Interaction between nitric oxide synthase inhibitor induced oscillations and the activation flow coupling response

Abstract

The role of nitric oxide (NO) in the activation-flow coupling (AFC) response to periodic electrical forepaw stimulation was investigated using signal averaged laser Doppler (LD) flowmetry. LD measures of calculated cerebral blood flow (CBF) were obtained both prior and after intra-peritoneal administration of the non-selective nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine (L-NNA) (40 mg/kg). Characteristic baseline low frequency vasomotion oscillations (0.17 Hz) were observed after L-NNA administration. These LD(CBF) oscillations were synchronous within but not between hemispheres. L-NNA reduced the magnitude of the AFC response (p<0.05) for longer stimuli (1 min) with longer inter-stimulus intervals (2 min). In contrast, the magnitude of the AFC response for short duration stimuli (4 s) with short inter-stimulus intervals (20 s) was augmented (p<0.05) after L-NNA. An interaction occurred between L-NNA induced vasomotion oscillations and the AFC response with the greatest increase occurring at the stimulus harmonic closest to the oscillatory frequency. Nitric oxide may therefore modulate the effects of other vasodilators involved in vasomotion oscillations and the AFC response.

Figure 1

Laser Doppler (LD) measurements in arbitrary units over the ipsilateral and contralateral somatosensory cortices prior to and one hour after administration of the non selective nitric oxide synthase (NOS) inhibitor- N^G-nitro-L-arginine (L-NNA). Before L-NNA no oscillations were observed when both probes were placed either adjacent to each other over the same hemisphere (A) or between hemispheres (B) for a single rat. L-NNA induced oscillations were seen within each hemisphere (C) and between hemispheres (D) for the same rat. Insets shows the FFT of unaveraged data from rats (n=15) before and after L-NNA. Prior to L-NNA the power spectrum is evenly distributed with no frequency dominating when the probes were placed either adjacent to each other (inset to Figure 1A) or bilaterally (inset to Figure 1B). After L-NNA a major peak at 0.17 Hz (P < 0.05) was present both unilaterally (inset to Figure 1C) and bilaterally (inset to Figure 1D). For all figures black lines are measurements from Probe #1 and gray lines are from Probe #2.

Figure 2

The activation flow coupling (AFC) response for periodic forepaw stimulation (1 minute) at longer inter-stimulus intervals (2 minutes) both prior to and after L-NNA. The early peak portion of the AFC response, as measured by % LD_CBF change from baseline, was diminished after L-NNA (n=5). Solid gray line shows the AFC response before L-NNA while solid black line shows the AFC response after L-NNA. The hashed lines represent the stimulus duration with all errors bars indicating standard deviation (SD).

Figure 3

The AFC response for periodic forepaw stimulation (4 seconds) at shorter inter-stimulus intervals (20 seconds) both prior to and after L-NNA. The magnitude of the AFC response was significantly augmented after L-NNA (n=5). The solid gray line shows the AFC response before L-NNA while the solid black line shows the AFC response after L-NNA. The hashed lines represent the stimulus and error bars indicate SD.

Figure 4

Interaction between AFC response for various inter-stimulus intervals and L-NNA induced vasomotion oscillations. Changes in LD_CBF expressed as a percentage of baseline for 4 second stimuli applied 16–20 seconds into a 24 second iteration (A) or randomly (B) within 24 second length iteration both before and after L-NNA (n=5). The AFC response was augmented after L-NNA for periodic stimuli (A) but cancelled for random stimuli (B). The solid gray line represents the AFC responses to 4 second stimulus before L-NNA for both conditions while the solid black line shows the AFC response for 4 second stimulus presented after L-NNA for both conditions. For Figure 4A the hashed lines represent the stimulus duration with all errors bars indicating SD. For Figure 4B the stimulus was randomly applied throughout the iteration.

Figure 5

Peak heights (PH) of the AFC response for 4 second stimuli obtained prior to and after L-NNA for various inter-stimulus intervals (n=5). After L-NNA administration, the PH was significantly increased for 4 second stimuli during the 16 second (*= p < 0.01) and 24 second (**= p < 0.05) length iterations. The PH was also increased, but not significantly (p = 0.11), for 4 second stimuli during the 36 second length iteration after L-NNA. Error bars indicate SD.

Figure 6

Spectral analysis using the FFT for various inter-stimulus intervals both before and after L-NNA. Power spectrum for 4 second stimuli for iterations lengths of 16 seconds (A), 24 seconds (B), and 36 seconds (C) (n=5). The FFT for 4 second stimuli applied every 16 seconds prior to L-NNA has a peak at 0.063 Hz, the fundamental frequency, with additional peak seen at stimulus harmonics of this frequency (0.125 Hz and 0.188 Hz). After L-NNA, the fundamental frequency and its harmonics were augmented with the second stimulus harmonic (0.188 Hz) having the greatest augmentation. Also after L-NNA a vasomotion oscillation peak was observed at 0.17 Hz (Fig. 6A). Similar results were seen with 4 second stimuli for both 24 seconds (Fig. 6B). As seen in Figure 6B the fundamental frequency was 0.042 Hz with peaks observed at stimulus harmonics of this frequency (0.083 Hz, 0.125 Hz, and 0.167 Hz). After L-NNA the fundamental frequency and its harmonics were augmented with the third stimulus harmonic (0.167 Hz) having the greatest augmentation. A vasomotion oscillation peak was also observed at 0.17 Hz. In Figure 6C for 4 second stimuli applied every 36 seconds, the fundamental frequency was 0.028 Hz with peaks observed at stimulus harmonics of this frequency (0.056 Hz, 0.083 Hz, 0.111 Hz, 0.139 Hz, 0.167 Hz and 0.194 Hz). After L-NNA there was no significant change in the fundamental frequency and its harmonics. The greatest change was at the fifth stimulus harmonic (0.167 Hz) but this was not significant. In all figures solid gray lines show the FFT prior to L-NNA while solid black lines show the FFT after L-NNA.