Download - Pharmacology of central nervous system (3)
By
Dr. Nehal Aly Afifi Professor of Pharmacology
Faculty of Vet. Med.
Cairo university
Pharmacology of Central
Nervous System
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CNS acting drugs have a major therapeutic & clinical importance
Drugs can produce diverse pharmacological &
psychological effects such as:
• Induction of Anesthesia
• Relief of Pain
• Prevention of Epileptic seizures
• Reduction of Anxiety
• Treatment of Depression
• TTT. of Alzheimer's disease & Parkinsonism
CNS Drugs : Sedatives, Tranquilizers, Hypnotics, Anesthetics,
Anticonvulsants , Analgesics, Psychotropic agents, Behavioral
modifing drugs
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Structure of CNS
1- Brain 2- Spinal cord
The Brain formed of 3 main parts:
ForebrainI. The
• Cerebrum
• Thalamus
• Hypothalamus
MidbrainII. The
HindbrainIII. The
• Cerebellum
• Pons
• Medulla oblongata
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Cerebrum: Largest part of brain, Surrounded by outer cortex
Cerebral cortex divided into 3 functional areas:
1. Sensory area (sensation)
2. Motor area (voluntary
movements) → ↑physical &
mental activities,↑↑ Convulsions
: Association area. 3
- Higher mental activities as
consciousness, thinking,
memory, behavior.
- ill developed In animal
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Thalamus : act as a relay station for
receiving N. impulses & sending to
sensory areas in C.C. Contain pain cent.
The hypothalamus:
Act as a control center for the A N S.
Contain heat regulating, appetite Cs.
The mid-brain : “bridge connect
cerebrum to cerebellum & Pons.
Cerebellum: Resp. for balance of
body & maintaining appropriate body
posture & equilibrium.
The medulla oblongata (M.O.) :
for passage of im. bet brain & SC
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Medulla oblongata contain 5 vital Cs.[ Vagal, Vasomotor,
Respiratory, Cough, Vomiting(chemoreceptor trigger zone (CTZ)
II- Spinal cord
A cylindrical mass of nerves extends from
the end of M.O. to the lower lumbar vertebrae
Impulses flow from and to brain through
descending & ascending tracts of spinal cord.
Contain sex centers (Erection & ejaculation)
+ sweat & micturation Cs.
Acts as a reflex station for receiving
impulses from and to all parts of body.
Pharmacology of Synaptic transmission
Receptor events
Propagation of action potentials
Calcium entry
transmitter release
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How brain cells communicate?
Neurotransmitter (NT): A chemical released by a
terminal nerve; has an excitatory or inhibitory effect on another
neuron.
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Transmission in CNS
A nerve impulse (electric current) passé along axon to
presynaptic membrane.
Release neurotransmitter into synaptic cleft.
NT interacts with receptors on effector cells to induce response.
NT released in response to action potentials is voltage dependent
& require calcium influx (Neuroregulators).
Transmission in C.N.S. occurs in 2 ways:
A- Release of Excitatory transmitter by Neuron
(1) Cause depolarization of postsynaptic membrane of neuron
(2) Cause Conduction of N impulse [postsynaptic excitation].
B- Release of Inhibitory Transmitter by Neuron
(!) Cause Hyper polarization of postsynaptic membrane .
(2) Block conduction of nerve impulse [postsynaptic inhibition].
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Neurotransmitters can be classified into:
1. Amino acids:
Excitatory A.A. : Glutamate & Aspartate.
• Location: interneuron at all brain , spinal cord
• Act on glutamate & aspartate subtype- receptor.
• Excitation due to depolarization
• 2. Inhibitory A.A. : GABA & Glycine.
Location: Spinal & supraspinal interneurons
Inhibition results from hyperpolarization.
3. Monoamines : Noradrenaline , Adrenaline , Dopamine,
5-HT
4- Choline ester: Acetylcholine
Amino acids & Amine Transmitters in CNS
Major Excitatory amino acids transmitters
Glutamate & aspartate exert an extremely powerful
excitatory effect on neurons in every region of CNS.
Excitation due to depolarization of postsynaptic mm .
By acting on glutamate & aspartate subtype receptors
Location: Interneuron at all levels (spinal & brain)
Glutamate antagonist (e.g., Ketamine ) hastherapeutic
role in treatment of “epilepsy” & induction of anaesithea
Glutamate and Aspartate
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Gamma amino butyric acid (GABA)
Major inhibitory transmitter at all levels of brain
GABA is a ligand gated cl- ion channel.
Stimulation of GABA receptors cause change in
postsynaptic membrane permeability to become
permeable to chloride (cl-) results in hyperpolarization of
the receptive neuron.
Benzodiazepines, Barbiturates, Sod. Valporate ,
Ivermectin increase binding GABA to Rs. (GABA Agonist)
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Picrotoxin: antagonize GABA by closing cl ion channel
inhibit presynaptic & postsynbtic inhibition of GABA causing
stimulant effect
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Acetylcholine ( Ach)
Ach is widely distributed in major parts of CNS
Both types of Cholinergic receptors (nicotinic & muscarinic)
occur in CNS
Muscarinic receptors (M1) are much more abundant than
nicotinic Rs. & mediate behavioral effects of Ach .
Ach in the brain plays a role in
1- Arousal. 2- Learning.
3- Short term memory. 4- Motor control.
Loss of cholinergic Ns. in memory site cause "Alzheimer’s dis
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Noradrenaline (NA) & Dopamine(DA)
NA found in all levels of the brain .
NA play important role in the regulation of :
1- Mood: Deficiency of NA cause depression.
2- Arousal: Increase release NA → wakefulness ,alertness
3- Bl. pressure regulation.
Dopamine acts as a “neurotransmitter” on dopaminergic Rs.
Dopamine play role in
1- motor activity: dopamine Deficiency→ Parkinson’s dis.
2- Behavioural effects: Control of behavior & emotion.
3- Vomiting: by stimulating dopamine receptors in CTZ in MO
Metoclopramide , phenothiazines are antiemetics drugs.
(dopaminergic antagonists)
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5-Hydroxytryptamine (5-HT) & Histamine
5-HT is an important CNS transmitter
5-HT in CNS involved in behavioral changes, mood,
hallucinations, sleep, and wakefulness.
5-HT plays a role in
1- Mood: ↑ 5-HT levels by giving tryptophan useful in
depressive states
2- Sensory transmission: 5-HT inhibits transmission of
pain impulses in SC. & brain and enhances morphine
analgesia.
3- Temperature control: hypothalamus rich in 5-HT
4- Vomiting: 5-HT3 blocker used as anti-emetic.
Histamine:
Blocking central H1- receptors have Sedative, Antiemetic effet
Drugs acting on CNS
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• Cerebral stimulants
• Medullary stimulants
• Spinal cord Stimulants
CNS
Stimulants
• Nerve Sedatives
• Tranquilizers
• Hypnotics
• Anticonvulsants
• Anaesthetics
• Analgesics
CNS Depressants
Cerebral stimulants
(Cerebral -cortex stimulants)
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Xanthines (Caffeine, Theophylline & Theobromine),
Amphetamine & Retaline.
Stimtimulate motor & sensory area so physical & mental a
Wakefulness, refreshment, fatigue.
Awake animals from anesthesia in Vet. Med.
Caffeine (coffee seeds), theophylline (tea leaves)
Theobromine (coca seeds) & methylated Xanthine.
MOA: Caffeine Inhibit Phosphodiesterase enz. so c-AMP
in cerebral cortex neurons.
Pharmacological Effects
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19 1- Smooth muscle relaxant (Spasmolytic) Aminophylline
2- Cardiac stimulant:↑ force of contraction (+ve inotropic)
↑ Heart rate (+ve chronotropic) → increase BL. Pressure
Coronary Vasodilatation & vagal stim.→ decres BL. pressur
Both effects lead to slight rise in BL. Pressure
Caffeine is contraindicated in hypertensive patients.
3- Secondary diuretic:
Increase renal Bl. Flow due to cardiac stimulant effect.
Renal vasodilatation increase glomerular filtration.
Inhibit release ADH decrease water reabsorption.
Decrease Na tubular reabsorption.
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4- GIT: caffeine increase gastric secretions improve
digestion.
Therapeutic uses:
Antagonize cortex depression & awake animal from
anesthesia.
Caffeine combined with Aspirin or Paracetamol for
treatment of Headache
For Oedema
Aminophylline used for bronchial asthma
For ttt billiary, renal & intestinal colic (spasmolytic).
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Differences Caffeine Theophylline Theobromine
Plant origin Coffee Tea Coca
Cerebral stimulant powerful Moderate Weak
Cardiac stimulant Weak Powerful Moderate
Diuretic Weak Powerful Moderate
Smooth ms. Relaxant Weak Powerful Moderate
Amphetamine (Benzedrine):
Powerful CNS stimulant; mental & muscular activities.
Non catecholamine, stimulate & receptors by inhibit MAO enz.
Increase basal metabolic rate for treatment of obesity.
Therapeutic uses: Illegally as a motor activator for race horse (doping).
For ttt. of CNS depression in barbiturate & narcotic poisoning.
Medullary stimulants
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Direct medullary stimulants
Picrotoxine
Nikethamide
Cardiazol
Bemegride
Doxapram
Reflex medullary stimulants
Camphor
Ammonia
Sol.
Medullary stimulants (Analeptics)
Drugs stimulate centers of M.O. especially depressed resp. C.
Awake anaesthetized animal by stim.brain & M.O. centers.
Picrotoxine:
Powerful stimulant on resp. & vasomotor centers.
Raised blood pressure & respiration.
MOA: 1- Antagonize GABA by closing cl¯ channel inhibit
pre- and postsynaptic inhibition caused by GABA.
2- Stim. chemo receptors at aortic sius in heart → stim. heart
For ttt. barbiturate poisoning (given i.v. or i.m)
Nikethamide (coramine): Given orally - has no effect on heart
Less effective & Less toxic than picrotoxine.
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Cardiazol (Leptazol):
Stim. resp. & vasomotor centers no effect on heart.
For ttt anesthesia & narcotic poisoning ( i.v.). L.D.→ convulsions
Bemegride :
Powerful respiratory stimulant in barbiturate poisoning.
MOA: Competitive antagonism with barbiturate due to chemical
similarity (given i.v.)
Doxapram:
The most powerful analeptic - has wide safety margin.
Act both centrally & peripherally.
Camphor & Ammonia solution (Reflex analeptic)
Camphor s/c → irritate cut. S. Ns. → reflexly stim. M.O. respt. C.
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Spinal cord Stimulants
Strychnine: Alkaloid from strychnus nux vomica seeds.
MOA:1- inhibit inhibitory transmitter glycine at post-synaptic N.E.
so increase reflex excitability to external stimuli.
2-On brain St. Sensory area of cerebral cortex sharp sensation
Large dose convulsions of sk.ms & diaphragm.
Aphrodisiac (for ttt sexual impotency in dogs).
General tonic & stomachic (for anorexia, weak depilating A.)
Neuromuscular purgative(stim. aurbach’s plexus of intestine)
Ruminal tonic ( For ruminal atony)
Used as Motor activator for race horses (doping) to increase
capacity for running
Strychnine Toxicity
Toxicity due to large toxic dose or Cumulative effect
Toxic symptoms : convulsions in all sk.ms & diaphragm.
Poisoned dog show ch. posture [ back become arched, limbs become
rigid & extended, head raised upward & backward & tail raised upward]
Convulsions interrupted by period of rest or relaxation( Tonic C . )
Death due to asphyxia as a result of prolonged contraction diaphragm m
Treatment: Dark Place – Artificial Respiration- Volatile Anesthetic
Specific Antidote: chloral hydrate (For horse). Barbiturates or
Bromides (dogs). Mephenesin(For Man , central sk. ms.relaxant).
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CNS Depressants
Classified according to the degree of depression
Tranquilizers Sedatives Sedative
Hypnotics
Anesthetics Anticonvulsants
Analgesics
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Sedatives and Tranquilizers
Tranquilizer :
Tranquilizer Reduce anxiety & induce calming without drowsiness
or loss of consciousness even with large dose .
Sedatives:
Drug-induced sedation has a more profound effect, produce
drowsiness and hypnosis.
Nerve Sedative: depress CNS, produce sedation, drowsiness,
unaware to surroundings , decrease loco motor activity and
reduce fear but animal remain conscious.
The large dose induces loss of consciousness & hypnosis so
called sedative hypnotics.
Drug cannot induce only one pharmacologic effect & Many
drugs cannot be categorized by only one pharmacologic
effect, i.e., as tranquilizers, sedatives, or analgesics.
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Tranquilizer relieve anxiety & mental tension in man.
In animals, quieting & calming effect and reduce fear
Phenothiazines in vet. Med. science 1950.as tranquilizers,
& as antipsychotic drugs.
The effect of Phenothiazine on CNS is called Neuroleptic .
Acepromazine maleate – Chlorpromazine hydrochloride
Promazine hydrochloride.
MOA: 1-Phenothiazine inhibit central dopamenergic
receptors (D2) which produce sedation & tranquilization.
2-Peripherally, Ph. block Nor epinephrine at alpha adrenergic Rs.
Indications:
Phenothiazine commonly administered for general sedation.
2- Used peri-aneathetically to reduce anxiety, reduce drugs doses for induction & maintenance.
Tranquilizers : 1- Phenothiazine derivatives
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Phenothiazines provide good muscle relaxation → used in
conjunction with anesthetics (e.g., Ketamine)
Ph. are antiemetic from block of dopamine Rs. in the CTZ within
the medulla.
Phenothiazines have little or No analgesic activity.
Used to treat psychiatric problems in people & animals
Other pharmacological effects of tranquilizers:
1- Anti-emetic 2 – Antihistaminic. 3- Hypotensive
4- Antispasmodic. 5- Sympatholytic.
Examples of tranquilizer: Phenothiazines , & azaperone.
Alpha–2-adrenergic Agonists: [Xylazine - Detomidine -
Medetomidine ,& Romifidine]
Produce profound sedation by activation of ∂-2-adrenergic Rs.
located in brainstem.
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Ruminants are the most sensitive spp. to sedative effect of ∂-2-
adrenergic agonists.
The duration of sedation & level of analgesia are dose- dependent
MOA: Alpha-2-adrenergic agonists produce activation of
central ∂-2-adrenergic Rs→ reduce sympathetic outflow
(decreased nor epinephrine)& so increase parasympathetic t
Alpha-2-agonists produce Musculoskeletal relaxation.
produce profound analgesia & administered parentally.
Vet. Products:
Xylazine (Rompun®) Injectable for use in cattle, dogs, cats,
horse, deers.
Detomidine: approved by FDA for use in horse.
Medetomidine & Dexmedetomidine: for use in dogs.
Romifidine: approved by FDA for use in horse
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Xylazine (Rompun)®
Act as sedative hypnotic & possesse Analgesic &
skeletal muscle relaxant effects
soluble in water and given by i.m. or i.v. injection to
domestic animals.
Xylazine is 2- adrenoceptor agonist
MOA: by stimulation of 2- receptors in sympathetic ns
Ruminants most respond to xylazine
Low dose sedation, high doses recumbence
Clinical Uses: Given by i.m. or i.v.
To Calm vicious animals before clinical examination
Preanesthetic medication.
For treatment of Spasmodic colic
Treatment of s convulsions as anticonvulsant.
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Bromides (potassium or sodium salts)
Pot. bromide act as a Nerve sedative & Anticonvulsant.
MOA: Bromide stabilize neuronal cell membranes by interfering
with chloride transport across cell mm & by potentiating the effect
of GABA via hyperpolarizing mm.(replacement with cl¯ in
extracellular fluid of N.C , so decrease cl¯ conc. causing sedation.
Bromides depress only motor area of cerebral cortex.
Bromide used as a first-choice AED in dogs with epilepsy
The elimination half-life is long (24 days) in dogs.
Bromide is really eliminated →Not used in renal disease
Bromide Not undergo hepatic metabolism so useful in dogs
with liver disease.
Br. formulated into a sol. of various conc. or into tablets .
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Chloral hydrate is a white crystalline powder soluble in water
Act as Nerve sedative at small doses
Hypnotic & General anesthetic at L.D. given mainly for horse by
i.v. injection.
MOA: in body Chloral hydrate reduced mainly in liver into
trichloroethanol → a direct depressant effect on CNS.
Disadvantages as a general anesthetic :
Narrow safety margin.
Must be administered in Large volume of freshly prepared sol.
Induction of anesthesia is slow & prolonged recovery period
Has irritant effect & can cause tissue necrosis.
Overdose cause respiratory failure & hypotension ( due to Chloral
hydrate cause depression of respiratory center & heart)
Chloral hydrate Lake analgesic effect .
Chloral hydrate
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Hypnotics produce normal sleep & deep sleep
( narcosis) or general anesthesia at L. D.
chloral hydrate
2 – adrenoreceptor
agonist
Barbiturates. Benzodiazepines
Chloral
hydrate Barbiturates
Benzodiazepine
- Diazepam
- Zolazepam
Sedative
hypnotics
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Barbiturates Derivative of Barbituric acid ,given as Sod. salt soluble in water. Barbiturates → gradual degree of CNS depression from Sedative
Hypnotic to General anesthetic
Low dose → a sedative action causing a reduction in
restlessness and tension.
A larger dose of Barbiturates → a hypnotic sleep inducing effect.
A further increased dosage → anesthesia
Good skeletal muscle relaxant, but lack analgesic effect.
MOA: act by interfering with ATPase in neurons of cerebral
cortex so prevent utilization of ATP → CNS depression
Highly irritant sol→ given i.v.
Classification.: according to period of sleep
Long acting barbiturates: Phenobarbital sod. 6-8hs.
Short acting : Pentobarbital sod. (Nembutal)® 2-4hrs.
Ultra short : Thiopental (Nesdonal)® ¼ -2 hrs.
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Sedative hypnotic Tranquilizer
Effect of high
doses
- Loss consciousness
- Anesthesia
- Resp. depression
- Conscious.
- only immobility
-NO resp.dep
Sensory
Reflexes
- Depressed - Normal
Effect on emesis - No effect -Antiemetic effect
(inhibition of CTZ)
Anticonvulsant
Effect
- General skeletal
muscle relaxants
Limited & specific
Preanaesthetic
Effect
-Duration not affected
- decrease the dose
Prolong duration
of anesthesia
Difference between hypnotics & tranquilizers
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Drugs for Treatment
Diazepam
Phenobarbital
Sodium IV
Propofol
Phenytoin
Drugs for Maintenance
Phenobarbital
PO
Bromide (potassium salt)
Valproic acid
Anticonvulsant Drugs
Anticonvulsants [Antiepileptic Drugs (AEDs]
AEDs used to stop an ongoing seizure or to decrease the
frequency or severity of anticipated future seizures.
Convulsions: result by stimulation of spinal cord or motor area of
cerebral cortex.
Epilepsy: acute prolonged recurrent seizures.
In status epileptic treatment is essential to prevent death from
hyperthermia, acidosis & hypoxia.
During a seizure', the route of administration for AEDs is IV .
Drugs Used for Treatment of Status Epilepticus:
1-Benzodiazepines [diazepam, clonazepam ,Midazolam ]
Diazepam (Valium®):Drug of choice to treat epileptics in small &
large animals[rapid onset of action to prevent spread of seizure]
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Diazepam is the most common benzodiazepine used in dogs and
cats to reduce motor activity .
Diazepam is used as IV bolus, the dose can be repeated 2–3
times at intervals of 5–10 min.
Diazepam is used IV for seizures in adult horses .
In foals, , slowly IV higher doses can be fatal to neonates. Clonazepam , Lorazepam , Midazolam can used in dogs.
MOA: Bz. binding to & activate (BZ)- receptor which located on
the GABA receptor subtype A (GABAA). This increase frequency of
opening of chloride ion channel leading to hyper polarization of post
synaptic neuron.
Indications: Bz. used in Vet. Med. 1- as anticonvulscent,
2-adjuncts to anesthetic agents, 3-skeletal muscle relaxants,
4- for behavioral modifying.
2- Phenobarbital : A long acting Barbiturates used for TTT. of
epileptics to prevent further convulsions.
A loading slow IV or IM, dose used.
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Phenobarbital has a long safety, efficacy, low cost .
• primarily use as anticonvulsant for long term sedation.
3- Propofol : Short-acting IV hypnotic anesthetic agent +
Anticonvulsant [ GABA-agonists, stabilizing GABA-inhibitory NT ]
• After IV inj, propofol rapidly cross BBB → onset of action<1 min.
• The duration of action after a single bolus is only ~2–5min.
• 4- Sod. Pentobarbital: for TTT of uncontrollable epilepticus in
dogs , cats, especially when diazepam & phenobarbital failed.
• Pentobarbital administered to effect for anesthesia.
• 5- Phenytoin: still used as a slow IV infusion in dogs to stop
seizure but has undesirable pharmacokinetics
• Too rapid metabolism reduces its effectiveness in dogs
•Too slow metabolism in cat→ increase toxicity (salivation,
vomiting, weight loss ).
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Maintenance Anticonvulsants
Both Phenobarbital & bromide are considered first-line
maintenance anticonvulsants in dogs, cats, horses,ruminants.
Gabapentin: Synthetic analogue of (GABA) inhibit seizures.
MOA: inhibition of neuronal sodium channels + potentiating of the
release & action of GABA neurotransmitter.
well absorbed after oral administration.
Valproic Acid:
Used as adjunct to phenobarbital in dogs with refractory seizures.
Also used to treat aggressive behavior problems (Psychotropic )
Clonazepam : used for oral maintenance therapy because
anticonvulsant tolerance develops less rapidly, its metabolism
reduce the elimination rate & it is more highly absorbed orally .
Carbamazepine: Not used in dogs[ rapid elimination, short half-
life] but used to treat aggressive behavior problems in cats.
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Psychotropic Drugs
Anxiolytics, Antipsychotics, Antidepressants, Mood stabilizers
Used to treat human behavioral disorders & used more
commonly in Vet. Med. for behavioral modification therapy.
Anxiolytics مضادات القلق used to treat generalized anxiety &
panic disorder in dogs, cats, as well as urine spraying in cats.
Benzodiazepines ( diazepam, oxazepam, clorazepate) act
by binding to (GABA) Rs. enhancing GABA-mediated cl influx.
Cause sedation , muscle relaxation & Anticonvulsant effect.
Azapirone (Buspirone): differ from benzodiazepines in MOA
MOA: Buspirone blocks serotonin pre-and postsynaptically.
Buspirone acts as a dopamine agonist
Has delayed onset of action and lack of sedative effect .
.
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Antipsychotics مضادات الذهان
Classified as low-potency agent (acepromazine, chlorpromazine)
and high-potency agents (haloperidol, fluphenazine,
trifluoperazine ).
All antipsychotics are used for nonselective tranquilization &
diminishing behavioral arousal.
Low-potency agents require larger doses, produce more sedation,
more anticholinergic side effects, & cardiovascular effects.
Acepromazine commonly used for anxietal episodes dogs, cats.
MOOD-STABILIZING Drugs االدوية المثبتة للحالة المزاجية
lithium, carbamazepine, & valproic acid are unrelated
chemical compounds, used in human medicine to treat
bipolar disorder, impulsivity, emotional reactivity, aggression.
Carbamazepine & valproic acid are also antiepileptic.
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Antidepressants المضادة لالكتئاب االدوية
Antidepressants are classified as
1- Tricyclic compounds (Tertiary amines, Secondary amines)
2- Selective serotonin-reuptake inhibitors(e.g., Fluoxetine,
Sertraline, Paroxetine )
3-Other antidepressants( Monoamine oxidase inhibitors, like
Selegiline.
They used to treat behavioral disorders, including obsessive-
compulsive behaviors & aggression
MOA: to block reuptake of serotonin and/or norepinephrine or
to reduce neurotransmitter turnover.