Gliotransmission and adenosinergic modulation: insights from mammalian spinal motor networks

Abstract

Astrocytes are proposed to converse with neurons at tripartite synapses, detecting neurotransmitter release and responding with release of gliotransmitters, which in turn modulate synaptic strength and neuronal excitability. However, a paucity of evidence from behavioral studies calls into question the importance of gliotransmission for the operation of the nervous system in healthy animals. Central pattern generator (CPG) networks in the spinal cord and brain stem coordinate the activation of muscles during stereotyped activities such as locomotion, inspiration, and mastication and may therefore provide tractable models in which to assess the contribution of gliotransmission to behaviorally relevant neural activity. We review evidence for gliotransmission within spinal locomotor networks, including studies indicating that adenosine derived from astrocytes regulates the speed of locomotor activity via metamodulation of dopamine signaling.

Fig. 1.

Established model of bidirectional signaling between neurons and glia at the tripartite synapse. Neurotransmitters activate astrocytic Gα_q-linked G protein-coupled receptors (GPCRs), stimulating the production of inositol trisphosphate (IP₃), activation of astrocytic IP₃ receptors (IP₃R2), and release of Ca²⁺ from internal stores. An increase in intracellular Ca²⁺ triggers release of gliotransmitters including glutamate, ATP, and d-serine by a vesicular or channel-mediated mechanism; these in turn bind to pre- or postsynaptic GPCRs, or in the case of d-serine, N-methyl-d-aspartate (NMDA) receptors, to modulate synaptic strength or neuronal excitability.

Fig. 2.

Adenosine derived from glial cells acts via A₁ receptors to modulate locomotor-related activity in a murine spinal cord preparation. A: an in vitro spinal cord preparation in which suction electrodes are used to recording compound action potentials from motoneurons in transected ventral roots. B: sample raw (top) and rectified/integrated (bottom) ventral root traces showing rhythmic locomotor-related bursting evoked by bath application of 5-hydroxytryptamine (10 µM), NMDA (5 µM), and dopamine (50 µM) in control conditions, following application of Thr-Phe-Leu-Leu-Arg-NH₂ trifluoroacetate salt (TFLLR; 10 µM) to selectively activate glial protease-activated receptor-1 (PAR1) and during washout. Ci: locomotor-burst frequency over 3 min during a control period, immediately following TFLLR application, and following a 20-min washout period. Individual data points are shown in gray, and means are represented by black lines. Error bars indicate ± SE Cii: time course plot of normalized data aggregated into 1-min bins showing a reduction in burst frequency upon application of TFLLR (n = 10 preparations). Di: locomotor-burst amplitude is unaffected by TFLLR application. Dii: time course plot showing no change in burst amplitude upon application of TFLLR. Ei: sample traces showing the effects of the nonselective adenosine receptor antagonist theophylline (20 µM) on locomotor-related activity. Eii: theophylline increases the frequency of locomotor-related bursting, revealing the modulatory role of endogenous adenosine derived from sources within the ventral horn. Values are means ± SE; n = 6. Eiii: the selective A₁-adenosine receptor antagonist cyclopentyl dipropylxanthine (DPCPX; 50 µM) also reveals modulation of locomotor-related activity by endogenous adenosine and prevents further frequency increases in the presence of theophylline, demonstrating that adenosine acts via A₁ receptors. Values are means ± SE; n = 6. Fi: traces showing the effects of the glial toxin fluoroacetate (FA; 5 mM) when coapplied with glutamine (1.5 mM) and a lack of modulation by endogenous adenosine following glial ablation, as revealed by application of theophylline (20 µM). Fii: theophylline has no effect on the frequency of locomotor activity when applied in the presence of the glial toxins FA and methionine sulfoximine. Values are means ± SE; n = 6. *P < 0.05; **P < 0.01. [B–D are adapted from Acton and Miles (2015). E and F are adapted from Witts et al. (2012).]

Fig. 3.

Schematics illustrating proposed release of ATP-adenosine from astrocytes and inhibition of D₁-like receptor signaling within locomotor networks. A: outline of the spinal locomotor CPG as a bilateral network comprising flexor and extensor and rhythm- and pattern-generating modules for the coordination of muscle activation during locomotion. Molecularly defined populations of postmitotic ventral horn neurons (V1, V2a, V2b, V0v, V0d, V3) are represented as blue circles, and some of their proposed interactions are indicated by arrows, signifying excitation, or bars, signifying inhibition. Astrocytes are proposed to modulate the rhythm-generating circuity by exerting an inhibitory effect, via secretion of ATP-adenosine, on an inhibitory population of interneurons that regulates locomotor frequency, possibly the V1 population (see text). For details of the roles of the neuronal populations indicated, see Goulding (2009) and Kiehn (2016). B: ATP is released from putative spinal cord astrocytes during network activity in response to an unidentified neuronal signal and upon experimental activation of the Gα_q-linked receptor PAR1 by TFLLR, which is proposed to mimic the endogenous action of neurotransmitters on astrocytic GPCRs. Extracellular ectonucleotidases mediate the hydrolysis of ATP to adenosine, which activates neuronal Gα_i-linked A₁ receptors to inhibit signaling through Gα_s-linked D₁-like receptors at the level of adenylyl cyclase. Reduced cAMP production by adenylyl cyclase results in reduced activation of PKA. PKA acts on unidentified targets, perhaps including ion channels and ionotropic receptors, to increase neuronal excitability; PKA inhibition results in a reduced frequency of locomotor-related activity. IN, interneuron; MN, motoneuron.