SOME EFFECTS OF HISTAMINE AND SEX HORMONES ON HISTAMINERGIC PATHWAYS IN THE RAT AND CHICK BRAIN
SOME EFFECTS OF HISTAMINE AND SEX HORMONES ON HISTAMINERGIC PATHWAYS IN THE RAT AND CHICK BRAIN
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Date
1992-10
Authors
ANUKA, JOSEPH AKPOJO
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Abstract
The behavioural effect of histamine in young
chicks revealed a triphasic response of initial
behavioural sleep which was followed by a longer
behavioural stimulation culminating in a very long
behavioural depression. HI and H2 receptors are
involved in these behavioural changes- The HI receptors
were found to be excitatory while the H2 receptors
were inhibitory. A blockade of the HI receptors
eliminates the excitatory component leaving the
inhibitory component (H2 receptors) unaffected for
endogenous histamine to act. The resulting effect is
inhibition. Likewise a blockade of the inhibitory
component by H2 receptor antagonist leads to
excitation. Chlorpheniramine (an HI receptor
antagonist) induced a biphasic response of
sedation/excitation.
Both HI and H2 receptors are involved in
thermoregulatory effect of histamine in young chicks.
Activation of these two receptors leads to hypothermia
while a blockage of H2 receptors produced
insiginificant increase in temperature. On the other
hand, a blockade of HI receptors leads to
insignificant fall in temperature. Neither Hi nor H2
receptor antagonists counteracted histamine induced
hypothermia but a concurrent administration of HI and (xi)
H2 receptor antagonists. Steroid sex hormones
(oestradiol 17B, testosterone and progesterone)
counteracted hypothermia induced by histamine. They
also counteracted hypothermia induced by HI and H2
receptor agonists, 2-thiazoly1-ethylamine and dimaprit
respectively- Chicks pretreated with hormone showed
fast recovery from hypothermia than the controls.
Antioestrogens (clomiphene and tamoxifen)
potentiated hypothermia induced by histamine and its
selective agonists on H1` and H2 receptors.
The role of histamine on seizure regulation was
investigated in chicks. Histamine offered 85%
protection against leptazol - induced seizure in
young chicks. Both HI and H2 receptor agonists, 2-
thiazoly1-ethylamine and dimaprit respectively also
offered protection against this seizure. Cimetidine
and triprolidine neither protected nor potentiated
this seizure. Histamine, its agonists and
antagonists were not convulsant at the dose level
used in this work but potentiated electroshock
seizures.
Intracerebroventricular (ICV) adminstration of
histamine was found to induce irregularities and at
times persistent oestrous cycle with older rats (xii)
exhibiting more cycling irregularities than the
younger ones. Cimetidine (H2 receptor antagonist)
produced a transient change in the cycle which
returned to normal rhythm after a day or two.
Triprolidine (HI receptor antagonist) showed no
changes in the oestrous rhythm while
chlorpheniramine, another HI receptor antagonist
produced a non-significant degree of changes in the
cycle.
Sex hormones have been found to regulate body
weight in rats through their influence on eating and
drinking behaviours. Histamine-induced drinking
behaviour was enhanced by sex hormones while their
influence on histamine — induced eating behaviour was
found to be variable. In this case, the eating
behaviour was enhanced by testosterone and
progesterone but depressed by oestradiol 17B.
Sex hormones particularly oestrogens affected
the affinity of histamine HI receptors labelled with
mepyramine. The affinity of (3H) mepyramine for
histamine HI receptors increased as the binding
capacity decreased depending on the level of sex
hormones. Ovariectomy decreased the affinity of the
receptors in all the brain areas examined especially (xiiii)
in the oestrogen concentrated regions (hypothalamus
and amygdala). Oestrogen replacement in
ovariectomized females showed binding affinity
reminiscent of proestrus.
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Keywords
SOME EFFECTS OF HISTAMINE AND SEX HORMONES ON, HISTAMINERGIC PATHWAYS IN THE RAT AND CHICK BRAIN