Somatostatin modulates generation of inspiratory rhythms and determines asphyxia survival.

Breathing and the activity of its generator (the pre-Bötzinger complex; pre-BötC) are highly regulated functions. Among neuromodulators of breathing, somatostatin (SST) is unique: it is synthesized by a subset of glutamatergic pre-BötC neurons, but acts as an inhibitory neuromodulator. Moreover, SST regulates breathing both in normoxic and in hypoxic conditions. Although it has been implicated in the neuromodulation of breathing, neither the locus of SST modulation, nor the receptor subtypes involved have been identified. In this study, we aimed to fill in these blanks by characterizing the SST-induced regulation of inspiratory rhythm generation in vitro and in vivo. We found that both endogenous and exogenous SST depress all preBötC-generated rhythms. While SST abolishes sighs, it also decreases the frequency and increases the regularity of eupnea and gasping. Pharmacological experiments showed that SST modulates inspiratory rhythm generation by activating SST receptor type-2, whose mRNA is abundantly expressed in the pre-Bötzinger complex. In vivo, blockade of SST receptor type-2 reduces gasping amplitude and consequently, it precludes auto-resuscitation after asphyxia. Based on our findings, we suggest that SST functions as an inhibitory neuromodulator released by excitatory respiratory neurons when they become overactivated in order to stabilize breathing rhythmicity in normoxic and hypoxic conditions.

Effects of riluzole and flufenamic acid on eupnea and gasping of neonatal mice in vivo.

The pre-Bötzinger complex (PBC), part of the ventral respiratory group that is responsible for inspiratory rhythm generation, contains at least two types of pacemaker neurons. In vitro studies have shown that bursting properties of one type of pacemaker relies on a riluzole-sensitive persistent sodium current, whereas bursting of a second type is sensitive to flufenamic acid (FFA), a calcium-dependent nonspecific cationic current blocker. In vitro, under control conditions, the PBC generates fictive eupneic activity that depends on both riluzole-sensitive and FFA-sensitive pacemaker neurons. During hypoxia the PBC generates fictive gasping activity and only riluzole-sensitive pacemaker neurons appear to be necessary for this rhythm. We carried out pharmacological experiments to test the role of respiratory pacemaker neurons in vivo by performing plethysmographic recordings on neonate mice. As reported in vitro, eupnea activity in vivo is abolished only if both FFA and riluzole are coadministered intracisternally, but not when either of them is administered independently. On the other hand riluzole, but not FFA, drastically reduced gasping generation and compromised the ability of mice to autoresucitate. Neither substance P nor forskolin was able to reestablish respiratory activity after riluzole and FFA coapplication. Our results confirm in vitro reports and suggest that eupnea generation in neonates requires a complex neuronal network that includes riluzole- and FFA-sensitive elements and that gasping activity depends mostly on a riluzole-sensitive mechanism.