Arlene S. Eison

Serotonergic mechanisms in the behavioral effects of buspirone and gepirone

The literature describing the role of serotonin (5-HT) in the mediation of anxiety is a controversial one. Serotonergic involvement in the mechanism of action of two nonbenzodiazepine anxiolytics, buspirone and gepirone, supports a role for serotonin in anxiety. The anticonflict effect of both drugs is blocked by serotonin lesions, and gepirone induces the serotonin syndrome. A shift in the gepirone dose-response curve to the left in serotonin lesioned rats suggests that this may be 5-HT-receptor mediated. Both buspirone and gepirone enhance the acoustic startle response and gepirones effect is attenuated in serotonin lesioned animals. While other components of buspirones mechanism of action may suppress the behavioral expression of its serotonergic interactions, results from these studies suggest that serotonin agonist-like activity may be an important mechanism in the actions of a clinically proven nonbenzodiazepine anxiolytic (buspirone), and anxiolytic candidate (gepirone).

Effects of antidepressants on 5-HT7 receptor regulation in the rat hypothalamus.

Recent evidence suggests that a novel serotonin receptor 5-HT7 localized in the hypothalamus downregulates in response to treatment with the antidepressant fluoxetine (Sleight et al. 1995). This receptor has also been implicated in the regulation of circadian rhythms (Lovenberg et al. 1993). Here, we show that several agents administered in a profile consistent with activity at the 5-HT7 receptor produce significant functional Fos immunoreactivity in the suprachiasmatic nucleus (SCN), an effect reduced upon chronic exposure. Furthermore, binding studies demonstrate that chronic administration of Fos-inducing agents produces a neuroadaptive downregulation of the 5-HT7 receptor in the hypothalamus. The current studies extend the previous observations to include several pharmacologically distinct antidepressants. In addition, these studies provide further evidence to support the role of the 5-HT7 receptor in the mechanism of antidepressant action and in the regulation of circadian rhythms controlled by the SCN.

Regulation of central 5-HT2A receptors: a review of in vivo studies

Numerous investigations have studied in vivo regulation of central 5-HT2A receptors. The majority of pharmacological studies point to non-classical regulation of this site. Serotonergic denervation does not modify 5-HT2A receptor density or second messenger responses (phosphoinositide hydrolysis). 5-HT2A receptor downregulation is produced by the chronic administration of 5-HT2A receptor agonists and uniquely among monoamine receptors by antagonists. Several classes of psychotherapeutic agents also downregulate 5-HT2A receptors with chronic administration including classical antidepressants and antipsychotics. 5-HT2A receptor downregulation produced by 5-HT2A antagonists and antidepressants occurs after presynaptic 5-HT denervation, suggesting that 5-HT2A receptors are postsynaptically localized and emphasizing that they are regulated differently than traditional monoaminergic receptors. Interestingly, the behavioral and biochemical effects of 5-HT2A receptor activation are modulated by activity at other 5-HT receptor subtypes (5-HT1A), as well as by stimulation of receptors for other neurotransmitters and hormones such as norepinephrine (beta-adrenergic) and melatonin. It is suggested that these diverse modulatory influences on 5-HT2A receptor regulation and function may meaningfully impact the therapeutic actions of drugs, including pharmacologically distinct antidepressants.

Serotonergic mechanisms in anxiety

1. Anatomical, behavioral, neurochemical and electrophysiological evidence collectively support a role for central 5-HT in the modulation of anxiety and the anti-anxiety action of the benzodiazepines. 2. The advent of selective agonists and antagonists for 5-HT receptor subtypes (5-HT1, 5-HT2, 5-HT3) has rekindled investigation of the role of 5-HT in anxiety mechanisms. 3. The azapirones represent a new class of agent which possesses affinity for 5-HT1A receptors (partial agonists) and is active in anxiolytic animal models as well as in the clinic (buspirone) 4. While preclinical data supporting the anxiolytic potential of 5-HT2 antagonists remains controversial, a recent clinical study supports ritanserins anxiolytic effects. 5. Several animal models support the anxiolytic potential of the 5-HT3 antagonist odansetron (GR38032F). Confirmation of its therapeutic utility awaits clinical results.

Melatonin binding sites are functionally coupled to phosphoinositide hydrolysis in Syrian hamster RPMI 1846 melanoma cells

Recent pharmacological studies demonstrate that Syrian hamster melanoma (RPMI 1846) cells possess a nanomolar-affinity binding site for melatonin. Inhibition of 2-[125I]-iodomelatonin binding to RPMI membranes by melatonergic ligands exhibit a rank order relationship similar to that observed in hamster hypothalamus. Biochemical studies indicate that the melatonin binding site in RPMI 1846 cells is not coupled in either a stimulatory or inhibitory fashion to adenylate cyclase as a second messenger. We now report that stimulation of RPMI 1846 melanoma cells with melatonergic agonists induces phosphoinositide hydrolysis in a concentration-dependent manner (EC50: N-acetylserotonin = 0.29 microM; 2-I-Melatonin = 0.39 microM; 6-Cl-Melatonin = 0.38 microM; Melatonin = 0.45 microM). Further, phosphoinositide hydrolysis induced by 2-I-melatonin and N-acetylserotonin was blocked by pre-incubation with the melatonin antagonist N-acetyltryptamine and prazosin, an antagonist which exhibits potency at the nanomolar affinity melatonin binding site. 2-I-melatonin and N-acetylserotonin-induced phosphoinositide hydrolysis were not blocked by the serotonin type 2 antagonist ketanserin or alpha-adrenergic antagonist, phentolamine. These data suggest that melatonin binding sites on RPMI 1846 cells are linked to phosphoinositide hydrolysis as a second messenger.

Reduction in cortical 5HT2 receptor sensitivity after continuous gepirone treatment.

Continuous administration (28 days) of gepirone, a potential anxiolytic, decreased rat cortical serotonin type 2 (5HT2) receptor density by 17 percent and reduced the frequency of head shakes induced by the serotonin agonist quipazine. These data support the results of previous studies confirming gepirones involvement with brain serotonin systems, and suggest that in addition to its preclinical anxiolytic profile, gepirone exhibits pharmacological properties common to clinically effective antidepressants.

Effect of chronic administration of antidepressant drugs on 5-HT2-mediated behavior in the rat following noradrenergic or serotonergic denervation

Chronic (14 day) administration of several pharmacologically-distinct antidepressant drugs resulted in marked reductions in the serotonin2 (5-HT2)-mediated quipazine-induced head shake response which were accompanied by significant reductions in the density of cortical Β-adrenergic and 5-HT2 binding sites. Noradrenergic (DSP4[N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine]-induced) and serotonergic (5,7-DHT[5,7-dihydroxytryptamine]-induced) lesions either attenuated or blocked antidepressant-induced reductions in 5-HT2-mediated behavior. DSP4- and 5,7-DHT lesions did not alter the down-regulation of 5-HT2 binding sites produced by imipramine, desipramine, phenelzine or iprindole. To a large extent, the antagonism of antidepressant-induced reductions in 5-HT2-mediated behavior was coincident with the blockade of down-regulation of Β-adrenergic binding sites by both noradrenergic and serotonergic denervation. The functional interrelationship of 5-HT2 and Β-adrenergic receptors suggested by the present findings may provide insight into a common mechanism underlying the action of pharmacologically-distinct antidepressant drugs.

Comparative effects of chronic 8-OH-DPAT, gepirone and ipsapirone treatment on the sensitivity of somatodendritic 5-HT1A autoreceptors

Chronic treatment with the full 5-hydroxytryptamine1A (5-HT1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), either by twice daily subcutaneous injection (0.2 or 2.0 mg/kg) or continuous subcutaneous administration via osmotic minipumps (0.4 or 4.0 mg/kg/day), for 14 days, had no effect on the dose-response curves for inhibition of 5-hydroxytryptophan (5-HTP) accumulation in rat cortex or hippocampus by 8-OH-DPAT or the partial 5-HT1A agonist BMY 7378. In contrast, chronic treatment with the nonbenzodiazepine putative anxiolytic gepirone via osmotic minipumps (20 mg/kg/day) resulted in a small but significant rightward shift (1.8-fold) in the dose-response curve to 8-OH-DPAT challenge in the cortex and a slightly larger shift (2.4-fold) in the hippocampus. Similarly, chronic treatment with another putative anxiolytic, ipsapirone, administered via twice daily subcutaneous injections (20 mg/kg), also resulted in a rightward shift (2.6-fold) in the dose-response curve to 8-OH-DPAT in the cortex and a slightly smaller shift (2.3-fold) in the hippocampus. Neither drug, however, decreased the maximal response. The present results are consistent with the suggestion that the clinical anxiolytic effects of gepirone and ipsapirone, and not of 8-OH-DPAT, may be related to their ability to desensitize somatodendritic 5-HT1A autoreceptors; other potential mechanisms are discussed.

Unique modulation of central 5-HT2 receptor binding sites and 5-HT2 receptor-mediated behavior by continuous gepirone treatment

The effect of continuous treatment with the selective 5-HT1A agonist gepirone upon 5-HT2-mediated behavior and cortical 5-HT2 receptor binding sites was examined in naive rats or rats receiving noradrenergic (DSP4) or serotonergic (5,7-DHT) lesions. Continuous administration of gepirone in non-lesioned rats for 3, 7, or 14 days enhanced the head shake response to the 5-HT agonist quipazine. This enhancement of 5-HT2-mediated behavior occurred despite concomitant down-regulation of cortical 5-HT2 binding sites. However, 28 days of gepirone administration significantly reduced behavioral responsiveness to quipazine. The gepirone-induced facilitation of 5-HT2-mediated behavior observed after 7 days of continuous treatment was blocked in both DSP4 and 5,7-DHT-lesioned rats. However, both noradrenergic and serotonergic denervation failed to modify the down-regulation of 5-HT2 receptor binding sites produced by continuous gepirone administration. These results suggest that the curious dissociation of behavioral and biochemical indices of 5-HT2 receptor function produced by continuous gepirone treatment may be the result of a dual yet separate action of the drug on central presynaptic noradrenergic and serotonergic mechanisms and postsynaptic 5-HT receptors. Furthermore, the postsynaptic action of gepirone which reduces the maximal number of cortical 5-HT2 receptor binding sites may be the result of gepirones agonist action at postsynaptic 5-HT1A receptors.

5-HT1A and 5-HT2 receptors mediate discrete behaviors in the Mongolian Gerbil

Although the ability of agonists at specific serotonin (5-HT) receptor subtypes to induce distinct behaviors has been well documented in the rat, similar studies have not been reported in the Mongolian gerbil. We have found that the 5-HT1A/5-HT2 agonist 5-methoxy,N-N dimethyltryptamine (5-MeODMT) (0.5-8 mg/kg, SC), the specific 5-HT1A agonist 8-hydroxy(di-n-propylamino)tetralin (8-OH-DPAT) (0.125-16 mg/kg, SC), and the 5-HT precursor L-5-hydroxytryptophan (L-5-HTP) (100-250 mg/kg, SC) all elicit a 5-HT syndrome in the gerbil. This syndrome, analogous to the 5-HT syndrome in the rat, consists of reciprocal forepaw treading (RFT), hindleg abduction (HA), body tremors (BT), and Straub tail (ST). The putative 5-HT1A antagonist NAN-190 (0.25-8 mg/kg, SC) when dosed 15 min prior to either 5-MeODMT (4 mg/kg, SC) or 8-OH-DPAT (16 mg/kg, SC) blocked both RFT and HA in a dose-dependent manner, suggesting these 5-HT syndrome behaviors are mediated via 5-HT1A receptor activation. We also identified a unique, dose-responsive behavior in the gerbil, induced selectively by 5-HT1A agonists such as quipazine (2-16 mg/kg, SC) and (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.125-8 mg/kg, SC). This reciprocal hindleg body scratch (RHBS) behavior is dose dependently inhibited by pretreatment with the selective 5-HT2 antagonist ritanserin (0.0125-0.2 mg/kg, SC). RHBS behavior is also potently inhibited by pretreatment with the selective 5-HT1A agonist 8-OH-DPAT (0.005-0.04 mg/kg, SC), demonstrating a 5-HT1A/5-HT2 receptor subtype interaction.(ABSTRACT TRUNCATED AT 250 WORDS)