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Reduction of baroreceptor-related sympathetic discharge during NMDA receptor antagonism
BRAIN RESEARCH ELSEVIER
Brain Research 665 (1994) 323-326
Short communication
Reduction of baroreceptor-related sympathetic discharge during NMDA receptor antagonism Anthony L. Sica *, Zafar A. Siddiqi Pediatric Research Center, Department of Pediatrics, Schneider Children's Hospital, Long Island Campus for Albert Einstein College of Medicine, New Hyde Park, NYl1042, USA Accepted 20 September 1994
Abstract
N-Methyl-D-aspartate receptor blockade by MK-801 produced changes in the baroreceptor related component of pre-ganglionic sympathetic discharges in kittens. Spectral analyses carried out on sympathetic and blood pressure signals demonstrated: (a) a marked reduction in amplitude of peaks, 2-6 Hz range, in sympathetic autopower spectra and (b) a substantial decrease of coherence between sympathetic and blood pressure signals in the 2-6 Hz range.
Keywords: N-Methyi-o-aspartate; MK-801; Sympathetic; Rhythm; Development
A common feature of pre- and post-ganglionic sympathetic nerves is the simultaneous presence of bursts of activity which are temporally related to the inspiratory (I) phase and to the cardiac cycle. Both forms of activity have been observed in developing animals as early as 16 h after birth [5,6]. Furthermore,there is evidence that pre ganglionic I activity may be mediated by glutamatergic receptors, since blockade of the Nmethyl-o-aspartate (NMDA) subtype of glutamate receptor by the non-competitive NMDA antagonist MK801 has been shown to exert different influences on pre-ganglionic nerve I activity depending upon the animal's age [5]. For example, in neonatal animals,sympathetic I activity was effectively abolished at doses of MK-801 which elicited a marked increase in the duration of the phrenic (PHR) discharge (i.e., apneusis); in contrast, those of older animals exhibited apneustic patterns identical to PHR discharge. Regardless of the type of change in sympathetic I discharge manifested following MK-801 infusions, an appreciable amount of tonic sympathetic activity remained, thereby suggesting that the cardiac component of sympathetic discharge may be unaffected by NMDA receptor block. If such in fact is the case, then spectral analyses should show that MK-801 does not significantly affect the degree of
* Corresponding author. 0006-8993/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 0 6 - 8 9 9 3 ( 9 4 ) 0 1 1 2 0 - 6
correlation (coherence) between sympathetic and blood pressure signals over the range of cardiac frequencies, i.e., 2-6 Hz. To test this hypothesis, we have carried out power and coherence spectral analyses on signals recorded from animals used in our earlier report [5]. Experiments were performed on six kittens (7-138 days old, 0.16-2.0 kg) anesthetized with sodium pentobarbital (35 mg/kg i.p.). Following induction of anesthesia, a femoral vein was cannulated for periodic infusions of saline (1-3 ml/kg), administration of supplemental doses of anesthetic (1-3 mg/kg i.v.) and for infusions of MK-801. A femoral artery was then cannulated in order to obtain measurements of arterial blood gas tensions and pH (Corning 178 p H / B l o o d Gas Analyzer). Animals were tracheotomized, vagotomized, paralyzed (pancuronium bromide 1 mg/kg i.v.; supplemental doses 0.3-0.4 mg/kg) and artificially ventilated with 100% 0 2. Intratracheal and arterial pressures were measured by transducers connected to tracheal and arterial cannulae, respectively. Rectal temperature was maintained at 37-38°C using a servo-regulated heating pad. Following the completion of surgery, arterial blood gas tensions and pH were measured periodically throughout the experiment and kept within normal limits. Adjustments of p C O 2 w e r e made by changes of either ventilatory rate or volume. If necessary, NaHCO 3 was given (i.v.) to correct metabolic acidosis.
324
A.L. Sica, Z.A. Siddiqi / Brain Research 665 (1994) 323-326
Animals were placed in a stereotaxic apparatus and rotated to the ventral position. The efferent C5 P H R root and cervical sympathetic (CS) nerve of the right side were dissected from surrounding tissues, cut, desheathed and immersed in paraffin oil. Monophasic recordings (bandpass 3 H z - 3 kHz) were made from the crushed central ends of nerves with bipolar platinum electrodes. MK-801 was dissolved in sterile saline at a low concentration of 5 0 / z g / m l and at a high concentration of 500/xg/ml; each solution was adjusted to a final pH of 7.4. Apneuses were produced according to the following protocol. Injections (0.1 ml of the low concentration) were made at intervals of 3 - 4 min until the I duration increased by at least 75% or until the animal had received a total of 10 injections of the low concentration of MK-801. If apneusis was not observed, additional injections were made at 3-4-rain intervals using the high concentration of MK-801 (0.1 ml per injection) until apneuses were produced. The total amount of drug given ranged from 20-2,000 /xg which was equivalent to dosages ranging from 0.04-2.5 mg/kg. Experimental runs were begun 0.5-1 h after surgery to allow for stabilization. Recordings were taken of activities before drug administration and after a sufficient amount of drug had been given to elicit apneuses. All neural signals and arterial blood pressure (BP) were stored on FM tape for off line computation of power spectra and signal average histograms. For both types of analysis, the CS and BP signals were low pass filtered (40 Hz) and acquired along with the integrated (100 ms time constant) PHR signal using a sampling rate of 128 Hz. To unambiguously demonstrate the coupling of sympathetic discharge to the cardiac cycle, power spectral and signal average histograms were constructed only from activities occurring during central expiratory (E) phases, thereby avoiding any distortion of cardiac related CS activity due to I-related CS activity. A fast Fourier transform (FFT) routine was carried out on E activities and the number of FFT points (256 or 512) was set to equal the duration of the E phase. The mean and any other trend were then subtracted from each data point; next each point was multiplied by a Hamming function. Power spectral estimates were then calculated for each segment and the final power spectrum was obtained by ensemble averaging of all individual segments (n = 20-35). To detect correlated (i.e., coherent) frequencies between CS and BP signals, the cross spectrum was first calculated followed by computation of the coherence spectrum: the ratio of the squared magnitude of the average cross-power estimate at each frequency to the product of the individual autopower spectra estimates at the same frequency [2]. As coherence could also result from cardiac-related artifacts mechanically coupled to nerve activity, it was
A.
B.
P R E MK-801
CS A V E R A G E D A C T I V I T Y :,
BP
--.
CS .... PRE --POST
,..,'
./
POST MK-801
o.os o11 o.~5 o12 0.25 0~ TIME(s)
250 ms Fig. 1. A: digital traces (sampling rate = 128 Hz) of low pass filtered (40 Hz) cervical sympathetic (CS) nerve and blood pressure (BP) signals before and after drug administration P R E MK-801 and POST MK-801, respectively. B: signal average histograms (n = 400 cardiac cycles) of low-pass filtered CS activity pre- and post-MK-810 infusion. The vertical line at time zero indicates the position of pulses (peak of systole) used to trigger the average.
necessary to illustrate the non-artifactual origin of sympathetic cardiac related discharge. This was accomplished by constructing signal average histograms of sympathetic activity, gated to the E phase and triggered by pulses positioned at the peak of systole. In all animals, CS discharges prior to infusion of MK-801 exhibited baroreceptor related activitiy, i.e., bursts of activity occurring during diastole. An example of rather strong coupling of such sympathetic discharge to the cardiac cycle is given in Fig. 1 (top set of digital traces in Panel A, PRE-MK-801). That the baroreceptor related discharge in massed sympathetic activity was a relatively constant feature is demonstrated by the presence of a large summation wave to the right of time zero (i.e., peak of systole) in the average histogram of Fig. 1 (Panel B, PRE). It is also important to note that this wave does not straddle the vertical line at time zero (i.e., the peak of the systolic component in the BP signal), thereby demonstrating the non-artifactual origin of the summation wave. Autopower and coherence spectra carried out on those signals demonstrated that the sympathetic discharge was dominated by cardiac-related activity as indicated by the large spectral peak at 4.0 Hz in both CS and BP autospectra (Fig. 2, left panel P R E MK801). Furthermore, the coherence estimate of 0.99 at 4.0 Hz (Fig. 2, left panel) indicated an exceptionally strong linear relationship between baroreceptor afferent inputs and the sympathetic rhythmn generating networks of this animal; such a marked degree of coupling is also manifested by the presence of large
A.L. Sica, Z.A. Siddiqi /Brain Research 665 (1994)323-326 POST MK-801
P R E MK-801 104F
i • 4.00 Hz
104
: ~ 3.25 Hz
BP
I0 °L
- -
,¢ c.
CS A
1010 0.0
BP-CS
10
15
20
00 0
5
10
15
20
F R E Q U E N C Y (Hz)
Fig. 2. Autopower and coherence spectra (0.25 bin width) of cervical sympathetic (CS) and blood pressure (BP) constructed from low pass filtered (40 Hz) signals before and after MK-801 infusion, PRE MK-801 and POST MK-801, respectively.
coherence values at the second and third harmonics, 0.94 and 0.38, respectively. Albeit the histograms of Figs. 1 and 2 represent optimal cases, each kitten exhibited an appreciable degree of correlation in their respective coherence spectra, ranging from 0.23 to 0.99 at the cardiac frequencies, 2.5-5.25 Hz (see Table 1). To ascertain the effect of NMDA receptor blockade on baroreceptor mediated influences in sympathetic discharges, we selected for data analyses only those portions of each record where PHR discharges were observed to have an apneustic discharge pattern. These epochs were selected because preliminary analyses demonstrated that the effect of NMDA antagonism on CS baroreceptor-related discharge occurred coincidentally with the observation of apneuses herein indicated by marked increases of discharge durations, i.e., from an average duration of 1.09 s before MK-801 infusion to one of 7.38 s after MK-801 infusion. As can be observed in the digital traces and signal averages of Table 1 Changes in values of coherence between cervical sympathetic (CS) and blood pressure (BP) signals following administration of MK-801 (n = 6 animals). In all comparisons the coherence estimate is referenced to the value at the cardiac frequency, i.e., location of peak in BP spectra locations (Hz) and values of coherence (Coh) for spectral peaks in CS and BP signals PRE MK-801
1. 2. 3. 4. 5. 6.
POST MK-801
CS
BP
Coh
CS
BP
Coh
3.00 4.00 5.00 2.50 3.50 5.25
4.00 5.00 5.00 2.50 4.00 4.75
0.99 0.53 0.88 0.89 0.29 0.59
3.50 3.50 5.00 4.50 3.50 5.00
3.00 4.50 5.00 3.75 4.50 4.75
0.17 0.19 0.36 0.88 0.07 0.24
325
post MK-801 activity in Fig. 1, NMDA receptor blockade produced a marked depression of the baroreceptor-related discharge in CS activity. Nevertheless, the CS autospectrum of Fig. 2 (POST MK-801) revealed the presence of a peak at the cardiac frequency (3.25 Hz). Upon comparisons of pre- and post-MK-801 spectra, however, several noteworthy features emerged as a consequence of drug administration: (a) a marked reduction in the amplitude of the peak at the cardiac frequency; (b) an apparent loss of synchronization at the cardiac frequency as indicated by the broad distribution of power in the CS autospectrum and (c) more to the point, a marked reduction in the value of the coherence estimate at the cardiac frequency, from 0.99 to 0.17. Such changes of frequency domain features were typical of 5 / 6 animals; in one case the coherence estimate was unchanged following MK-801 administration (0.89 vs. 0.88). This latter animal was not excluded from statistical comparisons, which demonstrated that NMDA receptor block produced a significant (p = 0.02, paired t-test) decrease in coherence values (from 0.70 + 0.11 pre-drug to 0.32 + 0.12 post-drug, mean + S.E.M.) at the cardiac frequencies (Table 1). On the other hand, MK-801 infusion produced no significant change in the mean arterial pressures of our animals, 75 + 8 mmHg during control conditions and 72 + 8 mmHg following MK-801 infusions. The results of this study when combined with those of our earlier report [5] (the same animals were used to generate both data sets) provide evidence that NMDA receptors are involved in the mediation of baroreceptor-related CS activities as well as respiration-related CS activities. However, unlike the CS respiration-related discharge, NMDA receptor block by MK-801 did not elicit any age-related changes in the baroreceptor related CS discharge. Rather, the overall effect of such anatagonism can be best described as an age-independent reduction of coherence at the cardiac-frequencies (2-6 Hz). The fact that such reduction in the values for coherence occurred regardless of the type of change in CS respiration-related discharge strongly suggests that this effect was not due to a change of state within the cardiorespiratory integrating networks. Rather, this result implies a strong involvement for NMDA receptors in the mediation of baroreceptor afferent inputs to central neural circuits involved in the shaping of sympathetic outflow. Perhaps, the threshholds for the production of apneusis and the diminuition of baroreceptor modulation are very similar, which might account for the contemporaneous occurrences of those events. The persistence of peaks in CS spectra at the cardiac frequencies, albeit markedly diminished, probably reflects modulation of baroreceptor inputs by nonNMDA as well as other receptor types. The finding of a substantial loss of power and coherence in pre-ganglionic sympathetic outflow may have
326
A.L. Sica, Z.A. Siddiqi / Brain Research 665 (1994) 323-326
functional implications, i.e., that this r e s i d u a l activity is u n a b l e to drive p o s t - g a n g l i o n i c activity and, consequently, t h a t t a r g e t o r g a n s no l o n g e r receive a p p r o p r i ate m o d u l a t o r y inputs. W h i l e this possibility was not s t u d i e d in the p r e s e n t investigation, studies of animals with spinal t r a n s e c t i o n s s h o w e d t h a t r e s i d u a l sympathetic nerve activity, even in cases w h e n f a c i l i t a t e d by i n t r a t h e c a l infusion of an excitatory a m i n o acid, was physiologically insignificant [4,7]. It is not known w h e t h e r the s y m p a t h e t i c activity r e c o r d e d in those studies c o n t a i n e d a 2 - 6 H z oscillation, since s p e c t r a l analyses w e r e not p e r f o r m e d on t h o s e signals. O n the o t h e r h a n d , studies o f s y m p a t h e t i c activity following s u p e r f u s i o n of t r a n s e c t e d spinal cords with kainic acid have d e m o n s t r a t e d a 2 - 6 H z oscillation; however, evid e n c e of c o h e r e n c e was not p r o v i d e d in those studies [1,3]. Thus, it r e m a i n s an u n a n s w e r e d q u e s t i o n as to w h e t h e r a lack of c o h e r e n c e can be i n t e r p r e t e d as an index of physiologically insignifcant activity. W e suggest that functionally a p p r o p r i a t e i n n e r v a t i o n o f t a r g e t o r g a n s by p o s t g a n g l i o n i c s y m p a t h e t i c nerves r e q u i r e s s y n c h r o n i z a t i o n by b r a i n stem s y m p a t h e t i c p r e m o t o r and h i g h e r - o r d e r neurons.
This r e s e a r c h was s u p p o r t e d by N I H G r a n t H L 41008 to A.L.S.
[1] Allen, A.M, Adams, J.M. and Guyenet, P.G., Role of the spinal cord in generating the 2- to 6-Hz rhythm in rat sympathetic outflow, Am. J. Physiol., 264 (1993) R938-R945. [2] Bendat, J.S. and Piersol, A.G., Random Data: Analysis and Measurement Procedures, 2nd edn., Wiley, New York, 1986, 566 pages. [3] Gebber, G.L., Barman, S.M. and Zviman, M., Sympathetic activity remains synchronized in presence of a glutamate antagonist, Am. J. Physiol., 256 (1988) R722-R732. [4] Hong, Y., Cechetto, D.F. and Weaver, L.C., Spinal cord regulation of sympathetic activity in intact and spinal rats, Am. J. Physiol., 266 (1994) H1485-H1493. [5] Sica, A.L., Siddiqi, Z.A. and Pisana, F., The effects of N-methylo-aspartate antagonism on the inspiratory activities of developing animals, Dev. Brain Res., 65 (1992) 281-283. [6] Sica, A.L., Siddiqi, Z.A., Hundley, B.W., Gootman, P.M. and Steele, A.M., Effects of G A B A A receptor antagonism on inspiratory activities in kittens, Neurosci. Lett., 160 (1993) 149-152. [7] Trostel, K.A. and Osborn, J.W., Does the spinal cord generate functionally significant sympathetic activity in the awake rat?, Am. J. Physiol., 266 (1994) R1102-R1110.
Brain Research 665 (1994) 323-326
Short communication
Reduction of baroreceptor-related sympathetic discharge during NMDA receptor antagonism Anthony L. Sica *, Zafar A. Siddiqi Pediatric Research Center, Department of Pediatrics, Schneider Children's Hospital, Long Island Campus for Albert Einstein College of Medicine, New Hyde Park, NYl1042, USA Accepted 20 September 1994
Abstract
N-Methyl-D-aspartate receptor blockade by MK-801 produced changes in the baroreceptor related component of pre-ganglionic sympathetic discharges in kittens. Spectral analyses carried out on sympathetic and blood pressure signals demonstrated: (a) a marked reduction in amplitude of peaks, 2-6 Hz range, in sympathetic autopower spectra and (b) a substantial decrease of coherence between sympathetic and blood pressure signals in the 2-6 Hz range.
Keywords: N-Methyi-o-aspartate; MK-801; Sympathetic; Rhythm; Development
A common feature of pre- and post-ganglionic sympathetic nerves is the simultaneous presence of bursts of activity which are temporally related to the inspiratory (I) phase and to the cardiac cycle. Both forms of activity have been observed in developing animals as early as 16 h after birth [5,6]. Furthermore,there is evidence that pre ganglionic I activity may be mediated by glutamatergic receptors, since blockade of the Nmethyl-o-aspartate (NMDA) subtype of glutamate receptor by the non-competitive NMDA antagonist MK801 has been shown to exert different influences on pre-ganglionic nerve I activity depending upon the animal's age [5]. For example, in neonatal animals,sympathetic I activity was effectively abolished at doses of MK-801 which elicited a marked increase in the duration of the phrenic (PHR) discharge (i.e., apneusis); in contrast, those of older animals exhibited apneustic patterns identical to PHR discharge. Regardless of the type of change in sympathetic I discharge manifested following MK-801 infusions, an appreciable amount of tonic sympathetic activity remained, thereby suggesting that the cardiac component of sympathetic discharge may be unaffected by NMDA receptor block. If such in fact is the case, then spectral analyses should show that MK-801 does not significantly affect the degree of
* Corresponding author. 0006-8993/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 0 6 - 8 9 9 3 ( 9 4 ) 0 1 1 2 0 - 6
correlation (coherence) between sympathetic and blood pressure signals over the range of cardiac frequencies, i.e., 2-6 Hz. To test this hypothesis, we have carried out power and coherence spectral analyses on signals recorded from animals used in our earlier report [5]. Experiments were performed on six kittens (7-138 days old, 0.16-2.0 kg) anesthetized with sodium pentobarbital (35 mg/kg i.p.). Following induction of anesthesia, a femoral vein was cannulated for periodic infusions of saline (1-3 ml/kg), administration of supplemental doses of anesthetic (1-3 mg/kg i.v.) and for infusions of MK-801. A femoral artery was then cannulated in order to obtain measurements of arterial blood gas tensions and pH (Corning 178 p H / B l o o d Gas Analyzer). Animals were tracheotomized, vagotomized, paralyzed (pancuronium bromide 1 mg/kg i.v.; supplemental doses 0.3-0.4 mg/kg) and artificially ventilated with 100% 0 2. Intratracheal and arterial pressures were measured by transducers connected to tracheal and arterial cannulae, respectively. Rectal temperature was maintained at 37-38°C using a servo-regulated heating pad. Following the completion of surgery, arterial blood gas tensions and pH were measured periodically throughout the experiment and kept within normal limits. Adjustments of p C O 2 w e r e made by changes of either ventilatory rate or volume. If necessary, NaHCO 3 was given (i.v.) to correct metabolic acidosis.
324
A.L. Sica, Z.A. Siddiqi / Brain Research 665 (1994) 323-326
Animals were placed in a stereotaxic apparatus and rotated to the ventral position. The efferent C5 P H R root and cervical sympathetic (CS) nerve of the right side were dissected from surrounding tissues, cut, desheathed and immersed in paraffin oil. Monophasic recordings (bandpass 3 H z - 3 kHz) were made from the crushed central ends of nerves with bipolar platinum electrodes. MK-801 was dissolved in sterile saline at a low concentration of 5 0 / z g / m l and at a high concentration of 500/xg/ml; each solution was adjusted to a final pH of 7.4. Apneuses were produced according to the following protocol. Injections (0.1 ml of the low concentration) were made at intervals of 3 - 4 min until the I duration increased by at least 75% or until the animal had received a total of 10 injections of the low concentration of MK-801. If apneusis was not observed, additional injections were made at 3-4-rain intervals using the high concentration of MK-801 (0.1 ml per injection) until apneuses were produced. The total amount of drug given ranged from 20-2,000 /xg which was equivalent to dosages ranging from 0.04-2.5 mg/kg. Experimental runs were begun 0.5-1 h after surgery to allow for stabilization. Recordings were taken of activities before drug administration and after a sufficient amount of drug had been given to elicit apneuses. All neural signals and arterial blood pressure (BP) were stored on FM tape for off line computation of power spectra and signal average histograms. For both types of analysis, the CS and BP signals were low pass filtered (40 Hz) and acquired along with the integrated (100 ms time constant) PHR signal using a sampling rate of 128 Hz. To unambiguously demonstrate the coupling of sympathetic discharge to the cardiac cycle, power spectral and signal average histograms were constructed only from activities occurring during central expiratory (E) phases, thereby avoiding any distortion of cardiac related CS activity due to I-related CS activity. A fast Fourier transform (FFT) routine was carried out on E activities and the number of FFT points (256 or 512) was set to equal the duration of the E phase. The mean and any other trend were then subtracted from each data point; next each point was multiplied by a Hamming function. Power spectral estimates were then calculated for each segment and the final power spectrum was obtained by ensemble averaging of all individual segments (n = 20-35). To detect correlated (i.e., coherent) frequencies between CS and BP signals, the cross spectrum was first calculated followed by computation of the coherence spectrum: the ratio of the squared magnitude of the average cross-power estimate at each frequency to the product of the individual autopower spectra estimates at the same frequency [2]. As coherence could also result from cardiac-related artifacts mechanically coupled to nerve activity, it was
A.
B.
P R E MK-801
CS A V E R A G E D A C T I V I T Y :,
BP
--.
CS .... PRE --POST
,..,'
./
POST MK-801
o.os o11 o.~5 o12 0.25 0~ TIME(s)
250 ms Fig. 1. A: digital traces (sampling rate = 128 Hz) of low pass filtered (40 Hz) cervical sympathetic (CS) nerve and blood pressure (BP) signals before and after drug administration P R E MK-801 and POST MK-801, respectively. B: signal average histograms (n = 400 cardiac cycles) of low-pass filtered CS activity pre- and post-MK-810 infusion. The vertical line at time zero indicates the position of pulses (peak of systole) used to trigger the average.
necessary to illustrate the non-artifactual origin of sympathetic cardiac related discharge. This was accomplished by constructing signal average histograms of sympathetic activity, gated to the E phase and triggered by pulses positioned at the peak of systole. In all animals, CS discharges prior to infusion of MK-801 exhibited baroreceptor related activitiy, i.e., bursts of activity occurring during diastole. An example of rather strong coupling of such sympathetic discharge to the cardiac cycle is given in Fig. 1 (top set of digital traces in Panel A, PRE-MK-801). That the baroreceptor related discharge in massed sympathetic activity was a relatively constant feature is demonstrated by the presence of a large summation wave to the right of time zero (i.e., peak of systole) in the average histogram of Fig. 1 (Panel B, PRE). It is also important to note that this wave does not straddle the vertical line at time zero (i.e., the peak of the systolic component in the BP signal), thereby demonstrating the non-artifactual origin of the summation wave. Autopower and coherence spectra carried out on those signals demonstrated that the sympathetic discharge was dominated by cardiac-related activity as indicated by the large spectral peak at 4.0 Hz in both CS and BP autospectra (Fig. 2, left panel P R E MK801). Furthermore, the coherence estimate of 0.99 at 4.0 Hz (Fig. 2, left panel) indicated an exceptionally strong linear relationship between baroreceptor afferent inputs and the sympathetic rhythmn generating networks of this animal; such a marked degree of coupling is also manifested by the presence of large
A.L. Sica, Z.A. Siddiqi /Brain Research 665 (1994)323-326 POST MK-801
P R E MK-801 104F
i • 4.00 Hz
104
: ~ 3.25 Hz
BP
I0 °L
- -
,¢ c.
CS A
1010 0.0
BP-CS
10
15
20
00 0
5
10
15
20
F R E Q U E N C Y (Hz)
Fig. 2. Autopower and coherence spectra (0.25 bin width) of cervical sympathetic (CS) and blood pressure (BP) constructed from low pass filtered (40 Hz) signals before and after MK-801 infusion, PRE MK-801 and POST MK-801, respectively.
coherence values at the second and third harmonics, 0.94 and 0.38, respectively. Albeit the histograms of Figs. 1 and 2 represent optimal cases, each kitten exhibited an appreciable degree of correlation in their respective coherence spectra, ranging from 0.23 to 0.99 at the cardiac frequencies, 2.5-5.25 Hz (see Table 1). To ascertain the effect of NMDA receptor blockade on baroreceptor mediated influences in sympathetic discharges, we selected for data analyses only those portions of each record where PHR discharges were observed to have an apneustic discharge pattern. These epochs were selected because preliminary analyses demonstrated that the effect of NMDA antagonism on CS baroreceptor-related discharge occurred coincidentally with the observation of apneuses herein indicated by marked increases of discharge durations, i.e., from an average duration of 1.09 s before MK-801 infusion to one of 7.38 s after MK-801 infusion. As can be observed in the digital traces and signal averages of Table 1 Changes in values of coherence between cervical sympathetic (CS) and blood pressure (BP) signals following administration of MK-801 (n = 6 animals). In all comparisons the coherence estimate is referenced to the value at the cardiac frequency, i.e., location of peak in BP spectra locations (Hz) and values of coherence (Coh) for spectral peaks in CS and BP signals PRE MK-801
1. 2. 3. 4. 5. 6.
POST MK-801
CS
BP
Coh
CS
BP
Coh
3.00 4.00 5.00 2.50 3.50 5.25
4.00 5.00 5.00 2.50 4.00 4.75
0.99 0.53 0.88 0.89 0.29 0.59
3.50 3.50 5.00 4.50 3.50 5.00
3.00 4.50 5.00 3.75 4.50 4.75
0.17 0.19 0.36 0.88 0.07 0.24
325
post MK-801 activity in Fig. 1, NMDA receptor blockade produced a marked depression of the baroreceptor-related discharge in CS activity. Nevertheless, the CS autospectrum of Fig. 2 (POST MK-801) revealed the presence of a peak at the cardiac frequency (3.25 Hz). Upon comparisons of pre- and post-MK-801 spectra, however, several noteworthy features emerged as a consequence of drug administration: (a) a marked reduction in the amplitude of the peak at the cardiac frequency; (b) an apparent loss of synchronization at the cardiac frequency as indicated by the broad distribution of power in the CS autospectrum and (c) more to the point, a marked reduction in the value of the coherence estimate at the cardiac frequency, from 0.99 to 0.17. Such changes of frequency domain features were typical of 5 / 6 animals; in one case the coherence estimate was unchanged following MK-801 administration (0.89 vs. 0.88). This latter animal was not excluded from statistical comparisons, which demonstrated that NMDA receptor block produced a significant (p = 0.02, paired t-test) decrease in coherence values (from 0.70 + 0.11 pre-drug to 0.32 + 0.12 post-drug, mean + S.E.M.) at the cardiac frequencies (Table 1). On the other hand, MK-801 infusion produced no significant change in the mean arterial pressures of our animals, 75 + 8 mmHg during control conditions and 72 + 8 mmHg following MK-801 infusions. The results of this study when combined with those of our earlier report [5] (the same animals were used to generate both data sets) provide evidence that NMDA receptors are involved in the mediation of baroreceptor-related CS activities as well as respiration-related CS activities. However, unlike the CS respiration-related discharge, NMDA receptor block by MK-801 did not elicit any age-related changes in the baroreceptor related CS discharge. Rather, the overall effect of such anatagonism can be best described as an age-independent reduction of coherence at the cardiac-frequencies (2-6 Hz). The fact that such reduction in the values for coherence occurred regardless of the type of change in CS respiration-related discharge strongly suggests that this effect was not due to a change of state within the cardiorespiratory integrating networks. Rather, this result implies a strong involvement for NMDA receptors in the mediation of baroreceptor afferent inputs to central neural circuits involved in the shaping of sympathetic outflow. Perhaps, the threshholds for the production of apneusis and the diminuition of baroreceptor modulation are very similar, which might account for the contemporaneous occurrences of those events. The persistence of peaks in CS spectra at the cardiac frequencies, albeit markedly diminished, probably reflects modulation of baroreceptor inputs by nonNMDA as well as other receptor types. The finding of a substantial loss of power and coherence in pre-ganglionic sympathetic outflow may have
326
A.L. Sica, Z.A. Siddiqi / Brain Research 665 (1994) 323-326
functional implications, i.e., that this r e s i d u a l activity is u n a b l e to drive p o s t - g a n g l i o n i c activity and, consequently, t h a t t a r g e t o r g a n s no l o n g e r receive a p p r o p r i ate m o d u l a t o r y inputs. W h i l e this possibility was not s t u d i e d in the p r e s e n t investigation, studies of animals with spinal t r a n s e c t i o n s s h o w e d t h a t r e s i d u a l sympathetic nerve activity, even in cases w h e n f a c i l i t a t e d by i n t r a t h e c a l infusion of an excitatory a m i n o acid, was physiologically insignificant [4,7]. It is not known w h e t h e r the s y m p a t h e t i c activity r e c o r d e d in those studies c o n t a i n e d a 2 - 6 H z oscillation, since s p e c t r a l analyses w e r e not p e r f o r m e d on t h o s e signals. O n the o t h e r h a n d , studies o f s y m p a t h e t i c activity following s u p e r f u s i o n of t r a n s e c t e d spinal cords with kainic acid have d e m o n s t r a t e d a 2 - 6 H z oscillation; however, evid e n c e of c o h e r e n c e was not p r o v i d e d in those studies [1,3]. Thus, it r e m a i n s an u n a n s w e r e d q u e s t i o n as to w h e t h e r a lack of c o h e r e n c e can be i n t e r p r e t e d as an index of physiologically insignifcant activity. W e suggest that functionally a p p r o p r i a t e i n n e r v a t i o n o f t a r g e t o r g a n s by p o s t g a n g l i o n i c s y m p a t h e t i c nerves r e q u i r e s s y n c h r o n i z a t i o n by b r a i n stem s y m p a t h e t i c p r e m o t o r and h i g h e r - o r d e r neurons.
This r e s e a r c h was s u p p o r t e d by N I H G r a n t H L 41008 to A.L.S.
[1] Allen, A.M, Adams, J.M. and Guyenet, P.G., Role of the spinal cord in generating the 2- to 6-Hz rhythm in rat sympathetic outflow, Am. J. Physiol., 264 (1993) R938-R945. [2] Bendat, J.S. and Piersol, A.G., Random Data: Analysis and Measurement Procedures, 2nd edn., Wiley, New York, 1986, 566 pages. [3] Gebber, G.L., Barman, S.M. and Zviman, M., Sympathetic activity remains synchronized in presence of a glutamate antagonist, Am. J. Physiol., 256 (1988) R722-R732. [4] Hong, Y., Cechetto, D.F. and Weaver, L.C., Spinal cord regulation of sympathetic activity in intact and spinal rats, Am. J. Physiol., 266 (1994) H1485-H1493. [5] Sica, A.L., Siddiqi, Z.A. and Pisana, F., The effects of N-methylo-aspartate antagonism on the inspiratory activities of developing animals, Dev. Brain Res., 65 (1992) 281-283. [6] Sica, A.L., Siddiqi, Z.A., Hundley, B.W., Gootman, P.M. and Steele, A.M., Effects of G A B A A receptor antagonism on inspiratory activities in kittens, Neurosci. Lett., 160 (1993) 149-152. [7] Trostel, K.A. and Osborn, J.W., Does the spinal cord generate functionally significant sympathetic activity in the awake rat?, Am. J. Physiol., 266 (1994) R1102-R1110.