Habituation: a condition which is characterized by a psychological craving for the effects of a drug and which is brought about by repeated (self)administration of the same drug. Unlike addiction, habituation is characterized by a desire to continue taking the drug for the pleasant feeling it produces, virtually no tendency to increase the dose, some degree of psychic dependence but no physical dependence and some detrimental effects on the drug user. Habituation is commonly observed with alcohol, tobacco and cannabis.

Haemostatic (hemostatic): a substance which stops bleeding from damaged blood vessels or tissues. Haemostatics mainly induce the coagulation of platelets but also include tranexamic acid (which inhibits fibrinolysis), desmopressin (an analogue of vasopressin which boosts factor VII levels in blood), aprotinin (a proteolytic enzyme and inhibitor of plasmin and kallidinogenase) and etamsylate (which reduces capillary bleeding by increasing platelet adhesiveness).

Half- life: also known as half-time (t_1/2), a measure of the time taken for a compound to decrease to half its initial concentration in the blood or plasma. Half-life is a useful kinetic parameter which expresses the relationship between clearance and volume of distribution. It is expressed in units of minutes or hours.

Hallucinogens: also known as psychedelic drugs and psychotomimetic agents, a group of drugs which induces hallucinations and changes in mood, perception, thinking and behaviour. Examples include lysergic acid diethylamide (LSD). The non-medical use of hallucinogens is common.

Haloperidol: a classical antipsychotic and butyrophenone compound first introduced in 1958 in Europe for the treatment of psychoses. It is a long-acting central dopamine receptor antagonist and is used clinically to treat hyperactive psychotic states and agitation and restlessness in the elderly. It also exhibits anti-emetic effects (because it blocks dopaminergic neurones in the chemoreceptor trigger zone) and is used in humans and animals to stop vomiting. It may also affect other neurotransmitter systems including NMDA receptors.

Halothane: a potent volatile inhalational anaesthetic used in human and veterinary medicine to induce general anaesthesia since its introduction in 1956. Halothane is a halogenated hydrocarbon liquid which is vaporized prior to breathing in. It was once widely used but is less commonly used these days in humans due to its hepatotoxicity. It acts by causing a general depression of the CNS but changes in the depth of anaesthesia can easily be achieved and recovery from anaesthesia is rapid.

Hammett relationship: a relationship between chemical substituents on a parent molecule and its reactivity. Named after Louis P. Hammett, a physical-organic chemist, in the middle 1930s after he studied the effects that substitutions on a benzene ring had on its reactivity. The Hammett relationship is used in quantitative structure-activity relationships (QSAR) to try to predict how the reactivity of a molecule changes according to the substituents it bears.

Hamster cheek pouch: an animal model of carcinogenesis used to investigate the induction of hyperplastic lesions and dysplastic and malignant lesions. Typically, Syrian golden hamsters are treated with a 0.5% solution of 7,12-dimethylbenz(a)anthracene (DBMA) three times a week for about 16 weeks. After about 2 weeks hyperplastic lesions develop in the cheek-pouch mucosa and dysplastic and malignant lesions appear from 4 to 8 weeks after treatment starts. This model is used to investigate antitumour compounds.
Giminez-Conti IB, Slaga TJ. The hamster cheek pouch carcinogenesis model. J Cell Biochem (1993) 17F (Supp); 83-90

Hanatoxins (HaTx): a family of two closely related 35 amino acid peptidyl potassium channel blockers isolated from the venom of the Chilean tarantula (Granulosa spatulata). Two main forms toxins are known, HaTX₁ and HaTX₂. They modify the gating energetics of voltage-dependent potassium channels.
Swartz KJ. Tarantula toxins interacting with voltage sensors in potassium channels. Toxicon. 2007 Feb;49(2):213-30

Hanes-Woolf plot: a linear form of the Michaelis-Menten equation for enzyme kinetics. The plot is made by plotting S/v on the y axis and S on the x axis. The slope of the line gives 1/V_max and the intercept is K_m/V_max.

Hanford-Moore miniature pigs: a variety of miniature pig which is used in cardiovascular research.
Smith AC, Spinale FG and Swindle MM. Cardiac function and morphology of Hanford miniature swine and Yucatan miniature and micro swine. Lab Animal Sci (1990) 40(1);  47-50

Hansch analysis: analysis of the quantitative relationship between the biological activity of a series of compounds and their physicochemical substituent including parameters representing hydrophobic, electronic and steric effects. Hansch analysis is used in quantitative structure-activity relationships (QSAR) and is an extremely useful tool in the identification and improvement of the pharmacological or toxicological profiles of xenobiotics. Named after Corwin Hansch.

Harassing agents: also known as antiriot agents.

Hard drug: a term used for drugs of abuse which are used for non-medical purposes. They often cause dependence and include heroin, cocaine and amphetamines. Hard drugs can be compared to soft drugs such as cannabis which are not generally considered to cause dependence.

Harmala alkaloid: a group of plant indole alkaloids comprising a β-carboline ring. The most abundant are harmine, harman, harmalol and harmaline. Some members of this group have hallucinogenic effects and harmine, once known as banisterine, was investigated for its potential use in the treatment of postencephalitic parkinsonism.

Hashish: another name for cannabis.

HaTx: hanatoxins

HC-3: Hemicholinium-3

Head twitch response (HTR): a form of behaviour observed in experimental animals where the animal twitches its head in response to stimulation of 5-HT₂ receptors which are thought to mediate this response. The head twitch response in rodents can be induced by the administration of 5-hydroxyptrytophan (5-HTP) and HTR is used as an animal model for the activation of the CNS 5-HT_2A receptors in the investigation of serotoninergic modulators.

Heidenhain pouch: an animal preparation named after Rudolf Heidenhain comprising a small sac or pouch formed from the stomach via an opening in the abdomen wall and used to measure the effects of drugs on stomach secretions in vivo. In this preparation the vagus nerve to the stomach is cut but the blood supply remains intact. The Heidenhain pouch received stimuli from circulating gastrin but no impulses from the brain which usually initiates the flow of blood after eating. A tube can be inserted into the pouch to collect the gastric juice without contamination by food and to determine the effects of drugs on the flow and acidity of gastric juice such as antihistamines and proton pump inhibitors.

Helminthicidals: antihelminthics

Helvolic acid: a tetracyclic triterpene antibiotic obtained from Aspergillus fumigatus which is effective against gram-positive bacteria.

Haematinics (hematinics): drugs which increase the haemoglobin content of blood and which are used to treat iron-deficiency anaemia. Examples include ferrous sulphate.

Hemicholiniums: a family of bisphenyacyl derivatives with neuromuscular blocking actions one of the most potent of which is hemicholinium 3.

Hemicholinium-3 (HC-3): a compound which blocks Na-dependent, high-affinity transport of choline into nerve terminals. Transport of choline into nerve terminals is a rate-limiting step in ACh formation so HC-3 can inhibit the synthesis of this neurotransmitter. It appears to compete with choline for the choline carrier in nerve terminal membranes. Hemicholinium is used as a pharmacological tool to investigate neuromuscular transmission of ACh. This compound is so named because it contains two choline-like moieties which, in solution, undergo hemiacetyl formation.

Henbane: a toxic plant (Hyoscyamus niger) which contains the alkaloids hyoscyamine (a racemic mixture of which is known as atropine) and hyoscine (scopolamine). Henbane was once used in soothsaying, magic and witchcraft and has been used to treat coughs, sensitive skin and pain because hyoscyamine is a non-subtype selective muscarinic antagonist and so dries up secretions into the airways.

Heparin: an acidic mucopolysaccharide derived from animal tissues and which is a natural inhibitor of blood coagulation. It prevents the formation of clots by preventing the conversion of prothrombin to thrombin and fibrinogen to fibrin. It is used clinically to treat the initial stages of deep vein thrombosis and pulmonary embolism.

Heparin-binding protein (HBP): a neutrophil azurophilic granule isolate . Recombinant HBP has been demonstrated to be a potent chemoattractant and to increase survival time of cultured monocytes.

Heparinoids: substances similar to heparin which are fibrinolytic and can improve blood supply to, for example, the skin. They can be used as a cream to reduce skin bruising and examples include danaparoid which is used clinically in the prophylaxis of deep-vein thrombosis in patients undergoing general or orthopaedic surgery.

Hepatic extraction ratio: the fraction of a drug removed from blood in one single passage through the liver. It is an important factor in drug pharmacokinetics.

Hepatic first-pass metabolism: the metabolism and, in most cases, a decrease in potency of a drug as it passes through the liver just after being absorbed from the gut (and being transported to the liver by the hepatic portal vein). Certain drugs undergo hepatic first-pass metabolism to a greater extent than other and examples of drugs where this effect is significant include oxprenolol, propranolol, labetolol, oestrogens and enalapril.

HEPES: a buffer (N-2-hydroxypiperazine-N-2-ethanesulphonic acid) commonly used in the pH range of 6.8 to 8.2.

Heroin: also known as diamorphine, diacetylmorphine (it is the 3,6-diacetyl derivative of morphine) and acetomorphine, an addictive narcotic analgesic and opium alkaloid which acts as a μ-opium agonist. It was originally synthesized from morphine in 1874 by CRA Wright in London, UK and introduced in 1898 for the treatment of cough (it is an antitussive) and morphine addiction (it was thought to be a non-addictive morphine substitute). It is rapidly converted in the body to morphine, is somewhat more active than morphine and eventually became the subject of addiction itself. Heroin causes euphoria, a sense of contentment and well-being and analgesia and is widely used as a drug of abuse to the extent that it was made illegal in the US in 1924. It is used clinically in the treatment of severe cough and pain particularly that associated with terminal illnesses such as cancer and is given in preference to morphine due to its greater solubility and is given orally as a syrup or linctus. Intravenous infusion is used in the case of severe pain. Heroin is actually a trade name originally given to diamorphine by Bayer (it means 'heroic treatment' and comes from the German word heroisch). It is known by a variety of slang names including China white, junk, babania, horse, brown, smack, black tar, big H, lady H, skag and juice amongst others.



Herxheimer reaction: also known as the Jarisch-Herxheimer reaction.

Heteroreceptor: a receptor which regulated the production and/or release of mediators other than its own ligand.

Hexahydrosiladifenidol: a selective M₃ muscarinic cholinoceptor antagonist.

Hexamethonium: a nicotinic receptor antagonist and ganglion blocker which has been used to treat hypertension.

5-HIAA: 5-hydroxyindoleacetic acid

High-ceiling diuretic: a type of diuretic.

High-throughput screening (HTS): a system of screening compounds for pharmacological effects where very small quantities of drug and very simple assay systems are used to screen up to 100,000 compounds for activity. There are several methods used in high-throughput screening; scintillation proximity assays, fluorescence resonance energy transfer, time-resolved fluorescence, fluorescence polarization and reverse yeast two-hybrid systems.

Hill, Sir Archibald V: English physiologist (1886-1977) and Noble Prize winner in 1922 which he shared with Otto Meyerhof for Physiology or Medicine "for his discovery relating to the production of heat in the muscle".
http://nobelprize.org/medicine/laureates/1922/hill- bio.html

Hill plot: a graphical method to determine the relationship between response and receptor occupancy. It is actually a plot of the log of (the proportion of receptor sites occupied)/(1-the proportion of receptors occupied) on the y (vertical) axis against the log of the drug concentration (log [D]) on the x (horizontal) axis. This type of plot gives a straight line where the slope of the line is given by n, the Hill coefficient (a Hill coefficient of 1.0 indicates independent binding of ligand to receptor, a value higher than 1.0 shows that binding of one ligand facilitates the binding of subsequent ligands (positive co-operativity) and a value of less than 1.0 indicates negative co-operativity). The value of n does not indicate the number of receptors on a cell. It is named after Archibald Vivian Hill who originally used it to determine the relationship between the percentage saturation of haemoglobin with oxygen and the partial pressure of oxygen). The Hill plot is also used in enzymology to determine the positive and negative co-operativity of enzymes.

Himbacine: a selective M₂ muscarinic receptor antagonist.

Hirudins: proteins obtained from medicinal leeches and having antithrombin anticoagulant effects. Examples include desirudin and lepirudin which are recombinant hirudins used clinically to treat heparin-induced thrombocytopaenia.

Hispid cotton rats: a moderately large, robust rat (Sigmodon hispidus) which has been used as a model of animal infections.
Niewiesk S and Prince G. Diversifying animal models: the use of hispid cotton rats (Sigmodon hispidus) in infectious diseases. Lab Anim (2002), 36(4); 357-72

Histaminase: also known as diamine oxidase.

Histamine: a biogenic amine (1H-imidazole-4-ethanamine), potent chemical mediator of inflammation and an important compound in the regulation of several physiological and pathological processes. Histaminergic neurons exist mainly in the periphery but exist centrally too in the tuberomamillary nucleus in the posterior hypothalamus where they appear to mediate sleep/wakefulness, cardiovascular control, food intake, thermoregulation, memory and hormone secretion.
Peripherally, most histamine is present in mast cells, enterochromaffin cells and basophils where it mediates disorders such as allergy, asthma and inflammation.

Histamine receptors: receptors for the biogenic amine histamine and currently divided into four subclasses H₁, H₂, H₃ and H₄ and utilize several signal transduction systems.

- H₁ receptor are found in the smooth muscle of the bronchus, intestinal smooth muscle and small arteries and veins. H₁ receptors mediate bronchiolar contraction, intestinal contractions, capillary permeability and vasodilation of small arteries and veins.

- H₂ receptors are found in the gut and vascular smooth muscle and mediate gastric acid secretion and vasodilation.

- H₃ are located presynaptically and modulates transmitter release.

- H₄ receptors are known to modulate IL-16 release and may play a role in the pathogenesis of asthma and autoimmune disease.

The table shows a summary of the agonists and antagonists at each receptor site. An asterisk denotes a receptor subtype-specific action.

S. J. Hill et al. Classification of Histamine Receptors. International Union of Pharmacology.
http://pharmrev.aspetjournals.org/cgi/content/full/49/3/253
http://www.tocris.com/catBrowser.php? &CatId=5011
http://www.iuphar-db.org/GPCR/ChapterMenuForward? chapterID=1287

Histamine receptor agonists: compounds which agonise histamine receptors and which mediate a broad range of tissue responses including gastric acid secretion, neurotransmission, inflammation and allergy. Examples are shown in the above table.

Histamine receptor antagonists: compounds which block the effects of histamine at H₁, H₂, H₃ and H₄ histamine receptor sites. They have a broad range of effects and are used clinically to treat allergy, inflammation, rhinitis and, gastrointestinal ulcers. Examples of these antagonists are shown in the table above.

Histamine N-methyl transferase: an enzyme (EC 2.1.1.8) which catalyses the transfer of a methyl group to histamine (ie, it N-methylates it) to form the largely inactive  N-(tau)-methylhistamine and so abolish the effects of histamine.

Histamine N-methyl transferase inhibitors: inhibitors of histamine N-methyl transferase and compounds which potentiate the actions of histamine by blocking its deactivation. Examples include chloroquine, metoprine and SKF 91488 dihydrochloride (which is a homologue of dimaprit).

Histaminergic: relating to the synthesis, storage and release of histamine.

Histidine decarboxylase: an enzyme (EC 4.1.1.22) widely distributed in tissues especially in histamine-storing tissues such as mast cells and gastric mucosal cells where it converts the dietary amino acid histidine to histamine by decarboxylation. It is one of two enzymes which carry out this conversion the other being aromatic amino acid decarboxylase (EC 4.1.1.26).

Histidine decarboxylase inhibitors: compounds inhibiting histidine decarboxylase and so the formation of histamine from the dietary amino acid histidine. Inhibitors of this enzyme such as brocresine have been developed for the treatment of histamine-related disorders such as urticaria.

Histone deacetylase (HDAC): an enzyme which removes an acetyl group from histones and allows them to bind to DNA thereby inhibiting gene transcription and so interrupting cell proliferation. This enzyme mediates the regulation of gene expression by changes in nucleosome conformation and deregulation of HDACs is linked to the formation of several forms of cancer.

Histone deacetylase inhibitor: inhibitors of histone deacetylase and compounds which are being targeted as anticancer agents. Examples include apicidin, trapoxin B, (-)-depudecin and scriptaid. Histone deacetylase inhibitors may also be of use as cognition enhancers.
Vigushin DM, Coombes RC. Histone deacetylase inhibitors in cancer treatment. Anticancer Drugs (2002)13(1);1-13

Histrionicotoxins (HTX): a family of dendrobatid alkaloids and toxic spirocyclic piperidine compounds found in South American poison dart frogs such as Dendrobates Histrionicus.

Hitchings, George H (1905-1998): an American chemist who shared the 1988 Nobel Prize for Physiology or Medicine with Elion and Black "for their discoveries of important principles for drug treatment".
http://nobelprize.org/medicine/laureates/1988/hitchings- autobio.html

HIV integrase: an integrase used by HIV to incorporate its genes into a host cell's DNA. (The DNA form of the viral genes is produced by reverse transcriptase.)

HIV integrase inhibitors: compounds which inhibit HIV integrase and so inhibit the incorporation of HIV genes into the DNA of the host thereby preventing HIV replication. Inhibition of HIV integrase is a useful therapeutic strategy since it protects healthy cells from infection with this virus. Examples include L-731, 988 and S-1360.
Barbaro G et al. Highly active antiretroviral therapy: current state of the art, new agents and their pharmacological interactions useful for improving therapeutic outcome. Curr Pharm Des (2005) 11(14);1805-43

HIV-1 protease: an aspartyl protease officially known as HIV-1 retropepsin (EC 3.4.23.16) which cleaves the viral precursor protein gp160 into proteins that make up a mature virion at the postintegration stage and prior to viral budding.

HIV-1 protease inhibitors: inhibitors of HIV protease and compounds which can inhibit the replication of HIV virus. Examples include indinavir and nelfinavir.

HIV reverse transcriptase: an enzyme used by HIV to transcribe its single-stranded RNA genome into single-stranded DNA. This is then used to construct a complementary strand of DNA to provide a DNA double helix capable of integration into host cell chromosomes.

HIV reverse transcriptase inhibitors: inhibitors of HIV reverse transcriptase and so compounds which can stop the replication of this virus. They are being developed for the treatment of AIDS and examples include alovudine, elvucitabine and racivir.

HMG-CoA reductase (3-hydroxy-3-methylglutaryl-coenzyme A reductase): an 887 amino acid, single chain enzyme (EC 1.1.1.88) present in the endoplasmic reticulum and which catalyzes the reduction of HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A which is produced from acetyl-CoA in 3 steps) to mevalonate. Mevalonate in turn is then converted via 30 steps to form cholesterol. HMG-CoA production is carefully controlled by three regulatory mechanisms one being a decrease in its production by cholesterol itself in a feedback mechanism and another is its rapid degradation. The production of mevalonate by HMG-CoA is an important step in cholesterol biosynthesis and inhibition of this enzyme leads to a lowering of circulating cholesterol levels as most serum cholesterol originates in the liver.

HMG-CoA reductase inhibitors: compounds which inhibit HMG-CoA reductase, the key regulatory enzyme in hepatic cholesterol biosynthesis. These are usually competitive inhibitors and include the orally active statins, eg simvastatin, pravastatin, lovastatin, cerivastatin, fluvastatin and atorvastatin. They decrease hepatic cholesterol production with consequent increased production of LDL receptors, increased clearance of LDL and a decrease in LDL-cholesterol in the blood. They are used in the treatment of hypercholesterolemia where dietary restrictions are themselves inadequate. They are used clinically to treat hypercholesterolaemia and hyperlipidaemia and are among the most commonly prescribed drugs.

Hodgkin, Sir Alan L (1914-1998): British physiologist who shared the 1963 Nobel Prize for Physiology or Medicine with Eccles and Huxley "for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane".
http://nobelprize.org/medicine/laureates/1963/hodgkin- bio.html

Hodgkin–Huxley model: a scientific model that describes how action potentials in neurons are initiated and propagated.
See wikipedia.

Hole poke activity: a test of exploratory behaviour allowing the investigation of curiosity and spatial working memory in mice or rats. The numbers of holes in a board which are explored by poking a paw or their nostrils into them, the time spent on this activity, the location of chosen holes and repetitions of hole poking can be studied. Hole poke activity is used to investigate the effects of drugs on spatial working memory

Hollenbach's deep vein thrombosis model: a novel rabbit model of quantitative and non-occlusive deep vein thrombosis which is used to test the effects of anticoagulants.
Hollenbach S et al. A comparative study of prothrombinase and thrombin inhibitors in a novel rabbit model of non-occlusive deep vein thrombosis. Thromb Haemost (1994) 71; 357-362

Holothurins: a group of ichthyotoxic marine toxins from sea cucumber (they cause erratic behavior, disease or death in small animals and fish).

Homologous desensitization: the desensitization of a receptor over a period of time. An example of this effect occurs with atrial natriuretic peptide at its ANP-A receptor site. This is the primary signaling receptor and ligand binding to ANP-A rapidly activates the guanylyl cyclase domain of this receptor but the rate of cGMP synthesis declines with time. This is partly mediated by receptor dephosphorylation.

Homomeric receptor: receptors which comprise two or more identical subunits.

Homovanillic acid (HVA): a metabolite of dopamine. Drugs which cause the release of dopamine cause an increase in HVA and this has often been taken as a measure of dopamine turnover.

Hoogsteen hydrogen bond: a hydrogen bond which holds complimentary nucleotide bases together.

Hot compounds: molecules which have been labelled with a radioactive atom.

Hot plate test: a animal test originally described in 1944 and used commonly to determine the effects of analgesics. In this test of allodynia (pain) an animal is placed on a plate which is maintained at an uncomfortably high temperature of about 55°C. After a while the animal shows signs of discomfort such as jumping or paw licking. The test compound is usually administered 30 minutes or so before the animal is placed on the plate and the length of time the animal endures the discomfort is a measure of its analgesic effects. Variants of this test include increasing the temperature of the hot plate.
Woolfe G and MacDonald AD. The evaluation of the analgesic action of pethidine hydrochloride (Demerol). J Pharmacol Exp Ther (1944) 80; 300-307
Hunskaar S et al. A modified hot-plate test sensitive to mild analgesics. Behav Brain Res (1986) 21; 101-108

h.s.: hour of sleep. Notation sometimes used on prescriptions to indicate that a medicine should be taken just prior to sleeping.

5-HT: 5-hydroxytryptamine, also known as serotonin.

HTS: high-throughput screening

Human tumor clonogenic assay: an assay used to measure the sensitivity of human tumors to anticancer drugs and to measure the cytotoxicity of lymphocytes. It is based on soft agar colony formation by human tumor cells.
Salmon SE. Human tumor clonogenic assays: growth conditions and applications. Cancer Genetics Cytogenetics (1986) 19; 21-8

Humectants: compounds added to cosmetics and topical aqueous drug preparations to stop them drying out after application to the skin. Examples include glycerol, polyethylene glycol and propylene glycol which are added to preparations at a concentration of about 5%.

HUVEC: human umbilical vein epithelial cells, cells often used to investigate the effects of drugs on cell proliferation and on cell adhesion.

Huxley, Sir Andrew F (1917- ): British physiologist who shared the Nobel prize in 1963 for Physiology or Medicine with Eccles and Hodgkin "for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane".
http://nobelprize.org/nobel_prizes/medicine/laureates/1963/huxley-bio.html

Hybrid drug: a drug having two or more therapeutic actions such as an antihypertensive agent which blocks β-receptors as well as causes vasodilation.

Hybrid mouse: a mouse which is a cross of two known inbred strains and which is referred to as an F1 hybrid. These mice are commonly use in research because, unlike inbred strains which exhibit inbreeding depression leading to smaller litters and significant neonatal death along with relatively low fertility and sterility, F1 hybrids show a high degree of hybrid vigor. This means that they are highly reproductive and show low within-strain genetic variability (so they are all homozygous in the same way). Common strains of hybrid mice include B6C3F1, CBAB6F1 and CB6F1 mice.

Hydantoins: compounds comprising a 5-membered hydantoin ring which have a similar structure to barbiturates. Phenylethylhydantoin was the first hydantoin to be used for the treatment of chorea in 1916. They have anticonvulsant effects and include phenytoin, mephenytoin and ethotoin. They act by reducing the spread of the seizure discharge from the focus of the seizure. Some hydantoins such as phenytoin have GABA-like effects on presynaptic and postsynaptic membranes and enhances presynaptic and postsynaptic inhibition.
Hydantoins are chemically related to other classes of antiepileptic drugs including the oxazolidindiones such as trimethadione and paramethadione and succinimides such as phensuximide and ethosuximide.

Hydralazine: a vasodilator which acts mainly on the arteries and arterioles. It is used clinically to treat moderate to severe hypertension, heart failure and hypertensive crisis.

Hydrazines: a class of monoamine oxidase inhibitors which have antidepressant effects. They include phenelzine, isocarboxazid, benmoxine, iproclozide, iproniazid and nialamide.

Hydrochlorthiazide: a thiazide diuretic that has been used for the treatment of oedema and hypertension.

Hydrocortisone: a steroid used clinically for adrenocortical insufficiency and shock.

Hydragogues: very strong drastics (another name for cathartics) which cause the loss of water from the bowel in the form of watery stools and are used to treat constipation. Many herbs and plants have such hydragogic actions and examples include japapa, scammonium and cambogia. Such plants are still used in traditional Chinese medicine.

Hydroxychloroquine: a quinolone-type antimalarial drug used in the prevention and treatment of malaria. It is also used to treat discoid or systemic lupus erythematosus and rheumatoid arthritis. Hydroxychloroquine is less oculotoxic than chloroquine.

6-Hydroxydopamine: a neurotoxin and pharmacological tool used in chemical sympathectomy.

Hydroxyindole O-methyltransferase (HIOMT): officially known as acetylserotonin O-methyltransferase, an enzyme (EC 2.1.1.4) which catalyzes the transfer of a methyl group to N-acetylserotonin to form N-acetyl-5-methoxytryptamine and an enzyme in the degradation pathway of serotonin.

5- Hydroxyindoleacetic acid (5-HIAA): the main breakdown product of serotonin. 5-Hydroxyindoleacetic acid (HIAA) is a potent smooth muscle stimulant formed by the deamination of serotonin by monoamine oxidase. 5-HIAA is excreted in the urine and is excreted in large quantities by individuals with carcinoid tumors particularly of mid-gut (eg, ileal) origin.

17 -hydroxylase-C_17,20-lyase: also known as steroid 17-alpha-hydroxylase/17,20 lyase but officially known as steroid 17-alpha-monooxygenase, an enzyme (EC 1.14.99.9) which catalyzes the transfer of a hydroxy group to the 17 position of a steroid and an enzyme essential in the biosynthesis of 4-androstenedione and testosterone from 17-á- hydroxyprogesterone.

17 -hydroxylase-C_17,20 lyase inhibitors: compounds which inhibit 17 -hydroxylase-C_17,20 lyase and so inhibitors of the production of testosterone and androgens from pregnane precursors. They are being developed for their potential use in treating prostate tumours.

Hydroxymethylglutaryl-CoA reductase: also known as HMG-CoA reductase.

11-β- Hydroxysteroid dehydrogenase (11-HSD): an enzyme which regenerates cortisol from inactive cortisone in the liver. There are two known isozymes, 1 and 2.

11-β- Hydroxysteroid dehydrogenase inhibitors: compounds which inhibit 11-β-hydroxysteroid dehydrogenase and which are being developed for their potential use in lowering intracellular cortisol concentrations. This enhances insulin sensitivity and hepatic lipid catabolism in type 2 diabetes, obesity, and hyperlipidemia. Examples include carbenoxolone.

Hyoscine: also known as scopolamine, an alkaloid obtained from solanaceous plants such as the thorn apple (Datura stramonium). Like atropine (from deadly nightshade (Atropa belladona)), it is a tertiary ammonium compound and muscarinic receptor antagonist used clinically for symptomatic relief of gastrointestinal disorders characterized by smooth muscle spasms, motion sickness and as a premedication for surgery to dry the flow of saliva.

Hyoscyamine: an alkaloid obtained from solanaceous plants such as Atropa belladona (deadly nightshade), Hyoscyamus niger (black henbane) and Datura stramonium (thorn apple). Atropine is a racemic mixture of (+) and (-) hyoscyamine.

Hyperswimming test: a test to observe the effects of centrally acting muscarinic receptor agonists since scopolamine (typically at 0.3 mg/kg ip and a centrally acting muscarinic antagonist) is known to significantly increases spontaneous swimming in rats.

Hypnotic: a drug which produces a state which is clinically identical to sleep by depressing the central nervous system. They tend to cause sleep at night and sedation during the day, however. Hypnotics are used clinically to treat all forms of insomnia. The main classes of hypnotics are the benzodiazepines such as nitrazepam, lorazepam and temazepan, zolpidem (an imidazopyridine) and zopiclone (a cyclopyrrole) which both act on benzodiazepine receptors, barbiturates and chlormethiazole. Side effects of all these compounds include impairment of judgment, paradoxical effects and increased reaction time.

Hypocholesterolaemic: a drug which lower blood levels of cholesterol such as the statins (HMG-CoA reductase inhibitors) and which are used in the treatment of heart diseases including atherosclerosis.

Hypocretins: also known as orexins (hypocretin 1 is the same as orexin A and hypocretin 2 is th same as orexin B), peptides found in the dorsolateral hypothalamus and which regulate arousal states, influence feeding and are implicated in the sleep disorder narcolepsy as low levels of hypocretin have been shown to cause narcolepsy.
Sutcliffe JG, de Lecea LT. The hypocretins: excitatory neuromodulatory peptides for multiple homeostatic systems, including sleep and feeding. J Neurosci Res. (2000) 15:62(2);161-8

Hypoglycaemics (hypoglycemics):  compounds which decrease blood sugar levels. Examples include insulin, insulin sensitizers such as pioglitazone and rosiglitazone and oral hypoglycaemics such as the sulphonylureas including glibenclamide, glipizide and tolbutamide. Other hypoglycaemics include acarbose (an intestinal α-glucosidase inhibitor) and nateglinide (which stimulates insulin release).

Hypolipidaemics (hypolipidemics): compounds which decrease blood lipid levels. Examples include

- anion-exchange resins such as colestyramine and colestipol

- compounds which inhibit the intestinal absorption of cholesterol such as ezetimibe

- fibrates such as benzafibrate, fenofibrate and ciprofibrate which decrease serum triglyceride levels, and

- statins such as atorvastatin, fluvastatin and pravastatin which mainly inhibit HMG-CoA and reduce serum cholesterol levels.

- Omega-3-acid ethyl esters and omega-3-marine triglycerides.

Hypophosphatemic mouse (Hyp): an animal model for human hypophosphataemic vitamin D-resistant rickets.

Hypotensives: drugs which lower blood pressure. Various mechanisms are involved in blood pressure regulation and hypotensives include:
- β-blockers such as propranolol, atenolol and labetalol,

- calcium antagonists such as amlodipine, diltiazem hydrochloride and nifedipine,

- vasodilators such as bosentan, minoxidil and sodium nitroprusside,

- ACE inhibitors such as captopril, enalapril and lisinopril,

- angiotensin II receptor antagonists such as the sartans including candesartan, irbesartan and valsartan,

- centrally acting hypotensives such as clonidine, methyldopa and moxonidine

- adrenergic neurone blockers such as guanethidine,

- α-adrenoceptor antagonists such as doxazosin, indoramin and prazocin, and

- nitrates such as glyceryl trinitrate, isosorbide dinitrate and isosorbide mononitrate