Rapid differential conditioning of the somatosensory evoked potential by changed patterns of brief innocuous tactile stimuli in waking rats is altered by atropine sulfate.

Air puffs delivered to the nose of an awake, lightly restrained rat every 15 s produced evoked potentials that changed gradually over time so that the averaged response to the last 40 stimuli was measurably different from the first 40. This habituation-like paradigm increased the size of an early component of the potential in several places. When measured with respect to the time of stimulus onset (there was a 21.6 ms delay in the time of arrival of the stimulus maximum at the nose), one of the largest increases occurred 46 ms later (39 ms latency to onset, and 55 ms latency to offset). As well, a late component of the waveform became more positive, showing a maximum between 156 and 185 ms (133 ms latency to onset, and more than 250 ms latency to offset). Changing the pattern but not the number of stimuli accelerated the rate of this positive shift with a maximum at 37 ms (21 ms latency to onset, and 42 ms latency to offset), but did not affect the rate of change in the late component. This effect of altering the temporal pattern of the stimuli was blocked by systemic injections of atropine sulfate, a blocker of central muscarinic receptors, whereas, neither saline injections nor atropine methyl nitrate injections (an atropine analog that does not cross the blood–brain barrier) could produce these changes. These observations suggest that the adaptive changes of the somatosensory evoked potential induced by novel patterns intercalated in otherwise monotonous repetitive somatic stimuli depend upon central muscarinic mechanisms.