Activity pattern of visceral cortex neurons during asphyxia

EXPERIMENTAL

NEUROLOGY

Activity

32, 12-21

Pattern

(1971)

of Visceral Cortex During Asphyxia A.

Neurons

PRZYBYLSKI

Exgerilnental and Clinical Medical Research Center, Polish Academy of Sciemes, Warsaw, Polartd Received

February

17, 1971

Upon stimulation of the vagns nerve, evoked responses and single unit activity were recorded from the orbital cortex of anesthetized and artificially ventilated rabbits. The neurons, based on single unit activity during asphyxia, could be separated into two groups: one whose activity disappeared with asphyxia in 54.6 set; and the other, in 213.4 sec. In both cases, they fired in bursts during asphyxia and their recovery was characterized by a transient increase in discharge frequency. Introduction

There is considerable experimental data on effects of hypoxia on the electrical activity of the brain. It has been established that disturbances in the spontaneous activity, induced by artificial ventilation of the animal with nitrogen and low oxygen content, passes through various stages: stage of activation. delta stage, and isoelectric stage. Although there is a possible relationship between the EEG and single unit activity (11, 12, 31), the behavior of single neurons under conditions of insufficient delivery and utilization of oxygen is still unknown. Experimental data are available for the effect of hypoxia and anoxia on neurons in the sensorimotor cortex and hypothalamus (4, 13, 14, 23, 24, 25, 28, 33, 39, 40), but data on changes in the activity of single neurons in other regions of the cortex under asphyxia and hypoxia are lacking. The study of the neuronal discharges in response to asphyxia recorded from the visceral cortex should be promising, since it has been shown that the cortical projection of the vagal afferents goes to the ventral orbital cortex (2, 3, 15, 16, 19, 26, 30, 37. 40. 41). The present paper examines changes of spontaneous activity of particular neurons in this region of the cortex during asphyxia and recovery. Methods

Experiments bital, paralyzed

were with

performed Flaxedil,

on 18 rabbits, anesthetized with Hexobarand artificially ventilated. Electrical activity 12

CORTICAL

13

ASI’HYSTA

of single nervous units was recorded extracellularly by means of glass micropipettes filled with 3 M KC1 in 1% agar-agar. Microelectrodes were introduced to the region of the vagal projection area in the ventral orbital cortex, This region was identified by the technique of averaged evoked potentials with a biological computer ANOPS 1 after stimulation of central cut end of the vagus nerve (rectangular pulses, OS-l.0 msec, 2-10 v ) The histograms of the spike intervals of the single neural units were analyzed also by ANOPS and mathematically elaborated by digital computer ODRA 120-I.’ Additionally the EEG ECG or the neuronogram of the phrenic nerve were recorded. nsphyxia was produced by switching off the respirator. Results

Twenty-five msec after stimulation of the central vagus ner\-e. in the ventral orbital cortex above the rhinal fissure, between stereotaxic from the bregma in the antero-postercoordinates : 7 1.0-2.5 mm anterior ior plane, 8-9 mm in the lateral plane, and at a depth of 2-4 mm from zero in the vertical plane, the averaged biphasic positive-negative evoked potentials of 50-150 ILV were recorded (Fig. 1). These potentials were strongly masked by oscillatory fluctuations. Therefore in the case of superposition technique without computer averaging these evoked potentials cannot he detected. The pattern of activity of single neural units in this region was recorded before and during asphyxia as well as during the period of recovery. Among 120 single units examined with regard to the type of change during asphyxiation one-third showed short-lasting periods of activity (54.6 set mean, ranging 25-90 set) with relatively small changes in the firing frequency (Fig. 2). Most of explored units revealed longer period of activity during asphyxia (213.4 set, ranging 110-420 set). Figure 3 represents the typical activity pattern of the neuron recorded in the ventral orhital cortex during asphysia. First, normal discharges of the neuron changed into regular volleylike activity. Next, slowing of the firing frequency and single irregularly appearing impulses together with the flattening of EEG occurred. Before complete cessation of spike activity, some transient increase in the frequency which correlated with oscillations of EEG was observed. 1 ANOPS : a computer manufactured by the Technical University 2 0DR.A : a computer constructed by ELWRO in Wrocfaw.

of \%‘arsaw.

3 Coordinates according stereota-xic atlas in “Electrophysiological Methods in Biological Research.” Eds. J. Bures, M. Petr6n. J. Zachar. =\cad. Pub. House CzechoSlovak Acad. Sci. Prague 1967. p. 653.

14

FIG. 1. The cal projection.

averaged (128 Positivity-upward.

repetitions) evoked potential recorded Calibrations : 100 pv, 100 msec.

in the vagal

corti-

After the respirator was switched on, the EEG background activity recovered, and the unit started firing. The unit activity progressed from single discharges through burst discharges to continuous and multifiring patterns of activity. As a rule enhancement of activity during asphysia and recovery was conxnon. Among the relatively resistant neurons there were units having more or less passive type of discharges (Fig. 2, 3) as well as active pattern of ac-

FIG. 2. The activity of sensitive type of neuron during N, spontaneous discharges and switching off the respirator recording; 45”, cessation of activity and its recovery after on 1’30”.

asphyxia and recovery. -P ; C, continuation of the pump was switched

CORTICAL

ASPHYXIA

15

tivity (Fig. 4). The volley character activity of the active pattern discharges changed during asphyxia toward uniform, monotonic activity. Both in the long-lasting (resistant) and short-lasting (sensitive) units after cessation of discharges produced by asphyxia, the recovery of activity was complete. The period of silence after the pump has been switched on till the first discharge in recovery was proportional to the duration of asphyxiation after disappearance of firing. The recovery was characterized by single discharges, followed by greater firing frequency (Fig. 2, 3, 4). The observed increase in firing frequency was correlated with a vollep-pattern of activity. It was observed in some experiments that in the course of recovery the unit activity spontaneously ceased after some time (after several up to 30 min of running) and it would reappear later. Additional anosic treatment at this moment resulted in quick cessation of activity. Before the complete recovery was reached (up to 30-60 min) the period of activity during asphyxia or nitrogen anoxia was evidently shorter (e.g., 45 set as compared to 195 set in the case of the first treatment). Mathematical elaboration of the firing frequency changes of the single

FIG. 3. (A) control recording. (B) 3 min after onset of asphyxia. (C) after onset of asphyxia. (D) 3-4 min after reoxygenation. Calibrations: I mv, 1 sec.

3-4 min 100 pv,

16

I’RZYBYLSKI

neural units during asphysia showed, in the interval histograms, distinct grouping of interspike intervals (Fig. 5 4, B, C) with it subsequent elongation and dissipation (Fig. SD). During recovery from asphyxia the restitution of the previous interspike intervals took place by means of grouping of discharges (Fig. 5E, F). Discussion

The present paper confirms Gellhorn’s (20) finding that the autonomic centers of the central nervous system are less sensitive to hypoxia than are somatic centers. If the time lapse of neural firing and EEG activity was in Creutzfeldt’s (14) experiments equal 29 -C 1.5 and 36 t 1.7 set, respec-

4’3 d

11’ CC) FIG. 4. (A) Activity from control recording c and after Flaxedil, F, and switching -P. (B) Activity 2.5-4.5 min afterwards. (C) Activity 4 min 50 set off the pump, afterwards when readmission of air, C, and switching on the pump, i-P, began. Calibrations: 1 mv, 1 sec.

CORTICAL

ASPIIYSIA

17

tively, the predominant number of neurons in the ventral orbital cortex preserved their function for as long as several minutes. The rest of the explored neurons stopped their activity in a relatively short time (54.6 set) , lvhich can be compared with thr period of activity of sensorimotor neurons in the vental orbital tortes there are populaduring hypoxia. Therefore. tions of relatively asphyxia sensitive as well as asphyxia resistant neurons. In the course of asphyxia induced activity pattern there is a period of enhancement of firing frequency. This increase has been observed both in sensitive and resistant neurons. This finding supports the previously reported data on a transient increase in excitability of neurons (11, 3.5 ) and of nerve fibers (27) followin g asphyxia. The frecluencv increment can be tlue, on one hand. to an increxe of afferentation from the reticular formation (5, 22) and chemoreceptors ( 10, 36), as well as to a decrease of the inhibitory influences from adjacent cortical neurons. The subsequent decrease in frequency in the course of asphyxia and hypoxia coultl be esplained by elongation of synaptic delay (33 ) and the abolishment of the reciprocal innervation because of the more severe damage to the inhibitory neurons (42). The reduction of inhibitory influences, the disturbances of reciprocal escitntory-inhibitory neural relationships with change of intercentral relations as well as the membrane features of various cortical neurons can cause the burst (volley ) character of the discharges during asphyxia and recovery. It has been established that during hypoxia and asphyxia, a decrease of membrane potential takes place (1, S, 9, 19, 25, 29, 34). In 1959 Kohnodin and Skoglund (25) showed, by direct microelectrode recordings, the initial phase of depolarization of neuron followed by its subsequent fast decline to the isoelectric point. Durin g the initial stage of hypoxia, the initial deI>olarization is to be connected with increment of escitability and spike freclurncy activity ( and probably also in asphyxia ). In Collewin’s and Van Harreveld’s (S) experiments on spinal motoneurons during acute asphyxia it WIS shown that in the first phase the membrane potential remained the same. or decreased, or increased. During asphyxia and hypoxia the intracellular K/N~ ratio decreased ( 32. 38‘). The full depression of electrical activity corresponds to the cessation of transmembrane ionic movement. It is likely, that such a degree of impairment of the neural function can be achieved only as a result of prolonged depression of tissue respiration and progressive exhaustion of the energy sources 1Koviding the processes of cellular repolarization. Taking into consideration the relative dissipation between discharge repolarization of the neuron we cat1 sullllose that the initial increase in the firing frequency during asphyxia results in the enhancement of this dissipation.

18

PRZYBYLSKI

P

I 14' N

04 (A)

812162024-

4x10

048121620 ms

(B)

32 4x10

+ ins

14.778 19.742 1.873 2.264

* 3240

((2 FIG.

(C) after AX,

4x10

ms

5. Interspike internal histograms. (A) Control. (B) 3 min of asphyxia. 3 min 15 set of asphyxia. (D) 3 min 30 set of asphyxia. (E) immediately reoxygenation. (F) 7 min after reoxygenation. MX, mean; SX, dispersion; excess of interspike distribution. asymmetry ; and EX,

The data of Lowry and associates (21) indicating the essential role of glucose and glycogen as energy substrates in ganglionic transmission and on substantial ATP and phosphocreatine levels, can be used to explain both the prevention of neural activity during hypoxia (asphyxia) and recovery from this state. The accumulation of nonoxidized products of glycolysis (lactate, pyruvate) after resupplying oxygen to the tissue which still keep the substantial level of macroergic compounds, starts the recovery firing discharges through transitory increment in the frequency. The fast burning of the incomplete oxidized metabolites (e.g., pyruvate) can oxidize some reduced nucleotides (20) and force processes of repolarization and generation of the action potential. The previously published data on membrane permeability by Chalazonitis and coworkers (1, 6)) and the more recent works (7, 34) concerning

CORTICAL

MX sx AX EX

0.E

(D) PA

= 39.167 = 19.737 = 249 = 1,500

l--L-

4

19

ASPHYXIA

8 12 16 20

1

+

40

x10

rns

MX SX z;

0 20

0.10

005

+

04815 IFI

ms

(El

04

8 121620

28

the direct effect of osygen and carbon dioxide and pH on neuron membrane, suggest the possible mechanism behind the initial depolarization of the neuron produced by hypoxia (asphyxia) as well as the fast recovery of its activity after short-lasting severe hypoxia (asphyxia). References 1. ARVANITAKI-CHALAZONITIS, A., G. ROX~EY, and H. TAKEUCHI. 1967. Variations de la relation caracteristique V/i de la neuromembrane en fonction de la PO,. Znzplicatiows fomtiorzellcs. c‘. R. Sot. Biol. 161 : 16291641. 2. AUBERT, M., and J. LEGROS. 1954. Projections du nerf vague sur le &o-cortex du chat. J. Physiol. (Pnvis) 55 : 109-110. 3. BAILEY, P., and F. BREIUER. 1938. Sensory cortical representation of vagus nerve with note on effects of low blood pressure on cortical electrogram. J. h’crrrophysiol. 1: 405412. 4. BAUMGARTNER, G., 0. CREUTZFELDT, and K. JUNG. 1961. Microphysiology of cortical neurons in acute anoxia and retinal ischemia, pp. 534. I+t “Cerebral anoxia and the Electroencephalogram.” J. S. Meyer, and H. Gastaut (eds.). Thomas, Springfield, Illinois. 5. BONVALLET, M., A. HUCELIN, and P. DELL. 1955. Sensibilite comparee du syst&e reticule activateur ascendant et du centre respiratuare aus gaz du sang et a l’adrenaline. J. Plz>~siol. (Paris). 47 : 651-54.

20 6.

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15. 16.

17. 18. 19. 20.

21.

22.

23.

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CORTICAI, 24.

KASAMATSU,

Neurone 166-166. 25.

26. 27. 28. 29.

30.

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34. 35. 36. 37. 38. 39.

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A.,

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;‘.SPHYSIA

21

1956. Die Tiitigkeit einzelner corticale bei reine Anoxie. k’lin. U’or&schr. 34 :

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