The emergence of tuning in newly generated tuberous electroreceptors.

Tuning curves of afferent electroreceptive fibers in the anterior lateral line nerve of the weakly electric fish, Sternopygus macrurus, indicate that the tuberous electroreceptors of each individual are well-tuned to its own electric organ discharge (EOD) frequency. In order to study how receptor tuning may develop, new receptor organs were induced to form in regenerating cheek skin, and their tuning properties were compared with those of intact receptors from the same fish. At 3 weeks after the onset of regeneration, new receptors of a given fish were broadly tuned with best frequencies (BFs) lower than that fish's EOD frequency and the BFs of its own intact tuberous receptors. Three weeks later, regenerated receptors of the same fish were indistinguishable from intact receptors in BF, although tuning curves were occasionally slightly broader than normal. To determine if the presence of an ongoing electric field is necessary for the genesis of proper tuning, receptors were allowed to regenerate in fish deprived of their EODs. At 6 weeks, tuning curves of these receptors also had BFs that were tuned similarly to intact receptors and to each individual's characteristic EOD frequency (determined by recordings of the pacemaker nucleus in the medulla). Thus, as regenerating receptors mature, they gradually become more sharply tuned and tuned to progressively higher frequencies until reaching the correct BF, which matches the EOD frequency; however, tuning to the appropriate EOD frequency occurs without reference to the ongoing electric field.