Opiate system influences central respiratory chemosensors

Brain Research, 211 (1981) 221-226 0 Elsevier/North-Holland Biomedical

221 Press

Opiate system influences central respiratory chemosensors

M. POKORSKI*,

P. GRIEB* and J. WIDEMAN

Department of Neurophysiology Sciences, Warsaw (Poland)

(Accepted November Key

words:

and Neurochemistry,

Medical

Research

Center Polish

Academy

of

27th, 1980)

central respiratory chemosensor - endogenous opiate respiratory output - ventral medulla

naloxone -

opiate receptor -

Central respiratory depressant action of opiates is reversed by the specific opiate antagonist naloxonesJ3. Endogenous opiates and opiate receptors have been localized in brain stem structures involved in the central control of respirationrJ4. Respiratory neurons are inhibited by iontophoretic application of opiate substancess. Recently, Lawson et a1.10,reported that intravenous naloxone stimulated efferent phrenic nerve activity, the main output of the central respiratory regulator, in cats that were deprived of respiratory chemical feedbacks. Holaday and Faden* found that bradypnea from spinal cord trauma was effectively reversed by naloxone in rats. All this is suggestive of the involvement of endogenous opiates in the regulation of respiration. The ventral surface of the medulla seems to be exceptionally sensitive to the respiratory depressant effects of opiates 59s. Since central respiratory chemosensors that respond to extracellular brain fluid acidity have been localized beneath this areais, an intriguing possibility is that the opiate system is involved in central respiratory chemosensitivity. In support of this hypothesis we present evidence that the morphinelike substance fentanyl depresses ventilation and the opiate antagonist naloxone stimulates ventilation when applied to the intermediate ‘chemosensitive’ areas (S) of the ventrolateral medulla of the cat. No such responses could be evoked from the neighboring areas. The evidence is based on experiments performed on 10 pentobarbitone anesthetized (30 mg/kg; i.p. plus supplementary doses as required), tracheostomized and spontaneously breathing cats of 2.6-3.5 kg. The central respiratory output was monitored by means of a phrenic electroneurogram. Action potentials were recorded from the cut central end of the right C5 phrenic root and integrated according to the method of Huszczuk and Widdicombes. Along with the phrenic activity, end-tidal COz and arterial blood pressure were recorded continuously. The acid-base conditions * Present address: Institute for Environmental Medicine, and Department of Pennsylvania School of Medicine, Philadelphia, Pa. 19104, U.S.A.

of Physiology, University

222 were monitored

in arterial

Rectal temperature medulla described acidosis

and

blood

samples

was maintained

constant

criteria

for the localization

elsewherer5Js. and/or

disturbances

All variables drugs. Topical inert gelatin

Experiments

assembly.

at 38 “C. Surgical exposure of the ventral

circulation steady-states

of drugs was achieved

sponge (Spongostan;

electrode

of the chemosensitive

were discontinued

in pulmonary

were assessed during

application

with a Radiometer

approximately

zones

if hypotension,

have been metabolic

became apparent. before and after application

by placing

pledgets

of

of chemically

3.5 x 1.5 x 1.5 mm) soaked

with

appropriate solutions on a given area. 50 ,uI of naloxone solution (0.4 mg/ml; Narcan, Endo) and 25 ,uI of fentanyl solution (0.5 mg/ml; Richter) were used. To test for nonspecific effects of the ‘carrier’,

control

experiments

were performed

with pledgets

soaked with saline. Both drugs were used as commercially available unbuffered solutions at pH 4.0-4.5. To exclude the effects of acidity, in two experiments naloxone was reconstituted from its salt with the animal’s own cerebrospinal fluid to the same concentration. No qualitative differences were noted in respiratory responses to either type of naloxone solution, and therefore all naloxone data were analyzed together. Chemosensitive caudal (L) and intermediate (S) areas, and neighboring nonchemosensitive pyramids were tested. The technique used excluded the possibility of testing the remaining rostra1 (M) chemosensitive zones, due to their far lateral localization. Results are summarized in Table 1. Fig. 1 shows the effects of bilateral application of naloxone (panel A) and fentanyl (panel B) to the S areas. Consistent and statistically significant respiratory responses to both fentanyl and naloxone were evoked only from the intermediate (S) areas. Both were of similar latency as shown in the first row of Table I. Although the net effects of fentanyl and naloxone on ventilation are reciprocal, the following differences are worth mentioning: (1) naloxone-induced effects were longer lasting than those induced by fentanyl and after removal of the ‘carrier’ containing naloxone ventilation returned to the control within one hour. Recovery following removal of the fentanyl ‘carrier’ was completed within 20 min; (2) naloxone-induced stimulation of ventilatory output was achieved by an increase in both amplitude and frequency of the integrated phrenic activity. (These are proportional to tidal volume and equal to respiratory frequency, respectively.) Fentanyl depressed the amplitude only - in some cases the frequency increased; and (3) arterial blood pressure decreased significantly following naloxone, but remained unaffected after fentanyl. Before interpreting these data in the context of possible involvement of opiates in chemical control of respiration, a few technical points deserve comment. Although the method used for introducing the drugs was not perfectly precise, there was no doubt that the affected area was restricted to the S zone. The most likely explanation is that the drugs had to penetrate beneath the surface to exert their actions. Apparently, during the maximum 10 min testing time a limited amount of solution was able to diffuse out of pledget and reached only the layers just beneath the pledget. The lack of respiratory stimulation as a consequence of applying the acidic naloxone solution to the area L indicates that the buffering capacity of cerebrospinal fluid at the medullary surface was able to maintain the surface pH within the normal range, despite diffusion

-

rn

Peak effect (min) Phrenic ampl. (arbitrary units) f (min-r) PCOZ(Torr) Psn (Torr)

(Torr)

-7

^.

* P