Quantification of dendritic and axonal growth after injury to the auditory system of the adult cricket Gryllus bimaculatus

Abstract

Dendrite and axon growth and branching during development are regulated by a complex set of intracellular and external signals. However, the cues that maintain or influence adult neuronal morphology are less well understood. Injury and deafferentation tend to have negative effects on adult nervous systems. An interesting example of injury-induced compensatory growth is seen in the cricket, Gryllus bimaculatus. After unilateral loss of an ear in the adult cricket, auditory neurons within the central nervous system (CNS) sprout to compensate for the injury. Specifically, after being deafferented, ascending neurons (AN-1 and AN-2) send dendrites across the midline of the prothoracic ganglion where they receive input from auditory afferents that project through the contralateral auditory nerve (N5). Deafferentation also triggers contralateral N5 axonal growth. In this study, we quantified AN dendritic and N5 axonal growth at 30 h, as well as at 3, 5, 7, 14, and 20 days after deafferentation in adult crickets. Significant differences in the rates of dendritic growth between males and females were noted. In females, dendritic growth rates were non-linear; a rapid burst of dendritic extension in the first few days was followed by a plateau reached at 3 days after deafferentation. In males, however, dendritic growth rates were linear, with dendrites growing steadily over time and reaching lengths, on average, twice as long as in females. On the other hand, rates of N5 axonal growth showed no significant sexual dimorphism and were linear. Within each animal, the growth rates of dendrites and axons were not correlated, indicating that independent factors likely influence dendritic and axonal growth in response to injury in this system. Our findings provide a basis for future study of the cellular features that allow differing dendrite and axon growth patterns as well as sexually dimorphic dendritic growth in response to deafferentation.

De novo assembly of a transcriptome for the cricket Gryllus bimaculatus prothoracic ganglion: An invertebrate model for investigating adult central nervous system compensatory plasticity

“…In addition, one could imagine Eph-Ephrin signaling being important for targeting growing dendrites to the appropriate region, identifying the point at which they should stop growing, and aiding in the formation of contralateral synapses. Previous work has shown a sexual dimorphism in male and female dendritic growth after deafferentation [ 13 ], with female dendrites growing only half as far as male dendrites. If Ephrin signaling were involved in stopping dendritic growth and facilitating synapse formation, one might predict a sexual dimorphism in Eph/Ephrin signaling after deafferentation.…”

“…In order to control for the results of future single cell RNA-Seq experiments, in which individual AN cells will be fluorescently labeled and collected, prothoracic ganglia were backfilled using conjugated biocytin Alexa-fluor 488 (green) from Invitrogen. Backfills of AN axons via the neck connectives were performed on control crickets 24 hours post-control-amputation as described previously [ 13 ]. Crickets were stored at 4°C in a moist chamber for 18–24 hours to allow the backfill to complete.…”

“…Removing auditory input to AN-2 (deafferentation) induces the AN-2 dendrites to grow across the midline, a boundary rarely crossed by innervated dendrites, and make functional synaptic connections with the contralateral auditory afferents ( Fig 1B ). This compensatory anatomical response is evident both during development [ 12 ] and in adulthood [ 11 , 13 ]. The evidence of anatomical neuronal plasticity in adults provides a unique and interesting point of inquiry into if and how mechanisms of plasticity shift between development and adults.…”

“…AN-2 dendrites sprout across the midline and become synaptically connected to the contralateral afferents, compensating for the loss of the ear. Fig adapted from [ 13 ].…”

Plasticity in invertebrate sensory systems

“…Despite their often being considered hard-wired, the nervous systems of invertebrates are in fact plastic, just as in vertebrates, both during development and in the adult. After injury, as reported in the article by Pfister et al (2012) neurite outgrowth occurs. For example deafferentated neurons in the auditory system of orthopteran insects undergo dendritic and axonal growth.…”