76 Section 2/ Drugs Acting on CNSDelta and kappa receptors can also con-
tribute to analgesia, particularly at spinal
level. Although morphine also acts on kappa
and delta sites but it is not clear that up to
what level they contribute in its analgesic
action.
Pharmacological Actions
Effect on CNS: The main action of mor-
phine is CNS depression which further re-
sults in analgesia, depression of respiratory
centre, cough centre and sleep. In addition
it causes euphoria or dysphoria and depen-
dence.
Analgesia: Morphine produces analge-
sia by elevation of pain threshold, thereby
reducing the perception of pain. It also al-
tered psychic reaction to pain which may be
associated with feeling of well being e.g.
euphoria. It also produces lethargy and
sleep, morphine relieves all types of pain,
but dull constant pain is relieved more ef-
fectively than sharp intermittent pain and
is very effective in visceral pain.
Morphine also produces a sense of anxi-
ety, known as dysphoria. The morphine pro-
duces euphoria (which makes morphine as
one of the main drugs of abuse) and analge-
sia by acting on higher centres and spinal
cord.
On intrathecal injection, it acts on substan-
tia gelatinosa of dorsal horn of spinal cord and
inhibit the release of excitatory transmitters.
At supraspinal sites, it acts on medulla, mid
brain, limbic and cortical areas.
Action on eye: Morphine causes con-
striction of pupil (miosis) due to action on
oculomotor nerve nucleus.
Action on respiration: Morphine de-
presses the medullary respiratory centre in
medulla oblongata and by reducing the sen-
sitivity of the medullary respiratory centre
to increased plasma CO 2. The rise in arterialTable 2.3.1: Classification of narcotic analgesics.
I. Natural alkaloids
Morphine (as sulphate; MORCONTIN) 0.2-0.8 mg/kg BD, IM/SC
Codeine (as phosphate) 20-60 mg/day
II. Semisynthetic compounds
Pholcodeine (ETHNINE) 10-30 mg/day
Diacetyl morphine (HEROIN) 3-5 mg/day IM/SC
Other semisynthetic agents are hydromorphone, oxymorphone, oxycodone, hydrocodone, dihydro-
codeine etc.
III.Synthetic compounds
Pethidine (as hydrochloride) 50-100 mg/day IM/SC
Fentanyl (as citrate; FENDROP) 50-100 μg/day IM/IV
Methadone (PYSEPTONE) 5-10 mg/day oral/SC
Dextropropoxyphene (DEXOVON) 65-130 mg/day
Tramadol (TRANDOL) 50-100 mg BD-TDS
Ethoheptazine (EQUAGESIC) 75-150 mg/day
Newer compounds include sufentanil, alfentanil etc.