Fig. 4.

Development and distribution of [³H]cytisine and [¹²⁵I]α-bungarotoxin binding sites in early (stages V, X), middle (stage XV), and late (stages XX, XXV) tadpole and adult (A) frog optic tectum. A: [³H]Cytisine binding was primarily constrained to the superficial, tectal layers in tadpoles and in adults. Binding was present in every stage examined, although at early stages it was restricted to the rostral half of the tectum. Invasion of binding of the caudal half of the tectum was gradual and progressive with high binding densities achieved only in adulthood. Binding was also evident in the thalamus at all stages examined. Nonspecific binding of [³H]cytisine (5 nM) was not detected (not shown). B: [¹²⁵I]α-Bungarotoxin (BtX) binding sites in the frog brain were distinct from those seen with [³H]cytisine. Consistent [¹²⁵I]α-bungarotoxin binding was seen in the superficial tectal layers throughout development. This binding also had a pattern of expression that paralleled the tectal maturation gradient by first presenting itself in the rostral tectum (stages V, X) and then, later in development, in the caudal tectum as well (stages XV and above). The levels of both total and nonspecific binding in the tectum noticeably increased between stage XXV and adult animals. Binding was also observed in the telencephalon and throughout the thalamus and midbrain tegmentum at all stages of development. C: Nonspecific binding, as determined by competition with nicotine (Nic; 10 μM), represented 10% to 30% of the total [¹²⁵I]α-bungarotoxin binding depending on the brain area sampled. Scale bar = 1 mm.