Involvement of 5-HT1A receptors in the anxiolytic action of S 14671 in the pigeon conflict test

Effects of buspirone on the immediate positive and delayed negative properties of intravenous cocaine as measured in the conditioned place preference test

In prior work, we have demonstrated that the behavioral effects of cocaine adhere to the predictions of the opponent-process theory of drug action. Animals develop conditioned place preferences for distinct locations paired with the immediate effects of IV cocaine, but learn to avoid places paired with the effects present 15-min post-injection. It was of interest to assess the putative role of 5-HT in producing the negative properties of cocaine since cocaine acts to inhibit the reuptake of serotonin (5-HT) and since such actions have been associated with anxiogenic consequences. Male rats were administered a reinforcing dose of cocaine (1.0 mg/kg IV) and then placed - either immediately or after a 15-min delay - into one side of a two-compartment (black-white) conditioned place preference (CPP) box for 5-min. On alternate days, the animals received IV saline injections and were placed in the opposite side of the CPP box. This continued for eight days after which animals had experienced 4 pairings of cocaine with one side (black or white) of the CPP apparatus, and 4 saline pairings with the opposite side. Other groups of rats were treated identically except that 30-min prior to placement into the apparatus, these animals received an IP injection of saline or buspirone (a partial 5-HT1A agonist) at a dose that we have shown to be anxiolytic (2.5 mg/kg IP). Control animals experienced either buspirone or saline pretreatments without cocaine. Our results confirm that animals increase the time spent on the side paired with the immediate effects of cocaine (compared to baseline), but tend to avoid the side paired with effects present 15-min post-injection. Buspirone had no effect on the immediate rewarding properties of cocaine, but completely reversed the negative properties present 15-min post-cocaine. These results are consistent with the view that attenuation of 5-HT neurotransmission (via the autoreceptor agonist properties of buspirone) can reverse the negative impact of IV cocaine.

Anxiolytic-like actions of buspirone in a runway model of intravenous cocaine self-administration

In previous work from our laboratory, rats traversing a straight alley for a reward of IV cocaine have been observed to develop ambivalence about entering the goal box. Over trials, animals repeatedly run toward the goal box, stop at the entry point, and then 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 subjects' concurrent positive (cocaine reward) and negative (cocaine-induced anxiety) associations with the goal box. Buspirone, a partial 5-HT(1A) agonist, has been reported to produce anxiolytic-like actions in the clinic, but has had mixed results in experimental tests of anxiety using animal subjects. Since most animal tests of conflict/anxiety employ the administration of foot-shock - a relatively strong aversive stimulus - it was of interest to determine whether buspirone would alter the more subtle approach-avoidance conflict observed in well-trained animals running a straight alley for single daily injections of 1.0 mg/kg IV cocaine. Runway testing consisted of single daily trials that continued until consistent approach-avoidance retreats were exhibited. Each animal was then pretreated 30 min prior to runway testing with vehicle and one of three doses of buspirone (0.0, 1.0, 2.5 or 5.0 mg/kg IP). Testing continued in a counterbalanced manner until all rats had experienced each dose of buspirone with 3 days of cocaine-only trials between each test day. The number of retreats exhibited on each trial served as an index of the approach-avoidance conflict present on that trial. Results clearly demonstrated that buspirone (at the two higher doses) attenuated the retreat behavior of animals approaching a goal box for IV cocaine -- an action consistent with its anxiolytic-like actions in the clinic.

Induction of burst firing in ventral tegmental area dopaminergic neurons by activation of serotonin (5-HT)1A receptors: WAY 100,635-reversible actions of the highly selective ligands, flesinoxan and S 15535

This study examined the influence of the highly selective 5-HT1A receptor ligands, flesinoxan, S 15535, and WAY 100,635, upon the electrical activity of dopaminergic neurons in the ventral tegmental area (VTA), as compared to serotonergic neurons in the dorsal raphe nucleus (DRN) of anesthetized rats. Flesinoxan, a high-efficacy agonist at both pre- and postsynaptic 5-HT1A receptors, dose-dependently (inhibitory dose (ID)50 = 19.5 microg/kg, i.v.) inhibited the firing of DRN serotonergic neurons. This action was abolished by WAY 100,635 (31 microg/kg i.v.) which is an antagonist at pre- and postsynaptic 5-HT1A receptors. S 15535, which behaves as an agonist and partial agonist at pre- and postsynaptic 5-HT1A receptors, respectively, similarly abolished DRN firing in a WAY 100,635-reversible fashion with an ID50 of 6.1 microg/kg, i.v. In contrast to these actions, both flesinoxan (> or = 500 microg/kg, i.v.) and S 15535 (> or = 125 microg/kg, i.v.) dose-dependently and monophasically increased the firing rate of dopaminergic neurons in the VTA, with maximal effects of 70.1 +/- 17.2% and 33.7 +/- 5.3%, respectively. Further, VTA dopaminergic neurons displaying a regular firing pattern were transformed into a bursting mode. This influence of flesinoxan and S 15535 on VTA cells was abolished by WAY 100,635. Administered alone, WAY 100,635 did not significantly modify the activity of either serotonergic or dopaminergic neurons. In conclusion, the present findings show that selective activation of 5-HT1A receptors not only inhibits serotonergic neurones but also elicits a (possibly related) increase in VTA dopaminergic output. A facilitatory influence of flesinoxan, S 15535, and other selective 5-HT1A receptor ligands upon mesocortical dopaminergic pathways may contribute to their putative antidepressant properties.

Interaction of the anxiogenic agent, RS-30199, with 5-HT1A receptors: modulation of sexual activity in the male rat

RS-30199 has been shown previously to have atypical interactions at 5-HT1A receptors. RS-30199 and RS-64459, an analogue of buspirone with a buspirone side chain, were compared with the classic, partial agonist at 5-HT1A receptors, 8-hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) and buspirone. At human (h) 5-HT1A receptors in CHO cells, RS-30199-193 (racemate) and its enantiomers (-197, -198) inhibited [3H]-8-OH-DPAT binding (RS-30199-198, ki, 29.7 +/- 11.7 nM; RS-30199-197, ki, 74.1 +/- 11.7 nM) as did RS-64459 (ki, 24.9 +/- 6.0 nM), but RS-30199-197 and -198 were almost full agonists in a [35S]-GTPgammaS binding assay, whereas RS-64459 was a partial agonist, resembling buspirone and 8-OH-DPAT. RS-64459 and the enantiomers of RS-30199 had weaker affinity for 5-HT2C and 5-HT7 receptors. These compounds did not induce the 5-HT behavioural syndrome in male rats. However, in a model where naive male rats were introduced to estrogen-progesterone primed, sexually receptive female rats, RS-30199-197 (0.1, 1, 10 mg/kg, s.c.) had a profound inhibitory effect on sexual behaviour score. Neither buspirone nor 8-OH-DPAT reduced the sexual behaviour score. Unlike 8-OH-DPAT, which shortens intromission latency, RS-30199 prolonged intromission latency. RS-30199 (10 mg/kg s.c.) fully inhibited the facilitation of sexual behaviour caused by the alpha2-adrenoceptor antagonist, delequamine (0.1 mg/kg, p.o.). In contrast, RS-64459 (100, 250, 1000 and 4000 microg/kg, s.c.) failed to modify the sexual behaviour score and did not modify intromission latency. The differences between the effects of RS-30199 and RS-64459 in binding and functional experiments are supported by molecular models of the receptor-ligand interaction, where the compounds interact in different ways with the receptor; a model is proposed for the allosteric interaction of different agents with the receptor, resulting in different functional profiles. RS-30199 can be considered an atypical agonist at 5-HT1A receptors.