Tim D. Ostrowski

Depressed GABA and glutamate synaptic signaling by 5-HT1A receptors in the nucleus tractus solitarii and their role in cardiorespiratory function

Serotonin (5-HT), and its 5-HT1A receptor (5-HT1AR) subtype, is a powerful modulator of the cardiorespiratory system and its sensory reflexes. The nucleus tractus solitarii (nTS) serves as the first central station for visceral afferent integration and is critical for cardiorespiratory reflex responses. However, the physiological and synaptic role of 5-HT1ARs in the nTS is relatively unknown. In the present study, we examined the distribution and modulation of 5-HT1ARs on cardiorespiratory and synaptic parameters in the nTS. 5-HT1ARs were widely distributed to cell bodies within the nTS but not synaptic terminals. In anesthetized rats, activation of 5-HT1ARs by microinjection of the 5-HT1AR agonist 8-OH-DPAT into the caudal nTS decreased minute phrenic neural activity via a reduction in phrenic amplitude. In brain stem slices, 8-OH-DPAT decreased the amplitude of glutamatergic tractus solitarii-evoked excitatory postsynaptic currents, and reduced overall spontaneous excitatory nTS network activity. These effects persisted in the presence of GABAA receptor blockade and were antagonized by coapplication of 5-HT1AR blocker WAY-100135. 5-HT1AR blockade alone had no effect on tractus solitarii-evoked excitatory postsynaptic currents, but increased excitatory network activity. On the other hand, GABAergic nTS-evoked inhibitory postsynaptic currents did not change by activation of the 5-HT1ARs, but spontaneous inhibitory nTS network activity decreased. Blocking 5-HT1ARs tended to increase nTS-evoked inhibitory postsynaptic currents and inhibitory network activity. Taken together, 5-HT1ARs in the caudal nTS decrease breathing, likely via attenuation of afferent transmission, as well as overall nTS network activity.

Frequency processing at consecutive levels in the auditory system of bush crickets (tettigoniidae)

We asked how processing of male signals in the auditory pathway of the bush cricket Ancistrura nigrovittata (Phaneropterinae, Tettigoniidae) changes from the ear to the brain. From 37 sensory neurons in the crista acustica single elements (cells 8 or 9) have frequency tuning corresponding closely to the behavioral tuning of the females. Nevertheless, one‐quarter of sensory neurons (approximately cells 9 to 18) excite the ascending neuron 1 (AN1), which is best tuned to the males song carrier frequency. AN1 receives frequency‐dependent inhibition, reducing sensitivity especially in the ultrasound. When recorded in the brain, AN1 shows slightly lower overall activity than when recorded in the prothoracic ganglion close to the spike‐generating zone. This difference is significant in the ultrasonic range. The first identified local brain neuron in a bush cricket (LBN1) is described. Its dendrites overlap with some of AN1‐terminations in the brain. Its frequency tuning and intensity dependence strongly suggest a direct postsynaptic connection to AN1. Spiking in LBN1 is only elicited after summation of excitatory postsynaptic potentials evoked by individual AN1‐action potentials. This serves a filtering mechanism that reduces the sensitivity of LBN1 and also its responsiveness to ultrasound as compared to AN1. Consequently, spike latencies of LBN1 are long (>30 ms) despite its being a second‐order interneuron. Additionally, LBN1 receives frequency‐specific inhibition, most likely further reducing its responses to ultrasound. This demonstrates that frequency‐specific inhibition is redundant in two directly connected interneurons on subsequent levels in the auditory system. J. Comp. Neurol. 518:3101–3116, 2010.

H2O2 induces delayed hyperexcitability in nucleus tractus solitarii neurons

Hydrogen peroxide (H₂O₂) is a stable reactive oxygen species and potent neuromodulator of cellular and synaptic activity. Centrally, endogenous H₂O₂ is elevated during bouts of hypoxia-reoxygenation, a variety of disease states, and aging. The nucleus tractus solitarii (nTS) is the central termination site of visceral afferents for homeostatic reflexes and contributes to reflex alterations during these conditions. We determined the extent to which H₂O₂ modulates synaptic and membrane properties in nTS neurons in rat brainstem slices. Stimulation of the tractus solitarii (which contains the sensory afferent fibers) evoked synaptic currents that were not altered by 10-500 μM H₂O₂. However, 500 μM H₂O₂ modulated several intrinsic membrane properties of nTS neurons, including a decrease in input resistance (R(i)), hyperpolarization of resting membrane potential (RMP) and action potential (AP) threshold (THR), and an initial reduction in AP discharge to depolarizing current. H₂O₂ increased conductance of barium-sensitive potassium currents, and block of these currents ablated H₂O₂-induced changes in RMP, Ri and AP discharge. Following washout of H₂O₂ AP discharge was enhanced due to depolarization of RMP and a partially maintained hyperpolarization of THR. Hyperexcitability persisted with repeated H₂O₂ exposure. H₂O₂ effects on RMP and THR were ablated by intracellular administration of the antioxidant catalase, which was immunohistochemically identified in neurons throughout the nTS. Thus, H₂O₂ initially reduces excitability of nTS neurons that is followed by sustained hyperexcitability, which may play a profound role in cardiorespiratory reflexes.

Processing of ultrasound in a bush cricket's brain

The processing and categorization of conspecific and heterospecific acoustic signals is an important task of the central nervous system. In orthopteran species, carrier frequency (besides temporal cues) is one of the major discriminators. In the bush cricket species Ancistrura nigrovittata Brunner von Wattenwyl (Phaneropteridae, Barbitistini), ultrasound has potentially different meanings and may elicit vastly different behaviours depending on the context it is perceived in. In the present study, data are presented of the morphology and neuronal responses of three local brain neurones (LBNs) that respond best to ultrasound. All neurones show dense arborizations in the lateral protocerebrum, where ascending interneurones terminate. The LBN2 and LBN9 neurones are entirely restricted to one side of the brain, whereas LBN5 crosses the midline, thereby linking both hemispheres. The response maxima for LBN2 overlap closely with the peak carrier frequencies found in a species‐specific duet, which consists of sonic (16 kHz, male), as well as ultrasonic (24–28 kHz, female) sound. By contrast, LBN9 responds only to ultrasound in the range of the female reply, whereas the male song induces exceptionally long‐lasting inhibition. The LBN5 neurone shows strongest spike activity to a broad range of ultrasonic frequencies, as long as the pulse duration remains short. All three brain neurones respond to ultrasound in a unique way and may be involved in the shaping of different behavioural outcomes.

Blunted Respiratory Responses in the Streptozotocin-Induced Alzheimer’s Disease Rat Model

Alzheimers disease (AD) is known for the progressive decline of cognition and memory. In addition to these disease-defining symptoms, impairment of respiratory function is frequently observed and often expressed by sleep-disordered breathing or reduced ability to adjust respiration when oxygen demand is elevated. The mechanisms for this are widely unknown. Postmortem analysis from the brainstem of AD patients reveals pathological alterations, including in nuclei responsible for respiratory control. In this study, we analyzed respiratory responses and morphological changes in brainstem nuclei following intracerebroventricular (ICV) injections of streptozotocin (STZ), a rat model commonly used to mimic sporadic AD. ICV-STZ induced significant astrogliosis in the commissural part of the nucleus tractus solitarii, an area highly involved in respiration control. The astrogliosis was identified by a significant increase in S100B-immunofluorescence that is similar to the astrogliosis found in the CA1 region of the hippocampus. Using plethysmography, the control group displayed a typical age-dependent decrease of ventilation that was absent in the STZ rat group. This is indicative of elevated minute ventilation at rest after STZ treatment. Peripheral chemoreflex responses were significantly blunted in STZ rats as seen by a reduced respiratory rate and minute ventilation to hypoxia. Central chemoreflex responses to hypercapnia, on the other hand, only decreased in respiratory rate following STZ treatment. Overall, our results show that ICV-STZ induces respiratory dysfunction at rest and in response to hypoxia. This provides a new tool to study the underlying mechanisms of breathing disorders in clinical AD.

H2O2 augments cytosolic calcium in nucleus tractus solitarii neurons via multiple voltage-gated calcium channels

Reactive oxygen species (ROS) play a profound role in cardiorespiratory function under normal physiological conditions and disease states. ROS can influence neuronal activity by altering various ion channels and transporters. Within the nucleus tractus solitarii (nTS), a vital brainstem area for cardiorespiratory control, hydrogen peroxide (H2O2) induces sustained hyperexcitability following an initial depression of neuronal activity. The mechanism(s) associated with the delayed hyperexcitability are unknown. Here we evaluate the effect(s) of H2O2 on cytosolic Ca2+ (via fura-2 imaging) and voltage-dependent calcium currents in dissociated rat nTS neurons. H2O2 perfusion (200 µM; 1 min) induced a delayed, slow, and moderate increase (~27%) in intracellular Ca2+ concentration ([Ca2+]i). The H2O2-mediated increase in [Ca2+]i prevailed during thapsigargin, excluding the endoplasmic reticulum as a Ca2+ source. The effect, however, was abolished by removal of extracellular Ca2+ or the addition of cadmium to the bath solution, suggesting voltage-gated Ca2+ channels (VGCCs) as targets for H2O2 modulation. Recording of the total voltage-dependent Ca2+ current confirmed H2O2 enhanced Ca2+ entry. Blocking VGCC L, N, and P/Q subtypes decreased the number of cells and their calcium currents that respond to H2O2 The number of responder cells to H2O2 also decreased in the presence of dithiothreitol, suggesting the actions of H2O2 were dependent on sulfhydryl oxidation. In summary, here, we have shown that H2O2 increases [Ca2+]i and its Ca2+ currents, which is dependent on multiple VGCCs likely by oxidation of sulfhydryl groups. These processes presumably contribute to the previously observed delayed hyperexcitability of nTS neurons in in vitro brainstem slices.

Response differences of intersegmental auditory neurons recorded close to or far away from the presumed spike-generating zone

Intracellular recordings may give valuable information about processing of a neuron and possibly its input from the network. Impalement with an electrode causes injury to the cell and depolarization from intrusion of extracellular fluid. Thus, penetration artefacts may contaminate recordings and conceal or even alter relevant information. These penetration artefacts may have the strongest impact close to the spike-generating zone near the dendrites. Recordings in axonal portions might therefore be less vulnerable while providing insufficient information about the synaptic input. In this study, we present data of five previously identified intersegmental auditory neurons of a bushcricket independently recorded in their dendrites (prothorax) and axon (brain). Generally, responses to acoustic pulses of the same parameter combination were similar within a neuronal class at the two recording sites. However, all neuronal classes showed significantly higher response variability and a tendency for higher spike activity when recorded in the dendrites. Unexpectedly, the combined activity of two neurons (Ascending Neurons 1 and 2) recorded in the brain provides a better fit to song recognition than when recorded in the thorax. Axonal recordings of T-shaped Neuron 1 revealed graded potentials originating in the brain and modulating its output in a potentially behaviourally relevant manner.