Retrograde tracing with a fluorescent dye (Fast Blue) combined with immunohistochemistry was used to identify putative neurotransmitter(s) at the phrenic motor nucleus in the cat. Fast Blue was injected bilaterally into the diaphragm of five cats, where each phrenic nerve enters the muscle. Seven days later the animals were perfusion fixed and tissue sections from the fourth, fifth, and sixth cervical spinal cord segments were analyzed using a fluorescence microscope. Retrogradely labeled fluorescent phrenic motor neuron cell bodies appeared in all of the segments but primarily in sections from the fifth segment. The same or adjacent transverse sections were then used for the demonstration of the distribution of the neurotransmitters 5-hydroxytryptamine (5HT), substance P, and thyrotropin-releasing hormone (TRH) in the area of the phrenic motor nucleus using the indirect immunofluorescence technique. The most conspicuous neurotransmitters found at the phrenic motor nucleus were 5-HT and substance P. We observed dense and diffuse fiber networks throughout the ventral horn which contains the phrenic motor nucleus. These fibers contained varicosities in close proximity to phrenic motor neurons. In addition to 5-HT-and substance Pcontaining nerve endings, some fibers containing TRH were also found in the area of the phrenic motor nucleus. These results are consistent with earlier physiological data suggesting that 5-HT, substance P, and TRH are important neurotransmitters and/or neuromodulators involved in central control of respiration.Our knowledge of central respiratory neurons is extensive with regard to their anatomical localization and their electrophysiological properties (Cohen, 1979; Mitchell and Berger, 1981). However, very little is known about the neurotransmitters of these neurons. One approach commonly used to obtain information about this point has been to inject a putative CNS neurotransmitter, an agonist or antagonist of the neurotransmitter or a pre-

No abstract

Respiratory responses to activation of gamma-aminobutyric acid (GABA) receptors in the hindbrain were measured in chloralose-anesthetized cats using a Fleisch pneumotachograph. GABA receptors were activated by intracisternal injections of muscimol and GABA. Muscimol (0.05--6.65 micrograms) administered to seven animals caused a depression of respiratory activity with apnea occurring in each animal. Before apnea occurred, a decrease in tidal volume was observed (from 25.7 +/- 0.9 to 14.7 +/- 1.1 ml). Respiratory rate and inspiratory and expiratory durations were unchanged. GABA (0.05--12.15 mg) administered to five animals produced the same effect as muscimol on respiratory activity. Apnea produced by both agents was reversed by intracisternal administration of the GABA-receptor antagonist drug, bicuculline. Administration of bicuculline to four naive animals increased tidal volume (from 31.3 +/- 1.7 to 36.5 +/- 0.7 ml) but had no effect on either respiratory rate or inspiratory duration. These results indicate that activation of GABA receptors causes respiratory depression and suggest that GABA may be an important neurotransmitter in CNS neural pathways involved in regulating respiratory activity.

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The aim of this study was to evaluate the cardiorespiratory effects of intravenously administered gamma-aminobutyric acid (GABA) alpha-(4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, THIP) and beta-(baclofen) receptor agonists and to locate the site of action of these drugs in the brain. THIP and baclofen were administered to alpha-chloralose-anesthetized cats while minute ventilation (VE), arterial blood pressure (AP), and heart rate were monitored. THIP, in doses of 0.5 to 2 mg/kg decreased VE, tidal volume (VT), and AP. No changes in respiratory rate (f) or inspiratory (TI) or expiratory (TE) duration were observed. Baclofen, in doses of 0.5 to 4 mg/kg, decreased VE, f, and AP. VT and TI increased and an "apneustic" breathing pattern was seen. THIP (9.5 micrograms), applied bilaterally to the glycine-sensitive area of the ventral medulla, reproduced the effects seen with intravenous administration. Application of 10 micrograms of bicuculline bilaterally to this area reversed the effects of intravenous THIP but not those of baclofen. Baclofen (5.6-56 micrograms), administered by the intracisternal route, produced the same respiratory effects seen with intravenous administration. We conclude that activation of GABA alpha- and beta-receptors produces cardiorespiratory depression. However, this is accomplished by different mechanisms and by actions exerted at different central nervous system sites.