The responses in respiratory outflow resulting from microstimulation and successive microlesions of the dorsal (DRG) and ventral (VRG) respiratory groups of neurons in the brainstem were studied in anesthetized, paralyzed, artificially ventilated cats. Microstimulation (2 to 120 Hz; 5 to 50 microA; 100 musec pulse duration) at almost every point within the DRG or VRG produced a bilateral short latency inhibition of phrenic nerve activity which had an onset latency of 4 to 9 msec and a duration of 4 to 24 msec. This global stereotyped phrenic inhibition was elicited by single pulses and often was accompanied by a postinhibitory excitation. In 48% (92/193) of the stimulation trials, trains of stimulus pulses during inspiration decreased the duration of inspiration. In 25% of the expiratory microstimulation trials, expiratory duration was increased and in 11%, expiration was shortened markedly by trains of pulses. Single shocks delivered to the right VRG or DRG produced a short latency excitation in the ipsilateral recurrent laryngeal nerve (RRL). This RRL excitation had an onset latency of 2 to 5 msec and a duration of 3 to 15 msec. Evidence suggests that the RRL excitation is due to a paucisynaptic activation of expiratory motoneurons in the caudal VRG. This activation is synchronous with the inhibition of inspiratory neurons in DRG and VRG. Despite the powerful short latency effects of microstimulation in VRG and DRG, extensive bilateral destruction of these neuronal populations had only modest effects on respiratory rhythm, while it decreased (or abolished) respiratory outflow in phrenic and recurrent laryngeal nerves. The combined results of the microstimulation and microlesion portions of this study suggest that a region (or regions) outside of the DRG and VRG might be important in the control of the respiratory pattern and that the DRG and VRG are important in determining the depth of inspiration; their role in generating respiratory rhythm needs to be critically re-examined.

The local neural circuitry underlying the control of breathing was studied by injecting nanoliter volumes of excitatory amino acids into discrete regions of cat brain stem. Experiments were performed on chloralose-urethane anesthetized, vagotomized, paralyzed, and artificially ventilated cats. Phrenic, intercostal, and recurrent laryngeal nerve discharges were recorded. Multibarrel pipettes were used for recording and pressure ejection of drugs or a dye for marking recording and ejection sites. Ejected volumes were directly monitored for every injection. Injections, proximal to neurons discharging with a respiratory periodicity, of as little of 200 fmol of L-glutamate in 200 pl of saline elicited marked, site-specific increases or decreases in respiratory motoneuronal discharge. N-Methyl-D-aspartic acid and homocysteic acid elicited similar site-specific alterations in respiratory motor output, although some details of the response could differ qualitatively. Responses to all the excitatory agents used were attenuated by concurrent injection of kynurenic acid, DL-2-aminoA-phosphonobutyric acid, or glutamic acid diethyl ester. There was no change in spontaneous phrenic nerve discharge in response to injections of equivalent or larger volumes of saline or lidocaine. These results indicate a heterogeneity in the spatial organization of the brain-stem neural circuitry underlying respiratory control, which has not been described previously. This injection technique may provide a mechanism for probing the neural circuitry underlying other behaviors.Brain function underlying behavior in mammals involves information transfer between neurons in complex networks. The nuclear and layered structure of the mammalian brain suggests that small aggregates of neurons act as functional units. It is of fundamental interest to determine the nature of this local information transfer and its consequences in terms of behavior. In this paper, we present an approach to study this problem using site-specific delivery of (sub)picomolar amounts of excitatory amino acids in nanoliter volumes. Using this approach, we present evidence of a neuronal mosaic underlying the control of breathing.Neurons that discharge with a respiratory periodicity are concentrated in the nucleus ambiguus-retroambigualis (ventral respiratory group, VRG) and the nucleus tractus solitarius of the medulla (Cohen, 1979; Feldman, in press;von Euler, 1983). Received Oct. 24, 1985; revised Jan. 21, 1986; accepted Jan. 22, 1986. The technical assistnce of Paul G. Bums and the helpful comments of Professor Oscar Hechter are gratefully acknowledged. We would like to thank Drs. Howard H. Ellenberger, Debra E. Weese-Mayer, and Jeffery C. Smith for their help in experiments at the latter stages of this study. This work was supported by NIH Grant NS-21036.Correspondence should be addressed to Jack L. The VRG extends rostrally from the spinomedullary junction to the level of the retrofacial nucleus. Neurons within the VRG can be divided into 2 populations based on their impuls...

Short latency phrenic motor responses to phrenic nerve stimulation were studied in anesthetized, paralyzed cats. Electrical stimulation (0.2 ms, 0.01-10 mA, 2 Hz) of the right C5 phrenic rootlet during inspiration consistently elicited a transient reduction in the phrenic motor discharge. This attenuation occurred bilaterally with an onset latency of 8-12 ms and a duration of 8-30 ms. Section of the ipsilateral C4-C6 dorsal roots abolished the response to stimulation, thereby confirming the involvement of phrenic nerve afferent activity. Stimulation of the left C5 phrenic rootlet or the right thoracic phrenic nerve usually elicited similar inhibitory responses. The difference in onset latency of responses to cervical vs. thoracic phrenic nerve stimulation indicates activation of group III afferents with a peripheral conduction velocity of approximately 10 m/s. A much shorter latency response (5 ms) was evoked ipsilaterally by thoracic phrenic nerve stimulation. Section of either the C5 or C6 dorsal root altered the ipsilateral response so that it resembled the longer latency contralateral response. The low-stimulus threshold and short latency for the ipsilateral response to thoracic phrenic nerve stimulation suggest that it involves larger diameter fibers. Decerebration, decerebellation, and transection of the dorsal columns at C2 do not abolish the inhibitory phrenic-to-phrenic reflex.

Systemic injection of MK-801, an N-methyl-D-aspartate (NMDA) receptor-associated channel blocker, induces an apneusis in vagotomized cats similar to that produced by pontine respiratory group (PRG) lesions, suggesting the possible involvement of NMDA receptors in the pontine pneumotaxic mechanism. Previous results from our laboratory indicate that the efferent limb of the pontine pneumotaxic mechanism is unlikely to require NMDA receptor-mediated neurotransmission. Therefore, the present study examined the potential involvement of PRG NMDA receptors in the pontine pneumotaxic mechanism. Experiments were conducted in decerebrate, paralyzed, and ventilated adult cats. The effects on inspiratory time (TI) of MK-801 microinjection into PRG were tested in 12 cats. Pressure microinjection of MK-801 (15 mM, 80-3,000 nl) significantly prolonged TI in all animals when lung inflation was withheld. TI progressively increased in most animals for > or = 30 min. After this period, partial recovery of the effect occurred in eight cats as TI shortened toward predrug levels. In three animals, microinjection of MK-801 induced a complete apneusis in the absence of lung inflation from which there was no detectable recovery. Microinjections into regions approximately 2 mm distant from PRG produced little or no effect. These results provide evidence that NMDA receptors located in the region of PRG play an important functional role in the control of the breathing cycle.

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