The paper is aimed at investigating the role of inhomogeneous heating of the neutral gas in a constricted glow discharge in argon. The discharge constriction may occur at intermediate pressures (tens and hundreds Torr), currents of 10 - 100 mA/cm and ionization degrees about 10-8 - 10-6. In the constricted argon discharge during the gas heating an ascent of plasma cord is also observed. In order to investigate the temperature field of neutral gas, optical methods based on the Michelson interferometer scheme were used. It allowed to measure the optical density of neutral atoms in a constricted discharge and to study the temperature field of neutral atoms, dynamics of both gas heating and ascent of the constricted cord.