Astroglial Control of Respiratory Rhythm Generating Circuits

Astrocytes, the most numerous glial cells of the central nervous system, are well known to provide neuronal circuits with essential structural and metabolic support. There is also evidence that astrocytes may modulate the activities of neuronal circuits controlling motor rhythms including those of the brainstem’s preBotzinger complex (preBotC) that generates the rhythm of breathing in mammals. However, the extent and mechanisms of active astroglial control of the respiratory rhythm-generating circuits remain unknown. The morphological features of astrocytes in this critical brainstem region are also unknown. In this dissertation, viral gene transfer approaches designed to block or activate astroglial signaling pathways were used to determine the role of preBotC astrocytes in the control of breathing using in vitro and in vivo experimental models. Computer-aided morphometric analyses were used to investigate the structural features of brainstem astrocytes potentially contributing to their functional role. The results from these complementary, multi-faceted experiments show that (i) morphologically, preBotC astrocytes are larger, have more branches, and longer processes when compared to astrocytes residing in other regions of the brainstem; (ii) in conscious adult rats, blockade of vesicular release mechanisms or ATP-mediated signaling in preBotC astrocytes by virally-induced bilateral expression of either the light chain of tetanus toxin (TeLC), the dominant-negative SNARE proteins (dnSNARE), or a potent ectonucleotidase – transmembrane prostatic acid phosphatase – results in a significant reduction of resting respiratory frequency and frequency of sighs, augmented breaths that engage preBotC circuits to increase inspiratory effort; (iii) hypoxic- and CO2-induced ventilatory responses are significantly reduced when vesicular release mechanisms in preBotC astrocytes are blocked; (iv) activation of preBotC astrocytes expressing Gq-coupled Designer Receptor Exclusively Activated by Designer Drug is associated with higher frequency of both normal inspirations and sighs; (v) blockade of vesicular release mechanisms (expression of TeLC or dnSNARE) in preBotC astrocytes is associated with a dramatic reduction of exercise capacity. These data suggest that astroglial mechanisms involving exocytotic vesicular release of signaling molecules (gliotransmitters), provides tonic excitatory drive to the inspiratory rhythm-generating circuits of the preBotC and contributes to the generation of sighs. The role of preBotC astrocytes in central nervous mechanisms controlling breathing becomes especially important in conditions of metabolic stress requiring homeostatic adjustments of breathing such as systemic hypoxia, hypercapnia, and exercise, when enhanced respiratory efforts are critical to support physiological and behavioral demands of the body.