Periaqueductal gray influence on anteroventral cochlear nucleus unitary activity and naloxone effects

The effect of periaqueductal gray (PAG) electrical stimulation on the response properties of auditory and 'spontaneously' firing units (abolished when the cochlea is destroyed) in the anteroventral cochlear nucleus (AVCN) was explored using extracellular recordings in acute guinea-pigs. Significant increases and decreases in firing rate were detected in both neuronal groups: only 4% of the sound-responding units were insensitive to PAG stimulation while the 'spontaneous' units showed significantly smaller changes in firing rate in response to PAG stimulation. The auditory AVCN neurons were categorized both by their sound post stimulus time (PST) histograms at their characteristic frequency (CF) and the changes in the probability of discharge after PAG stimulation while the tone burst was maintained constant. PAG was implicated in pain input modulation through enkephalin actions. Because enkephalins have been also observed at the CN level, a pharmacological approach administering naloxone was carried out. We observed that 1) naloxone abolished the unit discharge shifts observed after PAG stimulation and 2) when the drug was injected without PAG stimulation, it produced changes in the firing, increasing or decreasing, and shifts in the probability of discharge versus time, even in cases in which the firing rate was not altered. An involvement of the auditory efferent pathways to CN is postulated and a possible enkephalinergic factor is suggested as a modulator of the auditory input at this level. The probability of discharge observed in the PSTH at the AVCN is dependent on the auditory input plus the central efferent action to its neurons.