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Effects of Halothane and Enflurane on Schedule-controlled Behavior in the Rat

The effects of subanesthetic concentrations of halothane (0–0.310%) and enflurane (0–0.450%) on lever-pressing tasks were assessed in rats. Animals were trained to press a lever in two distinctive patterns, each resulting in access to evaporated milk for several seconds as a reward. The first pattern, requiring 30 successive presses to receive milk, is known as a fixed-ratio-30 (FR-30) schedule. This engenders a continuous, very rapid bar-pressing rate for the 20–40 s required for the animal to make the required number of presses. The second, where the first lever-press following a 5-min interval is rewarded similarly with milk is known as a fixed-interval-5-minute schedule (FI-5 min). This longer term component produces a very slow bar-pressing rate over the first several minutes, gradually building up to a rapid rate as the animal anticipates the reward at the 5-min point. These two lever-pressing components, the FR-30 and FI-5 min, alternated over a session during which subanesthetic concentrations were administered into a 75-1 sealed enclosure surrounding the rat chamber and monitored by infrared analyzer. The FR-30 showed progressive decrements in bar-pressing rate, significant above 0.2%, both with halothane and enflurane. The points at which responding ceased (0.310% halothane; 0.450% enflurane) bore the same ratio as their respective minimum alveolar concentration (MAC) values. The FI-5 min component showed no significant effects with either anesthetic except at concentrations approaching the point at which responding almost ceased. Thus, no behavioral excitatory effects, which would have increased the slow, initial bar-pressing rate, were observed. The effects of both anesthetics on the FR-30 and FI-5 min components most resemble those of barbiturates in doses that cause ataxia and incoordination. With the two volatile anesthetics, we found that these two lever-pressing tasks were affected long before the onset of ataxia or loss of righting reflex, and can be used to quantitate graded behavioral effects of inhaled agents possessing general anesthetic properties.