Abstract
The effects of the serotonergic (5-hydroxytryptamine, 5-HT) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.2 and 0.4 mg/kg i.p.) were examined in trace conditioning (Experiment 1) and overshadowing (Experiment 2) procedures. Both experiments used a fear conditioning procedure conducted off-the-baseline in water deprived male Wistar rats. 8-OH-DPAT was administered during conditioning and its effects were examined drug free as the suppression of an established licking response, both upon re-exposure to the cues provided by the conditioning chambers and upon presentation of experimental stimuli. There were no statistically significant effects of 8-OH-DPAT on conditioning to the discrete cue provided by a 5 s conditioned stimulus (CS), irrespective of the length of the trace interval used in Experiment 1, and irrespective of whether the CS took the form of a light alone, or a noise plus light compound in the Experiment 2 overshadowing procedure. The successful demonstration of overshadowing required the use of a second conditioning session which allowed further evaluation of the effects of 8-OH-DPAT in that neither a weak nor a strong overshadowing effect was modulated by either drug dose. Nonetheless conditioning to contextual cues was attenuated by treatment with 8-OH-DPAT at the 30 s trace interval. We therefore conclude that 8-OH-DPAT reduces competition from contextual but not discrete conditioning cues. This pattern of results lends further support to the view that contextual cue conditioning and discrete cue conditioning are modulated by different neuropharmacological mechanisms.
Highlights
The serotonergic (5-hydroxytryptamine, 5-HT) system is involved in a variety of cognitive and behavioural processes, including various aspects of learning and memory (Altman and Normile, 1988; McEntee and Crook, 1991; Meneses and Perez-Garcia, 2007)
In line with earlier findings, there were no clear effects of 8-OHDPAT on conditioning to the discrete cues provided by 5 s conditioned stimulus (CS), irrespective of the length of the trace interval used in Experiment 1, and irrespective of whether the CS took the form of a light alone, or a noise plus light compound in the Experiment 2 overshadowing procedure
The lack of effect on discrete cue conditioning is consistent with earlier reports contrasting effects of 8-OH-DPAT with those of other 5-HT compounds in other discrete cue conditioning procedures (Cassaday et al, 1993; Welsh et al, 1998; Inoue et al, 2011)
Summary
The serotonergic (5-hydroxytryptamine, 5-HT) system is involved in a variety of cognitive and behavioural processes, including various aspects of learning and memory (Altman and Normile, 1988; McEntee and Crook, 1991; Meneses and Perez-Garcia, 2007). Except under restricted conditions (e.g., Avanzi et al, 2003), fear conditioning with discrete experimental cues has been reported to be unaffected by treatment with 5-HT1A agonists (Inoue et al, 2011). Consistent with such compounds' potential efficacy as anxiolytics, at least in pre-clinical tests (Cheeta et al, 2001; Borsini et al, 2002), contextual fear conditioning is reliably reduced (Li et al, 2001, 2006), in some cases together with discrete cue conditioning (Youn et al, 2009). Overshadowing procedures use the relative intensity of competing CSs to
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