Buspirone also has lower affinities for the serotonin 5-HT2A, 5-HT2B, 5-HT2C, 5-HT6, and 5-HT7 receptors.In addition to binding to serotonin receptors, buspirone is an antagonist of the dopamine D2 receptor with weak affinity. It preferentially blocks inhibitory presynaptic D2 autoreceptors, and antagonizes postsynaptic D2 receptors only at higher doses. In accordance, buspirone has been found to increase dopaminergic neurotransmission in the nigrostriatal pathway at low doses, whereas at higher doses, postsynaptic D2 receptors are blocked and antidopaminergic effects such as hypoactivity and reduced stereotypy, though notably not catalepsy, are observed in animals. Buspirone has also been found to bind with much higher affinity to the dopamine D3 and D4 receptors, where it is similarly an antagonist.A major metabolite of buspirone, 1-(2-pyrimidinyl)piperazine (1-PP), occurs at higher circulating levels than buspirone itself and is known to act as a potent α2-adrenergic receptor antagonist. This metabolite may be responsible for the increased noradrenergic and dopaminergic activity observed with buspirone in animals. In addition, 1-PP may play an important role in the antidepressant effects of buspirone. Buspirone also has very weak and probably clinically unimportant affinity for the α1-adrenergic receptor. However, buspirone has been reported to have shown "significant and selective intrinsic efficacy" at the α1-adrenergic receptor expressed in a "tissue- and species-dependent manner".Unlike benzodiazepines, buspirone does not interact with the GABAA receptor complex. Pharmacokinetics
Buspirone has a low oral bioavailability of 3.9% relative to intravenous injection due to extensive first-pass metabolism. The time to peak plasma levels following ingestion is 0.9 to 1.5 hours.
In one clinical trial, buspirone was administered to healthy male volunteers at a dosage of 375 mg/day, and produced side effects including nausea, vomiting, dizziness, drowsiness, miosis, and gastric distress. In early clinical trials, buspirone was given at dosages even as high as 2,400 mg/day, with akathisia, tremor, and muscle rigidity observed. Deliberate overdoses with 250 mg and up to 300 mg buspirone have resulted in drowsiness in about 50% of individuals. One death has been reported in a co-ingestion of 450 mg buspirone with alprazolam, diltiazem, alcohol, cocaine. Interactions
Buspirone has been shown in vitro to be metabolized by the enzyme CYP3A4. This finding is consistent with the in vivo interactions observed between buspirone and these inhibitors or inducers of cytochrome P450 3A4 (CYP3A4), among others:
Itraconazole: Increased plasma level of buspirone
Rifampicin: Decreased plasma levels of buspirone
Nefazodone: Increased plasma levels of buspirone
Haloperidol: Increased plasma levels of buspirone
Carbamazepine: Decreased plasma levels of buspirone
Grapefruit: Significantly increases the plasma levels of buspirone. See grapefruit–drug interactions. Fluvoxamine: Moderately increase plasma levels of buspirone.Elevated blood pressure has been reported when buspirone has been administered to patients taking monoamine oxidase inhibitors (MAOIs). Pharmacology
Pharmacodynamics
Buspirone acts as an agonist of the serotonin 5-HT1A receptor with high affinity. It is a partial agonist of both presynaptic 5-HT1A receptors, which are inhibitory autoreceptors, and postsynaptic 5-HT1A receptors. It is thought that the main effects of buspirone are mediated via its interaction with the presynaptic 5-HT1A receptor, thus reducing the firing of serotonin-producing neurons.