PONTINE MICROINJECTION OF CARBACHOL DOES NOT RELIABLY ENHANCE PARADOXICAL SLEEP IN RATS

It has been repeatedly shown in cats that acute administration of carb achol into the pontine reticular formation (PRF) readily evokes a stat e that closely mimics natural paradoxical sleep (PS). Surprisingly, th ere are few corresponding studies in rats. In order to further charact erize the effects of pontine carbachol in rats, 151 injections of diff erent doses (from 3 mu g to 0.005 mu g in 0.1 mu l saline) of carbacho l were made at different sites. within the PRF of 70 rats. Sleep-wakin g states obtained in the 4 hours following carbachol administration we re compared to control values, obtained both under baseline condition (no injection) and following pontine injection of 0.1 mu l saline. On the one hand, from the whole set of carbachol injections, it appeared that: 1) most injections (112/151) did not significantly alter the sle ep-wake states; 2) when carbachol was effective, it induced either inc reased PS (20 injections) or increased waking (19 injections); and 3) effective injection sites were intermingled with noneffective sites. D ose-or site-dependency effects can account in part, but not totally, f or these discordant results. On the other hand, in accordance with pre vious rat studies, we found that: 1) the PRF medial and ventral to the motor trigeminal nucleus was the most effective region for carbachol to increase PS; 2) carbachol-induced PS enhancement was of moderate ma gnitude (+60% above control saline level over the 4-hour recording tim e); 3) latency to onset of the first PS episode was not shortened; and 4) only the number of PS episodes was increased, their duration was n ot prolonged. These characteristics of carbachol-induced PS enhancemen t strongly differ, both in terms of magnitude and timing, from those d escribed in cats. We suggest that the less reliable and weaker effects of pontine carbachol injection in rats compared to cats can be due to methodological problems inherent in the intracerebral microinjection technique and also to species-related differences in the mechanisms co ntrolling the PS state.