3. Umbruch 4.4..2005 - Online Pot

Cannabinoids: effects on vomiting and nausea in animal models 193
orally infused with a bitter-tasting quinine solution, they display a distinctive
pattern of rejection reactions (including gaping, chin rubbing and paw treading).
Interestingly, following pairing with a toxin, sweet solutions come to
elicit the rejection pattern of reactions. Therefore, conditioned rejection reactions
provide an alternative measure of flavor aversion learning to that of a
consumption test.
The consumption test is much less selective to emetic drugs than the TR
test, because most psychoactive drugs (with rewarding or emetic properties)
produce taste avoidance [84–87, 91]. In fact, rats will simultaneously avoid an
amphetamine-paired flavor while demonstrating a preference for the amphetamine-paired
place [92]. Considerable evidence indicates that not all taste
avoidance is accompanied by conditioned aversion (rejection reactions) to the
taste when assessed using the TR test. Stimuli that produce taste avoidance in
the absence of nausea do not produce an aversion to the taste. Lower-intestinal
discomfort [93], footshock [93], lesions of the lateral hypothalamus [94] as
well as reinforcing drugs [84–87] produce taste avoidance but do not produce
rejection reactions in the TR test. Conditioned rejection and conditioned
avoidance are mediated by qualitatively, not quantitatively, different processes.
Zalaquett and Parker [95] demonstrated that when the doses of lithium and
amphetamine are adjusted such that rats develop stronger avoidance of the
amphetamine-paired flavour, they still only reject the less-avoided
lithium-paired flavour. That is, only drugs with emetic effects produce conditioned
rejection reactions.
An early theory that attempted to explain paradoxical reports that reinforcing
drugs produce taste avoidance suggested that any novel change in physiological
state (be it hedonic or aversive) signals danger to the rat, a species that
cannot vomit [96]. A flavor paired with this change in state comes to signal
danger, resulting in subsequent avoidance of that taste. On the other hand,
shrews do not avoid a flavor paired with the rewarding drugs, amphetamine or
morphine; in fact, they develop a preference for a flavor paired with these
drugs [97]. Presumably, the emetic shrew does not need to be as wary as the
non-emetic rat about changes in state following ingestion, because it can
vomit.
The most reliable conditioned rejection reaction in the rat is that of gaping
[37, 87]. If conditioned gaping reflects nausea in rats, then anti-nausea drugs
should interfere with this reaction. Limebeer and Parker [88] demonstrated that
when administered prior to a saccharin/lithium pairing, the 5-HT 3 antagonist,
ondansetron, prevented the establishment of conditioned gaping in rats, but not
the establishment of conditioned taste avoidance. Presumably, ondansetron
interfered with lithium-induced nausea preventing the conditioned gaping; on
the other hand, it did not prevent the establishment of conditioned taste avoidance,
suggesting that nausea was not necessary to suppress saccharin consumption.
Ondansetron also interfered with the expression of previously established
conditioned gaping, but not taste avoidance. Since ondansetron did not
modify unconditioned gaping elicited by bitter quinine solution, the effect was