The dopamine D_ receptor is a 7-transmembrane G-protein coupled receptor associated with Gi proteins. In lactotrophs, stimulation of dopamine D_ receptor causes inhibition of adenylyl cyclase, which decreases intracellular cAMP concentrations and blocks IP3-dependent release of Ca_+ from intracellular stores. Decreases in intracellular calcium levels may also be brought about via inhibition of calcium influx through voltage-gated calcium channels, rather than via inhibition of adenylyl cyclase. Additionally, receptor activation blocks phosphorylation of p4_/p44 MAPK and decreases MAPK/ERK kinase phosphorylation. Inhibition of MAPK appears to be mediated by c-Raf and B-Raf-dependent inhibition of MAPK/ERK kinase. Dopamine-stimulated growth hormone release from the pituitary gland is mediated by a decrease in intracellular calcium influx through voltage-gated calcium channels rather than via adenylyl cyclase inhibition. Stimulation of dopamine D_ receptors in the nigrostriatal pathway leads to improvements in coordinated muscle activity in those with movement disorders.
Bromocriptine stimulates centrally-located dopaminergic receptors resulting in a number of pharmacologic effects. Five dopamine receptor types from two dopaminergic subfamilies have been identified. The dopaminergic D1 receptor subfamily consists of D1 and D5 subreceptors, which are associated with dyskinesias. The dopaminergic D2 receptor subfamily consists of D2, D3 and D4 subreceptors, which are associated with improvement of symptoms of movement disorders. Thus, agonist activity specific for D2 subfamily receptors, primarily D2 and D3 receptor subtypes, are the primary targets of dopaminergic antiparkinsonian agents. It is thought that postsynaptic D2 stimulation is primarily responsible for the antiparkinsonian effect of dopamine agonists, while presynaptic D2 stimulation confers neuroprotective effects. This semisynthetic ergot derivative exhibits potent agonist activity on dopamine D2-receptors. It also exhibits agonist activity (in order of decreasing binding affinity) on 5-hydroxytryptamine (5-HT)1D, dopamine D3, 5-HT1A, 5-HT2A, 5-HT1B, and 5-HT2C receptors, antagonist activity on _2A-adrenergic, _2C, _2B, and dopamine D1 receptors, partial agonist activity at receptor 5-HT2B, and inactivates dopamine D4 and 5-HT7 receptors. Parkinsonian Syndrome manifests when approximately 80% of dopaminergic activity in the nigrostriatal pathway of the brain is lost. As this striatum is involved in modulating the intensity of coordinated muscle activity (e.g. movement, balance, walking), loss of activity may result in dystonia (acute muscle contraction), Parkinsonism (including symptoms of bradykinesia, tremor, rigidity, and flattened affect), akathesia (inner restlessness), tardive dyskinesia (involuntary muscle movements usually associated with long-term loss of dopaminergic activity), and neuroleptic malignant syndrome, which manifests when complete blockage of nigrostriatal dopamine occurs. High dopaminergic activity in the mesolimbic pathway of the brain causes hallucinations and delusions; these side effects of dopamine agonists are manifestations seen in patients with schizophrenia who have overractivity in this area of the brain. The hallucinogenic side effects of dopamine agonists may also be due to 5-HT2A agonism. The tuberoinfundibular pathway of the brain originates in the hypothalamus and terminates in the pituitary gland. In this pathway, dopamine inhibits lactotrophs in anterior pituitary from secreting prolactin. Increased dopaminergic activity in the tuberoinfundibular pathway inhibits prolactin secretion making bromocriptine an effective agent for treating disorders associated with hypersecretion of prolactin. Pulmonary fibrosis may be associated bromocriptineês agonist activity at 5-HT1B and 5-HT2B receptors.
Completely metabolized by the liver, primarily by hydrolysis of the amide bond to produce lysergic acid and a peptide fragment, both inactive and non-toxic. Bromocriptine is metabolized by cytochrome P450 3A4 and excreted primarily in the feces via biliary secretion.
Symptoms of overdosage include nausea, vomiting, and severe hypotension. The most common adverse effects include nausea, headache, vertigo, constipation, light-headedness, abdominal cramps, nasal congestion, diarrhea, and hypotension.