Compensatory changes in GABAergic inhibition are differentially expressed in the respiratory network to promote function following hibernation

The respiratory network must produce consistent output throughout an animal's life. Although respiratory motor plasticity is well appreciated, how plasticity mechanisms are organized to give rise to robustness following perturbations that disrupt breathing is less clear. During underwater hibernation, respiratory neurons of bullfrogs remain inactive for months, providing a large disturbance that must be overcome to restart breathing. As a result, motoneurons upregulate excitatory synapses to promote the drive to breathe. Reduced inhibition often occurs in parallel with increased excitation, yet the loss of inhibition can destabilize respiratory motor output. Thus, we hypothesized that GABAergic inhibition would decrease following hibernation, but this decrease would be expressed differentially throughout the network. We confirmed that respiratory frequency was under control of GABAAR signaling, but after hibernation, it became less reliant on inhibition. The loss of inhibition was confined to the respiratory rhythm-generating centers: non-respiratory motor activity and large seizure-like bursts were similarly triggered by GABAA receptor blockade in controls and hibernators. Supporting reduced presynaptic GABA release, firing rate of respiratory motoneurons was constrained by a phasic GABAAR tone, but after hibernation, this tone was decreased despite the same postsynaptic receptor strength as controls. Thus, selectively reducing inhibition in respiratory premotor networks promotes stability of breathing, while wholesale loss of GABAARs causes non-specific hyperexcitability throughout the brainstem. These results suggest that different parts of the respiratory network select distinct strategies involving either excitation (motoneurons) or inhibition (rhythm generator) to minimize pathological network states when engaging plasticity that protects the drive to breathe.

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.

Lactate ions induce synaptic plasticity to enhance output from the central respiratory network

Lactate ion sensing has emerged as a process that regulates ventilation during metabolic challenges. Most work has focused on peripheral sensing of lactate for the control of breathing. However, lactate also rises in the central nervous system (CNS) during disturbances to blood gas homeostasis and exercise. Using an amphibian model, we recently showed that lactate ions, independently of pH and pyruvate metabolism, act directly in the brainstem to increase respiratory-related motor outflow. This response had a long washout time and corresponded with potentiated excitatory synaptic strength of respiratory motoneurons. Thus, we tested the hypothesis that lactate ions enhance respiratory output using cellular mechanisms associated with long-term synaptic plasticity within motoneurons. In this study, we confirm that 2 mM sodium lactate, but not sodium pyruvate, increases respiratory motor output in brainstem-spinal cord preparations, persisting for 2 h upon the removal of lactate. Lactate also led to prolonged increases in the amplitude of AMPA-glutamate receptor (AMPAR) currents in individual motoneurons from brainstem slices. Both motor facilitation and AMPAR potentiation by lactate required classic effectors of synaptic plasticity, L-type Ca²⁺ channels and NMDA receptors, as part of the transduction process but did not correspond with increased expression of immediate-early genes often associated with activity-dependent neuronal plasticity. Altogether these results show that lactate ions enhance respiratory motor output by inducing conserved mechanisms of synaptic plasticity and suggest a new mechanism that may contribute to coupling ventilation to metabolic demands in vertebrates. KEY POINTS: Lactate ions, independently of pH and metabolism, induce long-term increases in respiratory-related motor outflow in American bullfrogs. Lactate triggers a persistent increase in strength of AMPA-glutamatergic synapses onto respiratory motor neurons. Long-term plasticity of motor output and synaptic strength by lactate involves L-type Ca²⁺ channels and NMDA-receptors as part of the transduction process. Enhanced AMPA receptor function in response to lactate in the intact network is causal for motor plasticity. In sum, well-conserved synaptic plasticity mechanisms couple the brainstem lactate ion concentration to respiratory motor drive in vertebrates.

Synaptic up-scaling preserves motor circuit output after chronic, natural inactivity

Neural systems use homeostatic plasticity to maintain normal brain functions and to prevent abnormal activity. Surprisingly, homeostatic mechanisms that regulate circuit output have mainly been demonstrated during artificial and/or pathological perturbations. Natural, physiological scenarios that activate these stabilizing mechanisms in neural networks of mature animals remain elusive. To establish the extent to which a naturally inactive circuit engages mechanisms of homeostatic plasticity, we utilized the respiratory motor circuit in bullfrogs that normally remains inactive for several months during the winter. We found that inactive respiratory motoneurons exhibit a classic form of homeostatic plasticity, up-scaling of AMPA-glutamate receptors. Up-scaling increased the synaptic strength of respiratory motoneurons and acted to boost motor amplitude from the respiratory network following months of inactivity. Our results show that synaptic scaling sustains strength of the respiratory motor output following months of inactivity, thereby supporting a major neuroscience hypothesis in a normal context for an adult animal.

Neurons in the brain communicate using chemical signals that they send and receive across junctions called synapses. To maintain normal behavior over time, circuits of neurons must reliably process these signals. A variety of nervous system disorders may result if they are unable to do so, as may occur when neural activity changes as a result of disease or injury.

The processes underlying the stability of a neuron’s synapses is referred to as “homeostatic” synaptic plasticity because the changes made by the neuron directly oppose the altered level of activity. In one form of homeostatic plasticity, known as synaptic scaling, neurons modify the strength of all of their synapses in response to changes in neural activity.

There is substantial experimental evidence to show that in young animals, neurons that communicate using a chemical called glutamate undergo synaptic scaling in response to artificial changes in activity. It had not been directly shown that synaptic scaling protects the neural activity of adult animals in their natural environments, in part, because neural activity in most healthy animals generally only goes through small changes. However, the neurons in the brain that cause the breathing muscles of bullfrogs to contract are ideal for studying homeostatic plasticity because they are naturally inactive for several months when frogs hibernate in ponds during the winter. During this time, the bullfrogs do not need to use their lungs to breathe because enough oxygen passes through their skin to keep them alive.

Santin et al. have now observed synaptic scaling of glutamate synapses in individual bullfrog neurons that had been inactive for two months. Further experiments that examined the activity of the breathing control circuit in the brainstem provided evidence that synaptic scaling leads to sufficient amounts of neural activity that would activate the breathing muscles when frogs emerge from hibernation. Therefore neural activity after prolonged, natural inactivity relies on synaptic scaling to preserve life-sustaining behavior in frogs.

These results open up new questions: mainly, how do synaptic scaling and other forms of homeostatic plasticity operate in animals as they experience normal variations in neural activity? Determining how homeostatic plasticity works normally in an animal will help us to understand what happens when plasticity mechanisms go wrong, as is thought to occur in several human nervous system diseases including nervous system injury, Alzheimer’s disease, and epilepsy.