Blockade of neurokinin-1 receptors in the ventral respiratory column does not affect breathing but alters neurochemical release

Abstract

Substance P (SP) and its receptor, neurokinin-1 (NK1R), have been shown to be excitatory modulators of respiratory frequency and to stabilize breathing regularity. Studies in anesthetized mice suggest that tonic activation of NK1Rs is particularly important when other excitatory inputs to the pre-Bötzinger complex in the ventral respiratory column (VRC) are attenuated. Consistent with these findings, muscarinic receptor blockade in the VRC of intact goats elicits an increase in breathing frequency associated with increases in SP and serotonin concentrations, suggesting an involvement of these substances in neuromodulator compensation. To gain insight on the contribution to breathing of endogenous SP and NK1R activation, and how NK1R modulates the release of other neurochemicals, we individually dialyzed antagonists to NK1R (133, 267, 500 μM Spantide; 3 mM RP67580) throughout the VRC of awake and sleeping goats. We found that NK1R blockade with either Spantide at any dose or RP67580 had no effect on breathing or regularity. Both antagonists significantly (P < 0.001) increased SP, while RP67580 also increased serotonin and glycine and decreased thyrotropin-releasing hormone concentrations in the dialysate. Taken together, these data support the concept of neuromodulator interdependence, and we believe that the loss of excitatory input from NK1Rs was locally compensated by changes in other neurochemicals.

Effects on breathing of agonists to μ-opioid or GABA A receptors dialyzed into the ventral respiratory column of awake and sleeping goats

“…This instability contrasts to studies (Langer et al, 2016; Muere et al, 2013, 2015a; Muere et al, 2015b; Muere et al, 2015c) in which dialyses of antagonists to multiple excitatory receptors in the VRC did not destabilize breathing. The destabilization with DAMGO could be due to time-dependent changes in the direct effect of opioid-induced depression and the indirect effect of potential compensatory mechanisms.…”

“…The current and past dialysis studies in unanesthetized goats found neuromodulator compensation during perturbation of G protein-coupled receptors (Muere et al, 2013, 2015a; Muere et al, 2015b; Muere et al, 2015c). Herein we found what could be interpreted as overcompensation for increases in inhibitory ionotropic GABA receptor activity during muscimol dialysis.…”

“…Microdialysis of antagonists to excitatory, neuromodulatory, G protein-coupled receptors into the VRC of awake and sleeping goats (Langer et al, 2016; Muere et al, 2013, 2015a; Muere et al, 2015b; Muere et al, 2015c) support the concept of neuromodulator compensation, whereby decreases in excitatory receptor activity are compensated by increases in other (predominantly excitatory) neuromodulators to prevent a decrease in ventilation and breathing frequency (Doi and Ramirez, 2008, 2010). In other words, there is “compensation” defined as no change in ventilation with dialysis of excitatory antagonists and that this may be due to the observed concomitant increase in excitatory (SP) and decrease in inhibitory neurotransmitters (GABA).…”

“…This secondary/compensatory response may have been relatively greater with 100 μM compared to 50 μM which is why a significant depression in ventilation was found with 50 μM but not 100 μM DAMGO. A secondary/compensatory response is sufficiently rapid to prevent a detectable decrease in ventilation during dialysis of antagonists to excitatory neuromodulator receptors (Muere et al, 2013, 2015a; Muere et al, 2015b; Muere et al, 2015c). Compensation during DAMGO dialysis may also have been within the contralateral VRC.…”

“…We previously summarized (Muere et al, 2013, 2015a; Muere et al, 2015b; Muere et al, 2015c) the limitations and caveats of dialyses studies in the medulla of awake and sleeping goats. One limitation is that the microtubules were not all implanted at the same site ( Figure 1 ).…”