Synergistic Action of Presynaptic Muscarinic Acetylcholine Receptors and Adenosine Receptors in Developmental Axonal Competition at the Neuromuscular Junction

Abstract

Listen

Translate article

The development of the nervous system involves the initial overproduction of synapses, which promotes connectivity. Hebbian competition between axons with different activities leads to the loss of roughly half of the overproduced elements and this refines connectivity. We used quantitative immunohistochemistry to investigate, in the postnatal day 7 (P7) to P9 neuromuscular junctions, the involvement of muscarinic receptors (muscarinic acetylcholine autoreceptors and the M₁, M₂, and M₄ subtypes) and adenosine receptors (A₁ and A_2A subtypes) in the control of axonal elimination after the mouse levator auris longus muscle had been exposed to selective antagonists in vivo. In a previous study we analyzed the role of each of the individual receptors. Here we investigate the additive or occlusive effects of their inhibitors and thus the existence of synergistic activity between the receptors. The main results show that the A_2A, M₁, M₄, and A₁ receptors (in this order of ability) delayed axonal elimination at P7. M₄ produces some occlusion of the M₁ pathway and some addition to the A₁ pathway, which suggests that they cooperate. M₂ receptors may modulate (by allowing a permissive action) the other receptors, mainly M₄ and A₁. The continued action of these receptors (now including M₂ but not M₄) finally promotes axonal loss at P9. All 4 receptors (M₂, M₁, A₁, and A_2A, in this order of ability) are necessary. The M₄ receptor (which in itself does not affect axon loss) seems to modulate the other receptors. We found a synergistic action between the M₁, A₁, and A_2A receptors, which show an additive effect, whereas the potent M₂ effect is largely independent of the other receptors (though can be modulated by M₄). At P9, there is a full mutual dependence between the A₁ and A_2A receptors in regulating axon loss. In summary, postnatal axonal elimination is a regulated multireceptor mechanism that involves the cooperation of several muscarinic and adenosine receptor subtypes.