Adrenergic α₁ receptor activation is sufficient, but not necessary for phrenic long-term facilitation

Abstract

Acute intermittent hypoxia (AIH; three 5-min hypoxic episodes) causes a form of phrenic motor facilitation (pMF) known as phrenic long-term facilitation (pLTF); pLTF is initiated by spinal activation of Gq protein-coupled 5-HT2 receptors. Because α1 adrenergic receptors are expressed in the phrenic motor nucleus and are also Gq protein-coupled, we hypothesized that α1 receptors are sufficient, but not necessary for AIH-induced pLTF. In anesthetized, paralyzed, and ventilated rats, episodic spinal application of the α1 receptor agonist phenylephrine (PE) elicited dose-dependent pMF (10 and 100 μM, P < 0.05; but not 1 μM). PE-induced pMF was blocked by the α1 receptor antagonist prazosin (1 mM; -20 ± 20% at 60 min, -5 ± 21% at 90 min; n = 6). Although α1 receptor activation is sufficient to induce pMF, it was not necessary for AIH-induced pLTF because intrathecal prazosin (1 mM) did not alter AIH-induced pLTF (56 ± 9% at 60 min, 78 ± 12% at 90 min; n = 9). Intravenous (iv) prazosin (150 μg/kg) appeared to reduce pLTF (21 ± 9% at 60 min, 26 ± 8% at 90 min), but this effect was not significant. Hypoglossal long-term facilitation was unaffected by intrathecal prazosin, but was blocked by iv prazosin (-4 ± 14% at 60 min, -13 ± 18% at 90 min), suggesting different LTF mechanisms in different motor neuron pools. In conclusion, Gq protein-coupled α1 adrenergic receptors evoke pMF, but they are not necessary for AIH-induced pLTF.

Keywords: plasticity; respiratory motor neuron; spinal cord; α1 receptors.

Fig. 1.

Representative phrenic neurograms from five rats receiving episodic phenylephrine (PE, an α₁ receptor agonist), acute intermittent hypoxia (AIH), or prazosin (an α₁ receptor antagonist) before PE. Intrathecal applications of low-dose PE (1 μM) (A) did not alter phrenic amplitude (Phr). Intrathecal applications of higher PE doses (10 μM) (B) and (100 μM) (C) separated by 5 min caused phrenic motor facilitation (pMF), similar to phrenic long-term facilitation (pLTF) induced by AIH [three 5-min periods of hypoxia (Hx1, Hx2, and Hx3, respectively) separated by normoxia, and 20% DMSO vehicle preapplication] (D). E: PE-induced pMF was blocked by preapplication of prazosin (1 mM) 20 min prior to episodic PE (10 μM).

Fig. 2.

Group data show that high concentrations of PE induce pMF at 60 and 90 min, which were similar to AIH-induced pLTF and blocked by prazosin (an α₁ receptor antagonist). Both 10 (n = 7) and 100 (n = 7) μM PE significantly increased phrenic nerve amplitude at 60 and 90 min compared with baseline (**P < 0.01; ***P < 0.001) and were not different from vehicle + AIH (n = 8)-induced pLTF at either time point. Low doses of PE (1 μM; n = 8) did not evoke pMF at any time point and pMF was significantly reduced compared with 10 and 100 μM PE (^#P < 0.05; ^##P < 0.01). Preapplication of prazosin (1 mM; n = 6) caused a significant reduction in pMF induced by PE (10 μM) at 60 and 90 min (^@@P < 0.01; ^@P < 0.05, two-way repeated measures ANOVA, Fishers LSD post hoc).

Fig. 3.

Representative phrenic neurograms from two rats receiving either intrathecal preapplication of prazosin (an α₁ receptor antagonist, 1 mM) (A) or vehicle (20% DMSO) (B) 20 min prior to AIH [three, 5 min hypoxic episodes (Hx1, Hx2, Hx3) separated by normoxia]. Prazosin did not alter expression of pLTF. Time controls (n = 2, data not shown) did not show facilitation.

Fig. 4.

Intrathecal preapplication of prazosin (1 mM; n = 9) did not affect the hypoxia responses (A) (t-test) nor the magnitude of AIH-induced pLTF (B) compared with intrathecal vehicle preapplication (20% DMSO) + AIH (n = 7). Thus α₁ receptor activation is not necessary for AIH-induced pLTF (***P < 0.001, **P < 0.01 significantly different from baseline; ^###P < 0.001, ^##P < 0.01, #P < 0.05 significantly different from 30 min; ^@@@P < 0.001, ^@@P < 0.01 significantly different from 15 min; two-way repeated measures ANOVA, Fishers LSD post hoc test). In (B), significance symbols relative to vehicle data appear below data lines; significance symbols relative to prazosin appear above data lines.

Fig. 5.

Intravenous preapplication of prazosin (150 μg/kg; n = 6) did not alter development of AIH-induced pLTF (black line), but did significantly inhibit development of hypoglossal (XII) LTF (gray line). Thus α₁ receptor activation was not necessary for AIH-induced pLTF, but it was for hypoglossal LTF (***P < 0.001, **P < 0.01 significantly different from baseline; ^##P < 0.01, ^#P < 0.05 significantly different from 30 min; ^@P < 0.05 significantly different from 15 min; one-way repeated measures ANOVA Fishers LSD post hoc test).