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 1.0 mg/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.

Introduction

In addition to cocaine's well-known positive/rewarding properties, the drug produces a host of negative and/or aversive effects. So while cocaine is readily self-administered by both humans and animals (e.g., Comer et al., 2008, Fischman and Schuster, 1982, Haney, 2009, Porrino et al., 2004, Wolverton, 1992), produces preferences for places associated with its administration in animals (Aguilar et al., 2009, Bardo et al., 1995, Carr et al., 1989) and is described as having rewarding or euphoric properties by human users (Lynch et al., 2006, Rotheram-Fuller et al., 2007, Singha et al., 1999), the “crash” that follows its administration in human cocaine users is characterized by feelings of anxiety, agitation and craving for more drug (Gawin, 1991, Gawin and Kleber, 1986, Resnick and Resnick, 1984, Rohsenow et al., 2007, Williamson et al., 1997). Similarly, while animals develop preferences for places associated with the immediate positive effects of cocaine, they exhibit place aversions for locations paired with the effects present 15 min after an i.v. injection (Ettenberg and Bernardi, 2007, Ettenberg et al., 1999, Knackstedt et al., 2002). Cocaine has also been reported to enhance the anxiogenic response of animals to the presentation of a variety of learned and unlearned aversive stimuli (e.g., Costall et al., 1989, Dworkin et al., 1989, Hayase et al., 2005, Paine et al., 2002, Simon et al., 1994).

The mixed positive and negative properties of cocaine can be assessed concurrently in the same animal during the same trial. In our laboratory, rats trained to run a straight alley once each day for an i.v. injection of cocaine develop over trials a unique “retreat behavior” in which the subjects rapidly approach the goal, but then stop at the goal-box threshold, and turn and retreat back toward the start box (Ettenberg and Geist, 1991). This unique pattern of runway behavior closely resembles that of animals approaching a goal box with known positive and negative associations (e.g., food + foot-shock; Cohen et al., 2009, Geist and Ettenberg, 1997, Miller, 1944). Retreats are therefore thought to reflect a form of “approach–avoidance conflict” that stem from cocaine's immediate positive (cocaine reward) and delayed negative (cocaine-induced anxiety) properties (Ettenberg, 2004, Ettenberg, 2009). Indeed, like other forms of conflict behavior, retreat frequency is dose-dependently attenuated by pretreatment with anxiolytic drugs (Ettenberg and Geist, 1991, Guzman and Ettenberg, 2004, Knackstedt and Ettenberg, 2005).

Given the growing literature implicating 5-HT circuitry in the modulation of anxiety-related states (e.g., Abrams et al., 2004, Begg et al., 2005, Chaouloff, 2000, Hammack et al., 2009, Sena et al., 2003), it was of interest to investigate the role of 5-HT function in the anxiogenic response to self-administered cocaine. Toward that end, we have shown that the 5-HT_1A partial agonist buspirone (Eison and Temple, 1986, Jahanshahi et al., 2010) can reduce the approach–avoidance conflict of rats running an alley for i.v. cocaine (Ettenberg and Bernardi, 2006) and block the delayed aversive effects of cocaine as measured in a conditioned place preference test (Ettenberg and Bernardi, 2007). 5-HT_1A receptors are typically coupled to inhibitory G-proteins whose activation produces a hyperpolarization of cell membranes that results in an inhibition of neuronal activity (Aghajanian, 1995, Albert et al., 1997), thus the effects of buspirone could be accounted for by an inhibition of 5-HT neurotransmission. However, since buspirone has been shown to exert effects on multiple neurotransmitter systems (e.g., Eison and Temple, 1986, Jahanshahi et al., 2010), the current study adopted a more direct approach by examining the impact on cocaine-induced approach–avoidance conflict behavior of inactivating the dorsal raphé nucleus — the primary source of 5-HT in the rat forebrain (Azmitia and Segal, 1978, Vertes, 1991).