Distribution and development of nicotinic acetylcholine receptor subtypes in the optic tectum ofRana pipiens

Abstract

Acetylcholine allows the elicitation of visually evoked behaviors mediated by the frog optic tectum, but the mechanisms behind its effects are unknown. Although nicotinic acetylcholine receptors (nAChRs) exist in the tectum, their subtype has not been assessed. By using quantitative autoradiography, we examined the binding of [(3)H]cytisine and [(125)I]alpha-bungarotoxin in the laminated tectum. In mammalian systems, these radioligands bind with high affinity to alpha4 nAChR subunits and alpha7 nAChR subunits, respectively. [(3)H]Cytisine demonstrated high specific binding in adult frogs in retinorecipient layer 9, intermediate densities in layer 8, and low binding in layers 1-7 of the tectum. [(3)H]Cytisine binding was significantly higher in the tecta of adults than in those of tadpoles. Lesioning the optic nerve for 6 weeks decreased [(3)H]cytisine binding in layers 8/9 by 70+/-1%, whereas 6-month lesions decreased binding by 76+/-3%. Specific binding of [(125)I]alpha-bungarotoxin in adults was present only at intermediate levels in tectal layers 8 and 9, and undetectable in the deeper tectal layers. However, the nucleus isthmi, a midbrain structure reciprocally connected to the tectum, exhibited high levels of binding. There were no significant differences in tectal [(125)I]alpha-bungarotoxin binding between tadpoles and adults. Six-week lesions of the optic nerve decreased tectal [(125)I]alpha-bungarotoxin binding by 33+/-10%, but 6-month lesions had no effect. The pharmacokinetic characteristics of [(3)H]cytisine and [(125)I]alpha-bungarotoxin binding in the frog brain were similar to those demonstrated in several mammalian species. These results indicate that [(3)H]cytisine and [(125)I]alpha-bungarotoxin identify distinct nAChR subtypes in the tectum that likely contain non-alpha7 and alpha7 subunits, respectively. The majority of non-alpha7 receptors are likely associated with retinal ganglion cell terminals, whereas alpha7-containing receptors appear to have a different localization.

Nicotine exposure refines visual map topography through an NMDA receptor‐mediated pathway

“…Nicotine activates two classes of receptors: α‐bungarotoxin‐sensitive and α‐bungarotoxin‐insensitive nicotinic receptors. Both of these receptor classes are present in the optic tectum (Butt et al ., 2000). To determine which nicotinic receptor subtype mediated nicotine‐induced map refinement, we chronically exposed the tectum to either anatoxin‐a, an agonist selective for α‐bungarotoxin‐sensitive nicotinic receptors, or epiboxidine, an agonist selective for nicotinic receptors that are insensitive to α‐bungarotoxin.…”

“…Both α‐bungarotoxin‐sensitive and ‐insensitive receptors are present in the frog optic tectum (Butt et al ., 2000). Through the use of subtype‐selective agonists, we have shown that map refinement can be achieved through either nicotinic receptor subtype.…”

“…This selective stabilization process orders the map as the most effective depolarization is produced by retinal ganglion cells having neighbouring somata, overlapping receptive fields and correlated activity patterns. As in the mammalian visual system, retinal ganglion cell input is glutamatergic, visual plasticity requires postsynaptic N ‐methyl‐ d ‐aspartate (NMDA) receptor activity and nicotinic receptors are present (Butt et al ., 2000; Ruthazer & Cline, 2004).…”

Regulatory mechanisms that govern nicotinic synapse formation in neurons

“…Disruption of retinal inputs reduces nAChR ␣5, ␤2, and ␣7 subunit protein levels in the chick optic tectum neuropil (Britto et al, 1994;Torrao et al, 1996), and decreases binding of the nAChR ligands ␣-bgt and cytosine in the frog tectum (Sargent et al, 1989;Butt et al, 2000). These studies are complicated by the fact that nAChRs are expressed on both innervating presynaptic terminals and postsynaptic neurons found in the tectal neuropil, raising the possibility that degeneration of the lesioned nerve contributes to the loss of nAChR expression following denervation (Sargent et al, 1989;Torrao et al, 1996;Butt et al, 2000;Britto et al, 2001). Nonetheless, continued expression of nAChRs in mature CNS neurons appears to require regulatory signals from presynaptic afferents, just as in the PNS.…”

The effects of nicotinic and muscarinic receptor activation on patch-clamped cells in the optic tectum of rana pipiens

“…The optic tectum of the frog contains both nicotinic and muscarinic cholinergic receptors (Sargent et al, 1989;Birdsall et al, 1980;Butt et al, 2000Butt et al, , 2001) but knowledge of the physiological consequences of their activation is incomplete. Assessments of the effects of nicotinic receptor activation have focused on a facilitation of glutamate release from retinal ganglion cell terminals (Titmus et al, 1999;Kuras and Gutmaniene, 2001) while muscarinic receptors have been associated with tectal cell membranes following an enhancement of single unit recordings in response to ACh microiontophoresis (Fite and Wang, 1986).…”