Plasticity of electroreceptor tuning in the weakly electric fish,Sternopygus dariensis

Abstract

The tuning of electroreceptors inSternopygus was studied in control fish and in fish injected with 5α-dihydrotestosterone (DHT). DHT has been shown to cause long-term decreases in electric organ discharge (EOD) frequencies of this species (Meyer 1983), an effect which may underlie naturally occurring ontogenetic changes in EOD frequencies. As determined by impulse-evoked receptor oscillations and single unit recordings of primary electroreceptive afferents in untreated fish, electroreceptors of an individual fish are narrowly tuned to frequencies close to the EOD frequency of that animal. Sixty-six percent of all units had best frequencies (BF)s higher than the EOD frequency. Sharpness of tuning (Q1odb) was found to vary with the distance of the BF from the EOD frequency in each individual, and less so with BF proper: units with BFs above EOD frequency were more sharply tuned than those with BFs below. After treatment with DHT, along with expected decreases in EOD frequency, receptor oscillation frequencies and single unit BFs decreased and approached the new frequency of the EOD. DHT injections had no effect on unit thresholds, the sharpness of tuning, or the distribution of best frequencies around a given animal's mean best frequency. Units with higher BFs within a sample from an individual still tended to be the most sharply tuned. EOD frequency and mean recorded electroreceptor BF changed at approximately the same rate, although the change in receptor tuning lagged behind changes in EOD frequency. Thus, the tuning mechanism of electroreceptors in this species is labile, and this lability allows the electroreceptors to maintain their frequency sensitivities close to the naturally-changing values of the EOD frequency.