INTRODUCTION TO VETERINARY NEURO PHARMACOLOGY

INTRODUCTION TO VETERINARY NEURO PHARMACOLOGY

Central Nervous System

•       The central nervous system is the brain and spinal cord.

•       It is wrapped in 3 layers of membranes called meninges.

•       The brain contains fluid-filled ventricles that are continuous with the central canal of the cord.

Divisions of the Brain

•       Generally, many body functions involve cells in several areas of the brain. However, certain areas of the brain tend to be more important in some functions while other areas dominate the control of other functions.

•       Some major parts of the brain are listed below.

                Hindbrain: medulla oblongata, cerebellum, pons

                Midbrain

                Forebrain: thalamus, hypothalamus, cerebrum

Brain Structure	Function
Medulla oblongata	Vital functions such as breathing, heart rate, and blood pressure Reflexes such as vomiting, coughing, sneezing, hiccupping, swallowing, and digestion
Pons	Breathing, connects spinal cord, cerebellum and higher brain centers
Cerebellum	Motor coordination
Midbrain	Receives visual, auditory, and tactile information In mammals, this information is sent to the thalamus and higher brain centers.  In lower vertebrates, the information is further processed in the midbrain.
Thalamus	Relays sensory information to the cerebral cortex. Contains part of the reticular formation (controls arousal).
Hypothalamus	Maintains homeostasis, regulates the endocrine system Contains part of the Limbic system (controls emotion)
Cerebrum	Processes sensory information and produces signals that move the skeletal muscles.
Cerebral Cortex	This is the outer layer of the cerebrum. Thinking, intelligence, and cognitive functions are located here. Processing of sensory information and motor responses

DIFFERENCES BETWEEN CNS AND PNS

Central Nervous System

•       Mainly contains the brain and the spinal cord.

•       These two have a very specific bony protective covering, supplemented by the other soft tissues.

•       The brain is divided into forebrain, mid brain, hind brain.

•       Most of the functional mapping for the muscular movements and the sensory perceptions, as well as the executive functions, is distributed throughout the fore brain into discrete regions.

•       The mid brain constitutes a part of the brainstem, which is vital in keeping the person alive such as defensive physiological reflexes, respiration, and cardiac pacemaker control, whereas the hind brain is involved in the formation of the cerebellum, which is essential in maintaining balance of the body.

•       The spinal cord is limited by the brainstem atop and the first lumbar vertebrae and neuronal extensions (cauda equina) below. They are divided into separate areas that function as the control hub for information from the brain, which are to be transmitted to the peripheral nerves as well as to perform reflex functions of the peripheral organs.

Peripheral Nervous System

•       The peripheral nervous system is not protected by an osteolithic cavity or a blood brain barrier.

•       It contains all the nerves and the ganglia for the nerves.

•       There are various divisions of peripheral nerves. But to include all aspects, we can consider them as being, motor, sensory and autonomic.

•       The motor nerves are again divided as voluntary and involuntary, with the involuntary actions liaised with the autonomic functions.

•       The voluntary motor activities are produced through cranial nerves as well as spinal nerves, and they are transmitted from the cerebral cortex. The involuntary ones are mostly for visceral organs; thus, within the scope of the autonomic nervous system.

•       The sensory nerves are also divided as spinal and cranial, and they perceive sensations of touch, temperature, pressure, cold, vibration, proprioception, etc, which is transmitted to the sensory cortex in the brain.

•       The autonomic nerves that have distributions to cranial nerves, as well as forming spinal nerve plexi on occasion, are classified by their actions of sympathetic and parasympathetic and conducts the actions of visceral organs.

What are the differences between Central and Peripheral Nervous System?

•       Both these systems are managed by neurons, each having equal physiology and the mode of conducting information, and supported by similar structures. But the main differences lie at the varied differentiations, the proportions of the supportive structure, and the distributed chemical signatures.

•       CNS is protected by the bone and a blood brain barrier whereas the PNS is not. 

•       CNS is concerned with storing, comprehending and executing information appropriately, but the PNS is more about transmission to far away structures.

•       The main varieties in the PNS can be classified easily, but the CNS functions are of multiple levels and need greater understanding.

•       A damage to a PNS structure will cause only localized damage, but damage to a CNS structure can lead to global damage.

DIFFERENCES BETWEEN ANS AND SNS

	Somatic Nervous System	Autonomic Nervous System
Effectors	Skeletal muscle	Smooth muscle, Cardiac muscle, Glands
Anatomical
a) Ganglia outside CNS	None	YES: Paravertebral Prevertebral, Intramural
b) Pathway from CNS Effector Organ	Uninterrupted	Interrupted
c) Fibers	Myelinated	Preganglionic-myelinated; Postganglionic-unmyelinated
d) Peripheral plexus	None	Fibers branch and form a network
e) Reflex Arc	Spinal cord and brain stem	Spinal cord and brain stem
Functional
a) Peripheral Effect	Excitation	Excitation or inhibition
b) After Denervation	Paralysis (atrophy)	No paralysis
c) Control of environment	Adjusts body to external environment	Regulation of internal environment
Chemical
a) Neurotransmitter	Acetylcholine	Acetylcholine & norepinephrine
b) Co-transmitters		Vasoactive intestinal peptide (Parasympathetic) Neuropeptide Y (Sympathetic) ATP (Sympathetic)

What 4 areas control the ANS?

(1) Spinal cord = autonomic relexes (stretched bladder = urination)
(2) Brainstem = controls cardiovascular, respiration, tears, etc
(3) Hypothalamus = integrates ANS info. - hunger, thirst, sexual behavior
(4) Cerebral cortex and limbic system = stress, emotions, affect BP, HR, sweat etc

Differences between Sympathetic and Para-sympathetic NS

Item	Sympathetic Nervous System	Parasympathetic Nervous System
Outflow from CNS	Thoracolumbar	Craniosacral
Ganglia	Paravertebral; prevertebral close to CNS; a few intramural ganglia close to organ	Intramural ganglia-close to effector organ
Ratio: preganglionic - postganglionic	Preganglionic neurons synapse with many postganglionic neurons	Preganglionic neurons synapse with few postganglionic neurons
Function	“Fight or flight”	Conservation
Effectors	Smooth and cardial muscle, glands	Generally the same as sympathetic
Transmitters	Ach, NE, Co-T	Ach, Co-T

SELECTED EFFECTS

Structure	Sympathetic Activation	Parasympathetic Activation
Iris  (Radial Muscle)          (Sphincter Muscle)	Pupil dilated --------------	-------- Pupils constricted
Glands Lacrimal Salivary Sweat	---------- Stimulated (scanty thick, viscous secretion) Secretion, palms	Stimulated Stimulated (profuse, watery secretion) Generalized secretion
Heart Rate Force (Ventricles)	Increase Increase	Decrease ----------
Blood Vessels	Contraction (generally) Dilation (some)	Slight effect; Dilation in some
Bronchi Intestine	Relaxed Tone & motility decreased	Constricted Tone & motility increased
Adrenal Medulla	Secretion of Epi and NE	-------
Sex Organs	Vasoconstriction, Contraction of vas deferens, Seminal vesicle and Prostatic musculature (ejaculation)	Vasodilation & erection

HISTORICAL CONSIDERATIONS

- Lewandousky (1899) Administered extracts of the adrenal glands pointed towards the concept of chemical transmission

- Elliott (1904-05) Worked out similarities between stimulation of sympathetic nerves, administration of adrenal extracts and epinephrine

- Dixon (1907) Similarity between muscarine alkaloids and vagal stim.

- Sir Henry Dale (1914) Similarities between choline esters and Ach coined "muscarinic and nicotinic"

- Otto Loewi (1920) Obtained first real proof of chemical neurotransmissions Proof of Chemical Transmission of Vagal Impulses to the Frog’s Heart “Vagusstuff”

Nobel Laureates

- Dale (1936) Fundamental contributions to concept of chemical neurotransmission

- Loewi (1936) First proof of chemical neurotransmission

- Eccles (1963) Major electrophysiological contributions to nerve transmission

- Axelrod (1970) Established the major ways of inactivation of catecholamines

- von Euler ( 1970) Established NE as neurotransmitter of adrenergic nerves

- Katz (1970) Important electrophysiological observations and concept of quantal release

- Sutherland (1972) Co-discoverer of cyclic AMP

- Black (1988) β-blockers

- Gilman (1994) G-proteins

- Rodbel (1994) G-proteins

-Furchgott (1998) Nitric Oxide

-Murad (1998) Nitric Oxide

-Ignarro (1998) Nitric Oxide

-Carlsson (2000) CNS neurotransmission

-Greengard (2000) CNS neurotransmission

-Kandel (2000) CNS neurotransmission

The criteria adopted in assigning the role of transmitter within the CNS to a particular compound have been developed from the classical studies of Dale and his colleagues in their investigations of transmitters in the peripheral nervous system.

It must be shown that:

 I. the compound is present in neurones in CNS tracts;

2. the compound when administered produces effects similar to those of neuronal stimulation;

3. the compound is demonstrably released by electrical stimulation;

4. the neurones are capable of synthesizing the compound from precursor molecules;

5. means of terminating the activity of the com­pound, by enzymic degradation and/or uptake into neuronal or glial cells, exist;

6. inhibitors of enzymic breakdown or uptake produce effects similar to the administered trans­mitter and neuronal stimulation; and

7. specific receptor site antagonists of the com­pound block the actions of the transmitter following administration of the transmitter or its release by electrical stimulation.

NEURO HUMORAL TRANSMISSION

Nerve impulses elicit responses in smooth, cardiac and skeletal muscles and post synaptic neurons through liberation of specific chemical neurotransmitters.

Steps involved in Neurotransmission :

The sequence of events involved in neurotransmission is of particular importance pharmacologically, since the actions of a large number of drugs are altered directly to the individual steps.

1.       Axonal conduction :

It refers to the passage of an impulse along a nerve fibre.

The resting membrane potential is established by high Potassium (K⁺) permeability of axonal membrane and high axoplasmic concentration of this ion coupled with low sodium (Na⁺) permeability and its active extrusion. Stimulation or arrival of an electrical impulse causes a sudden increase in sodium permeability to the interior in relation to the potassium ion. Thus the membrane potential moves from  -85mv toward 0 and then overshoot to the extend that momentarily the inside of the fibre is positive in relation to the exterior of the cell (Depolarization).

Potassium ion then move out in the direction of their concentration gradient and repolarization occurs. Ionic distribution is normalized during the refractory period by the activation of Na⁺K⁺ pump.

Action potential thus generated sets up local circuit currents which activate ionic channels at the next excitable part of the membrane (next nodes of Ranvier in myelinated nerve – jumping / salutatory conduction) and action potential propagated with out decrements. Thus action potential is self propagating.

2.       Junctional transmission :

The arrival of the action potential at the axonal terminals initiates a series of events that trigger transmission of an excitatory / inhibitory impulse across the synapse or neuro-effctor junction.

a.       Storage and release of the transmitter :

                The non-peptide neurotransmitters are largely synthesized in the region of the axonal terminals and stored there in synaptic vesicles. Where as, peptide neurotransmitters are found in large dense core vesicles, which are transported down the axon from their site of synthesis in the cell body. During the resting state, there is a continuous slow release of isolated quanta of the transmitter which produces electrical responses at the post junctional membrane (Miniature End Plate Potential) that are associated with the maintenance of physiological responsiveness of the effector organ. As the action potential arrives at the nerve terminal, it facilitates an inward movement of Ca⁺⁺ , which triggers the discharge of neurotransmitters from the storage vesicles in to the synaptic cleft by causing the Excitation-secretion coupling phenomena (Influx of Ca⁺⁺ à into the axonal cytoplasm à increases the fusion of vesicular and axonal membranes à Exocytosis of the neurotransmitters and enzymes).

b. Combination of the transmitter with post-junctional receptors and production of post junctional potential:

The released transmitter combines with specific receptors on the post junctional membrane and depending on its nature induces an EPSP or an IPSP.

EPSP:  A generalized increase in the permeability to cations (notably Na⁺ and occasionally Ca⁺⁺), resulting in localized depolarization of the membrane, ie. Excitatory post synaptic potential.

IPSP:  I) a selective increase in permeability to anions, usually Cl^-, resulting in stabilization / actual hyperpolarisation of the membrane, which constitutes an Inhibitory post synaptic potential  (or)

              II) an  increased permeability to K⁺, because K⁺ can then exit the cell, hyperpolarization and stabilization of the membrane potential (IPSP).

MEPP:   In normal cicumstances, when there is no EPSP / IPSP there will be a constant release of small quantitites of the neurohumoral substances in the synaptic as well as neuro-effector junctions to sensitize the post junctional receptors there by they will be ready for the EPSP / IPSP, ie. Miniature endplate potential.

3.       Initiation of post junctional activity :

If an EPSP exceeds a certain threshold value, it initiates a propagated action potential in a post synaptic neuron / a muscle action potential in skeletal / cardiac muscle, in which propagated impulses are minimal, an EPSP may cause the rate of spontaneous depolarization and enhance muscle tone; in gland cells, it initiates secretion. An IPSP, which is found in neurons and smooth muscles, but not in skeletal muscles, will tend to oppose excitatory potentials initiated by other neuronal sources at the same time and site. The resultant response depends on the summation of all the potentials.

4.       Destruction / Dissipation of the transmitters :

                      Following its combination with the receptors and the post junctional activity the transmitter is either locally degraded (Ach E --- Ach) or is taken back in to prejunctional neurone by active (NEN) or diffuses away (GABA). Rate of termination of transmitter action governs the rate at which responses can be transmitted across a junction (1-1000/Sec.).

Events during neurochemical transmission:

•     electrical impulses from CNS

•     àá  in Na+ permeability

•     à local depolarization of neuronal membrane

•     àá in K+ permeability and repolarization

•     therefore, ion currents through distinct channels  action potential

•     action potential arrives at nerve terminal

•     à release of stored neurotransmitter by exocytosis

•     neurotransmitter diffuses across synaptic cleft

•     interacts with receptor on postganglionic cell body or effector organ

•     alters ion permeability and initiates action potential in post-ganglionic nerve cell body

•     or mediates a response in the end-organ (response is dependent on transmitter and receptor subtype)

Overview of Effect of Pharmacological Agents acting on Autonomic Nervous System

Central Nervous System Transmitters and Mediators

A neurotransmitter is a messenger released from a neuron at an anatomically specialised junction, which diffuses across a narrow cleft to affect one or sometimes two postsynaptic neurons, a muscle cell, or another effector cell.

A neuromodulator is a messenger released  from a neuron in the central nervous system, or in the periphery, that affects groups of  neurons, or effector cells that have the appropriate receptors. It may not be released at synaptic sites, it often acts through second messengers and can produce long-lasting effects. The release may be local so that only nearby neurons or effectors are influenced, or may be  more widespread, which means that the distinction with a neurohormone can become very blurred.

A neurohormone is a messenger that is released by neurons into the haemolymph and which may therefore exert its effects on distant peripheral targets. It may differ only in degree from a neuromodulator in the extent of its action.

Class	Putative or known transmitter	Location	Major functions and actions
Excitatory amino Acids	Glutamate	Interneurones at all levels	Excitation results from depolarization produced by increased sodium  (and other cation) conductance
	Aspartate	Interneurones at all levels
Inhibitory amino acids	Glycine	Spinal interneurones	Inhibition results from hyperpolarization produced by increased chloride conductance
	GABA	Supraspinal interneurones, glial cells
Quaternary choline ester	Acetyl choline	Cholinergic nerves present at all levels of brain and spinal cord, e.g. motoneurones	As in the peripheral nervous system excitation results from depolarization of postsynaptic membranes. Postsynaptic receptors may be muscarinic or nicotinic. Functions include motor control of skeletal muscle, arousal and learning
Monoamines	Noradrenaline	All levels	As in the peripheral nervous system the released transmitter may act on post­synaptic a- or b-receptors. Functions include the control of blood pressure, mood, arousal, locomotor activity and body temperature
	Adrenaline	Midbrain and brainstem to diencephalon	Roles poorly defined
	Dopamine	All levels, short, medium and long connections	Released transmitter stimulates postsynaptic receptors which have been classified into subtypes, DA1 DA2 etc. Specific functions include the regulation of behaviour, motor activity, prolactin secretion and vomiting
	5-Hydroxytryptamine (5-HT)	Midbrain and pons to all levels, pineal gland	Involved in the onset of sleep, sensory perception, temperature regulation and behaviour

CNS NEUROTRANSMITTERS

Neurotransmitter Classes

•      Monoamines – NE, Epinephrine, Dopamine, serotonin, Histamine

•      Amino acids – glycine, GABA, glutamate, Aspartate

•       Quarternry choline esters-Acetylcholine

•      Neuropeptide Transmitters – Enkephalins, Substance –P,VIP,  gastrin, CCK,Insulin, Glucagon,

•      Pitutary peptides- endorphins,  ά-MSH,LH,GH,ADH, oxytocin

•      Hypothalamic releasing factors

•      Other peptides- Angiotensin, Bradykinins.

•      Gases – nitric oxide,CO

I. Some (like glutamate) are excitatory, whereas others (like GABA) are primarily inhibitory.

II. In many cases (as with dopamine) it is the receptor which determines whether the transmitter is excitatory or inhibitory.

Transmission in the CNS

Processes involved in synaptic transmission in the CNS are similar to those operating peripherally.

            For convenience, transmission can be considered in five steps; these are important pharmacologically since drugs may act to enhance or depress transmission at one or more of the steps.

•      The initial event is the nerve action potential;

•      on arrival at the synaptic junction it leads to transmitter release;

•      the transmitter acts on postsynaptic receptors (and possibly presynaptic receptors also);

•      transmitter-receptor interaction leads either to altered intracellular meta­bolism, or to a sudden change in ionic permeability and flux;

•      and the transmitter is finally inactivated by uptake and/or degradative mechanisms.

Transmission in the CNS occurs in the following general ways:

•      Release of excitatory transmitter by neurone 1, causing depolarization of postsynaptic membrane of neurone 2 and conduction of nerve impulse by neurone 2 (postsynaptic excitation);

•      release of inhibitory transmitter by neurone 3, causing hyperpolarization of postsynaptic membrane by neurone 2 and block of conduction of nerve impulse by neurone 2 (postsynaptic inhibition); or

•      release of inhibitory transmitter by neurone 4, causing partial, small, long-lasting depolarization of nerve endings of an excitatory neurone (1). This in turn reduces the quantity of excitatory transmitter released on to the postsynaptic membrane of neurone 2 (presynaptic inhibition). Arrows indicate direction of impulse conduction.

•      The events following transmitter-receptor inter­action are similar for central neurones to those described for peripheral nerves. Either ionic fluxes, through channels in the neuronal membrane, lead to depolarization (e.g. nicotinic cholinoceptors, α-­adrenoceptors) or hyperpolarization (e.g. glycine and g-aminobutyric acid (GABA) receptors) or through enzymatic action secondary messengers are generated intracellularly (e.g. 3'5'-c-AMP for β-adrenoceptors and dopamine DA₁ receptors and 3'5'-c-GMP for muscarinic receptors).

Types of Ion Channels

Voltage-gated ion channels – open or close in response to changes in membrane potential (action potential)

Ligand-gated ion channels – open or close in response to a ligand (transmitter) binding to a receptor

–     Receptors on channels

–     Receptors that open ion channels via G proteins

Mechanically-gated ion channels

Types of Ligand-Gated Channels

Direct transmitter mediated

G protein / 2^nd messenger mediated

Neurotransmitters can activate excitatory or inhibitory pathways

•      Excitatory pathways depolarize postsynaptic membranes, mostly by opening Na⁺ and K⁺ channels

•      Inhibitory pathways hyperpolarize post-synaptic, membranes mostly by opening Cl^- channels

•      Drug selectivity comes from the fact that different pathways utilize different transmitters

Transmitter-affecting drugs act on either presynaptic or postsynaptic targets

•      Presynaptic drugs affect synthesis, storage, metabolism, uptake and release of neurotransmitters

•      Postsynaptic drugs affect transmitter receptors as agonists or antagonists

•      Ethanol and general anesthetics act on membrane lipids and proteins to produce effects

Amino Acid Transmitters

•      Inhibitory:

–     Glycine

–     GABA (g-amino butyric acid)

•      Excitatory:

–     Glutamate

–     (Aspartate)

GLYCINE

•      The simplest amino acid.

•      consisting of an amino group and a carboxyl (acidic) group attached to a carbon atom.

•      The glycine receptor, or GlyR, is the receptor for the amino acid neurotransmitter glycine.

•      It is one of the most widely distributed inhibitory receptors in the central nervous system and has important roles in a variety of physiological processes,

•      especially in mediating inhibitory neurotransmission in the spinal cord and brain stem.

•      Receptor linked to Cl^- channel. When released, glycine binds to a receptor resulting in opening of Cl^- ion channel. This hyperpolarizes the postsynaptic membrane.

•      Thus, glycine is an inhibitory transmitter.

•      The glycine receptor is primarily found in the ventral spinal cord.

•      It is inactivated by active transport (reuptake) back into the presynaptic membrane.

•      Receptor has 5 subunits: a₁, a₂, a₃, a₄, b

•      Agonists include

-          barbiturates and propofol

•      Antagonists include

-          strychnine, Tetanus toxin

•      The poison Strychnine is a glycine antagonist which can bind to the glycine receptor (i.e., it inhibits inhibition). This results in spinal hyperexcitability (causes convulsions)

•      Strychnine causes violent tetanic convulsions in which the body is arched and the head bent backward. After a minute the muscles relax. A touch, a noise or some other stimulus causes the convulsions to recur;

•      Activation and inactivation of the receptor can be

-          activated by a range of simple amino acids including  glycine, β-alanine and taurine,

-          Can be selectively blocked by the high-affinity competitive antagonist strychnine.

•      There are presently four known isoforms of the α-subunit (α1-4) of GlyR that are essential to bind ligands (GLRA1, GLRA2, GLRA3, GLRA4) and a single β-subunit (GLRB).

GAMMA AMINO BUTYRIC ACID (GABA)

•      GABA is the major inhibitory neurotransmitter of the brain.

•      It is most highly concentrated in the substantianigra & globus pallidus nuclei of the basal ganglia.

•      Like glycine, the GABA-A receptor is connected to a chloride ion channel. This hyperpolarizes the postsynaptic membrane.

•      GABA-B receptor is a G-protein coupled receptor.

•      Three receptor classes: GABA_A, GABA_B, GABA_C

•      GABA_A receptors composed of 5 subunits taken from a, b, g, d, e, and p – allows receptor selectivity & binding sites for ETOH, BZs, barbs, steroids

•      Brain GABA_A receptors are aabbg

•      GABA_B receptors are G-protein coupled, inhibit Ca²⁺ channels

•      GABA_C receptors are on Cl^- channels like the GABA_A receptors

•                                GABA receptors: Classification

•      GABA receptors are trans-membrane proteins that can classified into three  major subtypes: GABA_A, GABA_B  and GABA_C  receptors.

•      GABA_A receptors consist of five subunits that co-assemble to form an integral, ionotropic chloride channel. They belong to a superfamily of ligand-gated ion channels, modulate chloride anion permeability, and are responsible for most of the inhibitory neurotransmission in the CNS.

•      The GABA_B receptors, in contrast, is a single, G_i protein–coupled, 7-transmembrane, metabotropic receptor.

•      GABA_C receptors are ligand-gated chloride channels, with functional and pharmacological properties distinct from GABA_A and GABA_B receptors.

•      Benzodiazepines bind to the GABA_A (but not GABA_B or GABA_C) receptor  complex, at specific sites (distinct from the GABA-binding site) to enhance neuronal permeability to chloride anion and subsequent hyperpolarization.

•       Certain Benzodiazepine receptors are found in the CNS and in peripheral tissues (peripheral benzodiazepine receptors); their functions are distinct from those of  GABA receptors.

GABA_A receptor

ligand-gated ion channels responsible for mediating the effects GABA

In addition to the GABA binding site, the GABA_A receptor complex appears to have distinct allosteric binding sites for

•      Benzodiazepines

•      Barbiturates

•      Ethanol

•       inhaled anaesthetics

•      furosemide,

•      Kavalactones

neuroactive steroids and picrotoxin

GABA_A receptor ligands: Functions

Agonist effects (including anxiolysis, sedation, and hypnosis)--mediated by agonists, which

- bind to the GABA_A receptor complex at sites distinct from the GABA-binding site;  

-           act only as positive modulators of the receptor (i.e., they do not enhance neuronal

  chloride permeability in the absence of GABA); however, some agonists (e.g., certain

  barbiturates), at high concentrations, can bind to the GABA_A receptor complex and enhance 

  Cl^- permeability in the absence of GABA.

• Inverse agonist effects are mediated by inverse agonists, which

              - bind at the benzodiazepine site;

              - exert effects opposite to those of full agonists (as negative allosteric modulators);

Certain synthetic benzodiazepine-related compounds and some endogenous compounds (including β-carbolines) act as inverse agonists: they antagonize GABA-mediated chloride conductance and induce biological effects opposite those by benzodiazepines, including convulsions, and anxiety. Inverse agonists have intrinsic activity, whereas benzodiazepine antagonists do not.

• Benzodiazepine receptor antagonists  (e.g. flumazenil):

       - compete directly with benzodiazepines for biding to the GABA_A complex

       - lack intrinsic activity.

       - are useful in antidotal therapy (to reverse overdose effects of benzodiazepines);

       - do not antagonize the actions of  non-benzodiazepine agonists (e.g. ethanol, barbiturates).

Flumazenil antagonizes the effects (both electrophysiological and behavioral ) of agonists or inverse-agonists. It  does not antagonize the effects of barbiturates and other non-benzodiazepine CNS depressants.

Functional Diversity of the GABA_A Receptor Subunits

Studies (largely in knockout animals—with specific subunit deletions or animals with variant alleles of specific subunits) indicate (strongly suggest) functional specificity of different GABA_A subunits:

● α1 subunit-containing GABA_A receptors:  sedation

● α2 subunit-: anxiolysis.

● α3 subunit-: processing of sensory motor information related to a schizophrenia endophenotype.

● α4 subunit-: sedative, hypnotic and anesthetic effects of some agents in the thalamus.

● α5 subunit- (extrasynaptic): associative temporal and spatial  memory by inhibitory modulation of activities in the hippocampus.

● β3 subunit-: sedation, hypnosis and anesthesia by, e.g., pentobarbital, propofol and etomidate, but not by the neurosteroidal anesthetic alphaxalone).

Note: Due to overlaps, compound specificity, and redundancy with respect to subunit functions, no clear delineation may be drawn from a single subunit to a single pharmacological effect.

AGONIST

benzodiazepines (increase pore opening frequency; often the active ingredient of sleep pills and anxiety medications)

 barbiturates (increase pore opening duration; used as sedatives)

v   kavalactones (psychoactive compounds found in the roots of the kava plant)[

certain steroids, called neuroactive steroids[20]

Muscimol and gaboxadol (bind as agonists to the same site as GABA itself, used to distinguish GABA_A from GABA_B receptors.)

Antagonists

•      picrotoxin (non-competitive; binds the channel pore, effectively blocking any ions from moving through it)

•      bicuculline (competitive; transiently occupies the GABA binding site, thus preventing GABA from activating the receptor)

•      cicutoxin and oenanthotoxin,

•      poisons found in certain Northern Hemisphere plants that grow in boggy soils.

•      flumazenil (Pharmacological receptor antagonist)

•      which is used medically to reverse excessive effects of the benzodiazepines.

GABAB receptors (GABABR) are metabotropic transmembrane receptors for gamma-aminobutyric acid (GABA) that are linked via G-proteins to potassium channels.[1] These receptors are found in the central and peripheral autonomic nervous system

Functions

•      They can stimulate the opening of K+ channels which brings the neuron closer to the equilibrium potential of K+, hyperpolarising the neuron. This prevents sodium channels from opening, action potentials from firing, and VDCCs from opening, and so stops neurotransmitter release. Thus GABAB receptors are considered inhibitory receptors.

•      GABA_B receptors can also reduce the activity of adenylyl cyclase and decrease the cell’s conductance to Ca2+.

•      GABA_B receptors are involved in behavioral actions of ethanol, gamma-Hydroxybutyric acid (GHB), and possibly in pain. Recent research suggests that these receptors may play an important developmental role.

Agonists

•      GABA

•      Baclofen is a GABA analogue which acts as a selective agonist of GABA_B receptors, and is used as a muscle relaxant. However, it can aggravate absence seizures, and so is not used in epilepsy.

•      gamma-Hydroxybutyrate

•      Phenibut

•      CGP-35024: 3-Aminopropyl(methyl)phosphinic acid, CAS# 127729-35-5, 10x more potent than baclofen as GABAB agonist, but also GABAC antagonist

Antagonists

•      Saclofen

•      Phaclofen

•      SCH-50911

•      CGP-52432: 3-([(3,4-Dichlorophenyl)methyl]amino]propyl) diethoxymethyl)phosphinic acid, CAS# 139667-74-6

•      CGP-55845: (2S)-3-([(1S)-1-(3,4-Dichlorophenyl)ethyl]amino-2-hydroxypropyl) (phenylmethyl) phosphinic acid, CAS# 149184-22-5

•      Activation of GABA and GABA receptor is enhanced by agonists: diazepam, zolazepam, midazolam

•      Used for hypnotic, sedative, anti-convulsant, muscle relaxant, and pre-anesthetic effects

•      Antagonist - Flumazenil can reverse BZ facilitative effects