Differentiation of phencyclidine and sigma receptor types affecting the central inspiratory termination mechanism in cat

Inspiration-promoting vagal reflex under NMDA receptor blockade in anaesthetized rabbits

1. This study describes a novel vagal respiratory reflex in anaesthetized rabbits. In contrast to the well-known inspiratory (I) off-switching by vagal afferent excitation, this vagal reflex initiates and maintains the central I activity of phrenic nerve discharges in rabbits pre-treated with antagonists of N-methyl-D-aspartate-type excitatory amino acid receptors (NMDA-Rs). 2. Under NMDA-R blockade with either dizocilpine (0.025-0.3 mg kg-1), D-2-amino-5-phosphonopentanoic acid (AP5, 0.5-1 mg, i.c.v.) or ketamine (10 mg kg-1), vagal stimulation at low frequencies (5-40 Hz) during the I phase prevented or markedly delayed the spontaneous I termination. In contrast, stimulation of the same vagal afferent at the same intensity but at a higher frequency (100-160 Hz) during the I phase immediately terminated the I phase. 3. In non-vagotomized rabbits, maintaining the tidal volume at end-expiratory levels during the I phase prevented spontaneous I termination and maintained apneusis after NMDA-R blockade with dizocilpine. 4. Brief stimulation of vagal afferents at low frequency (5-40 Hz) during the expiratory (E) phase constantly initiated phrenic I discharge after NMDA-R block. 5. We conclude that low-frequency discharge of vagal pulmonary stretch receptor afferents, as when lung volume is near functional residual capacity, promotes central I activity under NMDA-R blockade.

Autoradiographic localisation of NMDA binding sites in brainstem cardiorespiratory areas of adult and newborn cats

Quantitative autoradiography was used to determine the distribution of [3H]1-[1-(2-thienyl)cyclohexyl] piperidine ([3H]TCP) binding sites in the brainstem in order to localize the possible targets activated by excitatory amino acids in adult and newborn cats during autonomic rhythmic functions. Medium to high densities of binding sites were found in the nucleus tractus solitarius complex and the laterodorsal part of the pontine tegmentum. In the kitten, there was an apparent higher density in cranial motor nuclei proximal to these structures. It is concluded that there is an overlapping between the high density of NMDA receptors and the localization of cardiorespiratory neurons in cat as well as in kitten.

The bulbar network of respiratory neurons during apneusis induced by a blockade of NMDA receptors

Our aim was to study the mechanisms producing the transition from the inspiratory phase to the expiratory phase of the breathing cycle. For this purpose we observed the changes affecting the discharge patterns and excitabilities of the different types of respiratory neurons within the respiratory network in cat medulla, after inducing an apneustic respiration with the N-methyl-D-aspartate (NMDA) antagonist MK-801 given systemically. Respiratory neurons were recorded extracellularly through the central barrel of multibarrelled electrodes, in the ventral respiratory area of pentobarbital-anesthetized, vagotomized, paralyzed and ventilated cats. Inhibitions exerted on each neuron by the pre-synaptic pools of respiratory neurons were revealed when the neuron was depolarized by an iontophoretic application of the excitatory amino-acid analogue quisqualate. Cycle-triggered time histograms of the spontaneous and quisqualate-increased discharge of respiratory neurons were constructed in eupnea and in apneusis induced with MK-801. During apneustic breathing, the activity of the respiratory neuronal network changed throughout the entire respiratory cycle including the post-inspiratory phase, and the peak discharge rates of all types of respiratory neurons, except the late-expiratory type, decreased. During apneusis, the activity of the post-inspiratory neuronal pool, the post-inspiratory depression of other respiratory neurons, and the phrenic nerve after-discharge were reduced (but not totally suppressed), whereas the discharge of some post-inspiratory neurons shifted into the apneustic plateau. The shortened post-inspiration (stage 1 of expiration) altered the organization of the expiratory phase. Late-expiratory neurons (stage 2 of expiration) discharged earlier in expiration and their discharge rate increased. The inspiratory on-switching was functionally unaffected. Early inspiratory neurons of the decrementing type retained a decrementing pattern followed by a reduced discharge rate in the apneustic plateau, whereas early-inspiratory neurons of the constant type maintained a high discharge rate throughout the apneustic plateau. Inspiratory augmenting neurons, late-inspiratory and "off-switch" neurons also discharged throughout the apneustic plateau. During the apneustic plateau, the level of activity was constant in the phrenic nerve and in inspiratory neurons of the early-constant, augmenting, and late types. However, progressive changes in the activity of other neuronal types demonstrated the evolving state of the respiratory network in the plateau phase. There was a slowed but continued decrease of the activity of early-inspiratory decrementing neurons, accompanied by an increasing activity and/or excitability of "off-switch", post-inspiratory and late-expiratory neurons. In apneusis there was a decoupling of the duration of inspiration and expiration.(ABSTRACT TRUNCATED AT 400 WORDS)