Electroreceptors of a weakly electric fish: II. Individually tuned receptor oscillations

Encoding phase spectrum for evaluating 'electric qualia'

The most broadly expressed and studied aspect of sensory transduction is receptor tuning to the power spectral density of the incoming signals. Temporal cues expressed in the phase spectrum are relevant in African and American pulse-emitting electric fish showing electroreceptors sensing the signals carried by the self- and conspecific-generated electric organ discharges. This article concerns the role of electroreceptor phase sensitivity in American pulse Gymnotiformes. These fish show electroreceptors sharply tuned to narrow frequency bands. This led to the common thought that most electrosensory information is contained in the amplitude spectra of the signals. However, behavioral and modeling studies suggest that in their pulses, Gymnotiformes electroreceptors also encode cues embodied in the phase spectrum of natural stimuli. Here, we show that the two main types of tuberous primary afferents of Gymnotus omarorum differentially respond to cues embodied in the amplitude and phase spectra of self-generated electrosensory signals. One afferent type, pulse markers, is mainly driven by the amplitude spectrum, while the other, burst coders, is predominantly sensitive to the phase spectrum. This dual encoding strategy allows the fish to create a sensory manifold where patterns of 'electric color' generated by object impedance and other potential sources of 'colored' images (such as large nearby objects and other electric fish) can be represented.

Peripheral sensory coding through oscillatory synchrony in weakly electric fish

Adaptations to an organism's environment often involve sensory system modifications. In this study, we address how evolutionary divergence in sensory perception relates to the physiological coding of stimuli. Mormyrid fishes that can detect subtle variations in electric communication signals encode signal waveform into spike-timing differences between sensory receptors. In contrast, the receptors of species insensitive to waveform variation produce spontaneously oscillating potentials. We found that oscillating receptors respond to electric pulses by resetting their phase, resulting in transient synchrony among receptors that encodes signal timing and location, but not waveform. These receptors were most sensitive to frequencies found only in the collective signals of groups of conspecifics, and this was correlated with increased behavioral responses to these frequencies. Thus, different perceptual capabilities correspond to different receptor physiologies. We hypothesize that these divergent mechanisms represent adaptations for different social environments. Our findings provide the first evidence for sensory coding through oscillatory synchrony.

Two Distinct Types of Noisy Oscillators in Electroreceptors of Paddlefish

Our computational analyses and experiments demonstrate that ampullary electroreceptors in paddlefish ( Polyodon spathula) contain 2 distinct types of continuously active noisy oscillators. The spontaneous firing of afferents reflects both rhythms, and as a result is stochastically biperiodic (quasiperiodic). The first type of oscillator resides in the sensory epithelia, is recorded as approximately 26 Hz and ±70 μV voltage fluctuations at the canal skin pores, and gives rise to a noisy peak at fe≈ 26 Hz in power spectra of spontaneous afferent firing. The second type of oscillator resides in afferent terminals, is seen as a noisy peak at fa≈ 30–70 Hz that dominates the power spectra of spontaneous afferent firing, and corresponds to the mean spontaneous firing rate. Sideband peaks at frequencies of fa± feare consistent with epithelia-to-afferent unidirectional synaptic coupling or, alternatively, nonlinear mixing of the 2 oscillatory processes. External stimulation affects the frequency of only the afferent oscillator, not the epithelial oscillators. Application of temperature gradients localized the feand faoscillators to different depths below the skin. Having 2 distinct types of internal oscillators is a novel form of organization for peripheral sensory receptors, of relevance for other hair cell sensory receptors.

Localization of nicotinamide adenine dinucleotide phosphate-diaphorase activity in electrosensory and electromotor systems of a gymnotiform teleost, Apteronotus leptorhynchus

The distribution of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity was determined in electrosensory and electromotor systems of the weakly electric gymnotiform teleost Apteronotus leptorhynchus as an indicator of putative nitric oxide synthase-containing cells. NADPH-d activity was detected in electroreceptors and in afferent nerves of both ampullary and type I and type II tuberous organs. All cell bodies within the anterior lateral line nerve ganglion were positive for NADPH-d activity, as were the primary afferent axons and termination fields in the medullary electrosensory lateral line lobe. In the corpus cerebelli and valvula cerebelli, NADPH-d label was present in Purkinje cell somata, mossy fiber synaptic glomeruli, granule cells, and parallel fibers. In the midbrain, NADPH-d activity was apparent in layer VIIIB of the torus semicircularis dorsalis and in electrosensory laminae of the optic tectum. NADPH-d was particularly associated with diencephalic electrosensory and electromotor nuclei, including the prepacemaker nucleus, the nucleus subelectrosensorius, and the central posterior nucleus of the thalamus. Intense NADPH-d activity was present in pacemaker and relay cells of the medullary pacemaker nucleus but was absent from a novel class of smaller cells in this structure. Relay cell axons and spinal electromotor neurons and their axons within the electric organ were positive for NADPH-d activity. These results indicate that putative nitric oxide synthase-containing neurons in Apteronotus are localized preferentially to electrosensory and electromotor structures, suggesting a role for nitric oxide in determining the activity of cells involved in detecting or generating weakly electric fields.

Hormone-induced and maturational changes in electric organ discharges and electroreceptor tuning in the weakly electric fish Apteronotus

Plasticity in the frequency of the electric organ discharge (EOD) and electroreceptor tuning of weakly electric fish was studied in the genus Apteronotus. Both hormone-induced and maturational changes in EOD frequency and electroreceptor tuning were examined. Apteronotus is different from all other steroid-responsive weakly electric fish in that estradiol-17 beta, rather than androgens, induces discharge frequency decreases. This result can account for the 'reversed' discharge frequency dimorphism found in Apteronotus in which, counter to all other known sexually dimorphic electric fish, females have lower discharge frequencies than males. Studies of electroreceptor tuning in Apteronotus indicate that electroreceptors are closely tuned to the frequency of the EOD. This finding was noted not only in adult animals, but also in juvenile animals shortly after the onset of their EODs. Tuning plasticity in Apteronotus, as in other species studied, is associated with altered EOD frequencies and was noted in both maturational EOD changes and in estrogen-induced changes. Thus, tuning plasticity appears to be a general phenomenon which occurs concurrent with a variety of EOD changes.

Proceedings of a conference on steroid hormones as trophic agents in neural development. UCLA, October 9 & 10, 1984

The electric communication system includes both special muscle-derived cells or electrocytes that produce species-typical electric signals, or electric organ discharges (EODs), and specialized sensory receptors, or electroreceptors, that encode the electric fields set up by EODs. Steroid hormones can influence the characteristic properties of both EODs and electroreceptors. Steroids appear to directly effect the anatomy and physiology of the electrocytes that generate an EOD. In contrast, the steroid effect on electroreceptors may be predominantly via an indirect mechanism whereby changes in the spectral characteristics of the EOD appear to induce changes in the spectral sensitivity of electroreceptors. Continued studies of electrosensory and electromotor systems will offer insights into the cellular bases for the development and evolution of steroid-sensitive pathways in the vertebrate nervous system.

Evidence for a direct effect of androgens upon electroreceptor tuning

Tuberous electroreceptors of individual wave type weakly electric fish are tuned to the fundamental frequency of that fish's electric organ discharge (EOD). EOD frequency and receptor best frequency (BF) are both lowered following systemic injection of 5-alpha-dihydrotestosterone (DHT). A previous study (Meyer et al. 1984) showed that the effect of DHT on the EOD generating circuitry was independent of an ongoing EOD and suggested that its effect on electroreceptor tuning was indirect, possibly mediated by the electric field. We have continued these studies to determine the factors which influence electroreceptor tuning. Baseline recordings of EOD frequency, receptor oscillations, and single afferent tuning curves were taken. After fish were electrically silenced by spinal cord transection they were injected daily with either DHT or saline or were implanted with either DHT-filled or empty silastic capsules. As previously reported, the EOD frequency (determined from pacemaker nucleus recordings) was lowered in DHT-treated, transected fish and increased in control fish. Similarly, receptor tuning was lowered in the DHT-treated, silenced fish. Oscillation frequencies decreased in both treated and control groups, but significantly more in the hormone group. Single afferent best frequencies were lowered in both DHT groups and raised in their respective control groups. In another series of experiments exogenous electric fields capable of driving receptors in a 1-to-1 phase-locked manner were placed around silenced fish. We were unable to elicit any shift in pacemaker frequency or electroreceptor tuning regardless of stimulus field geometry. Four transected fish were injected with DHT and placed in exogenous electric fields of higher frequency than their original EOD. Even in the presence of a higher frequency electric field, DHT lowered EOD frequency and afferent BF. We conclude that androgens produce effects both on the EOD generating circuitry, probably at the level of the pacemaker nucleus, and on electroreceptors, probably, ultimately, on receptor cell membrane conductances. These effects occur in parallel allowing the two parameters to remain well matched. In contrast to former predictions, exogenous electric fields alone appear unable to shift receptor tuning.

Tonotopic organization in the midbrain of a teleost fish

Recordings from small clusters of units within the auditory midbrain of the Japanese carp, Cyprinus carpio, provide the first evidence for tonotopy within the teleost brain. These findings suggest that place mechanisms of frequency coding and tonotopy, previously thought to have evolved first in amphibians, may be general and ancient features of the vertebrate auditory system.

Androgens Alter the Tuning of Electroreceptors

Weakly electric fish possess electroreceptors that are tuned to their individual electric organ discharge frequencies. One genus, Sternopygus, displays both ontogenetic and seasonal shifts in these frequencies, possibly because of endocrine influences. Systemic treatment with androgens lowers the discharge frequencies in these animals. Concomitant with these changes in electric organ discharge frequencies are decreases in electroreceptor best frequencies; hence the close match between discharge frequency and receptor tuning is maintained. These findings indicate that the tuning of electroreceptors is dynamic and that it parallels natural shifts in electric organ discharge frequency.

Temporal coding of species recognition signals in an electric fish

An electric fish in the African family Mormyridae recognizes members of its own species by "listening" to electric organ discharges, which are species-specific signatures. Reactions of fish in the field and of individual electroreceptors to both normal and modified computer-synthesized discharges emphasize the importance of the waveform (time-domain cues) in species recognition.