The present investigation was undertaken to study the
nature of neuro-melanophore junction in the white-spotted rabbit fish
Siganus cancliculatus. In vitro experiments using split
fin preparation indicated that melanophores of S. cancliculatus
are highly responsive to potassium ions and adrenergic agonists. Potassium
ions and the adrenergic agonists induced prompt melanosome aggregation
that could be competitively blocked by yohimbine (alpha-2 specific adrenergic
antagonist) and phentolamine (non-specific alpha adrenergic antagonist).
The melanophore responses to repeated potassium stimulation (up to 20
stimuli) did not show any sign of fatigue. However, statistically significant
enhancement was observed in responses to potassium that followed the first
five stimulations. Adrenergic agonists acted in a time and concentration-dependent
manner and their relative potency had the following rank order: clonidine
(alpha-2 specific agonist) > norepinephrine (non-specific adrenergic
agonist) > phenylephrine (alpha-1 specific agonist) > methoxamine
(alpha-1-specific agonist). Yohimbine exerted a more potent inhibiting
effect on norepinephrine induced melanosome aggregation compared to phentolamine.
Prazosine (alpha-1 specific antagonist) had no effect on such aggregation.
Chemically denervated melanophores displayed hypersensitivity to alpha-adrenergic
agonists but were refractive to potassium ion stimulation. The refractivity
of denervated melanophores to potassium indicates the effect of potassium
ion is not direct on melanophores but it is rather through depolarization
effect of potassium on the neuro-melanophore peripheral sympathetic fibers
and hence release of norepinephrine. In denervated melanophores, similar
to intact melanophores, only phentolamine and yohimbine but not prazosine,
significantly inhibited melanosome aggregation effect of norepinephrine,
indicating that norepinephrine effect is through postsynaptic alpha-2
adrenoceptors. The present data demonstrate that the nature of melanophore
innervation in this teleost is adrenergic and neuro-melanophore signals
mediating melanosome aggregation are transmitted through alpha-2 postsynaptic
M.H. Amiri , 2009. Postsynaptic Alpha 2-Adrenoceptors Mediate Melanosome Aggregation in Melanophores of the White-Spotted Rabbitfish (Siganus canaliculatus). Pakistan Journal of Biological Sciences, 12: 1-10.