Increased spinal adenosine impairs phrenic long-term facilitation in aging rats

Abstract

Moderate acute intermittent hypoxia (mAIH) elicits a form of spinal, respiratory motor plasticity known as phrenic long-term facilitation (pLTF). In middle-aged male and geriatric female rats, mAIH-induced pLTF is attenuated through unknown mechanisms. In young adults, mAIH activates competing intracellular signaling cascades, initiated by serotonin 2 and adenosine 2A (A2A) receptors, respectively. Since spinal A2A receptor inhibition enhances mAIH-induced pLTF, serotonin dominates, and adenosine constrains mAIH-induced plasticity in the daily rest phase, we hypothesized elevated basal adenosine levels in the ventral cervical spinal cord of aged rats shifts this balance, undermining mAIH-induced pLTF. A selective A2A receptor antagonist (MSX-3) or vehicle were delivered intrathecally at C4 in anesthetized young (3-6 months) and aged (20-22 months) Sprague-Dawley rats prior to mAIH (3,5-min episodes; arterial PO2=45-55mmHg). In young males, spinal A2A receptor inhibition enhanced pLTF (119 ± 5%) versus vehicle (55 ± 9%), consistent with prior reports. In old males, pLTF was reduced (25 ± 11%), but A2A receptor inhibition increased pLTF to levels greater than in young males (186 ± 19%). Basal adenosine levels in ventral C3-C5 homogenates are elevated 2-3-fold in old versus young males. These findings advance our understanding of age as a biological variable in phrenic motor plasticity and will help guide translation of mAIH as a therapeutic modality to restore respiratory and non-respiratory movements in older populations afflicted with clinical disorders that compromise movement.

Daily Acute Intermittent Hypoxia Elicits Age & Sex-Dependent Changes in Molecules Regulating Phrenic Motor Plasticity

“…Our finding of an age associated increase in A2a receptor ( Adora2a) expression supports a prior study (Marciante & Mitchell, 2023). Age-related increases in extracellular adenosine concentration (Castillo et al, 2009; Sebastião et al, 2000; Stockwell et al, 2017) increase A2a receptor activation and constrain the Q pathway to phrenic motor facilitation (Marciante & Mitchell, 2023). Downstream A2a receptor activation includes p38 MAP kinase activation in response to increased reactive oxygen species formation (Kralova et al, 2008).…”

“…To date, nearly all studies of key Q and S pathway molecules were based on investigations in young adult male rats. Information concerning Q or S pathway molecules in middle-aged or geriatric males or females are scarce (but see: Marciante et al, 2023c; (Marciante & Mitchell, 2023). Thus, one major goal of this study was to understand maturation effects (young to middle-aged) on key Q versus S pathway molecules in male and female rats, their association with serum estradiol levels, and their impact on dAIH-enhanced phrenic long-term facilitation.…”

“…Frozen serum samples were thawed on ice and estradiol levels assayed via competitive immunoassay in rat serum (BioVendor, Cat#RTC009R) (Marciante & Mitchell, 2023). The calibration range for the assay was 2.5-1280 pg/ml.…”

“…For example, with an injured spinal cord, heightened tissue hypoxia from systemic hypotension and/or pericyte capillary constriction (Li et al, 2017) may increase Q-S crosstalk inhibition, even at the same PaO 2 (Perim, Gonzalez-Rothi, et al, 2021; Perim et al, 2023). Time of the day (Marciante et al, 2023), age and sex (Behan et al, 2002; Behan et al, 2003; Zabka et al, 2001a, 2001b) also regulate pLTF due to changes in Q-S cross talk inhibition (Marciante & Mitchell, 2023; Marciante et al, 2023). Phrenic LTF elicited by AIH consisting of 1 minute hypoxic episodes is attenuated in the rodent active ( versus rest) phase due to diurnal variations in spinal adenosine levels (Marciante et al, 2023).…”

“…Although mechanisms whereby dAIH enhances phrenic LTF are unknown, they may relate to: 1) Q pathway amplification (MacFarlane et al, 2018; Satriotomo et al, 2012); or 2) minimizing S-Q crosstalk constraints (Hoffman et al, 2010; Mitchell & Baker, 2022). Since aging increases Q-to-S cross talk inhibition due to increased spinal adenosine in geriatric male and female rats (Marciante & Mitchell, 2023), differential age and sex effects of dAIH must be considered. Since pLTF is impacted greatly by the estrus cycle in females, it is important to account for estrogen effects on the Q and S pathways (Dougherty et al, 2017; Zabka et al, 2006).…”