Medullary electrosensory processing in the little skate

1. Previous studies have demonstrated that the resting activity of electrosensory ALLN fibers is modulated by the animal's own respiratory activity and that all fibers innervating a single ampullary cluster are modulated with the same amplitude and phase relationship to ventilation. We demonstrate that ALLN fibers in the skate are modulated in this common-mode manner bilaterally, regardless of receptor group, orientation, or position of the receptor pore on the body surface (Fig. 2). 2. Ascending efferent neurons (AENs), which project to the electrosensory midbrain from the DON, are modulated through a much smaller portion of their dynamic range. AENs give larger responses to an extrinsic local electric field than to the respiratory driving, indicating that a mechanism exists for suppressing ventilatory electrosensory reafference. 3. In paralyzed animals no modulation of resting activity or of responses of extrinsic electric fields could be observed with respect to the animal's respiratory motor commands in the absence of electrosensory reafference. 4. Cells of the dorsal granular ridge (DGR) project to medullary AENs via the DON molecular layer. A majority of proprioceptive DGR neurons are modulated by ventilatory activity, however, in a given fish the modulation is not in the same phase relationship to ventilation among DGR units. 5. The modulation of AENs during respiration was increased following transection of the contralateral ALLN (Fig. 9). Resting activity and responses to excitatory stimuli were inhibited by simultaneous stimulation of the transected contralateral ALLN indicating that a common-mode rejection mechanism is mediated via the commissural interconnections of the DONs.     

Medullary electrosensory processing in the little skate

1. Ampullary electroreceptors in elasmobranchs are innervated by fibers of the ALLN, which projects to the dorsal octavolateralis nucleus (DON). The purpose of this study is to examine the response characteristics of ALLN fibers and DON neurons to weak D.C. and sinusoidal electric field stimuli presented as local dipole fields. 2. ALLN fibers respond to presentation of D.C. fields with a phasic burst, followed by a more slowly adapting period of firing. Ascending efferent neurons (AENs) in the DON respond to stimuli with a similar initial burst, which adapts more quickly. 3. Type 1, 2, and 3 neurons are possible local interneurons or commissural DON neurons. Type 1 neurons demonstrate response properties similar to those of AENs. Type 2 cells demonstrated slowly adapting responses to excitatory stimuli, the duration of the response increased with the amplitude of the stimulus. Type 3 neurons demonstrated an increased rate of firing, but the response lacked any specific temporal characteristics. 4. ALLN fibers typically have receptive fields consisting of a single ampulla. The receptive field sizes of DON neurons exhibited varying degrees of convergence for different cell types. 5. Responses of ALLN fibers and DON neurons to weak sinusoidal stimuli demonstrated very similar frequency response characteristics for all cell types. The peak sensitivity of electrosensory neurons was between 5-10 Hz.     

Descending input from the vestibulolateral cerebellum suppresses electrosensory responses in the dorsal octavolateralis nucleus of the elasmobranch,Raja erinacea

1. The dorsal octavolateralis nucleus is the primary electrosensory nucleus in elasmobranchs and receives a major descending input from the dorsal granular ridge (DGR), a part of the vestibulolateral cerebellum. Removal of DGR altered the response properties of ascending efferent neurons (AENs), the projection neurons of the dorsal octavolateralis nucleus.
2. Elimination of DGR by lesion or lidocaine microinjection increased the excitability in AENs. Spontaneous activity increased by 680% and receptive fields became 1300% larger. The sensitivity of AENs to electric field stimuli increased by 560% and the time constant of adaptation increased by 300%, while threshold sensitivity remained unchanged.
3. Some electrosensory units responded to proprioceptive stimuli. In intact animals, the spontaneous activity of AENs was much less modulated by changes in fin position than primary electroreceptor afferents. Lesions to DGR appeared to increase the responsiveness of AENs to changes in fin position.
4. These results indicate that the action of DGR on the dorsal octavolateralis nucleus is primarily inhibitory and may function in a gain control mechanism. The possibility also exists for a mechanical-reafferent reduction mechanism in the electrosensory system of the elasmobranch that may be mediated by DGR.

     

The mechanism of SR95531 inhibition at GABA receptors examined in human alpha1beta1 and alpha1beta1gamma2S receptors

We examined the interaction of GABA and the competitive inhibitor SR95531 at human alpha1beta1gamma2S and alpha1beta1 GABA(A) receptors expressed in Sf9 cells. The efficacy and potency of inhibition depended on the relative timing of the GABA and SR95531 applications. In saturating (10 mM) GABA, the half-inhibitory concentrations of SR95531 (IC50) when coapplied with GABA to alpha1beta1gamma2S or alpha1beta1 receptors were 49 and 210 microM for the peak and 18 and 130 microM for the plateau current, respectively. Our data are explained by an inhibition mechanism in which SR95531 and GABA bind to two sites on the receptor where the binding of GABA allows channel opening but SR95531 does not. The SR95531 affinity for both receptor types was approximately 200 nM and the binding rate was found to be 10-fold faster than that for GABA. The dual binding-site model gives insights into the differential effects of GABA and SR95531 on the peak and plateau currents. The model predicts the effect of SR95531 on GABA currents in the synapse (GABA concentration approximately mM) and at extrasynaptic (GABA concentration < or = microM) sites. The IC50 (50-100 nM) for the synaptic response to SR95531 was insensitive to the GABA affinity of the receptors whereas the IC50 (50-800 nM) for extrasynaptic inhibition correlated with the GABA affinity.     

Commissural neurons of the electrosensory lateral line lobe of Apteronotus leptorhynchus: morphological and physiological characteristics

Extracellular injections of horseradish peroxidase were used to label commissural cells connecting the electrosensory lateral line lobes of the weakly electric fish Apteronotus leptorhynchus. Multiple commissural pathways exist; a caudal commissure is made up of ovoid cell axons, and polymorphic cells' axons project via a rostral commissure. Intracellular recording and labeling showed that ovoid cells discharge spontaneously at high rates, fire at preferred phases to the electric organ discharge, and respond to increased receptor afferent input with short latency partially adapting excitation. Ovoid cell axons ramify extensively in the rostro-caudal direction but are otherwise restricted to a single ELL subdivision. Polymorphic cells are also spontaneously active, but their firing is unrelated to the electric organ discharge waveform. They respond to increased receptor afferent activity with reduced firing frequency and response latency is long. Electrical stimulation of the commissural axons alters the behavior of pyramidal cells in the contralateral ELL. Basilar pyramidal cells are hyperpolarized and nonbasilar pyramidal cells are depolarized by this type of stimulation. The physiological results indicate that the ovoid cells participate in common mode rejection mechanisms and also suggest that the ELLs may function in a differential mode in which spatially restricted electrosensory stimuli can evoke heightened responses.Abbreviations ccELL caudal commissure of the ELL - CE contralaterally excited - DML dorsal molecular layer - ELL electrosensory lateral line lobe - EOD electric organ discharge - HRP horseradish peroxidase - IE ipsilaterally excited - MTI mouth-tail inverted - MTN mouth-tail normal - rcELL rostral commissure of the ELL - TRI transverse inverted - TRN transverse normal     

Dissociation of Muscimol, SR 95531, and Strychnine from GABAA and Glycine Receptors, Respectively, Suggests Similar Cooperative Interactions

Binding to gamma-aminobutyric acid-A (GABAA) receptors was studied in synaptosomal membranes of rat brain. Dissociation of [3H]muscimol and the GABAA antagonist [3H]2-(3-carboxypropyl)-3-amino-6-p-methoxyphenylpyridazinium bromide ([3H]SR 95531) binding elicited by 100-fold dilution was accelerated by excess of GABA or SR 95531. Control dissociation might be retarded by rebinding. The contribution of a rapid first phase of dissociation of the agonist [3H]muscimol was preferentially enhanced by SR 95531. In contrast, the dissociation of [3H]SR 95531 binding was preferentially accelerated by GABA. These opposite preferential accelerations can be explained by negative heterotropic cooperativity and a reversed affinity relationship of agonists and antagonists to GABAA binding sites with different affinities. Modification of the membranes by p-diazobenzenesulfonic acid (DSA) selectively decreased the accelerating effect of GABA on the dissociation of [3H]SR 95531 binding. [3H]Strychnine binding was studied in a membrane preparation of rat spinal cord. The dissociation of the antagonist [3H]strychnine elicited by dilution was preferentially accelerated by glycine. Again, pretreatment with DSA decreased selectively this negative heterotropic (i.e., allosteric) interaction. Chemical modification by DSA might be attributed to tyrosine residues responsible for similar allosteric interactions for the GABA- and glycine-gated chloride channels.     

Responses of neurons in the electrosensory lateral line lobe of the weakly electric fish <Emphasis Type="Italic">Gnathonemus petersii</Emphasis> to simple and complex electrosensory stimuli

Mormyrid fish use active electrolocation to detect and analyze objects. The electrosensory lateral line lobe in the brain receives input from electroreceptors and an efference copy of the command to discharge the electric organ. In curarized fish, we recorded extracellularly from neurons of the electrosensory lateral line lobe while stimulating in the periphery with either a local point stimulus or with a more natural whole-body stimulus. Two classes of neurons were found: (1) three types of E-cells, which were excited by a point stimulus; and (2) two types of I-cells, which were inhibited by point stimulus and responded with excitation to the electric organ corollary discharge. While all neurons responded to a point stimulus, only one out of two types of I-units and two of the three types of E-units changed their firing behavior to a whole-body stimulus or when an object was present. In most units, the responses to whole-body stimuli and to point stimuli differed substantially. Many electrosensory lateral line lobe units showed neural plasticity after prolonged sensory stimulation. However, plastic effects during whole body stimulation were often unlike those occurring during point stimuli, suggesting that under natural conditions electrosensory lateral line lobe network effects play an important role in shaping neural plasticity.     

Neurochemical correlates of gamma-aminobutyrate (GABA) inhibition in cat visual cortex

High affinity binding of [3H]gamma-aminobutyric acid (GABA) to neuronal membranes from different parts of cat visual cortex was tested for sensitivity to GABA(A) agonists isoguvacine and THIP, GABA(A) antagonist SR95531 and GABA(B) agonist baclofen. Some of the GABA(A)-binding sites were found to have a very low affinity for THIP, suggesting the presence and, possibly, uneven distribution of "non-synaptic" GABA(A) receptors in cat visual cortex. There were no differences in Km and Vmax values of high affinity uptake of GABA and in the potency of K(+)-stimulated release of GABA, between primary and association cortices. Consequently, the present results indicate that despite the anatomical and physiological differences between the primary and association feline visual cortices the neurochemical characteristics of GABAergic inhibition are very similar in the two regions.     

Time disparity sensitive behavior and its neural substrates of a pulse-type gymnotiform electric fish, Brachyhypopomus gauderio

Roles of the time coding electrosensory system in the novelty responses of a pulse-type gymnotiform electric fish, Brachyhypopomus, were examined behaviorally, physiologically, and anatomically. Brachyhypopomus responded with the novelty responses to small changes (100 μs) in time difference between electrosensory stimulus pulses applied to different parts of the body, as long as these pulses were given within a time period of ~500 μs. Physiological recording revealed neurons in the hindbrain and midbrain that fire action potentials time-locked to stimulus pulses with short latency (500–900 μs). These time-locked neurons, along with other types of neurons, were labeled with intracellular and extracellular marker injection techniques. Light and electron microscopy of the labeled materials revealed neural connectivity within the time coding system. Two types of time-locked neurons, the pear-shaped cells and the large cells converge onto the small cells in a hypertrophied structure, the mesencephalic magnocellular nucleus. The small cells receive a calyx synapse from a large cell at their somata and an input from a pear-shaped cell at the tip of their dendrites via synaptic islands. The small cells project to the torus semicircularis. We hypothesized that the time-locked neural signals conveyed by the pear-shaped cells and the large cells are decoded by the small cells for detection of time shifts occurring across body areas.     

Effects of global electrosensory signals on motion processing in the midbrain of <Emphasis Type="Italic">Eigenmannia</Emphasis>

Wave-type weakly electric fish such as Eigenmannia produce continuous sinusoidal electric fields. When conspecifics are in close proximity, interaction of these electric fields can produce deficits in electrosensory function. We examined a neural correlate of such jamming at the level of the midbrain. Previous results indicate that neurons in the dorsal layers of the torus semicircularis can (1) respond to jamming signals, (2) respond to moving electrosensory stimuli, and (3) receive convergent information from the four sensory maps of the electrosensory lateral line lobe (ELL). In this study we recorded the intracellular responses of both tuberous and ampullary neurons to moving objects. Robust Gaussian-shaped or sinusoidal responses with half-height durations between 55 ms and 581 ms were seen in both modalities. The addition of ongoing global signals with temporal-frequencies of 5 Hz attenuated the responses to the moving object by 5 dB or more. In contrast, the responses to the moving object were not attenuated by the addition of signals with temporal frequencies of 20 Hz or greater. This occurred in both the ampullary and tuberous systems, despite the fact that the ampullary afferents to the torus originate in a single ELL map whereas the tuberous afferents emerge from three maps.     

The role of amino acid neurotransmitters in the descending control of electroreception

The roles of amino acid neurotransmitters in determining the processing characteristics of the electrosensory lateral line lobe (ELL) in Apteronotus leptorhynchus were investigated by studying the responses of ELL output neurons to pressure ejection of various neurotransmitter agonists and antagonists alone and in combination with simple electrosensory stimuli.

1. Pressure ejection of L-glutamate into the ELL dorsal molecular layer caused either excitation or inhibition of ELL efferent neurons (pyramidal cells). The sign of these responses reversed with changes in the position of the pressure pipette. Histological verification of drug ejection sites relative to recorded cells and diffusion estimates indicate that excitatory and inhibitory responses result from glutamate activation of pyramidal cells and of inhibitory interneurons, respectively.
2. ELL output cells respond to both NMDA and non-NMDA glutamate agonists and the responses are attenuated by co-ejection of specific antagonists indicating that both AMPA/kainate and NMDA receptors exist on pyramidal cell apical dendrites.
3. Gamma-aminobutyric acid inhibits basilar and nonbasilar pyramidal cells when ejected near their apical dendrites and disinhibits them when ejected near surrounding inhibitory interneurons confirming the presence of GABA receptors on these cell types.
4. An NMDA antagonist did not alter pyramidal cell responses to electrosensory stimuli but a non-NMDA antagonist altered both responses to the stimuli and firing frequency shortly following stimulus cessation.

     

Response properties and biological function of the skate electrosensory system during ontogeny

This study examined the response properties of skate electrosensory primary afferent neurons of pre-hatch embryo (8–11 weeks), post-hatch juvenile (1–8 months), and adult (>2 year) clearnose skates (Raja eglanteria) to determine whether encoding of electrosensory information changes with age, and if the electro-sense is adapted to encode natural bioelectric stimuli across life history stages. During ontogeny, electrosensory primary afferents increase resting discharge rate, spike regularity, and sensitivity at best frequency. Best frequency was at 1–2 Hz for embryos, showed an upwards shift to 5 Hz in juveniles, and a downward shift to 2–3 Hz in adults. Encapsulated embryos exhibit ventilatory movements that are interrupted by a “freeze response”” when presented with weak uniform fields at 0.5 and 1 Hz. This phasic electric stimulus contains spectral information found in potentials produced by natural fish predators, and therefore indicates that the embryo electrosense can efficiently mediate predator detection and avoidance. In contrast, reproductively active adult clearnose skates discharge their electric organs at rates near the peak frequency sensitivity of the adult electrosensory system, which; facilitates electric communication during social behavior. We suggest that life-history-dependent functions such as these may shape the evolution of the low-frequency response properties for the elasmobranch electrosensory system. Accepted: 19 February 1998     

Neural simulations of adaptive reafference suppression in the elasmobranch electrosensory system

The electrosensory system of elasmobranchs is extremely sensitive to weak electric fields, with behavioral thresholds having been reported at voltage gradients as low as 5 nV/cm. To achieve this amazing sensitivity, the electrosensory system must extract weak extrinsic signals from a relatively large reafferent background signal associated with the animal's own movements. Ventilatory movements, in particular, strongly modulate the firing rates of primary electrosensory afferent nerve fibers, but this modulation is greatly suppressed in the medullary electrosensory processing nucleus, the dorsal octavolateral nucleus. Experimental evidence suggests that the neural basis of reafference suppression involves a common-mode rejection mechanism supplemented by an adaptive filter that fine tunes the cancellation. We present a neural model and computer simulation results that support the hypothesis that the adaptive component may involve an anti-Hebbian form of synaptic plasticity at molecular layer synapses onto ascending efferent neurons, the principal output neurons of the nucleus. Parallel fibers in the molecular layer carry a wealth of proprioceptive, efference copy, and sensory signals related to the animal's own movements. The proposed adaptive mechanism acts by canceling out components of the electrosensory input signal that are consistently correlated with these internal reference signals.Abbreviations AEN ascending efferent neuron - AFF primary afferent nerve fiber - DGR dorsal granular ridge - DON dorsal octavolateral nucleus - ELL electrosensory lateral line lobe - GABA -aminobutyric acid - IN inhibitory interneuron - ISI interspike interval - ST stellate cell     

Ionic and neuromodulatory regulation of burst discharge controls frequency tuning

Sensory neurons encode natural stimuli by changes in firing rate or by generating specific firing patterns, such as bursts. Many neural computations rely on the fact that neurons can be tuned to specific stimulus frequencies. It is thus important to understand the mechanisms underlying frequency tuning. In the electrosensory system of the weakly electric fish, Apteronotus leptorhynchus, the primary processing of behaviourally relevant sensory signals occurs in pyramidal neurons of the electrosensory lateral line lobe (ELL). These cells encode low frequency prey stimuli with bursts of spikes and high frequency communication signals with single spikes. We describe here how bursting in pyramidal neurons can be regulated by intrinsic conductances in a cell subtype specific fashion across the sensory maps found within the ELL, thereby regulating their frequency tuning. Further, the neuromodulatory regulation of such conductances within individual cells and the consequences to frequency tuning are highlighted. Such alterations in the tuning of the pyramidal neurons may allow weakly electric fish to preferentially select for certain stimuli under various behaviourally relevant circumstances.     

The functional characteristics of electroreceptive central neurons of the sea catfish Plotosus anguillaris

With the use of microelectrode techniques (extracellular recordings) and the method of post-stimulus histograms, the functional characteristics of medulla oblongata neurons of sea catfish Plotosus were investigated under stimulation of electroreceptors by a homogeneous electric field of different duration, intensity, and direction. Two types of the cells possessing, accordingly, tonic or phase activity were registered among 66 neurons investigated. The mode of responses (inhibition or acceleration) of tonic neurons to the direction of the applied electric current is typical for central neurons of fresh-water catfish connected with ampullae's electroreceptors. Neurons showing a substantial response to fields of an intensity less than 1 microV/cm were registered. The reactions were most pronounced with the duration of electric stimuli in the range of 20-200 ms; however, particularly sensitive neurons showed distinct responses to stimuli of duration of 5 and even 2 ms. Thus, for the first time a high sensitivity of ampullae's electroreceptors to high-frequency stimulus was discovered, which allows one to expand the range of studying electric signals used by weakly electric fish for electrolocation and communication.     

Neuroethology and life history adaptations of the elasmobranch electric sense.

The electric sense of elasmobranch fishes (sharks and rays) is an important sensory modality known to mediate the detection of bioelectric stimuli. Although the best known function for the use of the elasmobranch electric sense is prey detection, relatively few studies have investigated other possible biological functions. Here, we review recent studies that demonstrate the elasmobranch electrosensory system functions in a wide number of behavioral contexts including social, reproductive and anti-predator behaviors. Recent work on non-electrogenic stingrays demonstrates that the electric sense is used during reproduction and courtship for conspecific detection and localization. Electrogenic skates may use their electrosensory encoding capabilities and electric organ discharges for communication during social and reproductive interactions. The electric sense may also be used to detect and avoid predators during early life history stages in many elasmobranch species. Embryonic clearnose skates demonstrate a ventilatory freeze response when a weak low-frequency electric field is imposed upon the egg capsule. Peak frequency sensitivity of the peripheral electrosensory system in embryonic skates matches the low frequencies of phasic electric stimuli produced by natural fish egg-predators. Neurophysiology experiments reveal that electrosensory tuning changes across the life history of a species and also seasonally due to steroid hormone changes during the reproductive season. We argue that the ontogenetic and seasonal variation in electrosensory tuning represent an adaptive electrosensory plasticity that may be common to many elasmobranchs to enhance an individual's fitness throughout its life history.     

GABAA Receptor Populations Bind Agonists and Antagonists Differentially and with Opposite Affinities

Pretreatment of synaptosomal membranes with a diazo-coupling reagent and the presence of Cl- ions were used to differentiate high- and low-affinity populations of postsynaptic gamma-aminobutyric acid (GABAA) receptors. The super-low-affinity GABAA receptors were characterized by the enhancing effect of GABA on [3H]diazepam binding. The GABA antagonists 2-(3-carboxypropyl)-3-amino-4-methyl-6-phenylpyridazinium chloride (SR 95103) and 3-alpha-hydroxy-16-imino-5 beta-17-aza-androstan-11-one (R 5135) shifted and suppressed the dose-response curve of GABA on diazepam binding. SR 95103 displaced the lower affinity [3H]GABA binding with higher potency. Dissociation of the binding of the antagonist 2-(3-carboxypropyl)-3-amino-6-p-methoxyphenylpyridazinium bromide ([3H]SR 95531) was polyphasic. Displacing potencies of SR 95531 and GABA were examined on the major (85%) rapid and minor slower phases of dissociation separated kinetically. The slower phase corresponded to higher affinity binding of SR 95531 which was displaced by GABA with about 10 times less potency. Photoaffinity labeling with muscimol decreased the number of [3H]muscimol binding sites by 27%. It decreased the displacing potency of GABA by 72%, but not that of bicuculline methiodide. These findings can be explained by a preferential binding of antagonists to hydrophobic accessory sites around low-affinity GABAA receptors.     

Phantoms in the brain: Ambiguous representations of stimulus amplitude and timing in weakly electric fish

In wave-type weakly electric fish, two distinct types of primary afferent fibers are specialized for separately encoding modulations in the amplitude and phase (timing) of electrosensory stimuli. Time-coding afferents phase lock to periodic stimuli and respond to changes in stimulus phase with shifts in spike timing. Amplitude-coding afferents fire sporadically to periodic stimuli. Their probability of firing in a given cycle, and therefore their firing rate, is proportional to stimulus amplitude. However, the spike times of time-coding afferents are also affected by changes in amplitude; similarly, the firing rates of amplitude-coding afferents are also affected by changes in phase. Because identical changes in the activity of an individual primary afferent can be caused by modulations in either the amplitude or phase of stimuli, there is ambiguity regarding the information content of primary afferent responses that can result in ‘phantom’ modulations not present in an actual stimulus. Central electrosensory neurons in the hindbrain and midbrain respond to these phantom modulations. Phantom modulations can also elicit behavioral responses, indicating that ambiguity in the encoding of amplitude and timing information ultimately distorts electrosensory perception. A lack of independence in the encoding of multiple stimulus attributes can therefore result in perceptual illusions. Similar effects may occur in other sensory systems as well. In particular, the vertebrate auditory system is thought to be phylogenetically related to the electrosensory system and it encodes information about amplitude and timing in similar ways. It has been well established that pitch perception and loudness perception are both affected by the frequency and intensity of sounds, raising the intriguing possibility that auditory perception may also be affected by ambiguity in the encoding of sound amplitude and timing.     

GABA能抑制调制大棕蝠下丘神经元对串声刺激的强度敏感性

本文采用不同重复率的串声刺激,模拟大棕蝠回声定位不同阶段听到的调频声纳信号,利用电生理方法和微电泳技术研究不同重复率串刺激条件下GABA能抑制对下丘神经元强度敏感性的影响。结果发现,随串刺激重复率的增加,有的神经元强度敏感性增强,有的神经元强度敏感性则降低。在不同串刺激条件下,微电泳荷包牡丹碱,神经元放电率均增加,随重复率增加强度敏感性增强或减弱的趋势消失,提示GABA能抑制调制下丘神经元对不同重复率串刺激反应的强度敏感性。串刺激强度在最低闽值附近时,微电泳荷包牡丹碱导致放电率增加的百分率最大,随串刺激强度增加,放电率增加的百分率逐渐减小。提示刺激强度较低时,GABA能抑制对下丘神经元强度敏感性的影响更有效。     

Neurochemical architecture of the central complex related to its function in the control of grasshopper acoustic communication

The central complex selects and coordinates the species- and situation-specific song production in acoustically communicating grasshoppers. Control of sound production is mediated by several neurotransmitters and modulators, their receptors and intracellular signaling pathways. It has previously been shown that muscarinic cholinergic excitation in the central complex promotes sound production whereas both GABA and nitric oxide/cyclic GMP signaling suppress its performance. The present immunocytochemical and pharmacological study investigates the question whether GABA and nitric oxide mediate inhibition of sound production independently. Muscarinic ACh receptors are expressed by columnar output neurons of the central complex that innervate the lower division of the central body and terminate in the lateral accessory lobes. GABAergic tangential neurons that innervate the lower division of the central body arborize in close proximity of columnar neurons and thus may directly inhibit these central complex output neurons. A subset of these GABAergic tangential neurons accumulates cyclic GMP following the release of nitric oxide from neurites in the upper division of the central body. While sound production stimulated by muscarine injection into the central complex is suppressed by co-application of sodium nitroprusside, picrotoxin-stimulated singing was not affected by co-application of this nitric oxide donor, indicating that nitric oxide mediated inhibition requires functional GABA signaling. Hence, grasshopper sound production is controlled by processing of information in the lower division of the central body which is subject to modulation by nitric oxide released from neurons in the upper division.