Interactions between brainstem and trigeminal neurons detected by cross-spectral analysis

Cells in the nucleus raphe magnus that are inhibited by noxious skin stimuli (off-cells) have been postulated to suppress pain by continuously inhibiting spinal and trigeminal nociceptive neurons. To test this hypothesis, spontaneous activity was simultaneously recorded from off-cells ( n=15) and wide-dynamic range cells ( n=27) of the trigeminal complex (subnucleus interpolaris), in rats anesthetized with pentobarbital. Most off-cells ( n=14) had rhythmic interspike intervals, their modes averaging 106 ms. No trigeminal cell fired rhythmically. Rhythmic firing was defined quantitatively: the autospectrum's peak power had to exceed 1.75 times its asymptote. This formula was obtained by generalizing from a natural cut-off in the theoretical autospectrum for serially uncorrelated, gamma-distributed intervals, whose firing can be varied from Poissonian to highly regular by adjusting one parameter. It encompasses the qualitative judgement of autocorrelograms commonly made in neurophysiology. Cross-correlograms ( n=29) appeared noisy and otherwise featureless. However, their power spectra (cross-periodograms) sometimes showed significant peaks, compared with simulated non-interactive distributions. The latter were generated by interchanging the raphe interval sequences at one random point (as in cutting a deck of cards), thus retaining most of their serial correlation. Of 29 cross-periodograms, 21 were significant at 1 to 13 frequencies (100 points, 0.4 to 39 Hz). These frequencies were often near the peak raphe power, and sometimes near its harmonics too. Furthermore, cross-spectral phase angles at peak power were non-uniform, most falling between 0 and 180 degrees (unit vector sum 60°, n=20). To understand why the frequency domain gave better detection, cross-spectra and cross-correlations were modeled theoretically by convolving idealized input autocorrelations and synaptic response functions. This demonstrated that rhythmic firing is insufficient for better frequency-domain detection, and that serially correlated input intervals or non-additive synaptic responses are necessary. The conclusion was confirmed by stochastic simulation of a simple non-additive synapse, that required successful input spikes to fall within a specified interval of the preceding spike. Experimentally, serial correlation was found in 12 of the 15 raphe cells, and in 20 of the 27 trigeminal cells. It is proposed that the weak experimental cross-correlograms arise because many asynchronous raphe inputs converge on each trigeminal cell, possibly to optimize the resting suppression of pain. The distribution of cross-spectral phase angles at peak raphe power suggested that raphe spikes arriving at the synapses' preferred interval cause a fall in trigeminal activity. In general, cross-spectral analysis can sometimes uncover influences hidden in cross-correlograms, but the firing of one neuron must be rhythmic and non-renewal, or else certain input intervals must be favored in synaptic transmission

Spatial and temporal variation of microstimulation thresholds for inhibiting the tail-flick reflex from the rat's rostral medial medulla

Suppression of the tail-flick reflex by microstimulation of the rostral medial medulla in rats lightly anesthetized with barbiturates was studied with regard to spatial and temporal variations in electrical threshold. Trains of constant-current pulses with linearly descending amplitudes (called ‘ramps’) were passed through the extracellular brain microelectrode during noxious heating of the tail. The pulse amplitude at the time of the reflex, after allowance for conduction and reaction latencies, was taken as the threshold reading. This new method revealed a range of vertical electrode positions corresponding roughly to the nucleus raphe magnus, where the thresholds tended to be lowest (a mean of 4.1 μA for 0.4-ms pulses delivered at 50 Hz). In confirmation of the technique's validity neither the duration of the ramp nor its starting amplitude, within their useful range, significantly affected the measured threshold. Pronounced temporal fluctuation was seen in thresholds measured every 2 min. Spatial variability within the low-threshold region and differences between preparations were statistically much smaller sources of variation. The temporal fluctuation appeared to have a stationary mean for at least 20 min under constant conditions of anesthesia. In some experiments, action potentials from single neurons were recorded through the stimulating electrode, and classified into those inhibited during the tail-flick (off-cells), those excited (on-cells), and those unaffected (neutral cells). The thresholds where off-cells exhibited their maximum action potential were on average significantly lower than corresponding thresholds for on-cells. Short-range (<0.2mm) spatial variations in the threshold appeared however to be uncorrelated with the distance to an individual recorded off-cell or on-cell. The temporal fluctuations are suggested to reflect the profound cyclical changes in firing state of off-cells and on-cells that have been reported previously from barbiturate-anesthetized rats