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Jan-Marino Ramirez

Publications and source records attributed to Jan-Marino Ramirez.

26 records · Page 2Linked to original sources
PubMed2003

Stabilization of bursting in respiratory pacemaker neurons.

Synaptic and endogenous pacemaker properties have been hypothesized as principal cellular mechanisms for respiratory rhythm generation. This rhythmic activity is thought to originate in the pre-Bötzinger complex, an area that can generate fictive respiration when isolated in brainstem slice preparations of mice. In slice preparations, external potassium concentration ([K+]o) is typically elevated from 3 to 8 mm to induce rhythmic population activity. However, elevated [K+](o) may not simply depolarize respiratory neurons but also change rhythm-generating mechanisms by inducing or altering pacemaker properties. To test this, we examined the membrane potential (V(m)) of nonpacemaker neurons and endogenous bursting properties of pacemaker neurons before and after blockade of excitatory and inhibitory synaptic input in 3 mm [K+]o artificial CSF (aCSF). Most pacemaker neurons (82%) ceased to burst in 3 mm [K+]o aCSF after blockade of glutamatergic transmission. In all of these, endogenous bursting was restored on additional blockade of glycinergic and GABAergic inhibition. Thus, bursting properties are suppressed by endogenous synaptic inhibition, the level of which may determine whether network rhythmicity is generated in 3 mm (n = 12) or 8 mm (n = 40) [K+]o aCSF. In 3 mm [K+]o aCSF, synaptically isolated pacemaker neurons (n = 22) continued to burst over a wide range of imposed V(m). Furthermore, the V(m) of synaptically isolated pacemaker neurons was not significantly affected (p = 0.1; n = 10) when [K+]o was changed from 8 to 3 mm, whereas isolated nonpacemakers hyperpolarized (p < 0.001; n = 14). We conclude that respiratory pacemaker neurons possess membrane properties that stabilize their bursting against changes in [K+]o and imposed changes in V(m).

Animals↗
PubMed2003

Response of the respiratory network of mice to hyperthermia.

Most mammals modulate respiratory frequency (RF) to dissipate heat (i.e., panting) and avoid heat stroke during hyperthermic conditions. During hyperthermia, the RF of intact mammals increases and then declines or ceases (apnea). It has been proposed that this RF modulation depends on the presence of higher brain structures such as the hypothalamus. However, the direct effects of hyperthermia on the respiratory neural network have not been examined. To address this issue, the respiratory neural network [i.e., ventral respiratory group (VRG)] was isolated in a brain stem preparation taken from the medulla of mice (P0 -P6). Integrated population activity, predominated by inspiratory neurons, was recorded extracellularly from VRG neurons. The bath temperature was then heated from 30 to 40 degrees C, resulting in a biphasic frequency response in VRG activity. Following an initial six- to sevenfold increase and subsequent decline, fictive RF was maintained at a frequency that was higher than baseline frequency; at 40 degrees C, the RF was maintained at about two to four times that at 30 degrees C. The inspiratory burst amplitude and duration were significantly reduced during hyperthermic conditions. An increase in RF and decrease in VRG burst amplitude and duration also occurred when heating from 37 to 40 degrees C. Fictive apnea typically occurred during cooling to the control temperature. Furthermore, changes in hypoglossal motor nucleus activity paralleled those of the VRG, suggesting that temperature modulation of the VRG is likely to have a behaviorally relevant impact on respiration. We conclude that the VRG activity itself is modulated during hyperthermia and the respiratory network is particularly sensitive to temperature changes.

Animals↗
PubMed2002

Endogenous activation of serotonin-2A receptors is required for respiratory rhythm generation in vitro.

Endogenous amines and peptides continuously modulate the activity of neuronal networks and are required even for their normal operation. The respiratory rhythm generator, localized in the pre-Bötzinger complex, is not an exception. This network is modulated by various neurotransmitters, including serotonin (5-HT). In this study, we isolated the respiratory network in brainstem slices and demonstrate that the endogenous activation of 5-HT(2A) is required for the generation of the respiratory rhythm in vitro. At the network level, activation of 5-HT(2A) receptors with 4-iodo-2,5-dimethoxyamphetamine or the 5-HT uptake blocker alaproclate increased the frequency of respiratory activity. Blockade of endogenously activated 5-HT(2A) receptors with three different antagonists decreased the frequency, amplitude, and regularity of respiratory population activity, an effect that was blocked by protein kinase C (PKC) activators. At the cellular level, blockade of 5-HT(2A) receptors reduced the action potential discharge in all examined respiratory neurons, which was associated with a reduction in the fast and the persistent sodium current. Continuous application of 5-HT(2A)-receptor antagonists differentially affected pacemaker neurons. Pacemaker activity was eliminated in cadmium-insensitive pacemaker neurons. In cadmium-sensitive pacemaker neurons, the frequency of pacemaker activity was unaffected and the amplitude of pacemaker bursts was enhanced. It is assumed that cadmium-insensitive pacemakers rely on the persistent sodium current, whereas cadmium-sensitive pacemakers depend on the activation of calcium currents. We conclude that endogenously activated 5-HT(2A) receptors are required for maintaining fictive respiratory activity in the brainstem slice by modulating sodium conductances via a PKC pathway.

Action Potentials↗
PubMed2002

Thermal preconditioning and heat-shock protein 72 preserve synaptic transmission during thermal stress.

As with other tissues, exposing the mammalian CNS to nonlethal heat stress (i.e., thermal preconditioning) increases levels of heat-shock proteins (Hsps) such as Hsp70 and enhances the viability of neurons under subsequent stress. Using a medullary slice preparation from a neonatal mouse, including the site of the neural network that generates respiratory rhythm (the pre-Bötzinger complex), we show that thermal preconditioning has an additional fundamental effect, protection of synaptic function. Relative to 30 degrees C baseline, initial thermal stress (40 degrees C) greatly increased the frequency of synaptic currents recorded without pharmacological manipulation by approximately 17-fold (p < 0.01) and of miniature postsynaptic currents (mPSCs) elicited by GABA (20-fold) glutamate (10-fold), and glycine (36-fold). Thermal preconditioning (15 min at 40 degrees C) eliminated the increase in frequency of overall synaptic transmission during acute thermal stress and greatly attenuated the frequency increases of GABAergic, glutamatergic, and glycinergic mPSCs (for each, p < 0.05). Moreover, without thermal preconditioning, incubation of slices in solution containing inducible Hsp70 (Hsp72) mimicked the effect of thermal preconditioning on the stress-induced release of neurotransmitter. That preconditioning and exogenous Hsp72 can affect and preserve normal physiological function has important therapeutic implications.

Animals↗
PubMed2002

Respiratory rhythm generation: converging concepts from in vitro and in vivo approaches?

The timing and activation pattern of breathing movements are determined by the respiratory network. This network is amenable to a variety of in vivo and in vitro approaches, which offers a unique opportunity to investigate multiple organizational levels. It is only recently, however, that concepts obtained under in vivo and in vitro conditions are being integrated into a coherent model of breathing behavior. For example, the pre-Bötzinger complex as an essential site for rhythm generation was first identified in vitro, but has since been verified in vivo. Conversely, timing signals provided by other central and peripheral neuronal areas have so far been investigated in vivo, but it is now possible to address these issues with more complex in vitro preparations. Several key issues remain unresolved. For example, to what extent is the respiratory pattern controlled independently of the underlying rhythm? Answers to this and other questions require a dissection of mechanisms that is only possible through a complementary combination of experimental approaches.

Animals↗
PubMed2002

Long-term modulation of respiratory network activity following anoxia in vitro.

Neural networks that produce rhythmic behaviors require flexibility to respond to changes in the internal and external state of the animal. It is important to not only understand how a network responds during such perturbations but also how the network recovers. For example, the respiratory network needs to respond to and recover from temporary changes in oxygen level that can occur during sleep, exercise, and respiratory disorders. During a temporary decrease in oxygen level, there is an increase in respiratory frequency followed by a depression that can lead to complete apnea. Here we used a mouse brain stem slice preparation as a model system to examine the recovery of respiratory network activity after brief episodes of anoxia. We found the respiratory network recovers from a single anoxic episode with a transient increase in fictive respiratory frequency. Although repetitive anoxia does not elicit a greater frequency increase, it does elicit a longer lasting frequency increase persisting < or =90 min. Thus there is a centrally mediated long-lasting influence on the respiratory network elicited by decreased oxygen levels. This modulation occurs as a prolonged facilitation of fictive respiratory frequency after brief repetitive but not single anoxic exposure. These data are important to consider in the context of disorders such as sleep apnea in which brief periodic anoxic episodes are experienced.

Animals↗
PubMed2002

Long-term deprivation of substance P in PPT-A mutant mice alters the anoxic response of the isolated respiratory network.

The aim of this study was to elucidate the role of the neuromodulator substance P and its related tachykinin neurokinin A (NKA) in the homeostasis of respiratory activity. Respiratory activities, in form of fictive eupneic and sigh activities, were recorded extracellularly from the preBötzinger complex (PBC) in normoxic and anoxic conditions using medullary slice preparations. The effect of a blockade of endogenous substance P was assessed by an acute pharmacological blockade of the receptors with spantide in wild-type animals and by the use of preprotachykinin-A (PPT-A) mutants. These mutants lack from birth the PPT-A gene, which codes for the precursor of substance P and NKA. Spantide treatment reduced frequency (-37%, n = 9) and regularity (twofold) of eupneic-like respiratory activity under normoxic conditions, whereas in PPT-A mutants, eupneic-like activity was under normoxic conditions not significantly different from the wild-type mice (WT). The response to short anoxic episodes (5 min) was characterized in the WT by an increase in respiratory frequencies at the onset of anoxia (ratio anoxic/control frequency = 1.9 +/- 0.2, n = 18). This anoxic ratio was unaltered in the presence of spantide (ratio = 2.3 +/- 0.4, n = 8) but increased in the mutant (ratio = 4.1, n = 15). We conclude that endogenously released substance P is important for the maintenance of regular respiratory activity. Short-term blockade of substance P receptors decreases the frequency and regularity of rhythmic activity. Long-term deficiency in substance P leads to compensatory mechanisms that result in an apparently normal respiratory activity under normoxic conditions but a significantly altered response of the respiratory network during anoxia.

Animals↗