Auto-regulatory Role of Novel Histamine H3 like Receptor (H3R) and Subsequent Modulation of Adrenergic Induced Aggregation in the Pigmentary Responses of Oreochromis mossambicus
Ayesha S. Ali
Sharique A. Ali
Background and purpose: H3 receptor plays an essential role in the integration of histaminergic signal transmission in central and peripheral nervous systems. Recently histaminergic innervation and molecular expression of L-histamine decarboxylase has been described in zebrafish brain. Pre-synaptically H3Rs have been described as auto-receptors, mediating negative feedback of histamine release. Subsequently, it is reported that H3Rs are not just restricted to histaminergic neurons but also serves as peripheral postsynaptic hetero-receptors, regulating the release of other neurotransmitters. Although, histaminergic innervation of zebrafish CNS resembles that of other vertebrates, little is known about their pharmacological profiles and functional implications. Also, peripheral tissue distribution of heterogeneously distributed H3Rs in different mammalian species has been described but there is limited information on the presence of H3Rs in lower vertebrates. Keeping the aforementioned facts we investigated the existence of H3Rs at the neuro-melanophore junctions of teleost fish Oreochromis mossambicus. Understanding the putative role of this monoaminergic system in teleostean pigmentary effector cells; melanophores would be a considerable step to unravel the complex and fragmentary picture of post-synaptic histaminergic system in the process of skin pigmentation and highlight its evolutionary disposition. Results and conclusion: The auto-regulation of histamine release via stimulation of H3Rs in dorsal skin melanophores was studied at the neuro melanophore junction. Histamine emanated a dual response within the cells. At low concentration histamine caused pigment aggregation while high doses resulted in pigment dispersion. Immethridine a specific H3R agonist caused slight pigment granule dispersion. Thioperamide, a specific H3R antagonist blocked the dispersion caused by Immethridine. Compound 48/80 also elicited a significant increase in release of histamine that was significantly inhibited by Immithridine. Thioperamde prevented and effectively antagonized the inhibition caused by Immethridine resulting into pigment aggregation. Yohimbine a specific alpha adrenergic antagonist attenuated the aggregation caused by thioperamide suggesting a subsequent involvement of adrenergic receptors. These data suggest that the melanophores may have histamine H3 receptors and that histamine probably modulates its own release through the stimulation of H3 receptors and subsequently regulates the liberation of adrenaline which via alpha adrenergic receptors induces the pigmentary responses leading to paling of the skin.
Central histaminergic functions are not just confined to brain but have
been extended in the periphery. The presence of multitudinous histaminergic
targets across the periphery makes this system one of the most important regulatory
systems that not just controls the activity of brain but other neurotransmitter
systems. The functions of histamine and its disposition in the CNS have been
well understood in mammals, however the exact target localization of its peripheral
concourse is still not completely known. Also the presence of this monoaminergic
system in lower vertebrates has been hardly explored. Recently it has been reported
that mast cells of most evolutionary advanced fish contains histamine and is
involved in the immunological responses (Mulero et al.,
2007). The primary physiological bearing of histamine as a mediator of immunological
response has been well accepted. However its other subsidiary functions in the
peripheral tissues have been beginning to emerge. One such function is in skin
pigmentation (Salim et al., 2011). Skin pigmentation
in vertebrates is a very perplexed phenomenon governed by an array of signal
transduction pathways. Involvement of histaminergic receptors in the pigmentary
responses of amphibian and piscean pigment cells, the melanophores have been
brought to light by Ali (1983) Ali
et al. (1993, 1998) Peter
et al. (1996), and Salim et al. (2011)
unpublished data. Emergence of histamine as a mediator in skin pigmentation
in vertebrates presents this biogenic amine in a very interesting and novel
The histamine H3-Receptors (H3R) are primarily located
on the pre-synaptic membranes of histaminergic neurons as autoregulators, negatively
regulating the synthesis and release of histamine (Haas
et al., 2008). In addition, H3Rs are also located on non-histaminergic
neurons, acting as hetero-receptors to regulate the release of other neurotransmitters
such as norepinephrine, GABA, dopamine and serotonin (Yoshimoto
et al., 2006). Although the CNS has highest density of H3
receptors, but in situhybridization studies have revealed that these receptors
do exist in dorsal root ganglia, spinal cord and some of the peripheral tissues
including skin (Pollard et al., 1993; Heron
et al., 2001; Pillot et al., 2002).
Interestingly, it has been reported that Zebra fish brain contains a widespread
histaminergic system with the existence of histamine H1, H2
and H3 receptor genes in zebra fish. These histamine receptors resemble
those of higher vertebrates and they provide a useful model for pharmacological
and behavioral studies for characterizing the functions of histamine in more
detail (Peitsaro et al., 2007). Also in addition,
the actions of histamine in the immune cells of most evolutionary advanced fish
has been reported to be mediated through well conserved H1 H2
and H3 receptors (Mulero et al., 2007).
The histamine receptor is fishes have been linked to a variety of neurological
functions such as the control of arousal, attention, sensory processing and
cognition. Histamine also plays a role in pituitary hormone secretion, appetite
control and, potentially, regulation of vestibular reactivity (Choich
et al., 2004), however the peripheral function of histamine apart
from its immunological aspect has still been incognito. In the case of fishes,
only a few investigators have explored the role of histamine in the process
of skin pigmentation. Interestingly, Ali (1983) had
reported that histamine H1 and H2 receptors are present
in the melanophores of teleost Channa punctatus contributing to the substantial
involvement of histaminergic receptors in pigmentary responses of teleosts.
Besides a few reports there has been no studies undertaken so far, with the
revelation of new member receptors to the histamine family. The present study
was conducted to reveal the presence of histamine H3 like receptors
in the periphery at the neuro-melanophore junction and their putative involvement
in the regulation of skin pigmentary responses in an important member of teleosts;
Oreochromis mossambicus. The present finding would abet in bringing out
the disposition of histaminergic system in lower vertebrates and signify in
underscoring their evolutionary implication.
MATERIALS AND METHODS
(Peters) commonly known as Tilapia mossambica
of both sexes, 10-12 cms in length weighing 25-30 g were purchased from
the local commercial sources. The fishes were kept in laboratory aquaria for
2-3 days for acclimatization with 12:12 h of light and dark phase with temperatures
between to 22-25°C. Fishes with signs of disease were discarded and any
chances resulting to cause stress were minimized.
The experiments were conducted on the isolated scale melanophores. The scales
were carefully plucked with the help of fine forceps from the dorso-lateral
region of the fish according to the method described by Spaeth
(1913). Thereafter the scales were immediately immersed into freshly prepared
physiological saline solution (PS) of the following concentration (mM): NaCL,
125.3; KCL, 2.7; CaCL2, 1.8; MgCL2, 1.8; D-glucose, 5.6
After 10 min of equilibration in PS the scales containing approx. 50-100 melanophores were incubated with known concentrations of drugs for 7-10 min. All drugs were dissolved freshly in doubled distilled water and their solutions were added to the Petri-dishes containing the PS, the total volume of which was kept constant (10 mL). For experiments using antagonists scales were first incubated in antagonist solution for 10 min and then treated with agonist for 10 min.
Drugs: Histamine dihydrochloride (>98%) was purchased from Sigma Aldrich (USA), Immethridine, Ranitidine hydrochloride and Thioperamide were generous gifts from Dr. Leurs, Netherlands. Mepyramine maleate (>98%) was a generous gift from Dr. Samreen Arshad, Sanofi- Aventis, Bridgewater, NJ (USA), Yohimbine (>98%) was purchased from Sigma Aldrich USA. Compound 48/80 was purchased from Sigma Aldrich (USA).
Responses of melanophores in terms of mean melanophore size index assay:
The relationship between concentration of an agonist and the magnitude of the
response elicited was studied by exposing the preparation of solutions of various
strengths in cumulative and increasing order. The responses of control as well
as of those melanophores that were incubated in 10 mL PS containing various
concentrations of receptor specific agonists were measured according to the
method by Bhattacharya et al. (1976)
by Ali et al. (1998)
based on Hogben
and Slome (1931)
melanophore index. In this modified method, the individual
melanophore was measured with the help of Leitz ocular micrometer (Erma, Japan)
calibrated by Stage micrometer, by marking the maximum vertical and horizontal
diameters. Ten such randomly selected melanophores from each scale were measured.
When the melanophores disperse i.e., the melanin pigment granules within the
melanophores move to the periphery, the diameter of the cells increases and
vice versa. The calculated value is the mean melanophore size index and expressed
Statistical data analysis: Statistical data analyses are presented as
Mean±SEM (standard error of the mean) represented by vertical bars and
n represents the number of experiments carried on different animals (n = 7).
Comparisons were made between treated and control groups by use of Students
t-test (Lewis, 1971). All data were analyzed using Graph
Pad Prism software (UK). p<0.05 indicates statistically significant difference.
Histamine per se displays dual response in the melanophores of O.
The melanophores of Oreochromis mossambicus
considerable sensitivity to histamine. Histamine was found to cause pigment
aggregation in a dose dependent manner. The threshold dose that could result
in aggregation within the melanophores was found to be as low as 1x10-8
. This effect remained consistent with the increasing dose
concentration. The melanophores showed considerable pigment aggregation and
appeared significantly aggregated at dose concentration of 4x10-5
. At this stage the MMSI was observed to be (1.0±0.1241,
p<0.007). Later, it was observed that subsequent increase in concentration
of histamine resulted in gradual pigment dispersion. The melanophores showed
progressive pigment dispersion with increasing histamine concentration. The
dendritic processes of melanophores sequentially spread out and attained complete
reticulated state. This state was observed at dose concentration of 6.4x10-4
of histamine. The pigment granules within the cells extended
towards the peripheral dendrites and the cells appeared expanded with the MMSI
measured at (9.98±0.321, p<0.0001) (Fig. 1
To investigate the putative mechanism of this dual response exhibited by melanophores towards histamine and the underlying cellular receptors involved in this process, several receptor specific potent and selective agonists and antagonists were employed. We hypothesized that the dual response exhibited by histamine on pigment cell melanophores could be most likely mediated by histaminergic receptors present at the neuro-melanophore junction that demonstrate an auto regulatory mechanism of action regulating the innervating histaminergic neurons and controlling subsequent histamine release from the encompassing mast cells.
Action of compound 48/80 per se on the pigmentary responses in the melanophores of Oreochromis mossambicus: Compound 48/80 displayed dual responses within the melanophores of O. mossambicus at low and high concentration respectively. Compound 48/80 exerts mast cell degranulation and releases histamine. The effect of comp 48/80 on melanophores was quite interesting. The immediate response of melanophores to compound 48/80 was seen in the form of pigment aggregation. The minimum dose concentration that resulted in a noticeable pigment translocation towards the centre of cells was found to be (1x10-7 g mL-1) with the MMSI (4.71±0.40). Later the aggregating effect was consistent with the increase in concentration of compound 48/80 (Fig. 1). The melanophores showed substantial degree of sensitivity to this compound and confirms the underlying involvement of histaminergic receptors in the process. Later with further increase in concentration of compound 48/80, there was a noticeable change in pigment movement. The pigment granules gradually drifted towards the peripheral processions and showed a considerable expansion in the melanophore index. The MMSI at the highest concentration (6.4x10-4 g mL-1) of compound 48/80 was recorded to have elevated to (9.15±0.49, p<0.0001) from the control MMSI of (4.92±0.006, p<0.065). This dual response that was exhibited by compound 48/80 on the melanophores further reinforces the likelihood of a self regulatory system in the pigmentary responses of melanophores incurring to histamine (Fig. 1).
Antagonistic effects of mepyramine and ranitidine on the action of histamine:
Histamine exerts its pleiotropic effect via stimulation of post synaptic cellular
receptors. The two major classes of these receptors i.e., H1R and
H2R have been identified to be present on melanophores of Oreochromis
mossambicus (Salim et al., 2011 unpublished
data). The effect of histamine was challenged with H1R antagonist
mepyramine (2x10-6 g mL-1) and H2R antagonist
Ranitidine (4x10-6 g mL-1). It was observed that histamine
in high dose range from 1x10-5 g mL-1 to 6.4x10-4
g mL-1 showed discernible pigment dispersion. When this concentration
of histamine was challenged by specific H1R agonist mepyramine (2x10-6
g mL-1) and H2R antagonist ranitidine (4x10-6
g mL-1), it was observed that the effect of histamine was feebly
attenuated to the first four concentrations, the attenuation in response to
histamine was observed by a shift in the dose response curve towards right.
Dose response curves showing the effect of Histamine
per se and compound 48/80 per se on the isolated scale melanophores
of Oreochromis mossambicus. Note the variation in responses with
the increase in concentrations of histamine and compound 48/80, respectively.
At low dose range the melanophores show a decrease in MMSI, whereas with
the shift of concentration towards higher side the MMSI show considerable
increase. Abscissa: Concentrations of histamine and compound 48/80 per
se in g mL-1. Ordinate: Responses of melanophores in terms
of MMSI. Vertical bars represent standard error. The p-value signifies level
of significance; *p<0.0013, **p = 0.0005,***p = 0.0001
Dose response curve showing the blocking effect of
mepyramine (2x10-6 g mL-1) and ranitidine (4x10-6 g mL-1) against the action of histamine (at high dose range)
on the isolated scale melanophores of Oreochromis mossambicus. Note
the blockage demonstrated at the first four concentrations of histamine,
however with the subsequent increase in histamine concentration the melanophores
show considerable dispersion with the increase in MMSI. Abscissa: Concentrations
of histamine per se in g mL-1. Ordinate: Responses of melanophores
in terms of MMSI. Vertical bars represent standard error. The p-value signifies
level of significance; *p<0.0016, **p = 0.0001
However the combined action of mepyramine and ranitidine could not effectively
block the dispersion caused by subsequent increase in concentration of histamine.
The melanophores showed dispersion with the shift in MMSI from the control value
4.78±0.14 to 9.12±0.20, (p<0.0001) at the concentration 6.4x10-4g
mL-1 of histamine (Fig. 2). The conjecture emanated
with this finding whether there is any other receptor class belonging to the
histaminergic family that is regulating the pigmentary responses of melanophores.
Specific h3r agonist immethridine per se and combined action of immethridine along with compound 48/80: Specific and potent H3R agonist Immethridine was employed and its effect was tested on the melanophores of Oreochromis mossambicus. It was found that Immethridine per se resulted in pigment dispersion in a dose dependent manner. The extent of pigment dispersion in the dose range 1x10-6 g mL-1 to 6.4x10-5 g mL-1 was quite feeble with the MMSI recorded at 7.71±0.360 at dose concentration 6.4x10-5 g mL-1 from the control value 4.84±0.127 (Fig. 3).
Dose response curves showing the effect of Immethridine
per se and the blocking action of thioperamide against Immethridine on the
isolated scale melanophores of Oreochromis mossambicus. Note the
action of Thioperamide against Immethridine result into a partial attenuation
of dispersion exerted by Immethridine only up to first four concentrations.
Abscissa: Concentrations of Immethridine per se in g mL. Ordinate:
Responses of melanophores in terms of MMSI. Vertical bars represent standard
error. The p-value signifies level of significance; *p<0.0001, **p =
0.09, ***p = 0.0032
Dose response curves showing the effect of Immethridine
along with compound 48/80. Note the attenuation in the aggregatory response
of compound 48/80 on Immethridine pretreated cells. Thioperamide caused
considerable reversal in response against Immethridine along with comp 48/80
and resulted into considerable pigment aggregation. Abscissa: Concentrations
of compound 48/80 on cells pretreated with Immethridine in g mL-1.
Ordinate: Responses of melanophores in terms of MMSI. Vertical bars represent
standard error. The p-value signifies level of significance; *p<0.0001,
**p = 0.09, ***p = 0.0032
As reported that H3 agonists modulate the process of mast cell degranulation
and henceforth release of histamine (Theoharides, 1998).
We investigated if there is any regulatory mechanism involved in the release
of histamine from mast cells encompassing the melanophores. Compound 48/80 was
employed and found to cause pigment aggregation as discussed in the above section.
Cells were pre-incubated with Immethridine (8x10-6 g mL-1)
and challenged with increasing concentration of compound 48/80. It was found
that Immethridine attenuates the action of compound 48/80 (1x10-6-6.4x10-5
g mL-1) and blocks the aggregation completely (Fig.
4). Earlier compound 48/80 per se in the dose range 1x10-6-6.4x10-5
g mL-1 showed a considerable pigment aggregation within the melanophores
indicating the involvement of histaminergic receptors. But the marked degree
of debilitation displayed by cells pretreated by Immethridne further corroborates
the likelihood of H3R involvement.
Effect of Immethridine and compound 48/80 along with H3R specific
antagonist thioperamide: To affirm the involvement of H3Rs in
the observed pigmentary responses by specific agonist Immethridine we employed
a H3R antagonist thioperamide. Earlier pilot experiments were conducted
and it was found that thioperamide showed pigment aggregation per se
in a dose dependent manner. Thioperamide in dose concentration 8x10-6
g mL-1 was selected to be the antagonist dose. The cells were pretreated
with thioperamide and combined effect of Immethridine along with compound 48/80
was analyzed. It was found that pretreatment of cells with thioperamide resulted
into pigment aggregation within the cells (Fig. 4). We found
earlier that melanophores of Oreochromis mossambicus possess both H1
and H2 receptors, with the predominance of H1 over
H2 (Unpublished data). The observed pigment aggregation therefore
might be a result of H1 (Gq/11) receptor stimulation, that brings
about pigment aggregation due to calcium influx within the cells (Oshima
et al., 1988; Miyashita and Moriya, 1990).
To further investigate this contemplation we used specific H1 and
H2 receptor antagonists and examined the subsequent variance in responses.
Action of Immethridine with H1R antagonist- mepyramine and H2R antagonist-ranitidine along with compound 48/80: The effect of compound 48/80 per se showed considerable pigment aggregation. The synergistic actions of combined H1 and H2 receptor antagonists; Mepyramine (2x10-6 g mL-1) and Ranitidine (4x10-6 g mL-1) were examined against cells pretreated with Immethridine against increasing concentrations of compound 48/80. It was quite interesting to observe that the melanophores showed slight pigment dispersion. This effect was dose dependent and increased with increasing concentrations of compound 48/80. The degree of pigment dispersion was noted and the MMSI at the highest dose concentrations (6.4x10-6 g mL-1) was found to be 7.92±0.477 (p<0.0001). The dispersion observed clearly indicates that Immethridine is resulting into nullifying the aggregatory effects of compound 48/80. The masking of H1 and H2 receptors by mepyramine and ranitidine further emphasizes upon the role of histaminergic receptors of H3 type (Fig. 5).
Action of Immethridine with synergistic blocking effects of mepyramine,
ranitidine and thioperamide along with compound 48/80: Cells pretreated
with specific antagonists; H1R- mepyramine (2x10-6 g mL-1),
H2R- Ranitidine (4x10-6 g mL-1) and H3R-thioperamide
(8x10-6 g mL-1) along with Immethridine against comp 48/80
showed a marked decrease in MMSI from the control value.
Dose response curves showing the effect of Immithridine
along with mepyramine (2x10-6 g mL-1) and ranitidine
(4x10-6 g mL-1) against compound 48/80. The synergistic
blockage demonstrated by reversal in responses exhibited by combined action
of mepyramine (2x10-6 g mL-1), ranitidine (4x10-6 g mL-1)and thioperamide (8x10-6 g mL-1)
with cells pretreated with Immithridine is shown by decrease in MMSI. Note
the complete attenuation to the action of compound 48/80 by synergistic
blockage effect of yohimbine (4x10-7 g mL-1) along
with mepyramine, ranitidine and thioperamide. Abscissa: Concentrations of
compound 48/80 on cells pretreated with Immethridine in g mL-1.
Ordinate: Responses of melanophores in terms of MMSI. Vertical bars represent
standard error. The p-value signifies level of significance; *p<0.0001,
**p = 0.0123, ***p = 0.00352, ****p = 0.0005
Putative mechanism of Histamine H3 Receptor in regulation
of Pigmentary responses within the melanophores of Oreochromis mossambicus.
Release of histamine from surrounding mast cells by Comp 48/80 is inhibited
by stimulation of H 3R by Immeth ridine. The feed back inhibition
loop further inhibits the release of adrenaline from encompassing sympathetic
nerve terminals and results in subsequent pigment dispersion
The MMSI recorded at highest dose concentration of compound 48/80 was 0.92±0.105
(p<0.001).The marked potentiation in aggregatory responses of melanophores
by masking H1, H2 and H3 receptors in presence
of immethridine is clearly observed with the pigment movement towards the perikaron
and the fall in MMSI (control 5.15±0.160) (Fig. 5).
The unexpected exaggeration in pigment aggregation when compared with the dose
response curve of compound 48/80 per se (Fig. 1) in
the same dose range and Immethridine along with H1 and H2R
antagonists with compound 48/80 implies the likelihood of an auxiliary involvement
of sympathetic system. The observed findings indicate that the histaminergic
systems further expands in regulating the release of other neurotransmitters
most likely adrenaline.
Action of Immethridine and antagonistic effect of alpha 2 adrenergic blocker;
yohimbine (4x10-7 g mL-1), mepyramine, ranitidine and
thioperamide along with comp 48/80: The unexpected potentiation in aggregatory
responses exhibited by the cells on blocking the H1, H2
and H3 receptors along with compound 48/80 emanated the likelihood
of adrenergic involvement in pigment responses. We assumed that the association
of pre-junctional H3 like receptors at the neuro-effector alliance
might be further modulating the release of adrenaline. Adrenergic innervation
has been earlier reported to be present in teleosts (Xu
and Xi, 2011; Jacobowitz and Laties, 1968) with
the presence of alpha 2 adrenergic receptors in Oreochromis mossambicus
(Acharya and Ovais, 2007). The effect of compound 48/80
when examined against cells pre-treated with mepyramine, ranitidine, thioperamide
resulted into pigment aggregation.
The blockage of H3R with specific antagonist thioperamide
resulted in pigment aggregation most likely due to subsequent stimulation
of alpha adrenergic receptors present at the neuro melanophore junction
When the cells were again preincubated with H1, H2 and
H3 receptor antagonists along with yohimbine an alpha adrenergic
blocker, it was found that the earlier aggregatory response is completely abolished
and blocked (Fig. 5). The dose response curve of yohimbine
treated cells showed a shift towards right, almost in line with the control,
demonstrating considerable blockage of aggregatory response. This finding confirms
that the histaminergic system is involved in modulating the pigment motility
within the cells and post synaptic H3 like receptor is playing a
regulatory role in controlling the regulation of adrenergic receptors.
The characterization of histaminergic receptors in fish by other workers
has revealed that histamine receptors are linked to numerous neurological and
neuro behavioral responses (Santos et al., 2003
Romaguera and Mattioli, 2008
et al., 2010
). Also, the endocrinal employment of histamine extends
into thermo regulation and gastric acid secretion (Green
and Lomax, 1976
; Holstein, 1986
). In addition,
the fish brain is well characterized structurally and the existence of the H3
receptor has been documented recently in zebrafish (Peitsaro
et al., 2007
). Also phylogenetic information on the central histaminergic
system in the brain of a teleost, jack mackerel (Trachurus trachurus
has been reported (Inagari et al., 1991
to date there is little information about specific tissue localization and functioning
of this histamine receptor in fish. The functional aspect of H3
in mammalian system has been explored quite significantly; nonetheless the exact
localization of this receptor in lower vertebrates in different peripheral systems
is still necessitated.
Interestingly, there is a well conserved 60% amino acid similarity between
fish and mammalian homologues for H1, H2 and H3
receptors (Peitsaro et al., 2007). Teleost fish
are phylogenetically closer to the basic vertebrate blueprint than higher vertebrates
and appear to have a simpler histaminergic system in terms of fiber density,
area of innervation (Inagari et al., 1991).
Our findings clearly implicate that the peripheral H3 receptors exist
on certain types of sensory nerves that innervate the melanophores and the activation
of these receptors regulates/inhibits histamine release. Thereafter, subsequent
downstream signaling impulses result in pigment translocation governing the
pigmentary responses of the fishes. Although, histamine is not stored in neurons
outside of CNS but mast cell derived histamine can modify peripheral sensory
nerve function (Hough and Leurs, 2006). Most importantly,
profound pigmentary responses within the animals might result due to acute states
of stress, immunological reactions or peripheral nerve cell injury suggesting
a definite participation of mast cell histamine in the process (Hough
and Leurs, 2006). The present finding indicates upon the putative involvement
of mast cells derived histamine in the process of cellular pigmentary responses.
The presence of these receptor classes in lower vertebrates and the possible
role therein might clarify some inexplicable concepts about the evolution of
these receptors classes from lower to higher vertebrates.
Activation of the prejunctional histamine H3 receptor modulates
sympathetic control in smooth muscles in mammals (McLeod
et al., 1993). The sympathetic control of melanophores has already
been reported in a number of species including Oreochromis mossambicus
(Acharya and Ovais, 2007) where the melanophores respond
by pigment aggregation (Yamada et al., 1984).
It is known that mast cells are distributed on either side of pigment cell layer
(Roberts et al., 1971) and compound 48/80 is
a liberator of histamine from mast cells. In our present study it was observed
that compound 48/80 released histamine that elicited a dual response within
the melanophores on a concentration dependent manner. This finding entailed
a prospective engagement of a signaling route pertaining to histamine that may
possess a distinct mechanism of action, most likely self regulatory in operation.
Henceforth we examined the effect of Immethridine a specific H3R
ligand and found that Immethridine nullifies the action of compound 48/80, indubitably
connoting the involvement of H3 receptors in the process (Fig.
6). The total attenuation expected with the employment of thioperamide against
Immethridine was fairly ineffective; rather a surprising potentiation in the
aggregatory response was observed within the melanophores. This finding gave
an interesting direction in the study and presented the plausible adrenergic
nexus in the course (Fig. 7). We later employed yohimbine
a specific alpha adrenergic blocker and found that the aggregation caused by
compound 48/80 along with Immethridine and thioperamide is completely and effectively
blocked by the synergistic attenuation with yohimbine. The distinguishing feature
of H3 receptors mediating the regulation of other neurotransmitter
systems; adrenaline in this regard has been clearly noticed. It is quite possible
that there is a cross interaction between the histaminergic receptors inhibiting
the innervating neurons at the neuro melanophore junction that are regulating
the release of adrenaline. The release of adrenaline and subsequent stimulation
of adrenoceptors resulting into pigment aggregation could be significant point
crucial to the resultant cascade of signals preceded by histaminergic signaling.
It is concluded that there is a plausible role of histaminergic H3
like receptors in the melanophore responses of the teleost, Oreochromis mossambicus
at the neuro-melanophore junction. The presence of distinct class of H3
like receptors on certain nerve endings innervating the melanophores is categorically
determined. The engagement of these receptors seems to be more of self regulatory
in operation depending upon the concentration of histamine at the encompassing
milieu. Since histamine as a neurotransmitter may be associated to a plethora
of neurobiological and neurobehavioral aspects, the consequential stimulation
of histaminergic receptors may result into a cascade of signaling pathways leading
to distinct pigmentary responses in the animal. The characteristic attribute
receptors mediating the regulation of other neurotransmitter
systems; adrenaline in this regard has also been divulged. The release of adrenaline
and subsequent stimulation of adrenoceptors resulting into pigment aggregation
has been a sequential switching point in the entailing cascade preceded by histaminergic
signaling. The presence of histamine receptors at the neuro-melanophore junction
has been a very interesting finding. The central operation of histaminergic
neurons towards mediation of auxiliary neuro transmitter systems needs to be
further investigated. Since vertebrate melanophores have been reported to have
a numerous classes of metabotropic receptors (Salim and
) and that vertebrate skin pigmentation in an incredibly complex
mechanism, the probable association of other receptor classes may answer some
unexplained apprehensions in the phenomenon of skin pigmentation.
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