tectum where the late component was found. Intracellular recordings demonstrated postsynaptic potentials in the middle and deeper layers, which is consistent with results from mammalian superior colliculus in earlier studies. Additional stimulation of the optic tract demonstrated that some somatosensory neurons had bimodal responses. The responses of those in the middle layers appeared to participate in avoidance behavior, based upon previous CSD analysis of the tectum using optic tract stimulation. All somatosensory responses elicited in these neurons were IPSPs. The findings imply that the somatosensory input to the optic tectum gives a suppressive effect on avoidance behavior. A somatosensory effect on prey-catching behavior could not be found in the present small number of intracellular data.

The nucleus isthmi and dual modulation of the receptive field of tectal neurons in non-mammals

The nucleus isthmi in the dorsolateral tegmentum had been one of the most obscure structures in the nonmammalian midbrain for eight decades. Recent studies have shown that this nucleus and its mammalian homologue, the parabigeminal nucleus, are all visual centers, which receive information from the ipsilateral tectum and project back either ipsilaterally or bilaterally depending on species, but not an auditory center as suggested before. On the other hand, the isthmotectal pathways exert dual, both excitatory and inhibitory, actions on tectal cells in amphibians and reptiles. In birds, the magnocellular and parvocellular subdivisions of this nucleus produce excitatory and inhibitory effects on tectal cells, respectively. The excitatory pathway is mediated by glutamatergic synapses with AMPA and NMDA receptors and/or cholinergic synapses with muscarinic receptors, whereas the inhibitory pathway is mediated by GABAergic synapses via GABA(A) receptors. Further studies have shown that the magnocellular and parvocellular subdivisions can differentially modulate the excitatory and inhibitory regions of the receptive field of tectal neurons, respectively. Both the positive and the negative feedback pathways may work together in a winner-take-all manner, so that the animal could attend to only one of several competing visual targets simultaneously present in the visual field. Some behavioral tests seem to be consistent with this hypothesis. The present review indicates that the tecto-isthmic system in birds is an excellent model for further studying tectal modulation and possibly winner-take-all mechanisms.

Influence of muscarinic ligands on the amplitudes of the evoked surface potential's late components in the optic tectum of the urodele Plethodon jordani

Recordings of field potentials from the tectal surface of an urodele amphibian were obtained in an in vitro preparation under influence of various muscarinic drugs. Bath applied acetylcholine (ACh) led to no change in the amplitudes or the shape of the evoked potentials. If the ACh-esterase blocker (-)-physostigmine was applied synchronously, the late components of the surface potential increased in amplitude. The non-selective cholinergic agonist carbachol showed a similar effect which was partially diminished by the nicotinic antagonist d-tubocurarine chloride (d-TC) and the muscarinic antagonist atropine sulfate. The application of the non-selective muscarinic agonist (+)-pilocarpine hydrochloride led to an increase of the late oligo- and polysynaptic events. This effect was reduced by the M(1)-antagonist pirenzepine dihydrochloride. The presented findings suggest that muscarinic receptors play a more important role in tectal processing than assumed in previous studies which emphasized the role of nicotinic receptors.

Current source density analysis of ON/OFF channels in the frog optic tectum

In the vertebrate retina, it is well known that an ON/OFF dichotomy is present. In other words, ON-center and OFF-center cells participate in segregated pathways morphologically and physiologically. However, there is no doubt that integration of both channels is necessary to generate the complicated response properties of visual neurons in higher optic centers. So far, functional organization of the ON and OFF channels in the optic centers has not been demonstrated at the level of neuronal populations. In this review article, we summarize our experimental approaches to demonstrate functional organization of the ON and OFF channels using current source density (CSD) analysis in the frog optic tectum. First, we show that one-dimensional CSD analysis, assuming constant conductivity, is applicable in the tectal laminated structure. The CSD depth profile of a response to electrical stimulation of the optic tract is composed of three current sinks (A, B, and D) in the retinorecipient layers and two current sinks (C and E) below those layers. This result is in agreement with previous morphological and physiological findings, and shows that CSD analysis is very useful to demonstrate the flow of visual information processing. Second, CSD analysis of tectal responses evoked by diffuse light ON and OFF stimuli reveals obviously different distributions of synaptic activity in the laminar structure. Two or three current sinks (I, II and III) are generated in response to ON stimulation only in the retinorecipient layers, while up to six current sinks (IV, V, VI, VII, VIII and IX) to OFF stimulation throughout the tectal layers. Based on well known properties of retinal ganglion cells of the frog, possible neuronal mechanisms underlying each current sinks and their functional roles in visually guided behavior are considered.

Effects of glutamatergic, cholinergic and gabaergic antagonists on tectal cells in toads

The present paper using microiontophoresis analysis describes transmitters and their receptor subtypes used in retinotectal and isthmotectal transmission, and suggests several modes converging retinotectal and isthmotectal afferents on tectal neurons in toads (Bufo bufo gargarizans). Neuronal responses of tectal cells were extracellularly recorded to both visual stimulation and electrical stimulation of the nucleus isthmi, and effects of glutamatergic, cholinergic, GABAergic and glycinergic antagonists on these responses examined. Visual responses in 80% of tectal cells were reversibly blocked by the N-methyl-D-aspartate antagonist 3-Rs-2-carboxypiperazin-4-yl-propyl-1-phosphonic acid, and those of the remaining 20% of cells by the muscarinic antagonist atropine, suggesting that there may be at least two kinds of retinotectal synapse that use glutamate and N-methyl-D-aspartate receptors, and acetylcholine and muscarinic receptors, respectively. Electrical stimulation of the nucleus isthmi elicited excitatory responses in 67% of tectal cells, excitatory-inhibitory responses in 16% of cells, and inhibitory responses in 17% of cells examined. The excitatory responses were reversibly abolished by atropine, but not affected by either 3-Rs-2-carboxypiperazin-4-yl-propyl-1-phosphonic acid or the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, whereas the inhibitory responses were released by the GABA receptor A antagonist bicuculline, but not influenced by the GABA receptor B antagonist 2-hydroxysaclofen and glycinergic antagonist strychnine. Excitatory and inhibitory components in the excitatory-inhibitory responses were blocked by atropine and bicuculline, respectively. It appears that glutamatergic and cholinergic afferents from the retina, and cholinergic and GABAergic afferents from the nucleus isthmi may converge on tectal neurons in at least five modes of synaptic connections, in agreement with the heterogeneous populations of tectal cells in amphibians.

ON and OFF channels of the frog optic tectum revealed by current source density analysis

The spatiotemporal patterns of excitatory synaptic activity in response to diffuse lightON and OFF stimuli were examined by means of current source density (CSD) analysis. The qualitative and quantitative analyses obtained from 24 depth profiles for each stimulus revealed obviously different distributions of synaptic activity in the laminar structure. Two or three dominant current sinks I, II, and III were evoked in response to diffuse light ON stimulation. Sink I was observed at the bottom of the retinorecipient layer. Both sinks II and III, showing an identical spatial pattern, were observed just above sink I. On the other hand, diffuse light OFF stimulation elicited up to six current sinks IV, V, VI, VII, VIII, and IX. Sink IV was observed at the bottom of the retinorecipient layer. Sink V was observed in the most superficial layer. Both sinks VI and VIII were located between the two preceding sinks. Finally, sinks VII and IX occurred below the retinorecipient layer. Five electrically evoked current sinks A, B, C, D, and E, characterized in our previous study, were also recognized in the present quantitative analysis. A statistical analysis revealed that, in visually evoked responses, statistical differences in the spatial distribution were not present between sinks I and IV, and sinks II and VIII (P < 0.05). The analysis also showed that, in electrically evoked responses, only a pair of sinks C and E exhibit virtually identical spatial distribution (P < 0.05). Based on well-known properties of the retinal ganglion cells, possible neuronal mechanisms underlying each of current sinks in the ON and OFF channels and their functional meanings were considered. Sink I reflects the excitatory monosynaptic activity derived from R3 retinal ganglion cells. Sink IV reflects the excitatory monosynaptic activity derived from both R3 and R4 cells. Sinks V, VI, VII, and IX may be composed of successive polysynaptic excitatory potentials derived from convergence of inputs from both R3 and R4 cells. We concluded that the early four sinks play in particular an important role in eliciting avoidance behavior. On the other hand, sinks II, III, and VIII reflect excitatory synaptic activities derived from - retinal fibers of another type having slow conduction velocity. These late current sinks were suggested to mediate prey catching and its facilitation.

Principal neuronal organization in the frog optic tectum revealed by a current source density analysis

In the frog optic tectum, the spatiotemporal pattern of neuronal activity evoked by electrical stimulation of the optic tract was examined by means of a current source density (CSD) analysis. The CSD depth profile was highly reproducible in different experiments. In all seven CSD profiles, three current sinks A, B, and D were observed in the retinorecipient layers. Four out of the seven profiles show additional two sinks C and E below the retinorecipient layers. Very small and short lasting sinks related to afferent fiber activities precede sinks A and B by about 1 ms, which could be accounted for by monosynaptic delay, in the corresponding depth region. The earliest prominent sink A at the bottom of the retinorecipient layers reflects only excitatory monosynaptic activities derived from R3 and/or R4 retinal ganglion cells. The second prominent sink B in the superficial retinorecipient layer is composed partly of excitatory monosynaptic activity from medium-sized myelinated optic fibers. It may involve excitatory monosynaptic activity from unmyelinated optic fibers and further polysynaptic activity. The fourth prominent sink D in the intermediate retinorecipient layer partially reflects excitatory monosynaptic activity derived from unmyelinated optic fibers. It may also involve further polysynaptic activity. In contrast with these three sinks, the third prominent sink C and fifth sink E exclusively reflect intratectal polysynaptic activity that has not been reported in any previous CSD studies in the frog optic tectum. These sinks almost overlap spatially in the tectal layer. We also measured the intratectal resistance changes and computed inhomogeneous CSD depth profiles to show that the results from homogeneous CSD computation assuming constant conductivity are valid for our present study. Finally, we compared the present results with previously reported CSD studies on the frog optic tectum and discuss consistencies and discrepancies among these experiments.

Rapid activity-dependent sprouting of optic fibers into a local area denervated by application of beta-bungarotoxin in goldfish tectum

The retinotectal projection is known to be capable of extensive long-term expansion of connections, but it is not known how fast such changes can occur or what triggers sprouting of terminals. We studied sprouting of optic fibers into an area denervated by local microinjection of beta-bungarotoxin (beta-BTX), a specific presynaptic neurotoxin with phospholipase A2 activity that destroys nerve terminals at the neuromuscular junction. After injection of 0.1 pmol of beta-BTX, the optic terminals fired spontaneously with decreasing amplitude and became silent within 1 to 2 h. Outside the injection zone, the retinotectal map was normal, so the silent zone was associated with a scotoma in the visual field. Horseradish peroxidase (HRP) staining of the entire optic nerve showed a denervated region at the injection site with beaded, degenerating fibers at its edge. Between 3 and 9 days later, optic units were recorded within the injection zone whose receptive fields lay just outside the scotoma in the visual field, indicating that intact surrounding terminals had sprouted into the area. These sprouts made functional connections, as indicated by field potential recordings and current source-density analysis. At this time, HRP staining also demonstrated retinal innervation within the injection zone. By 12 days, normal maps with no scotoma were recorded and HRP staining was normal at the injection site, indicating that the beta-BTX-damaged fibers had regenerated to reclaim their tectal sites. The results show that the retinotectal projection of goldfish is very dynamic, since intact optic fibers can sprout into adjacent vacant postsynaptic territory within 2 to 3 days, much faster than previously reported. In a final experiment, we showed that this sprouting is activity-dependent, since it could be prevented by blocking retinal activity with intraocular tetrodotoxin (TTX) during the first 2 days postinjection, even though TTX block of activity does not block regeneration in this system. One possible mechanism for this rapidly triggered sprouting is that arachidonic acid liberated by beta-BTX acts as a sprouting factor to attract surrounding healthy fibers into the denervated region but requires activity at the terminals to be effective.

Distribution of [3H]QNB and [125I]alpha-bungarotoxin binding and acetylcholinesterase activity in visual system and hippocampal structures of eleven mammalian species

This study assessed interspecies differences in regional brain distribution of [3H]QNB binding, [125I]alpha-bungarotoxin binding and acetylcholinesterase activity, by autoradiographic and histochemical methods. Eleven mammalian species were examined, including carnivores (cat, dog), a lagomorph (rabbit), and rodents (squirrel, guinea pig, gerbil, hamster, vole, lemming, rat, mouse). Comparisons were based on primary visual system structures (superior colliculus, lateral geniculate nucleus, primary visual cortex) and the hippocampal formation. The two radioligands differed greatly in the degree of interspecies variation: while the pattern of [3H]QNB binding was quite similar across species, [125I]alpha-bungarotoxin showed striking interspecies diversity. This contrast was most obvious in laminar patterns of the visual cortex and hippocampal formation. Regional distributions of acetylcholinesterase staining were fairly diverse, and were unlike the patterns of either [3H]QNB or [125I]alpha-bungarotoxin. The two ligands showed more consistency in overall levels across species than did acetylcholinesterase. Possible correlates of the differences in interspecies diversity are discussed.

Analysis of cyclic and acyclic nicotinic cholinergic agonists using radioligand binding, single channel recording, and nuclear magnetic resonance spectroscopy

The relationship between the structure and function of a series of nicotinic cholinergic agonists has been studied using radioligand binding, single channel recording, and nuclear magnetic resonance spectroscopy. The cyclic compound 1,1-dimethyl-4-acetylpiperazinium iodide and its trifluoromethyl analogue (F3-PIP) interact with nicotinic acetylcholine receptors (nAChRs) from both Torpedo electroplaque and BC3H-1 cells at lower concentrations than the acyclic derivatives, N,N,N,N'-tetramethyl-N'-acetylethylenediamine iodide and its fluorinated analogue (F3-TED). The magnitude of the difference in potencies depends on the type of measurement. In binding experiments, the differences between the two classes of compounds depends mainly on the conditions of the experiment. In measurements of the initial interaction with the nAChR, the PIP compounds have an affinity approximately one order of magnitude higher than that of the TED compounds. Longer incubations indicated that the PIP compounds were able to induce a time-dependent shift in receptor affinity consistent with desensitization, whereas the TED compounds were unable to induce such a shift. The activation of single channel currents by the cyclic compounds occurs at concentrations approximately two orders of magnitude lower than for the acyclic compounds, but the TED compounds exhibit a larger degree of channel blockade than the PIP compounds. Previous work (McGroddy, K.A., and R.E. Oswald. 1992. Biophys. J. 64:314-324) has shown that the TED compounds can exist in two energetically distinct conformational states related by an isomerization of the amide bond. 19F nuclear magnetic resonance experiments suggest that the higher energy population of the TED compounds may interact preferentially with the ACh binding sites on the nAChRs and that a significant fraction of the difference between the initial affinity of the PIP and TED compounds may be accounted for by the predominance in solution of a conformational state less able to interact with the ACh binding sites on nAChRs.

Adult rat barrel cortex plasticity occurs at 1 week but not at 1 day after vibrissectomy as demonstrated by the 2-deoxyglucose method

Stimulation of a single facial vibrissa in rats receiving [14C]2-deoxyglucose leads to increased local cerebral glucose utilization in the corresponding contralateral barrel of lamina IV of the first somatosensory cortex (SmI). In the adult rat, the metabolic representation of such a barrel enlarges 2 months after removal of all other vibrissal follicles but enlargement is prevented by prior removal of SmI norepinephrine. Here, the early time course of such enlargement and how this was affected by cortical norepinephrine manipulations were examined in adult rats. One day after total vibrissal follicle removal with sparing of the central (C3) vibrissa, neither the areal extent nor absolute glucose utilization in the stimulated, spared C3 cortical barrel were changed. However, 7 days after follicle removal, the spared C3 barrel was enlarged by 41%, although absolute glucose utilization remained constant. This delayed onset of enlargement is compatible with either a structural or neurochemical change in barrel circuitry following vibrissal deafferentation. With ipsilateral locus coeruleus lesions but intact vibrissae, there was progressive enlargement of stimulated C3 barrel areas with increasing cortical norepinephrine depletion (r = 0.864) suggesting a suppressive effect of norepinephrine on activity spread in barrels with intact vibrissal afferents. Previously shown blockade of chronic (2 month) vibrissectomy-induced barrel enlargement by norephinephrine depletion suggested an additional effect on plasticity. Even though acute (1 day) follicle removal here produced no change in spared C3 barrel area, addition of norepinephrine depletion produced a surprising 40% decrease in barrel area. Thus, barrel plasticity assessed by 2-deoxyglucose reflects a complex interaction between barrel metabolic activity and the extent of vibrissal and noradrenergic afferent input.

Activity-dependent plasticity in the visual systems of frogs and fish

The retinotectal system of lower vertebrates has provided considerable insight into the cellular mechanisms underlying the development and maintenance of orderly visual projections to the brain. This review will briefly summarize some of the data on the activity-dependent components of these mechanisms and incorporate the data into a model for selective synapse stabilization of coactive synapses. The model, based on the Hebbian synapse, is similar to models of long-term potentiation (LTP) of synaptic transmission, which are thought to account for the increased synaptic efficacy observed after associative conditioning paradigms. However, more recent data from two studies, one using confocal microscope analysis of migrating retinal arbors in vivo and the other investigating the requirement for protein kinase activity in map formation, point to a possible divergence in the cellular events underlying synapse stabilization in the developing visual system of the frog and LTP in the mammalian hippocampus.

The long latency component of retinotectal transmission: enhancement by stimulation of nucleus isthmi or tectobulbar tract and block by nicotinic cholinergic antagonists

The optic tectum of teleosts contains high concentrations of nicotinic and muscarinic acetylcholine receptors and receives putative cholinergic innervation from both nucleus isthmi in the tegmentum and a population of intrinsic tectal cells. Using in vitro preparations of goldfish brain, we have examined the effects of cholinergic antagonists and stimulation of nucleus isthmi on the tectal response to optic nerve stimulation. Our results show that: (1) a long latency component of the retinotectal field potential is polysynaptic in origin and occurs in isolated tectum; (2) this component can spread across the tectum from a beam of stimulated fibers and can appear in areas where the monosynaptic response is small or absent; (3) both monosynaptic and long latency components of the field potential are enhanced by prior stimulation of nucleus isthmi or the tectobulbar tract (15-300 ms); (4) both the long latency component of the field potential and the effects of stimulation of nucleus isthmic or tectobulbar tract are blocked by low concentrations of nicotinic antagonists; and (5) in deeper tectum a second polysynaptic response uncovered by pharmacological block of inhibition is not blocked by nicotinic antagonists. These results indicate that the cholinergic neurons intrinsic to tectum have a role in the spread of retinotectal excitation by nicotinic actions, and that stimulation of nucleus isthmi or tectobulbar tract facilitates activity in this system. There is in addition a separate recurrent excitatory circuit in tectum.

Pharmacologic evidence for NMDA, APB and kainate/quisqualate retinotectal transmission in the isolated whole tectum of goldfish

The optic tectum of goldfish with intact optic and toral marginal fiber tracts was isolated in a perfusion chamber where the effectiveness of antagonists was tested on synaptic field potential responses to stimulation of each afferent system. There were 3 main conclusions about excitatory synapses. First, monosynaptic activation of retinotectal synapses was not detectably antagonized by D-tubocurarine, implying there is no nicotinic cholinergic component to optic transmission nor strong cholinergic gating of optic terminals. Second, a significant component of retinotectal transmission was shown to be mediated by kainate and quisqualate receptors since 6,7-dinitroquinoxaline-2,3-dione and kynurenate strongly suppressed the optic field potential. In addition, activation of these synapses involves two previously undescribed N-methyl-D-aspartate (NMDA) and APB receptor subtypes since optic field potentials were partially suppressed by 2-amino-5-phosphonovalerate (APV), 2-amino-4-phosphonobutyrate (APB) and MK-801. This is the first evidence that APB receptors exist in the visual system central to the retina. Together, these results are consistent with the possibility that retinal ganglion cells use multiple glutamate receptor subtypes. Third, the optic tectum contains a population of intrinsic glutaminergic synapses activated by a non-optic input, the marginal fibers, which can be suppressed by both APV and kynurenate. The existence of tectal NMDA receptors which are not at primary optic synapses implies that APV used to interfere with rearrangement of optic fibers during development may act not only at afferent synapses but also at a more central component of the circuit.

Ultrastructure of the crossed isthmotectal projection in Xenopus frogs

The nucleus isthmi (NI) of frogs is a relay for input from the eye to the ipsilateral tectum; each NI receives retinotopic input from one tectum and sends retinotopic output to both tecta. The crossed isthmotectal projection in Xenopus displays tremendous plasticity during development. Physiological and anatomical studies have suggested that the location at which a developing isthmotectal axon will terminate is determined by the correlation of its visually evoked activity with the activity of nearby retinotectal terminals. What structures could mediate such communication? We have examined quantitatively the ultrastructural characteristics of crossed isthmotectal axons and synapses in order to determine whether retinotectal axons communicate directly with isthmotectal axons via axo-axonic synapses or whether the communication is indirect, e.g., via common postsynaptic dendrites. Our results support the conclusion that isthmotectal axons interact with retinotectal axons indirectly and that tectal cell dendrites are the critical site of interaction.

Nucleus isthmi: its contribution to tectal acetylcholinesterase and choline acetyltransferase in the frog Rana pipiens

The distribution of acetylcholinesterase and the activity of choline acetyltransferase was studied in the tecta of normal frogs and frogs without retinal and/or nucleus (n.) isthmi inputs. In normal animals acetylcholinesterase activity is found primarily in three bands in the outer layers of the tectum-lamina A, laminae C-F, and lamina G. After retinal and contralateral n. isthmi deafferentation three distinct bands of tectal acetylcholinesterase activity are still present. After bilateral n. isthmi deafferentation there is loss of activity in lamina G and reduced activity in lamina A. With retinal and ipsilateral n. isthmi deafferentation, activity is seen only in lamina A. With retinal and bilateral n. isthmi deafferentation there is virtually no acetylcholinesterase activity in the outer tectal layers. Following unilateral retinal deafferentation there is no statistically significant difference in choline acetyltransferase specific activity between intact and deafferented tectal lobes after two, four and nine weeks. With unilateral nucleus isthmi lesions and survival times of between 10 and 40 days, choline acetyltransferase specific activity in the tectal lobe ipsilateral to the ablation is approximately 38% of the specific activity of the contralateral lobe. With bilateral n. isthmi lesions there is a strong correlation between amount of n. isthmi ablated and reduction of choline acetyltransferase activity. In vitro tectal acetylcholine synthesis was also determined in animals with unilateral n. isthmi ablation. On average, tectal lobes ipsilateral to the ablated n. isthmi synthesize acetylcholine at a rate which is approximately 58% of that of contralateral tecta. Collectively, these results imply that n. isthmi is the sole cholinergic input to the frog optic tectum, with ipsilaterally projecting isthmotectal fibers accounting for the greater share.

Actions of the insecticide 2(nitromethylene)tetrahydro-1,3-thiazine on insect and vertebrate nicotinic acetylcholine receptors

The nitromethylene heterocyclic compound 2(nitromethylene)tetrahydro)1,3-thiazine (NMTHT) inhibits the binding of [125I]alpha-bungarotoxin to membranes prepared from cockroach (Periplaneta americana) nerve cord and fish (Torpedo californica) electric organ. Electrophysiological studies on the cockroach fast coxal depressor motorneuron (Df) reveal a dose-dependent depolarization in response to bath-applied NMTHT. Responses to ionophoretic application of NMTHT onto the cell-body membrane of motorneuron Df are suppressed by bath-applied mecamylamine (1.0 x 10(-4) M) and alpha-bungarotoxin (1.0 x 10(-7) M). These findings, together with the detection of a reversal potential close to that estimated for acetylcholine, provide evidence for an agonist action of this nitromethylene on an insect neuronal nicotinic acetylcholine receptor. The binding of [3H]H12-histrionicotoxin to Torpedo membranes was enhanced in the presence of NMTHT indicating an agonist action at this vertebrate peripheral nicotinic acetylcholine receptor. NMTHT is ineffective in radioligand binding assays for rat brain GABAA receptors, rat brain L-glutamate receptors and insect (Musca domestica) L-glutamate receptors. Partial block of rat brain muscarinic acetylcholine receptors is detected at millimolar concentrations of NMTHT. Thus nitromethylenes appear to exhibit selectivity for acetylcholine receptors and exhibit an agonist action at nicotinic acetylcholine receptors.

Kappa-bungarotoxin blockade of nicotine electrophysiological actions in cerebellar Purkinje neurons

The agonistic actions of nicotine in cerebellum were selectively blocked by kappa-bungarotoxin depending on the cell type studied. Nicotine-induced Purkinje cell inhibitions were antagonized by the simultaneous application of this toxin. In contrast, nicotine-induced cerebellar interneuron excitations were unaltered. These findings suggest that kappa-bungarotoxin may be used as a selective pharmacological tool for the study of nicotine actions which are dependent on ganglionic-like receptors, which have been associated with Purkinje cells in previous studies.

Identification of a novel nicotinic acetylcholine receptor structural subunit expressed in goldfish retina

A new non-alpha (n alpha) member of the nicotinic acetylcholine receptor (nAChR) gene family designated GFn alpha-2 has been identified in goldfish retina by cDNA cloning. This cDNA clone encodes a protein with structural features common to all nAChR subunits sequenced to date; however, unlike all known alpha-subunits of the receptor, it lacks the cysteine residues believed to be involved in acetylcholine binding. Northern blot analysis shows multiple transcripts hybridizing to the GFn alpha-2 cDNA in goldfish retina but undetectable levels of hybridizable RNA in brain, muscle, or liver. S1 nuclease protection experiments indicate that multiple mRNAs are expressed in retina with regions identical or very similar to the GFn alpha-2 sequence. In situ hybridization shows that the gene encoding GFn alpha-2 is expressed predominantly in the ganglion cell layer of the retina.

Protein phosphorylation of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAcChR) is a ligand-gated ion channel found in the postsynaptic membranes of electric organs, at the neuromuscular junction, and at nicotinic cholinergic synapses of the mammalian central and peripheral nervous system. The nAcChR from Torpedo electric organ and mammalian muscle is the most well-characterized neurotransmitter receptor in biology. It has been shown to be comprised of five homologous (two identicle) protein subunits (alpha 2 beta gamma delta) that form both the ion channel and the neurotransmitter receptor. The nAcChR has been purified and reconstituted into lipid vesicles with retention of ion channel function and the primary structure of all four protein subunits has been determined. Protein phosphorylation is a major posttranslational modification known to regulate protein function. The Torpedo nAcChR was first shown to be regulated by phosphorylation by the discovery that postsynaptic membranes contain protein kinases that phosphorylate the nAcChR. Phosphorylation of the nAcChR has since been shown to be regulated by the cAMP-dependent protein kinase, protein kinase C, and a tyrosine-specific protein kinase. Phosphorylation of the nAcChR by cAMP-dependent protein kinase has been shown to increase the rate of nAcChR desensitization, the process by which the nAcChR becomes inactivated in the continued presence of agonist. In cultured muscle cells, phosphorylation of the nAcChR has been shown to be regulated by cAMP-dependent protein kinase, a Ca2+-sensitive protein kinase, and a tyrosine-specific protein kinase. Stimulation of the cAMP-dependent protein kinase in muscle also increases the rate of nAcChR desensitization and correlates well with the increase in nAcChR phosphorylation. The AcChR represents a model system for how receptors and ion channels are regulated by second messengers and protein phosphorylation.

The alpha-bungarotoxin receptor purified from a human neuroblastoma cell line: biochemical and immunological characterization

The pharmacological and electrophysiological characteristics of the alpha-bungarotoxin receptor present on the human neuroblastoma cell line IMR-32 indicate that this receptor is not associated with an acetylcholine-operated ionic channel. In this paper we report its biochemical purification and immunological characterization. This molecule has a standard sedimentation coefficient of 10S and sodium dodecyl-sulphate gel electrophoresis shows that it is made up of three polypeptide chains of molecular weights of 67,000, 60,000 and 52,000. Ligand binding to blots of purified receptor revealed that only the polypeptide of molecular weight 52,000 is bound by [125I]alpha-bungarotoxin. The purified alpha-bungarotoxin receptor was bound by polyclonal antibodies raised against purified fetal calf, Torpedo and chick optic lobe nicotinic receptors and by the sera of myasthenic patients. Furthermore, despite the fact that a number of different immunological techniques were used, it was impossible to label this alpha-bungarotoxin receptor with mAb 35, a monoclonal antibody which binds some neuronal nicotinic receptors. Rabbit antisera against the purified alpha-bungarotoxin receptor were used to compare this protein with other known nicotinic receptors and, once again, it was demonstrated that there is some immunological cross-reactivity between the alpha-bungarotoxin receptor present on neuroblastoma cells and Torpedo, fetal calf and chick optic lobe nicotinic receptors. All these immunological data, together with previously published pharmacological and molecular biology data, demonstrate that the alpha-bungarotoxin receptor present in nerve cells is neither a muscular nor a neuronal nicotinic receptor, although it has similarities with both.

Binding pattern of alpha-bungarotoxin on horizontal cells of a marine teleost retina

A conjugate of alpha-bungarotoxin and a fluorescent marker (fluorescein isothiocyanate) has been used to localize "nicotinic" acetylcholine receptors on neurons in the outer plexiform layer of marine teleost retina. Toxin binding was confined to bipolar cell dendrites and to intermediate horizontal cells. The arrangement of labeled horizontal cells appears irregular in the whole retina, with a peak density in the ventral and dorsal quandrants. Alpha-bungarotoxin receptors on horizontal cells differ from those on bipolar cells and from those on dendrites in the inner plexiform layer in their sensitivity to agonists and antagonists such as d-tubocurarine and nicotine. They constitute a different type of "nicotinic" receptor that probably has a different function.

Regulation of nicotinic acetylcholine receptors by protein phosphorylation

Neurotransmitter receptors and ion channels play a critical role in the transduction of signals at chemical synapses. The modulation of neurotransmitter receptor and ion channel function by protein phosphorylation is one of the major regulatory mechanisms in the control of synaptic transmission. The nicotinic acetylcholine receptor (nAcChR) has provided an excellent model system in which to study the modulation of neurotransmitter receptors and ion channels by protein phosphorylation since the structure and function of this receptor have been so extensively characterized. In this article, the structure of the nAcChR from the electric organ of electric fish, skeletal muscle, and the central and peripheral nervous system will be briefly reviewed. Emphasis will be placed on the regulation of the phosphorylation of nAcChR by second messengers and by neurotransmitters and hormones. In addition, recent studies on the functional modulation of nicotinic receptors by protein phosphorylation will be reviewed.

kappa-Bungarotoxin: binding of a neuronal nicotinic receptor antagonist to chick optic lobe and skeletal muscle

kappa-Bungarotoxin, a snake venom kappa-neurotoxin, is a potent neuronal nicotinic receptor antagonist. kappa-Neurotoxins are structurally related to the long-type alpha-neurotoxins (including alpha-bungarotoxin), which often fail to block neuronal nicotinic transmission, but which are potent antagonists of nicotinic receptors found on vertebrate skeletal muscle. The binding of kappa-bungarotoxin has now been examined in homogenates of chick skeletal muscle and optic lobe. In muscle, kappa-bungarotoxin binds to nicotinic receptors with 200-fold lower affinity than does alpha-bungarotoxin. The weakest known alpha-neurotoxin, L.s. III, is found to be 6.5-fold more potent than kappa-bungarotoxin. These findings support the conclusion that kappa-neurotoxins are selective for neuronal nicotinic receptors. In the optic lobe, 125I-alpha-bungarotoxin and 125I-L.s. III. A second nicotinic site, detected with high affinity by both alpha-neurotoxins, is only weakly bound by kappa-bungarotoxin. No evidence for a unique 125I-kappa-neurotoxin site is observed. Furthermore, kappa-bungarotoxin does not recognize the high affinity L-[3H]nicotine binding site in chick optic lobe which is distinct from the alpha-neurotoxin binding sites. Three subtypes of nicotinic sites can thus be defined in chick optic lobe, although which of these subtypes is involved in nicotinic transmission in the lobe remains to be conclusively determined.

Epileptic discharges induced by intermittent light stimulation in photosensitive baboons: a current source density study

The current source density (CSD) method was applied to the study of paroxysmal discharges (PDs) induced by intermittent light stimulation (ILS) in Papio papio baboons made photosensitive by a subconvulsant dose of allylglycine. CSD was studied in the motor and premotor areas (4 and 6). Laminar profiles of sinks and sources are similar in both areas. Nevertheless, the motor area seems to become involved first since it shows the earliest and most prominent sink in layer III. Such a sink, correlated with the PD spike, moves progressively upward to the cortical surface. The localization and other experimental arguments obtained by the same method suggest that this sink could be mainly of dendritic origin. The cortico-cortical afferents to the superficial layers of the motor area might thus determine the generation of this sink. A smaller sink, detected at the same latency between layers V and VI could correspond to synaptic activations due to thalamo-cortical afferents probably arriving on the pyramidal cells which project to the spinal cord. Intense sinks correlated with the PD wave in layer V could be passive, due to active sources lying just above and/or below, because in previous studies an inhibition of the cellular discharges was always observed in correlation with the wave. It is suggested that ILS triggered PDs involve visual cortico-cortical afferents directed mainly to the superficial layers of the motor area provoking an intense synaptic activation of the cellular elements situated at this level.

The development and distribution of alpha-bungarotoxin binding sites in rat tectal transplants

The development and distribution of alpha-bungarotoxin (alpha-BTX) binding sites in tectal grafts was examined autoradiographically using the radioligand [125I]-alpha-BTX. High alpha-BTX binding was observed in localized areas within grafts; these areas corresponded to regions which contained high acetylcholinesterase activity and received retinal input. Receptor differentiation also occurred in the absence of specific host afferents. The graft data show that, as in normal superior colliculus, development of high alpha-BTX binding is limited to areas containing presumptive superficial layer cells.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

Cholinergic innervation of the optic tectum in the frog Rana pipiens

An immunohistochemical method for choline acetyltransferase (ChAT) identifies presumably cholinergic axons in two retino-receptive laminae in the optic tectum of the frog Rana pipiens. Following eye enucleation there is no loss of immunoreactive axons in the optic tectum. Following unilateral ablation of the nucleus isthmi there is a near-total loss of ChAT-positive axons in the superficial cholinergic lamina contralaterally and in the deeper cholinergic lamina ipsilaterally. Thus, the cholinergic innervation of the tectum appears to derive from the nucleus isthmi. However, ChAT-positive staining of the basal optic nucleus does depend upon an intact retinal input and could derive from either retinal axons or some system trophically dependent on them.

Lophotoxin: selective blockade of nicotinic transmission in autonomic ganglia by a coral neurotoxin

Lophotoxin is a diterpene lactone isolated from gorgonian corals. The toxin has previously been shown to bind with high affinity to an acetylcholine recognition site located on skeletal muscle nicotinic receptors, producing an essentially irreversible blockade of neuromuscular transmission. Lophotoxin has also been shown to block nicotinic transmission in autonomic ganglia of the frog and in ileal strips of guinea pig and rabbit. The effects of lophotoxin have now been examined on neuronal nicotinic receptors in autonomic ganglia of the chick and rat. Low concentrations of lophotoxin (1 microM) produce a blockade of neuronal nicotinic transmission which is partially reversed by 3-5 h of washing out the toxin. The blockade produced by higher concentrations of lophotoxin (up to 32 microM) is not reversed during a similar washout period. Prior exposure to d-tubocurarine, a competitive nicotinic antagonist, can partially protect ganglia against exposure to lophotoxin. In contrast the local anesthetic QX-314, a noncompetitive nicotinic antagonist, does not protect ganglia against lophotoxin exposure. Lophotoxin binds to a site in ganglia identified by [125I]kappa-bungarotoxin which appears to be on the neuronal nicotinic receptor. Intracellular recordings reveal that lophotoxin has no effect on either muscarinic responses or on responses to gamma-aminobutyrate in autonomic ganglia. Passive and active membrane properties of the neurons are unaffected by lophotoxin except for the blockade of nicotinic responses. It is concluded that lophotoxin is a selective, high-affinity antagonist at the neuronal nicotinic receptor. The long-term nature of the blockade with lophotoxin suggests that the toxin will be of considerable value as a probe for characterizing the ganglionic nicotinic receptor.

Acetylcholine receptor synthesis in retina and transport to optic tectum in goldfish

Previous studies have suggested that the retinotectal system of the goldfish contains a nicotinic acetylcholine receptor (nAChR) that is sensitive to alpha-bungarotoxin. Extracellularly recorded field potentials elicited in response to visual stimulation can be blocked by alpha-bungarotoxin, and alpha-bungarotoxin can interfere with the maintenance of retinotectal synaptic connections. Whether the transmission between the retinal ganglion cells and the tectal cells is mediated by acetylcholine and whether nAChR's exist on the dendrites of tectal cells are questions that remain. The experiments described in this report were designed to determine the site of synthesis of the nAChR's associated with the goldfish retinotectal projection. Radioactive (35S-labeled) methionine was injected into either the eye or the tectal ventricle, and the incorporation of radioactivity into the nAChR was measured by immunoprecipitation. The use of this technique provides evidence that an nAChR associated with the goldfish retinotectal projection is synthesized in the retina and transported to the optic tectum, which suggests a presynaptic site of acetylcholine action on retinal terminals.

Alpha-bungarotoxin binding to a high molecular weight component from lower vertebrate brain identified on dodecyl sulfate protein-blots

The binding of [125I]iodo-alpha-bungarotoxin [( 125]alpha-BuTX) to the dissociated alpha-subunit of Torpedo acetylcholine receptor (AChR) can be readily demonstrated in a modified 'protein-blot' analysis utilizing electrophoretically transferred, dissociated subunits immobilized onto positively charged nylon membranes which are then incubated directly with [125I]alpha-BuTX. We report here the use of the protein-blotting technique to detect the alpha-BuTX binding site present in the central nervous system of lower vertebrates and to characterize some of the physicochemical properties of the toxin binding site. High molecular weight (Mr greater than or equal to 200,000 and greater than or equal to 120,000) alpha-BuTX-binding components can be readily demonstrated in avian and fish brain extracts upon protein-blotting with [125I]alpha-BuTX following lithium dodecyl sulfate PAGE. Neither extensive reduction with dithiothreitol nor prior reduction followed by alkylation with iodoacetamide alter the mobility of the CNS-derived BuTX-binding sites. In contrast to our findings with Torpedo AChR or muscle AChR derived from a number of different species, no binding is observed in the molecular weight range of the alpha-subunit (Mr = 40,000) nor is any binding at any molecular weight observed in similar fractions prepared from adult, mammalian (rat, guinea pig) brain using this technique. These results demonstrate the existence in lower vertebrate brain of a BuTX binding site comparable in size to the AChR oligomeric complex of electric organ and muscle. They also suggest, however, striking structural differences between muscle AChR and the central neuronal BuTX-binding complex as well as a considerable difference between the neuronal BuTX-binding sites derived from lower and higher vertebrate brain.

Effect of enucleation on choline acetyltransferase activity in layers of goldfish optic tectum

Choline acetyltransferase (ChAT) activity was determined in layers of optic tectum in control goldfish and in goldfish 4-20 days following unilateral enucleation. Significant changes in activity were found in the periventricular (PV) and superficial gray and white (SGW) layers. Within 4 days, ChAT activity in the PV layer on the lesioned side was about 75% of that on the control side. By 20 days, ChAT specific activity in the SGW layer on the lesioned side was about 150-160% of that on the control side. This increase in specific activity in the SGW layer was accounted for by the decrease in volume and in density of the layer after enucleation, so that the total amount of activity in the layer did not change significantly, indicating that the optic terminals contain little to no ChAT activity. ChAT activity in the optic tract was very low and did not decrease after enucleation. These data strongly indicate that the retinotectal pathway in goldfish is not cholinergic and, therefore, that the ChAT activity in the SGW layer is related to sources other than retinal ganglion cells. It is suggested that one such source might be neurons with somata in the PV layer.

Choline acetyltransferase immunoreactivity suggests that ganglion cells in the goldfish retina are not cholinergic

Published evidence that ganglion cells in the retinae of nonmammalian species are cholinergic is strong but indirect. In this paper we report results of attempts to demonstrate choline acetyltransferase immunoreactivity in ganglion cells of goldfish retina using two different antisera against choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme. We obtained ChAT-immunoreactive staining of amacrine and displaced amacrine cells in the retina and type XIV cells in the tectum, but we obtained no direct immunocytochemical evidence that ganglion cells in the goldfish retina are cholinergic. Thus, ganglion cells identified by retrograde transport of propidium iodide were never ChAT-immunoreactive. Intraocular injections of colchicine did not result in the appearance of a population of ChAT-immunoreactive neurons in the ganglion cell layer. ChAT-immunoreactive axons were not observed in intact, ligated, or transected optic nerves. And finally, the ChAT immunoreactivity of cells and fibers in the optic tectum was unaffected by deafferentation. These experiments provide no positive evidence that any ganglion cells in goldfish retina contain the acetylcholine-synthesizing enzyme, ChAT. While it is possible that our method is too insensitive to detect the enzyme in ganglion cell somata or too specific to recognize the form of ChAT present in these cells, the fact that we can stain putatively cholinergic retinal amacrine cells and tectal neurons makes these alternative explanations improbable. We conclude that it is unlikely that any of the ganglion cells in the retina are cholinergic and that alternative explanations should be sought for previously published results that suggest that they are.

Interaction of monoclonal antibodies to electroplaque acetylcholine receptors with the alpha-bungarotoxin binding site of goldfish brain

Polyclonal and monoclonal antibodies raised against acetylcholine receptors from Torpedo californica and Electrophorus electricus electroplaque were tested for interaction with the [125I]alpha-bungarotoxin binding protein of goldfish brain. A subset of monoclonal antibodies which recognize the main immunogenic region of the alpha subunit of the Electrophorus acetylcholine receptor interacted at high affinity with the [125I]alpha-bungarotoxin binding protein. Using immunofluorescence, these antibodies were shown to label the same layers of the optic tectum as [125I]alpha-bungarotoxin.

Irreversible autonomic actions by lophotoxin suggest utility as a probe for both C6 and C10 nicotinic receptors

The marine natural product lophotoxin has produced a non-reversible antagonism of parasympathetic and sympathetic functions that are known to be mediated by C6 sub-type nicotinic receptors. Transmission through anuran paravertebral ganglia was eliminated in 20-40 min by 10-30-min treatments with 16-32 microM lophotoxin, in a time course resembling the onset of block of C10 sub-type nicotinic receptors at the neuromuscular junction and in cultured BC3H-1 cells. The action persisted through 16 h of washout. Nerve conduction was unaffected. Somewhat longer treatments (80 min) of in vitro ileal sections resulted in loss of sensitivity to nicotine, but not to acetylcholine, for at least 5 h. These data indicate that lophotoxin can serve as a more universal nicotinic receptor probe than the alpha-neurotoxins, which may bind to both C6 and C10 sub-types, but block only the C10.

Purification and characterization of the alpha-bungarotoxin binding protein from rat brain

The alpha-bungarotoxin (BGT) binding protein from rat brain has been purified and its polypeptide chain composition has been examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Polypeptide chains with Mrs of 55,000, 53,500 and 49,000 have been identified as constituents of the protein. The affinity ligand [3H]maleimidobenzyl trimethylammonium bromide ([3H]MBTA), used to identify the ligand binding site on neuromuscular junction acetylcholine receptors (NMJ AChRs), binds to the 55,000 dalton polypeptide chain. Using a technique where ligands are bound to the protein while the protein is immobilized on alpha-cobratoxin-Sepharose 4B, it was established that the brain BGT binding protein, like NMJ AChRs, possesses two binding sites for BGT. These experiments reinforce previous evidence that the brain BGT binding protein is closely related but not identical to NMJ AChRs.

Formation of retinotopic connections: selective stabilization by an activity-dependent mechanism

During regeneration of the optic nerve in goldfish, the ingrowing retinal fibers successfully seek out their correct places in the overall retinotopic projection on the tectum. Chemospecific cell-surface interactions appear to be sufficient to organize only a crude retinotopic map on the tectum during regeneration. Precise retinotopic ordering appears to be achieved via an activity-dependent stabilization of appropriate synapses and is based upon the correlated activity of neighboring ganglion cells of the same receptive-field type in the retina. Four treatments have been found to block the sharpening process: (a) blocking the activity of the ganglion cells with intraocular tetrodotoxin (TTX), (b) rearing in total darkness, (c) correlating the activation of all ganglion cells via stroboscopic illumination and (d) blocking retinotectal synaptic transmission with alpha-bungarotoxin (alphaBTX). These experiments support a role for correlated visually driven activity in sharpening the diffuse projection and suggest that this correlated activity interacts within the postsynaptic cells, probably through the summation of excitatory postsynaptic potentials (EPSPs). Other experiments support the concept that effective synapses are stabilized: a local postsynaptic block of transmission causes a local disruption in the retinotectal map. The changes that occur during this disruption suggest that each arbor can move to maximize its synaptic efficacy. In development, initial retinotectal projections are often diffuse and may undergo a similar activity-dependent sharpening. Indirect retinotectal maps, as well as auditory maps, appear to be brought into register with the direct retinotopic projections by promoting the convergence of contacts with correlated activity. A similar mechanism may drive both the formation of ocular dominance patches in fish tectum and kitten visual cortex and the segregation of different receptive-field types in the lateral geniculate nucleus. Activity-dependent synaptic stabilization may therefore be a general mechanism whereby the diffuse projections of early development are brought to the precise, mature level of organization.

Eye-specific segregation of optic afferents in mammals, fish, and frogs: the role of activity

Eye-specific patches or stripes normally develop in the visual cortex and superior colliculus of many (but not all) mammals and are also formed, after surgically produced binocular innervation, in the optic tectum of fish and frogs. The segregation of ocular dominance patches or columns has been studied using a variety of anatomical pathway-tracing techniques, by electrophysiological recording of postsynaptic units or field potentials, and by the 2-deoxyglucose method following visual stimulation of only one eye. In the tectum of both fish and frogs and in the cortex and colliculus of mammals, eye-specific patches develop from initially diffuse, overlapping projections. Of the various mechanisms that might cause such segregation, the evidence favors an activity-dependent process that stabilizes synapses from the same eye because of their correlated activity. First, several environmental manipulations affect the segregation of afferents in visual cortex: strabismus and alternate monocular exposure apparently enhance segregation, whereas dark rearing slows the segregation process, and monocular deprivation causes the experienced eye to form larger patches at the expense of those of the deprived eye. Second, blocking activity in both eyes is effective in preventing the segregation both in the tectum of fish and frog and in the visual cortex of cat. With the eyes blocked, alternate stimulation of the optic nerves permits the segregation of ocular dominance, at least onto single cells in the cat visual cortex. These findings are discussed in terms of an activity-dependent stabilization of those synapses having correlated activity (those from neighboring ganglion cells within one eye) but not of those lacking correlated activity (those from left and right eyes). We suggest that the eye-specific patches represent a compromise between total segregation of the projections from the two eyes and the formation of a single continuous retinotopic map across the surface of the cortex or tectum.

Diffusion from an injected volume of a substance in brain tissue with arbitrary volume fraction and tortuosity

When a substance is pressure-injected from a micropipette into the extracellular space of the brain it may either form a cavity or it may infiltrate the extracellular space. In either case subsequent diffusion is governed by the volume fraction and tortuosity of the brain tissue as well as the diffusion coefficient of the substance itself. Appropriate equations, solutions and approximations to these problems are discussed. The results are relevant to the interpretation of studies on neuropharmacology and in situ electrochemistry.

Stroboscopic illumination and dark rearing block the sharpening of the regenerated retinotectal map in goldfish

Blocking activity with intraocular tetrodotoxin prevents the sharpening of the retinotectal map formed during regeneration of the optic nerve. If (under normal conditions) the initially diffuse map sharpens because of correlated activity in neighboring but not distant ganglion cells, then sharpening should also be prevented merely by disrupting the spatiotemporal correlation in the pattern of activity. To test this idea, fish were exposed during regeneration to stroboscopic illumination in a featureless environment, or were maintained in complete darkness. The regenerating cells remained visually responsive after axotomy, and the xenon strobe effectively drove each ganglion cell at a constant latency. The maps formed in the strobe-reared fish were normally oriented, but the multiunit receptive fields were greatly enlarged, averaging 32 degrees. In control regenerates, multiunit receptive fields averaged only 11-12 degrees, nearly the same as for single units. Dark rearing, which allows only spontaneous activity, also resulted in enlarged multiunit receptive fields, averaging more than 28 degrees. Both effects parallel those reported previously with tetrodotoxin block. The mature projection did not become diffuse as a result of the strobe rearing, and the sensitive period corresponded to the early stage of synaptogenesis (20-34 days). Periods of normal visual exposure after 35 days produced very little sharpening of the diffuse maps produced during either strobe or dark rearing. The results are attributed to an activity-dependent stabilization of developing synapses. The correlated firing of neighboring ganglion cells could allow postsynaptic summation of their responses, and the retention of those more effective, retinotopically placed synapses might then occur via a Hebbian mechanism.