α4βδ-GABA _A receptors in dorsal hippocampal CA1 of adolescent female rats traffic to the plasma membrane of dendritic spines following voluntary exercise and contribute to protection of animals from activity-based anorexia through its localization at excitatory synapses

Abstract

In hippocampal CA1 of adolescent female rodents, α4βδ-GABA _A receptors (α4βδ-GABA _ARs) suppress excitability of pyramidal neurons through shunting inhibition at excitatory synapses. This contributes to anxiolysis of stressed animals. Socially isolated adolescent female rats with 8 days of wheel access, the last 4 days of which are restricted of food access, have been shown to exhibit excessive exercise, choosing to run instead of eat (activity-based anorexia, ABA). Up-regulation of α4βδ-GABA _ARs in the dorsal hippocampal CA1 (DH), seen among some ABA animals, correlates with suppression of excessive exercise. We used electron microscopic immunocytochemistry to show that exercise alone (EX), but not food-restriction alone (FR), also augments α4βδ-GABA _AR expression at axo-spinous excitatory synapses of the DH (67%, P= 0.027), relative to socially isolated controls without exercise or food-restriction (CON). Relative to CON, ABA animals’ synaptic α4βδ-GABA _AR elevation was modestly elevated (37%), but, this level correlated strongly and negatively with individual differences in ABA vulnerability – i.e., food-restriction-evoked hyperactivity (Pearson’s R=-0.902, P=0.002) and weight changes ( R=0.822, P=0.012). These correlations were absent from FR and EX brains or ventral hippocampus of ABA brains. Comparison to CON of α4βδ-GABA _AR location in the DH indicated that ABA induces trafficking of α4βδ-GABA _AR from reserve pools in spine cytoplasm to excitatory synapses. Pair-housing control animals reduced cytoplasmic α4βδ-GABA _AR without reducing synaptic α4βδ-GABA _AR. Thus, exercise induces trafficking of α4βδ-GABA _ARs to excitatory synapses, while individual differences in ABA vulnerability are linked most strongly to trafficking of α4βδ-GABA _ARs in the reverse direction - from excitatory synapses to the reserve pool during co-occurring food-restriction.

Graphical abstract

EM-immunocytochemnistry reveals that trafficking of α4βδ−GABA _A receptors to synaptic clefts of excitatory synapses in the hippocampus is influenced by food restriction (FR), voluntary exercise (EX), and social isolation (SI). These α4βδ−GABA _A receptors contribute towards adolescent female rats’ resilience to activity-based anorexia (ABA), an excessive EX behavior induced by FR.