Inactivity and Ca2+ signaling regulate synaptic compensation in motoneurons following hibernation in American bullfrogs.

Neural networks tune synaptic and cellular properties to produce stable activity. One form of homeostatic regulation involves scaling the strength of synapses up or down in a global and multiplicative manner to oppose activity disturbances. In American bullfrogs, excitatory synapses scale up to regulate breathing motor function after inactivity in hibernation, connecting homeostatic compensation to motor behavior. In traditional models of homeostatic synaptic plasticity, inactivity is thought to increase synaptic strength via mechanisms that involve reduced Ca2+ influx through voltage-gated channels. Therefore, we tested whether pharmacological inactivity and inhibition of voltage-gated Ca2+ channels are sufficient to drive synaptic compensation in this system. For this, we chronically exposed ex vivo brainstem preparations containing the intact respiratory network to tetrodotoxin (TTX) to stop activity and nimodipine to block L-type Ca2+ channels. We show that hibernation and TTX similarly increased motoneuron synaptic strength and that hibernation occluded the response to TTX. In contrast, inhibiting L-type Ca2+ channels did not upregulate synaptic strength but disrupted the apparent multiplicative scaling of synaptic compensation typically observed in response to hibernation. Thus, inactivity drives up synaptic strength through mechanisms that do not rely on reduced L-type channel function, while Ca2+ signaling associated with the hibernation environment independently regulates the balance of synaptic weights. Altogether, these results point to multiple feedback signals for shaping synaptic compensation that gives rise to proper network function during environmental challenges in vivo.

Characterization of laryngeal motor neuron properties in the American bullfrog, Lithobates catesbieanus.

Motor neurons represent the final output from the central respiratory network. American bullfrogs, Lithobates catesbieanus, have provided insight into development and plasticity of the breathing control system, yet cellular aspects of bullfrog motor neurons are not well-described. In this study, we characterized properties of laryngeal motor neurons that produce motor outflow to the glottal dilator, a muscle that gates airflow to the lungs of anurans. To this end, we measured several intrinsic membrane properties of labeled laryngeal motor neurons in brain slices. Using unsupervised clustering analyses, we identified two broad classes of motor neurons: those with high firing rates and strong adaptation (∼70 %), and those with lower firing rates and less adaptation (∼30 %). These results suggest that two neuronal cell types innervate the glottal dilator, roughly aligning with the composition of fast and slower twitch fibers of this muscle. In sum, these data reinforce the need to consider cell-type when assessing motor neuron function in the respiratory network.

Neuromodulation or energy failure? Metabolic limitations silence network output in the hypoxic amphibian brainstem.

Hypoxia tolerance in the vertebrate brain often involves chemical modulators that arrest neuronal activity to conserve energy. However, in intact networks, it can be difficult to determine whether hypoxia triggers modulators to stop activity in a protective manner or whether activity stops because rates of ATP synthesis are insufficient to support network function. Here, we assessed the extent to which neuromodulation or metabolic limitations arrest activity in the respiratory network of bullfrogs-a circuit that survives moderate periods of oxygen deprivation, presumably, by activating an inhibitory noradrenergic pathway. We confirmed that hypoxia and norepinephrine (NE) reduce network output, consistent with the view that hypoxia may cause the release of NE to inhibit activity. However, these responses differed qualitatively; hypoxia, but not NE, elicited a large motor burst and silenced the network. The stereotyped response to hypoxia persisted in the presence of both NE and an adrenergic receptor blocker that eliminates sensitivity to NE, indicating that noradrenergic signaling does not cause the arrest. Pharmacological inhibition of glycolysis and mitochondrial respiration recapitulated all features of hypoxia on network activity, implying that reduced ATP synthesis underlies the effects of hypoxia. Finally, activating modulatory mechanisms that dampen neuronal excitability when ATP levels fall, KATP channels and AMP-dependent protein kinase, did not resemble the hypoxic response. These results suggest that energy failure-rather than inhibitory modulation-silences the respiratory network during hypoxia and emphasize the need to account for metabolic limitations before concluding that modulators arrest activity as an adaptation for energy conservation in the nervous system.

The Inhibitory Effect of Essential Oils on Rhizopus stolonifer, Trichophyton mentagrophytes, and Microsporum gypseum.

BACKGROUND: Rhizopus stolonifer is an opportunistic fungus that causes respiratory infections, sinusitis, and otomycosis. Trichophyton mentagrophytes and Microsporum gypseum cause athlete's foot, ringworm, and nail infections. Previous research has shown that some essential oils inhibit fungal growth. METHODS: We investigated the effects of 0.5% and 1% concentrations of Rosmarinus officinalis, Cinnamomum verum, Citrus paradisi, and Capsicum annuum extracts on the growth of R. stolonifer and T. mentagrophytes. The combined effects also were determined at 0.5% concentration for these fungal species and for M. gypseum. Fungal species were grown in flasks, along with growth media, for 7, 14, and 21 days, with each species of essential oil or combination of oils and mass determined and compared to its control. RESULTS: Rosmarinus officinalis, Cinnamomum verum, and Citrus paradisi had overall significant inhibitory effects (P ≤.05) on R. stolonifer and T. mentagrophytes. C. annuum significantly (P ≤.05) inhibited the growth of T. mentagrophytes and enhanced that of R. stolonifer. The combination of essential oils resulted in inhibition of growth (P ≤.05) at similar percentages as R. officinalis, C. verum, and C. paradisi only. CONCLUSION: Essential oils such as Rosmarinus officinalis, Cinnamomum verum, and Citrus paradisi may be useful for treating opportunistic and dermatophytic fungal diseases.