Analysis of alpha-bungarotoxin binding in the goldfish central nervous system

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.

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.

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.