Jay B. Dean

Depolarization and stimulation of neurons in nucleus tractus solitarii by carbon dioxide does not require chemical synaptic input.

The effects of elevated CO2 (i.e. hypercapnia) on neurons in the nucleus tractus solitarii were studied using extracellular (n = 82) and intracellular (n = 33) recording techniques in transverse brain slices prepared from rat. Synaptic connections from putative chemosensitive neurons in the ventrolateral medulla were removed by bisecting each transverse slice and discarding the ventral half. In addition, the response to hypercapnia in 20 neurons was studied during high magnesium-low calcium synaptic blockade. Sixty-five per cent of the neurons (n = 75) tested were either insensitive or inhibited by hypercapnia. However, 35% (n = 40) were depolarized and/or increased their firing rate during hypercapnia. Nine out of 10 CO2-excited neurons retained their chemosensitivity to CO2 in the presence of high magnesium-low calcium synaptic blockade medium. Our findings demonstrate that many neurons in the nucleus tractus solitarii were depolarized and/or increased their firing rate during hypercapnia. These neurons were not driven synaptically by putative chemosensitive neurons of the ventrolateral medulla since this region was removed from the slice. Furthermore, because chemosensitivity persisted in most neurons tested during synaptic blockade, we conclude that some neurons in the nucleus tractus solitarii are inherently CO2-chemosensitive. Although the function of dorsal medullary chemosensitive neurons cannot be determined in vitro, their location and their inherent chemosensitivity suggest a role in cardiorespiratory central chemoreception.

CO2 decreases membrane conductance and depolarizes neurons in the nucleus tractus solitarii

SummaryTo identify central sites of potential CO2/H+-chemoreceptive neurons, and the mechanism responsible for neuronal chemosensitivity, intracellular recordings were made in rat tissue slices in two cardiopulmonary-related regions (i.e., nucleus tractus solitarii, NTS; nucleus ambiguus, AMBc) during exposure to high CO2. When the NTS was explored slices were bisected and the ventral half discarded. Utilizing such “dorsal” medullary slices removed any impinging synaptic input from putative chemoreceptors in the ventrolateral medulla. In the NTS, CO2-induced changes in firing rate were associated with membrane depolarizations ranging from 2–25 mV (n = 15). In some cases increased e.p.s.p. activity was observed during CO2 exposure. The CO2-induced depolarization occurred concomitantly with an increased input resistance ranging from 19–23 MΩ (n = 5). The lower membrane conductance during hypercapnia suggests that CO2-induced depolarization is due to a decreased outward potassium conductance. Unlike neurons in the NTS, AMBc neurons were not spontaneously active and were rarely depolarized by hypercapnia. Eleven of 12 cells tested were either hyperpolarized by or insensitive to CO2. Only 1 neuron in the AMBc was depolarized and it also showed an increased input resistance during CO2 exposure. Our findings suggest that CO2/H+-related stimuli decrease potassium conductance which depolarizes the cell and increases firing rate. Although our in vitro studies cannot guarantee the specific function of these cells, we believe they may be involved with brain pH homeostasis and cardiopulmonary regulation.

Neurophysiological Aspects of Thermoregulation

The ability to react to thermal challenges and regulate body temperature depends on the ability of the nervous system to sense temperature both in the environment and deep within the body core. The basis of the neural control of thermoregulation is the synaptic communication between peripheral and central thermal receptors. Peripheral cutaneous thermoreceptors relay ambient temperature (Ta) information over afferent pathways to central neurons in the lower brain stem and hypothalamus. Many of these central neurons are, themselves, thermosensitive, sensing changes in core temperature that occur, for example, during exercise or under drastic environmental conditions. Perhaps most important is the ability of these neurons to integrate this central thermal information with afferent peripheral thermal information. As a result of this integration, efferent signals are produced that select the most appropriate responses to maintain a constant central temperature.

Experimentally induced postinhibitory rebound in rat nucleus ambiguus is dependent on hyperpolarization parameters and membrane potential

Postinhibitory rebound (PIR), a transient depolarization subsequent to release from experimental hyperpolarization, was identified and characterized in 81% of the cells studied in the nucleus ambiguus in slices from medulla of rat. Hyperpolarizing current pulses were administered via the recording microelectrode in the bridge-balanced mode to test for PIR. The voltage trajectory was characterized by a depolarizing sag during the pulse, rebound depolarization (PIR) after the pulse and increased input resistance during rebound. The amplitude and time course of PIR were dependent on prepulse membrane potential, pulse amplitude and pulse duration. These results suggest a potential role of PIR in respiratory rhythmogenesis.

A diencephalic slice preparation and chamber for studying neuronal thermosensitivity

An in vitro slice preparation and electrophysiological recording chamber for studying neuronal thermosensitivity throughout the hypothalamus are described. A series of eight, 300 microns thick, horizontal tissue slices encompassing most of the hypothalamus are prepared from male Sprague-Dawley rats. Horizontal slice maps showing the major nuclei and fiber tracts are provided. Horizontal tissue slices contain many hypothalamic nuclei as well as the medial forebrain bundle, a large fiber tract interconnecting these nuclei. Three water-perfused thermodes directly beneath the tissue slices are used to produce discrete thermal stimulations of rostral, middle, and caudal nuclear regions. Fine thermocouples monitor slice temperature over each thermode. Limiting microelectrode explorations to regions directly over a thermode eliminates the problems of temperature gradients and permits more accurate manipulation of temperature at the recording site. While this preparation is ideal for characterizing hypothalamic neuronal thermosensitivity, it is also appropriate for electrophysiological studies of other hypothalamic functional systems.

Excitatory and inhibitory effects on respiration of l-glutamate microinjected superficially into the ventral aspects of the medulla oblongata in cat

L-Glutamate (4-40 nmol) was microinjected at superficial depths beneath the ventral surface of the medulla oblongata in cats. Injections (100-300 microns beneath the surface) made rostromedial to the hypoglossal nerve, less than 1.5 mm lateral to the pyramidal tract, caused stimulation of phrenic nerve activity. Injections (100-500 microns beneath the surface) up to 1 mm further lateral caused a marked increase in arterial pressure and depression of phrenic nerve activity. These findings support the existence of two cell groups in the ventral medulla that are involved in regulation of respiration; when activated, one (medial group) causes facilitation and the other (lateral group) inhibition of respiration.