How can i use a variable from form1 with a User Control?

I created in my project a User Control: Right click on project name > Add > User Control.

Then in the User Control designer i added some controls. And in the User Control constructor i did:

public DopplerEffect()
        {
            InitializeComponent();

            InitGifFile = @"C:\Users\me\AppData\Local\mws\mws\temp_directory\file000050.gif";
            pen = new Pen(Color.Red, 3);
            formloadfirsttime = true;
            numericUpDown1.Value = 200;
            brush = new SolidBrush(Color.Red);
            trackBar2.Minimum = 1;
            trackBar2.Enabled = false;
            bmpnew = new Bitmap(512, 512);
            label1.Text = "";
            path_exe = Path.GetDirectoryName(Application.LocalUserAppDataPath);
            ScanClouds = Path.Combine(path_exe, ScanClouds);
            if (!Directory.Exists(ScanClouds))
            {
                Directory.CreateDirectory(ScanClouds);
            }
            ConvertedBmpDir = Path.Combine(ScanClouds, ConvertedBmpDir);
            if (!Directory.Exists(ConvertedBmpDir))
            {
                Directory.CreateDirectory(ConvertedBmpDir);
            }               
            b = new Bitmap(InitGifFile);
            b1 = new Bitmap(InitGifFile);
            pictureBox1.Image = b;
            ConvertedBmp = ConvertTo24(InitGifFile);
            mymem = ToStream(ConvertedBmp, ImageFormat.Bmp);
            startButton.Enabled = true;
            pauseButton.Enabled = false;

        }

For now i'm using a static directory for a gif file:

The problem is when i'm dragging the User Control to form1 designer it will work on the file: file000050.gif

But i want it to use a file i have in form1 somewhere in my form1 code i have a string file called: next_file

So when i'm dragging the User Control in the form1 designer i need it somehow to get automatic the next_file so InitGifFile instead be file000050.gif should be next_file

How can i do it ?