Does taking otc melatonin supplements shuts off its natural production permanently in brain????

It's possible. By UPTODATE: Melatonin's MT1 and MT2 receptors are highly susceptible to "desensitization," their activity decreasing markedly after exposure to supranormal concentrations of the hormone. This is relevant to the prolonged use of high melatonin doses to promote sleep, particularly among older adults with insomnia who might inadvertently purchase excessively large doses of the hormone