Klaus Schildberger

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.

Assessment strategy of fighting crickets revealed by manipulating information exchange

The assessment strategy used by adult male crickets, Gryllus bimaculatus (de Geer), during conspecific aggression was deduced from the effects of handicaps (disabled mandibles, blackened eyes, size) and altered aggressive motivation (after defeat and flying) on aggressiveness, fight duration and win probability under laboratory conditions. The contestants received either the same (symmetrical) or different (asymmetrical) treatments. Despite handicaps, all crickets showed essentially normal escalating fighting behaviour. In symmetrical contests, fights became even harsher and longer with multiple handicaps. In asymmetrical contests, neither disabling the mandibles nor blackening the eyes significantly influenced win probability. However, ‘blinded’ crickets nearly always defeated opponents with disabled mandibles that could not inflict bites. Furthermore, small crickets and losers showed physical fighting more frequently when prevented from seeing. Handicap effects became more pronounced when both contestants were flown, to maximize aggressive motivation, before fighting. Our findings conform to the cumulative assessment hypothesis. We conclude, first, that contestants need longer to accumulate the cues required to surpass a threshold to flee when visual and physical inputs are reduced. Second, the decision to flee is based solely on the opponents actions. Third, flying enhances aggressiveness by reducing disparities in motivation, making other disparities, such as in weaponry, more decisive. Thus, a crickets aggressiveness is a trade-off between its inherent, experience-dependent agonistic motivation and the accumulated perceived actions of the adversary.

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

SUMMARY 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.

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 opponents 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. J. Comp. Neurol. 519:1677–1690, 2011.

Female crickets are driven to fight by the male courting and calling songs

Crickets have traditional sex roles, where males compete aggressively for access to selective polyandrous females. However, in a laboratory experiment, we found that normally nonaggressive female Gryllus campestris fought each other vigorously in the presence of a courting male, resulting in a dominant female that gained a greater probability of receiving the spermatophore. Female–female fights included the same series of characteristic actions known from male–male fights, which demonstrates that the females can perform the full repertoire of agonistic actions except for the production of the aggressive rival song. Since females remained nonaggressive towards each other in the vicinity of a muted male, but were induced to fight each other in the complete absence of a male by the auditory experience of the courtship song, this song is both sufficient and necessary to induce female competition for males. Calling song was as effective as courtship song at inducing female aggression, whereas rival song was least effective. We therefore speculate that the calling and courtship songs may signal a males resource value.