Motor control of the jamming avoidance response of Apteronotus leptorhynchus: evolutionary changes of a behavior and its neuronal substrates

The two closely related gymnotiform fishes, Apteronotus and Eigenmannia, share many similar communication and electrolocation behaviors that require modulation of the frequency of their electric organ discharges. The premotor linkages between their electrosensory system and their medullary pacemaker nucleus, which controls the repetition rate of their electric organ discharges, appear to function differently, however. In the context of the jamming avoidance response, Eigenmannia can raise or lower its electric organ discharge frequency from its resting level. A normally quiescent input from the diencephalic prepacemaker nucleus can be recruited to raise the electric organ discharge frequency above the resting level. Another normally active input, from the sublemniscal prepacemaker nucleus, can be inhibited to lower the electric organ discharge frequency below the resting level (Metzner 1993). In contrast, during a jamming avoidance response, Apteronotus cannot lower its electric organ discharge frequency below the resting level. The sublemniscal prepacemaker is normally completely inhibited and release of this inhibition allows the electric organ discharge frequency to rise during the jamming avoidance response. Further inhibition of this nucleus cannot lower the electric organ discharge frequency below the resting level. Lesions of the diencephalic prepacemaker do not affect performance of the jamming avoidance response. Thus, in Apteronotus, the sublemniscal prepacemaker alone controls the change of the electric organ discharge frequency during the jamming avoidance response.     

The control of pacemaker modulations for social communication in the weakly electric fish Sternopygus

Summary Nearly sinusoidal electric organ discharges (EODs) of the weakly electric fish Sternopygus, occur at a regular rate within a range from 50 to 200 Hz and are commanded by a medullary pacemaker nucleus (Pn). During courtship and aggression, the rate of EODs is modulated as smooth EOD-frequency rises or brief EOD-interruptions (Hopkins 1974b). The present study examines the control of such modulations. Rises were elicited by L-glutamate stimulation of the diencephalic prepacemaker nucleus, the only previously known source of input to the Pn. We demonstrate an additional input to the Pn, the sublemniscal prepacemaker nucleus (SPPn). L-glutamate stimulation of this area caused EOD-interruptions.The Pn contains electrotonically coupled pacemaker cells which generate the rhythm of the EODs, as well as relay cells which transmit the command pulse to the spinal motor neurons that innervate the electric organ. Pacemaker cells recorded intracellularly during EOD-interruptions continued firing at their regular frequency but with slightly increased jitter. Relay cells, on the other hand, were strongly depolarized and fired spikelets at a greatly increased frequency during EOD-interruptions. Thus EOD-interruptions were caused by SPPn input to relay cells that caused their massive depolarization, blocking the normal input from pacemaker cells without greatly affecting pacemaker cell firing characteristics.Application to the Pn of an antagonist to NMDA-type glutamate receptors blocked EOD-frequency rises and EOD-interruptions. Antagonists to quisqualate/ kainate receptor-types were ineffective.Abbreviations EOD Electric Organ Discharge - JAR Jamming Avoidance Response - Pn pacemaker nucleus - PPn diencephalic prepacemaker nucleus - SPPn sublemniscal prepacemaker nucleus     

Neuronal control of behavioral plasticity: the prepacemaker nucleus of weakly electric gymnotiform fish

 Gymnotiform fish of the genera Apteronotus and Eigenmannia provide an excellent vertebrate model system to study neural mechanisms controlling behavioral plasticity. These teleosts generate, by means of an electric organ, quasi-sinusoidal discharges of extremely stable frequency and waveform. Modulations consisting of transient rises in discharge frequency are produced during social encounters, and play an important role in communication. These so-called “chirps” exhibit a remarkable sexual dimorphism, as well as an enormous seasonal and individual variability. Chirping behavior is controlled by a subset of neurons in the complex of the central posterior/prepacemaker nucleus in the diencephalon. It is hypothesized that the plasticity in the performance of chirping behavior is, at least in part, governed by two mechanisms: first, by seasonally induced structural changes in dendritic morphology of neurons of the prepacemaker nucleus, thus leading to pronounced alterations in excitatory input. Second, by androgen-controlled changes in the innervation pattern of the prepacemaker nucleus by fibers expressing the neuropeptide substance P. In addition to these two dynamic processes, cells are generated continuously and at high number in the central posterior/prepacemaker nucleus during adulthood. This phenomenon may provide the basis for a “refreshment”, thus facilitating possible changes in the underlying neural network. Accepted: 17 September 1990     

Distinct mechanisms of modulation in a neuronal oscillator generate different social signals in the electric fishHypopomus

Summary The medullary pacemaker nucleus of the gymnotiform electric fish,Hypopomus, is a relatively simple neuronal oscillator which contains pacemaker cells and relay cells. The pacemaker cells generate a regular discharge cycle and drive the relay cells which trigger pulse-like electric organ discharges (EODs). The diencephalic prepacemaker nucleus (PPN) projects to the pacemaker nucleus and modulates its activity to generate a variety of specific discharge patterns which serve as communicatory signals (Figs. 2 and 3).While inducing such signals by microiontophoresis of L-glutamate to the region of the PPN (Fig. 4) of curarized animals, we monitored the activity of neurons in the pacemaker nucleus intracellularly. We found that pacemaker cells and relay cells were affected differently in a manner specific to the type of EOD modulation (Figs. 5–10). The normal sequence of pacemaker cell and relay cell firing was maintained during gradual rises and falls in discharge rate. Both types of cells ceased to fire during interruptions following a decline in discharge rate. During sudden interruptions, however, relay cells were steadily depolarized, while pacemaker cells continued to fire regularly. Short and rapid barrages of EODs, called chirps, were generated through direct and synchronous activation of the relay cells whose action potentials invaded pacemaker cells antidromically and interfered with their otherwise regular firing pattern.Abbreviations EOD electric organ discharge - HRP horseradish peroxidase - NMDA N-Methyl-D-Aspartate - PPN prepacemaker nucleus     

Electrical stimulation of the preoptic area in Eigenmannia: evoked interruptions in the electric organ discharge

The functional role of the basal forebrain and preoptic regions in modulating the normally regular electric organ discharge was determined by focal brain stimulation in the weakly electric fish, Eigenmannia. The rostral preoptic area, which is connected with the diencephalic prepacemaker nucleus, was examined physiologically by electrical stimulation in a curarized fish. Electrical stimulation of the most rostral region of the preoptic area with trains of relatively low intensity current elicits discrete bursts of electric organ discharge interruptions in contrast to other forebrain loci. These responses were observed primarily as after-responses following the termination of the stimulus train and were relatively immune to variations in the stimulus parameters. As the duration and rate of these preoptic-evoked bursts of electric organ discharge interruptions (approximately 100 ms at 2 per s) are similar to duration and rate of natural interruptions, it is proposed that these bursts might be precursors to natural interruptions. These data suggest that the preoptic area, consistent with its role in controlling reproductive behaviors in vertebrates, may be influencing the occurrence of electric organ discharge courtship signals by either direct actions on the prepacemaker nucleus or through other regions that are connected with the diencephalic prepacemaker nucleus. Accepted: 16 October 1999     

Interruption of pacemaker signals is mediated by GABAergic inhibition of the pacemaker nucleus in the African electric fish <Emphasis Type="Italic">Gymnarchus niloticus</Emphasis>

The wave-type African weakly electric fish Gymnarchus niloticus produces electric organ discharges (EODs) from an electric organ in the tail that is driven by a pacemaker complex in the medulla, which consists of a pacemaker nucleus, two lateral relay nuclei and a medial relay nucleus. The prepacemaker nucleus (PPn) in the area of the dorsal posterior nucleus of the thalamus projects exclusively to the pacemaker nucleus and is responsible for EOD interruption behavior. The goal of the present study is to test the existence of inhibition of the pacemaker nucleus by the PPn. Immunohistochemical results showed clear anti-GABA immunoreactive labeling of fibers and terminals in the pacemaker nucleus, but no apparent anti-glycine immunoreactivity anywhere in the pacemaker complex. GABA injection into the pacemaker nucleus could induce EOD interruptions that are comparable to the interruptions induced by glutamate injection into the PPn. Application of the GABA_A receptor blocker bicuculline methiodide reversibly eliminated the effects of stimulation of the PPn. Thus the EOD interruption behavior in Gymnarchus is mediated through GABAergic inhibition of the pacemaker nucleus by the PPn.     

Ultrastructural evidence of GABA-ergic inhibition and glutamatergic excitation in the pacemaker nucleus of the gymnotiform electric fish,Hypopomus

The medullary pacemaker nucleus of Hypopomus triggers each electric organ discharge (EOD) by a single command pulse. It consists of electrotonically coupled pacemaker cells, which generate the rhythm, and relay cells, which follow the pacemaker cells and excite the spinal motoneurons of the electric organ. The pacemaker cells receive two inputs from the complex of the diencephalic prepacemaker nucleus (PPn), a GABA-ergic inhibition and a glutamatergic excitation. Relay cells, on the other hand, receive two glutamatergic inputs, one from a subnucleus of the PPn, the PPn-C, and a second from the sublemniscal prepacemaker nucleus (SPPn).We have labelled afferents to the pacemaker nucleus by injecting HRP to specific sites of the prepacemaker complex. By using immunogold-labelled antibodies and en-grid staining techniques, we demonstrated GABA and glutamate immunoreactivity in labelled synaptic profiles of ultra-thin sections of the pacemaker nucleus. The two types of synapses were interspersed on the surfaces of pacemaker cells, with GABA-immunoreactive synapses apparently representing the GABA-mediated input of the PPn-I, an inhibitory subdivision of the PPn, and glutamate-immunoreactive synapses representing the input of the PPn-G, an excitatory subdivision of the PPn. Only glutamate-immunoreactive synapses were found on relay cells.Abbreviations AMPA -Amino-3-hydroxy-5-methylisoxazole-4-propionic acid - CP central posterior nucleus - EOD electric organ discharge - GABA -aminobutyric acid - GAD L-glutamate decarboxylase - HRP horseradish peroxidase - JAR jamming avoidance response - NMDA N-methyl-D-aspartate - PPn (diencephalic) prepacemaker nucleus - SPPn sublemniscal prepacemaker nucleus     

Development of a sexual dimorphism in a central pattern generator driving a rhythmic behavior: The role of glia‐mediated potassium buffering in the pacemaker nucleus of the weakly electric fish Apteronotus leptorhynchus

Central pattern generators play a critical role in the neural control of rhythmic behaviors. One of their characteristic features is the ability to modulate the oscillatory output. An important yet little‐studied type of modulation involves the generation of oscillations that are sexually dimorphic in frequency. In the weakly electric fish Apteronotus leptorhynchus, the pacemaker nucleus serves as a central pattern generator that drives the electric organ discharge of the fish in a one‐to‐one fashion. Males discharge at higher frequencies than females—a sexual dimorphism that develops under the influence of steroid hormones. The two principal neurons that constitute the oscillatory network of the pacemaker nucleus are the pacemaker and relay cells. Whereas the number and size of the pacemaker and relay cells are sexually monomorphic, pronounced sex‐dependent differences exist in the morphology, and subcellular properties of astrocytes, which form a syncytium closely associated with these neurons. In females, compared to males, the astrocytic syncytium covers a larger area surrounding the pacemaker and relay cells and exhibits higher levels of expression of connexin‐43 expression. The latter indicates a strong gap‐junction coupling of the individual cells within the syncytium. It is hypothesized that these sex‐specific differences result in an increased capacity for buffering of extracellular potassium ions, thereby lowering the potassium equilibrium potential, which, in turn, leads to a decrease in the oscillation frequency. This hypothesis has received strong support from simulations based on computational models of individual neurons and the whole neural network of the pacemaker nucleus.     

c-fos expression in the putative avian suprachiasmatic nucleus

c-fos induction was investigated as a potential component in the avian photic entrainment pathway and as a possible means of locating the central pacemaker in birds. In both quail (Coturnix coturnix japonica) and starlings (Sturnus vulgaris) exposure to 1 h of light induced Fos-lir in the visual suprachiasmatic nucleus but not in the medial suprachiasmatic nucleus. However, the degree of c-fos induction in the visual suprachiasmatic nucleus was similar at different circadian times despite the fact that the light pulses caused differential phase shifts in the locomotor rhythm. For golden hamsters the same experiment resulted in significantly different levels of Fos-lir in the suprachiasmatic nucleus, as well as different phase shifts. Starlings and hamsters were also entrained to T-cycles that caused a large daily phase shift (T = 21.5 h in starlings, T = 22.67 hours in hamsters), or no daily phase shift (T = free running period). No difference in the induced levels of Fos-lir in the visual suprachiasmatic nucleus region was observed between the two groups of starlings, but in hamsters there were significantly different levels of Fos-lir in the suprachiasmatic nucleus between the two groups. Accepted: 15 November 1996     

The jamming avoidance response ofEigenmannia: properties of a diencephalic link between sensory processing and motor output

Summary Brain regions participating in the control ofEigenmannia's electric organ discharge frequency were localized by electrical microstimulation and anatomically identified by means of horseradish peroxidase deposition. A diencephalic region was found which, when stimulated, caused electric organ discharge (EOD) frequency increases of similar magnitude and time course as the frequency increases seen during the jamming avoidance response. Single unit recordings from this region revealed one cell type which preferentially responded to stimuli that cause the acceleration phase of the jamming avoidance response (electric organ discharge frequency increase). A second cell type responded preferentially to stimuli which cause EOD frequency decrease, and both cell types were tuned to stimuli which evoked maximal jamming avoidance behaviors.The results of the horseradish peroxidase experiments showed that the recording and stimulation sites correspond to the previously described nucleus electrosensorius. Our results confirm the earlier finding that this nucleus receives output from the torus semicircularis and we also found that the N. electrosensorius projects to the mesencephalic prepacemaker nucleus. The prepacemaker projects to the medullary pacemaker nucleus which generates the commands that evoke electric organ discharges.The anatomical and physiological results described here establish this diencephalic region as a link between the major sensory processing region for the jamming avoidance response, the torus semicircularis, and a mesencephalic pre-motor region, the prepacemaker nucleus.Abbreviations AM amplitude modulation - DF Delta F - ELLL electrosensory lateral line lobe - EOD electric organ discharge - JAR jamming avoidance response - NE nucleus electrosensorius - PPN prepacemaker nucleus - PN pacemaker nucleus     

From distributed sensory processing to discrete motor representations in the diencephalon of the electric fish,Eigenmannia

Summary During their jamming avoidance response (JAR), weakly electric fish of the genusEigenmannia shift their electric organ discharge (EOD) frequency away from a similar EOD frequency of a neighboring fish. The behavioral rules and neural substrates for stimulus recognition and motor control of the JAR have been extensively studied (see review by Heiligenberg 1986). The diencephalic nucleus electrosensorius (nE) links sensory processing within the torus semicircularis and optic tectum with the mesencephalic prepacemaker nucleus which, in turn, modulates the medullary pacemaker nucleus and hence the EOD frequency. Two separate areas within the nE responsible for JAR-related EOD frequency rises and frequency falls, respectively, were identified by iontophoresis of the excitatory amino acid L-glutamate. Bilateral lesion of the areas causing EOD frequency rises resulted in elimination of JAR-related frequency rises above a baseline frequency obtained in the absence of a jamming stimulus. Similarly, bilateral lesion of the areas causing frequency falls resulted in a loss of JAR-related frequency falls below the baseline frequency. Whether these areas are also responsible for non-JAR-related frequency shifts is not known. The strength of response and spatial extent of the areas causing frequency shifts varied among fish and also varied in individual fish, reflecting the strength of JAR-related frequency shifts and the balance of activities in frequency-rise and frequency-fall areas. Local application of bicuculline-methiodide or GABA demonstrated a tonic inhibitory input to each area and suggests a reciprocal inhibitory interaction between the two ipsilateral areas, possibly accounting for much of the individual plasticity.The nE thus is a site for neuronal transformation from distributed, topographically organized processing within the laminated structures of the torus and tectum to discrete cell clusters which control antagonistic motor responses.Abbreviations EOD electric organ discharge - JAR jamming avoidance response - Df difference frequency between jamming signal and the fish's own EOD - nE nucleus electrosensorius - PPn prepacemaker nucleus     

Modulation of Drosophila heartbeat by neurotransmitters

The heart of Drosophila melanogaster is a simple muscular tube with a posterior pulsatile portion and a thoracic-cranial vessel. The pacemaker, located caudally, is myogenic. Its rate of firing is modulated by neurotransmitters. Serotonin, octopamine, norepineph-rine, dopamine, and acetylcholine accelerate the heart, in that order of potency. Dihydroxyphenylalanine, γ-aminobutyric acid, glutamate, and glycine have no effect. Generally, the regularity of the heartbeat is not adversely affected by treatment with any of these neurotransmitters. We show here that amnesiac, a neurological mutation, and Dihydroxyphenylalanine decarboxylase ^{temperature sensitive}, a mutation that interferes with synthesis of dopamine, norepinephrine, and serotonin, result in slower heart rate and reduced regularity across a normal range of temperatures for these flies. Dopamine-N-acetyltransferase, which is on the catabolic route to dopamine, serotonin, and octopamine, has no effect. hypoactiveC reduces the rate of the heart, but its mechanism of action is unknown. Accepted: 5 August 1996     

Interruption of pacemaker signals by a diencephalic nucleus in the African electric fish, Gymnarchus niloticus

The African electric fish Gymnarchus niloticus rhythmically emits electric organ discharges (EODs) for communication and navigation. The EODs are generated by the electric organ in the tail in response to the command signals from the medullary pacemaker complex, which consists of a pacemaker nucleus (PN), two lateral relay nuclei (LRN) and a medial relay nucleus (MRN). The premotor structure and its modulatory influences on the pacemaker complex have been investigated in this paper. A bilateral prepacemaker nucleus (PPn) was found in the area of the dorsal posterior nucleus (DP) of the thalamus by retrograde labeling from the PN. No retrogradely labeled neurons outside the pacemaker complex were found after tracer injection into the LRN or MRN. Accordingly, anterogradely labeled terminal fibers from PPn neurons were found only in the PN. Iontophoresis of l-glutamate into the region of the PPn induced EOD interruptions. Despite the exclusive projection of the PPn neurons to the PN, extracellular and intracellular recordings showed that PN neurons continue their firing while MRN neurons ceased their firing during EOD interruption. This mode of EOD interruption differs from those found in any other weakly electric fishes in which EOD cessation mechanisms have been known.     

A novel fission yeast gene, kms1 +, is required for the formation of meiotic prophase-specific nuclear architecture

In the meiotic prophase nucleus of the fission yeast Schizosaccharomyces pombe, chromosomes are arranged in an oriented manner: telomeres cluster in close proximity to the spindle pole body (SPB), while centromeres form another cluster at some distance from the SPB. We have isolated a mutant, kms1, in which the structure of the meiotic prophase nucleus appears to be distorted. Using specific probes to localize the SPB and telomeres, multiple signals were observed in the mutant nuclei, in contrast to the case in wild-type. Genetic analysis showed that in the mutant, meiotic recombination frequency was reduced to about one-quarter of the wild-type level and meiotic segregation was impaired. This phenotype strongly suggests that the telomere-led rearrangement of chromosomal distribution that normally occurs in the fission yeast meiotic nucleus is an important prerequisite for the efficient pairing of homologous chromosomes. The kms1 mutant was also impaired in karyogamy, suggesting that the kms1 ⁺ gene is involved in SPB function. However, the kms1 ⁺ gene is dispensable for mitotic growth. The predicted amino acid sequence of the gene product shows no significant similarity to known proteins. Received: 5 September 1996 / Accepted: 21 November 1996     

Species-specific differences in sensorimotor adaptation are correlated with differences in social structure

Here, we report a species difference in the strength and duration of long-term sensorimotor adaptation in the electromotor output of weakly electric fish. The adaptation is produced by changes in intrinsic excitability in the electromotor pacemaker nucleus; this change is a form of memory that correlates with social structure. A weakly electric fish may be jammed by a similar electric organ discharge (EOD) frequency of another fish and prevents jamming by transiently raising its own emission frequency, a behavior called the jamming avoidance response (JAR). The JAR requires activation of NMDA receptors, and prolonged JAR performance results in long-term frequency elevation (LTFE) of a fish’s EOD frequency for many hours after the jamming stimulus. We find that LTFE is stronger in a shoaling species (Eigenmannia virescens) with a higher probability of encountering jamming conspecifics, when compared to a solitary species (Apteronotus leptorhynchus). Additionally, LTFE persists in Eigenmannia, whereas, it decays over 5–9 h in Apteronotus.     

Food web analysis of southern California coastal wetlands using multiple stable isotopes

Carbon, nitrogen, and sulfur stable isotopes were used to characterize the food webs (i.e., sources of carbon and trophic status of consumers) in Tijuana Estuary and San Dieguito Lagoon. Producer groups were most clearly differentiated by carbon, then by sulfur, and least clearly by nitrogen isotope measurements. Consumer ¹⁵N isotopic enrichment suggested that there are four trophic levels in the Tijuana Estuary food web and three in San Dieguito Lagoon. A significant difference in multiple isotope ratio distributions of fishes between wetlands suggested that the food web of San Dieguito Lagoon is less complex than that of Tijuana Estuary. Associations among sources and consumers indicated that inputs from intertidal macroalgae, marsh microalgae, and Spartina foliosa provide the organic matter that supports invertebrates, fishes, and the light-footed clapper rail (Rallus longirostris levipes). These three producers occupy tidal channels, low salt marsh, and mid salt marsh habitats. The only consumer sampled that appears dependent upon primary productivity from high salt marsh habitat is the sora (Porzana carolina). Two- and three-source mixing models identified Spartina as the major organic matter source for fishes, and macroalgae for invertebrates and the light-footed clapper rail in Tijuana Estuary. In San Dieguito Lagoon, a system lacking Spartina, inputs of macroalgae and microalgae support fishes. Salicornia virginica, S. subterminalis, Monanthochloe littoralis, sewage- derived organic matter, and suspended particulate organic matter were deductively excluded as dominant, direct influences on the food web. The demonstration of a salt marsh–channel linkage in these systems affirms that these habitats should be managed as a single ecosystem and that the restoration of intertidal marshes for endangered birds and other biota is compatible with enhancement of coastal fish populations; heretofore, these have been considered to be competing objectives. Received: 24 April 1996 / Accepted: 24 October 1996     

Molecular and biochemical characterization of a proteasome subunit from rice and carrot cells

Proteasomes function mainly in the ATP-dependent degradation of proteins that have been conjugated with ubiquitin. We isolated a cDNA clone for a rice protein that exhibited high homology to subunit C2 of proteasomes. Southern blot analysis revealed that the corresponding gene was present as a single copy in the rice genome. After fractionation of a crude extract from cultured cells, a 35-kDa protein that cross-reacted with antibodies against the C2 subunit was recovered in the peak fraction of both 20 S and 26 S complexes. The same antibodies cross-reacted with two proteins in seedlings, one of which was the same as that detected in cultured cells. The level of the protein was reduced in roots under conditions of high salinity. The 35-kDa protein was not detected in the nuclei of rice or carrot cells. However, during somatic embryogenesis of carrot cells, the C2 subunit was found in the nucleus at the globular stage, and it gradually disappeared in the period from the heart to the torpedo stage. Cells at the globular stage are proliferating rapidly, thus, it is possible that proteasomes are associated with the proliferation of plant cells. Received: 11 November 1996 / Accepted: 28 January 1997     

Effect of the removal of cleaner fish on the abundance and species composition of reef fish

The ecological significance of cleaner fish on coral reefs was investigated. I removed all cleaner fish, Labroides dimidiatus, from eight small reefs, measured the subsequent effect on the abundance and species composition of all reef fish after 3 and 6 months, and compared it with eight control reefs with cleaner fish. The removal of cleaner fish had no detectable effect on the total abundance of fish on reefs and the total number of fish species at both times. Multivariate analysis by non-metric multidimensional scaling and ANOSIM pairwise tests based on 191 fish species revealed no effect of cleaners on the community structure of fish. Similar results were obtained using principal components analysis on subsets of the data using the 33 most common fish species and the 15 most abundant species (≥5 individuals per reef ) with both log10 (x + 1) transformed data and with fish numbers standardized for abundance. This study demonstrates that the removal of cleaner fish for 6 months did not result in fish suffering increased mortality nor in fish leaving reefs to seek cleaning elsewhere. Received: 28 October 1996 / Accepted: 7 February 1997     

An in vitro physiological preparation of a vertebrate communicatory behavior: chirping in the weakly electric fish, Apteronotus

1. An in vitro preparation of the medullary pacemaker nucleus of the weakly electric fish Apteronotus leptorhynchus was studied which fires regularly and synchronously at the fish's characteristic frequency of electric organ discharge (EOD). Upon bipolar stimulation of tissue regions through which pass prepacemaker nucleus afferents to the pacemaker, a brief, transient increase in discharge frequency ensued at short-latency (Fig. 1A). 2. Intracellular recordings revealed that the acceleration was accompanied by a depolarization and decline in action potential amplitude. The magnitude of these changes was both phase- (Fig. 5) and amplitude-dependent, with the latter showing an evident threshold effect (Figs. 4 and 12). The response was reversibly blocked by high Mg2+ saline (Fig. 1B), and the magnitude of the accelerations showed marked facilitation during repeated stimulation (Fig. 6). 3. Optical and histological identification allowed characteristically different responses in the intracellular recordings to be attributed to the two cell types of the pacemaker nucleus: pacemaker and relay cells (Figs. 2 and 3). Similar responses have been observed at these respective recording locations in the intact animal during chirping (Dye and Heiligenberg 1987). 4. Simultaneous recordings of pairs of cells revealed a transient change in the phase relationship of firing during the accelerations which was most marked between relay and pacemaker cells (Fig. 7). These dual recordings also revealed that the relay cells depolarize and accelerate more than pacemaker cells (Fig. 10), suggesting that they are the principal effectors of this behavioral modulation. 5. Trains of pulses additionally elicited a long-lasting frequency elevation which occurred at a slightly higher threshold than the brief accelerations. This slow frequency change relaxed back to baseline following a biexponential time course which closely resembled that of a distinct behavior seen in intact fish, termed 'yodeling' (Dye 1987).