Expression of Bcl-2 extends the survival of olfactory receptor neurons in the absence of an olfactory bulb.

In the olfactory neuroepithelium, the number of olfactory receptor neurons (ORNs) is maintained at a relatively constant level by a precise balance between the elimination of mature receptors and proliferation of their precursors. However, little is known of the mechanisms that couple alterations in receptor death rates to changes in precursor proliferation. To investigate this relationship, we generated a line of mice expressing Bcl-2, a protein with anti-apoptotic properties, in mature olfactory receptor neurons using the Olfactory Marker Protein (OMP) promoter. OMP-bcl-2 transgenic mice showed selective expression of Bcl-2 in mature sensory neurons of the olfactory neuroepithelium (ONE) and vomeronasal organ. Olfactory bulbectomy (OBX) resulted in the death of mature receptor neurons followed by the sustained proliferation of their precursors in wild-type and OMP-bcl-2 transgenic mice. The persistently enhanced proliferation of olfactory neuroblasts that followed bulbectomy was indistinguishable between transgenic and non-transgenic mice. However, receptor neurons that were subsequently born in the absence of the bulb had longer life spans in OMP-bcl-2 mice. The increased proliferation of neuroblasts and extended life spans combined to restore near normal numbers of olfactory receptors in bulbectomized OMP-bcl-2 mice. A model is proposed to explain the dissociation of death and proliferation in OMP-bcl-2 transgenic mice.

Synaptic activity-independent persistent plasticity in endogenously active mammalian motoneurons.

Potentiation and depression of glutamate receptor function in hippocampal, cerebellar, and cortical neurons are examples of persistent changes in synaptic function that underlie important behavioral adaptations such as learning and memory. Persistent changes in synaptic function relevant for motor behaviors have not been demonstrated in mammalian motoneurons. We demonstrate that adaptive changes in (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA) receptor function at endogenously active synapses occur in motoneurons in neonatal rodents. We found a form of serotonin (5-HT)-dependent synaptic plasticity in hypoglossal (XII) motoneurons, which control tongue muscles affecting upper airway function, that is metamodulated by metabotropic glutamate receptors. Episodic, but not continuous, activation of postsynaptic 5-HT type 2 (5-HT(2)) receptors on hypoglossal (XII) motoneurons leads to long-lasting increases in their AMPA receptor-mediated respiratory drive currents and associated XII nerve motor output. Antagonism of group-I metabotropic glutamate receptors blocks induction of the 5-HT-induced increase in excitability. We propose that this activity-independent postsynaptic 5-HT-mediated plasticity represents the cellular mechanism underlying long-term facilitation, i.e., persistent increases in respiratory motor output and ventilation seen in humans and rodents in response to episodic hypoxia. Loss of activity in XII motoneurons is common during sleep causing snoring and, in serious cases, airway obstruction that interrupts breathing, a condition known as obstructive sleep apnea. These results may provide the basis for rationale development of therapeutics for obstructive sleep apnea in humans.

Opioid-induced quantal slowing reveals dual networks for respiratory rhythm generation.

Current consensus holds that a single medullary network generates respiratory rhythm in mammals. Pre-Bötzinger Complex inspiratory (I) neurons, isolated in transverse slices, and preinspiratory (pre-I) neurons, found only in more intact en bloc preparations and in vivo, are each proposed as necessary for rhythm generation. Opioids slow I, but not pre-I, neuronal burst periods. In slices, opioids gradually lengthened respiratory periods, whereas in more intact preparations, periods jumped nondeterministically to integer multiples of the control period (quantal slowing). These findings suggest that opioid-induced quantal slowing results from transmission failure of rhythmic drive from pre-I neurons to preBötC I networks, depressed below threshold for spontaneous rhythmic activity. Thus, both I (in the slice), and pre-I neurons are sufficient for respiratory rhythmogenesis.

Dynamic modulation of inspiratory drive currents by protein kinase A and protein phosphatases in functionally active motoneurons.

Plasticity underlying adaptive, long-term changes in breathing behavior is hypothesized to be attributable to the modulation of respiratory motoneurons by intracellular second-messenger cascades. In quiescent preparations, protein kinases, including cAMP-dependent protein kinase A (PKA), potentiate glutamatergic inputs. However, the dynamic role of protein kinases or phosphatases in functionally active and behaviorally relevant preparations largely remains to be established. Rhythmic inspiratory drive to motoneurons innervating inspiratory muscles is mediated by the release of glutamate acting predominantly on AMPA receptors. In rhythmically active brainstem slices from neonatal rats, we investigated whether synaptic AMPA receptor function could be modulated by changes in intracellular PKA activity, affecting inspiratory drive in hypoglossal (XII) motoneurons. Intracellular perfusion of the catalytic subunit of PKA potentiated endogenous synaptic and (exogenously applied) AMPA-induced currents in XII motoneurons. Conversely, when a peptide inhibitor of PKA was perfused intracellularly, inspiratory drive currents were depressed. Intracellular perfusion with microcystin, a potent phosphatase 1 and 2a inhibitor, increased both endogenous and exogenous AMPA receptor-mediated currents, further supporting a role of phosphorylation in modulating motoneuronal excitability affecting behaviorally relevant synaptic inputs. These findings suggest that PKA is constitutively active in XII motoneurons in vitro. Thus, endogenous synaptic AMPA currents in XII motoneurons are influenced by phosphorylation, specifically by PKA, and dephosphorylation. The role of this modulation may be to keep the activity of motoneurons within a dynamic range that aids in responding to different physiological challenges affecting breathing, such as exercise, hypoxia, and sleep.