Mechanisms of experience dependent control of aggression in crickets

Aggression is a highly plastic behaviour, shaped by numerous experiences, and potential costs and benefits of competing, to optimize fitness and survival. Recent studies on crickets provide insights into how nervous systems achieve this. Their fighting behaviour is promoted by physical exertion, winning disputes and possession of resources. These effects are each mediated by octopamine, the invertebrate analogue of noradrenaline. Submissive behaviour, in less well understood. It is induced when the accumulated sum of the opponent's agonistic signals surpass some critical level, and probably mediated by nitric oxide, serotonin and other neuromodulators. We propose that animals can make the decision to fight or flee by modulating the respective behavioural thresholds in response to potentially rewarding and aversive attributes of experiences.

Neuronal organization of a fast-mediating cephalothoracic pathway for antennal-tactile information in the cricket (Gryllus bimaculatus DeGeer)

Crickets use their long antennae as tactile sensors. Confronted with obstacles, conspecifics, or predators, antennal contacts trigger short-latency motor responses. To reveal the neuronal pathway underlying these antennal-guided locomotory reactions we identified descending interneurons that rapidly transmit antennal-tactile information from the head to the thorax in the cricket Gryllus bimaculatus. Antennae were stimulated with forces approximating those of naturally occurring antennal contacts. Responding interneurons were individually identified by intracellular axon recordings in the pro-mesothoracic connective and subsequent tracer injection. Simultaneous with the intracellular recordings, the overall spike response in the neck connectives was recorded extracellularly to reveal the precise response-timing of each individual neuron within the collective multiunit response. Here we describe four descending brain neurons and two with the soma in the subesophageal ganglion. All antennal-touch elicited action potentials apparent in the neck connective recordings within 10 ms after antennal-contact are generated by these six interneurons. Their dendrites ramify in primary antennal-mechanosensory neuropils of the head ganglia. Each of them consistently generated action potentials in response to antennal touching and three of them responded also to different visual stimulation (light-off, movement). Their descending axons conduct action potentials with 3-5 m/s to the thoracic ganglia where they send off side branches in dorsal neuropils. Their physiological and anatomical properties qualify them as descending giant fibers in the cricket and suggest an involvement in evoking fast locomotory reactions. They form a fast-mediating cephalo-thoracic pathway for antennal-tactile information, whereas all other antennal-tactile interneurons had response latencies exceeding 40 ms.

Octopamine and occupancy: an aminergic mechanism for intruder-resident aggression in crickets

Aggression is a behavioural strategy for securing resources (food, mates and territory) and its expression is strongly influenced by their presence and value. While it is known that resource holders are generally highly aggressive towards intruding consexuals and usually defeat them, the underlying neuronal mechanisms are not known. In a novel intruder-resident paradigm for field crickets (Gryllus bimaculatus), we show that otherwise submissive losers of a preceding aggressive encounter readily fight and often defeat aggressive winners after occupying an artificial shelter. This aggression enhancing effect first became evident after 2 min residency, and was maximal after 15 min, but absent 15 min after shelter removal. The residency effect was abolished following non-selective depletion of biogenic amines from the central nervous system using reserpine, or semi-selective depletion of octopamine and dopamine using α-methyl-tyrosine, but not following serotonin depletion using α-methyl-tryptophan. The residency effect was also abolished by the treatment with phentolamine, an α-adrenergic receptor antagonist, or epinastine, a highly selective octopamine receptor blocker, but not by propranolol, a ß-adrenergic receptor antagonist, or by yohimbine, an insect tyramine receptor blocker. We conclude that crickets evaluate residency as a rewarding experience that promotes aggressive motivation via a mechanism involving octopamine, the invertebrate analogue of noradrenaline.

A muscarinic cholinergic mechanism underlies activation of the central pattern generator for locust flight

A central question in behavioural control is how central pattern generators(CPGs) for locomotion are activated. This paper disputes the key role generally accredited to octopamine in activating the CPG for insect flight. In deafferented locusts, fictive flight was initiated by bath application of the muscarinic agonist pilocarpine, the acetylcholine analogue carbachol, and the acetylcholinesterase blocker eserine, but not by nicotine. Furthermore, in addition to octopamine, various other amines including dopamine, tyramine and histamine all induced fictive flight, but not serotonin or the amine-precursor amino acid tyrosine. However, flight initiation was not reversibly blocked by aminergic antagonists, and was still readily elicited by both natural stimulation (wind) and pilocarpine in reserpinized, amine-depleted locusts. By contrast, the muscarinic antagonists atropine and scopolamine reversibly blocked flight initiated by wind, cholinergic agonists, octopamine, and by selective stimulation of a flight-initiating interneurone (TCG). The short delay from TCG stimulation to flight onset suggests that TCG acts directly on the flight CPG, and accordingly that TCG, or its follower cell within the flight generating circuit, is cholinergic. We conclude that acetylcholine acting via muscarinic receptors is the key neurotransmitter in the mechanism underlying the natural activation of the locust flight CPG. Amines are not essential for this, but must be considered as potential neuromodulators for facilitating flight release and tuning the motor pattern. We speculate that muscarinic activation coupled to aminergic facilitation may be a general feature of behavioural control in insects for ensuring conditional recruitment of individual motor programs in accordance with momentary adaptive requirements.

Octopamine and experience-dependent modulation of aggression in crickets

Intraspecific aggression is influenced in numerous animal groups by the previous behavioral experiences of the competitors. The underlying mechanisms are, however, mostly obscure. We present evidence that a form of experience-dependent plasticity of aggression in crickets is mediated by octopamine, the invertebrate counterpart of noradrenaline. In a forced-fight paradigm, the experience of flying maximized the aggressiveness of crickets at their first encounter and accelerated the subsequent recovery of aggressiveness of the normally submissive losers, without enhancing general excitability as evaluated from the animals' startle responses to wind stimulation. This effect is transitory and concurrent with the activation of the octopaminergic system that accompanies flight. Hemocoel injections of the octopamine agonist chlordimeform (CDM) had similar effects on aggression but also enhanced startle responses. Serotonin depletion, achieved using alpha-methyl-tryptophan, enhanced startle responses without influencing aggression, indicating that the effect of CDM on aggression is not attributable to increased general excitation. Contrasting this, aggressiveness was depressed, and the effect of flying was essentially abolished, in crickets depleted of octopamine and dopamine using alpha-methyl-p-tyrosine (AMT). CDM restored aggressiveness in AMT-treated crickets, indicating that their depressed aggressiveness is attributable to octopamine depletion rather than to dopamine depletion or nonspecific defects. Finally, the flight effect was blocked in crickets treated with the octopamine receptor antagonist epinastine, or with the alpha-adrenoceptor and octopamine receptor antagonist phentolamine, but not with the beta-adrenoceptor antagonist propranolol. The idea that activity-specific induction of the octopaminergic system underlies other forms of experience-dependent plasticity of aggressive motivation in insects is discussed.

FMRFamide modulates potassium currents in circadian pacemaker neurons of Bulla gouldiana

The peptide FMRFamide (Phe-Met-Arg-Phe-NH(2)) is known to modulate the circadian pacemaker found in the eye of the marine snail Bulla gouldiana. In the present study, we investigated the cellular mechanisms underlying this modulation by examining the effects of FMRFamide on the membrane properties of the circadian pacemaker cells, known as basal retinal neurons in this preparation. Bath application of FMRFamide (0.1-1 microM) increased the membrane conductance, and hyperpolarized the membrane potential of these neurons. Next, perforated-patch recordings were used to demonstrate that FMRFamide reversibly increased the outward current amplitude due to an augmentation of a non-inactivating calcium-independent current. Reversal potential of the tail currents and its dependence on extracellular potassium concentration suggested potassium ions as the charge carrier for this current. The peptide-modulated outward current was blocked by 54% after bath application of the potassium channel blocker tetraethylammonium chloride and completely blocked by substituting cesium for intracellular potassium. Voltage dependence, activation kinetics and tail current kinetics of the FMRFamide-modulated current were consistent with values found for the delayed rectifier current.Overall, our data suggest that FMRFamide modulates a delayed rectifier potassium current and at least one other, less voltage-dependent conductance. This provides a mechanistic explanation for FMRFamide's ability to both shift the phase and attenuate light-induced phase shifts of the circadian pacemaker in B. gouldiana.

The fight and flight responses of crickets depleted of biogenic amines

Aggressive and escape behaviors were analysed in crickets (Orthoptera) treated with either reserpine, a nonspecific depleter of biogenic amines, or the synthesis inhibitors alpha-methyltryptophan (AMTP) and alpha-methyl-p-tyrosine (AMT) to specifically deplete serotonin, respectively dopamine and octopamine. Standard immunocytochemical techniques were used to verify depletion from central nervous tissue, and determine the effective dosages. Reserpinized crickets became exceedingly lethargic and had severely depressed escape responses. However, they were still able to express all the major elements of the escalating sequences of stereotype motor performances that typifies normal aggressive behavior in the cricket. AMT and AMTP treatment had opposing influences on escape behavior, being enhanced by serotonin depletion, but depressed by dopamine/octopamine depletion. AMTP-induced serotonin depletion had no influence on aggressive or submissive behaviors. AMT-treated crickets could normally only be brought to fight by coaxing. Though capable of expressing aggressive behavior per se, agonistic encounters between AMT-treated crickets were shorter, and rarely involved actual physical interactions. Hence, although amines seem to have similar actions on escape behavior in insects and crustaceans, the aminergic control of aggression seems to be fundamentally different in these arthropods groups. We conclude that amines are not in principle required for the initiation and operation of the motor circuits underlying aggression in the cricket. However, octopamine and/or dopamine seem necessary for establishing a level of excitability sufficient for aggressive behavior to become overt in response to appropriate natural releasing stimuli.

Gating of sensory responses of descending brain neurones during walking in crickets

Single descending brain neurones were recorded and stained intracellularly in the neck connectives of crickets while they walked upon a styrofoam ball under open-loop conditions. The animal's translational and rotational velocities were measured simultaneously, and various stimuli were used to investigate the neuronal response characteristics. Stimulation with a moving grating or an artificial calling song of 5 kHz induced optomotor behaviour and positive phonotaxis. An acoustic stimulus of 20 kHz elicited negative phonotaxis. <P> We report the first clear evidence for behaviourally dependent gating of sensory responses of identified descending brain neurones. Most descending cells only responded to visual stimuli or to an artificial calling song of 5 kHz while the animal was walking, indicating that the responses to these stimuli were gated by the walking activity of the animal. In contrast to this, responses to stimuli that elicit negative phonotaxis, such as acoustic stimuli of 20 kHz, were not gated. This indicates that the gating of sensory responses in these cells depends on the behavioural context of the stimulus. <P> From these findings, we conclude that significant information about the properties of sensory processing in higher-order neurones can only be gained from tests in behaviourally relevant paradigms. Important characteristics might otherwise be missed, thus leading to misinterpretations regarding their function. <P>

Local interneurons associated with the mushroom bodies and the central body in the brain of Acheta domesticus

Within the central part of the brain in male crickets (Acheta domesticus) 150 local interneurons were recorded intracellularly and consequently stained with either Lucifer Yellow CH or cobaltous sulphide. The shapes of the injected cells were reconstructed from serial sections and drawn to scale using known brain regions as landmarks. Several types of neurons were encountered, which differ in their topographical relationships to the mushroom bodies and the central body. Extensive branching was often found in the calyces or the lobes of the mushroom bodies, within the lateral protocerebral neuropil and in the columns of the central body. These neurons connect different parts of the brain with the neuronal assemblies of the mushroom bodies and the central body, structures, which are considered to participate in the control of complex behavior. The role of the mushroom body with respect to its connectivity to other brain areas and various feedback loops is discussed.