Treatment of Depressed Dark Nasal Line
It is best to analyze what characteristics of this line make it more noticeable. Treatments to lighten dark pigmentation include topical bleaching creams, chemical peeling, or laser resurfacing (fraxel or pulsed dye lasers). The depression itself may create a shadow effect, making the skin look darker. Filling in the depressed line with a hyaluronic acid filler such as Restylane or Juvederm, may help minimize visibility for a year or two and create a smoother skin surface that looks better. Any of these treatments are worth a try, easy to go through, and should help.
It looks like there is an actual crease or indentation that could be addressed with either a Juvederm or some type of filler that would lessen it being so noticeable. You may want to try this, it's not permanent but if you like the results you can continue with it. Best of Luck to you!
Treating Pigmentation on the Face
There are many ways to treat pigmentation depending on the cause of the darkness. If this is hormonally caused than Cosmelan is an extremely efficient way of instantly treating this pigmentation. Sun damage can also be reversed with the Fraxel or mini-Fraxel (Clear and Brilliant). Please consult a board certified dermatologist for the best treatment options.
Allergic salute treatment
The horizontal line found on your nose is related to allergic rhinitis and is from routinely wiping your runny nose. Preventative treatments include allergy nose sprays and allergy medications that are best prescribed by your primary care doctor, your ENT, or your allergist. Once the crease has developed, hyaluronic acid fillers can help fill in the depressed skin, however extreme care must be taken using fillers in the nasal tip. There are a very high number of small blood vessels in the nose and improper filling techniques can lead to skin necrosis - a much worse problem than the horizontal crease. Pigmentary changes can be addressed with cover up or makeup as well as a number of topicals, such as hydroquinone. Laser treatments also can help build collagen and decrease hyperpigmented (darkened) areas.
Treatment for dark horizontal line on nos
Without on person examination I might recommend micro needling 3 sessions with Restylane HA filler. Fee $1,000.