Inactivation of the dorsal raphé nucleus reduces the anxiogenic response of rats running an alley for intravenous cocaine

Abstract

Rats traversing a straight alley once a day for delivery of a single i.v. injection of cocaine develop over trials an ambivalence about entering the goal box. This ambivalence is characterized by the increasing occurrence of "retreat behaviors" where animals leave the start box and run quickly to the goal box, but then stop at the entry point and "retreat" back toward the start box. This unique pattern of retreat behavior has been shown to reflect a form of "approach-avoidance conflict" that stems from the animals' concurrent positive (cocaine reward) and negative (cocaine-induced anxiety) associations with the goal box. Cocaine blocks reuptake of the serotonergic (5-HT) transporter and serotonin has been implicated in the modulation of anxiety. It was therefore of interest to determine whether inactivation of the serotonergic cell bodies residing in the dorsal raphé nucleus (DRN) and projecting to brain areas critical for the modulation of anxiety, would alter the anxiogenic state exhibited by rats running an alley for single daily i.v. injections of 1.0mg/kg cocaine. Reversible inactivation of the DRN was accomplished by intracranial application of a mixed solution of the GABA agonists baclofen and muscimol. While DRN inactivation had no impact on the subjects' motivation to initiate responding (i.e., latencies to leave the start box were unaffected) it reliably reduced the frequency of approach-avoidance retreat behaviors (conflict behavior). These data suggest that inactivation of the dorsal raphé reduces the conflict/anxiety otherwise present in experienced cocaine-seeking animals.

Figure 1

Locations of injection cannula placements within the dorsal raphé nucleus. Numbers represent mm from bregma. Redrawn from Paxinos and Watson (2007).

Figure 2

The development of approach-avoidance conflict (i.e. retreat behaviors) in a group of ten rats running a straight alley for single daily injections of 1.0 mg/kg i.v. cocaine. The data are expressed as the means (±S.E.M.) frequency of retreat behaviors per trial.

Figure 3

Effects of dorsal raphé nucleus (DRN) inactivation on the start latencies (panel A), run times (panel B) and retreat frequency (panel C) of animals running an alley for a single i.v. injection of 1.0 mg/kg cocaine. Each animal was tested under two experimental conditions on two consecutive days in a counterbalanced order – the shaded bars represent performance following an intra-DRN infusion of baclofen/muscimol (“DRN inactivated”) and the open bars represent the performance of the same animals following an intracranial infusion of vehicle (“DRN intact”). * indicates a statistically significant difference between “intact” and “inactivated” runway performance (P<.05)

Figure 4

Two spatiotemporal records depicting the runway behavior of a single representative animal during control (panel A) and DRN-inactivated (panel B) conditions. The x-axis indicates time (in sec) and the y-axis represents locations within the runway with the number “2” being just outside the start box door, number “12” being at the threshold of the goal door, and number “13” being inside the goal box (and resulting in the closure of the goal door and the delivery of the i.v. cocaine reinforcer). Thus the curves reflect the path and speed of the animal’s behavior from start to finish during each of the two trials. Retreat behaviors (a measure of approach-avoidance conflict about goal-box entry) are represented by the peaks in the records. Dorsal raphé inactivation produced a clearly distinguishable pattern of responding that involved a reduction in the frequency of retreat behaviors and a shorter run time (i.e., the rat entered the goal box sooner) compared to the vehicle “intact” trial.