The ammonium cation is a positively charged polyatomic ion with the chemical formula NH4+. Ammonium ions are a waste product of the metabolism of animals. In fish and aquatic invertebrates, it is excreted directly into the water. In mammals, sharks, and amphibians, it is converted in the urea cycle to urea, because urea is less toxic and can be stored more efficiently. In birds, reptiles, and terrestrial snails, metabolic ammonium is converted into uric acid, which is solid and can therefore be excreted with minimal water loss. Ammonium is an important source of nitrogen for many plant species, especially those growing on hypoxic soils. However, it is also toxic to most crop species and is rarely applied as a sole nitrogen source. The ammonium ion (NH4+) in the body plays an important role in the maintenance of acid-base balance. The kidney uses ammonium (NH4+) in place of sodium (Na+) to combine with fixed anions in maintaining acid-base balance, especially as a homeostatic compensatory mechanism in metabolic acidosis. When a loss of hydrogen ions (H+) occurs and serum chloride (Cl–) decreases, sodium is made available for combination with bicarbonate (HCO3–). This creates an excess of sodium bicarbonate (NaHCO3) which leads to a rise in blood pH and a state of metabolic alkalosis. The therapeutic effects of Ammonium (as Ammonium Chloride) depend upon the ability of the kidney to utilize ammonia in the excretion of an excess of fixed anions and the conversion of ammonia to urea by the liver, thereby liberating hydrogen (H+) and chloride (Cl–) ions into the extracellular fluid.

Ensaculin is related to naturally occurring benzopyranones like scoparone. The compound is a potent functional antagonist of excitatory amino acid-induced convulsions and mortality. In receptor-binding studies, Ensaculin showed high affinity to dopaminergic (D2, D3), serotoninergic (5-HT1A, 5-HT7), and adrenergic (A1a, A1b) receptors in the nanomolar range. Ensaculin antagonizes NMDA responses in a voltage-dependent manner. Various studies support the notion that this compound could indeed have a broad range of nootropic properties. Although few patients presented postural hypotension and dizziness after receiving ensaculin in phase I clinical trials, this drug candidate was further discontinued in phase III due to potential side effects.

Metitepine, a psychotropic agent was developed as a non-selective antagonist of serotonin, dopamine, and adrenergic receptors. Metitepine has never been marketed.

Mezilamine is a dopamine antagonist, it blocks presynaptic but stimulates postsynaptic alpha-adrenoceptors. Potential antipsychotic drug.

Spiramide (AMI-193) is a spiperone derivative, a selective 5-HT2A, (Ki = 2 nM) 5-HT1A (Ki = 50 nM), and D2 receptor (Ki = 3 nM) antagonist, with negligible affinity for the 5-HT2C receptor (Ki = 4300 nM). The ability of Spiramide to serve as a functional 5-HT2A antagonist in behavioral studies was demonstrated through studies in which Spiramide blocked the discriminative - stimulus effects of the 5HT2A agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM).

Mespiperone C-11 (3-N-[11C] methylspiperone) is a radiolabeled 3-N-methylspiperone. 3-N-methylspiperone is high-affinity D2/3 dopamine and 5-HT2A serotonin receptor antagonist. It has been studied as a positron emission tomography (PET) tracer for imaging D2/3 and 5HT2A receptor densities.

Sumanirole is a novel dopamine agonist with high affinity and efficacy at D2 dopamine receptors and has a substantial degree of selectivity for the D2 receptor over other dopamine receptor subtypes. It had been in clinical trials for the treatment of Parkinson’s disease and restless leg syndrome but these studies were terminated for the efficacy reason.

PNU-96391A (known as OSU6162) is a weak dopamine (DA) D(2) receptor antagonist with behavioral stabilizing properties. OSU6162 seem to act as stabilizers not only on dopaminergic, but also on serotonergic brain signaling (partial agonist on 5-HT2A receptor). OSU6162 in a phase II European clinical trial in treatment of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. One of the isomer of OSU 6162, has promise for treating Parkinson's disease, Huntington's disease and schizophrenia, but both enantiomers of OSU 6162 had dual effects on behavior, stimulating locomotor activity in 'low activity' animals and inhibiting locomotor activity in 'high activity' animals.

Adrogolide is a chemically stable prodrug of the dopamine D1 receptor agonist A-86929. Adrogolide is rapidly converted in plasma to A-86929. A-86929 has high affinity and functional selectivity for the dopamine D1 receptor. Adrogolide has been in phase II clinical trials for the treatment of Parkinson's disease and cocaine abuse. However, this research has been discontinued. The adverse events associated with its use of adrogolide were of mild-to-moderate severity and included injection site reaction, asthenia, headache, nausea, vomiting, postural hypotension, vasodilitation, and dizziness.

JNJ-37822681 is a novel, potent, specific, centrally active, dopamine D2 receptor antagonist, which was developed by Johnson & Johnson. This drug is in the phase II of clinical trial for the treatment of schizophrenia. JNJ-37822681 has optimal brain disposition and somnolence was the most frequently reported adverse effect.