The first thing you should do is consult a feline neurologist. Specialists are usually found at 24 hr. ER vet centers and veterinary teaching hospitals and universities. I've found this information which explains degenerative disc disease (it pertains to dogs, but the physical description is the same): (From: http://www.petnews.com/clinicroom2.html)
"The result of a ruptured disc usually is soreness, pain and instability of the back. Other symptoms include weakness, lack of coordination, loss of feeling and inability to move. In extreme cases, complete paralysis, total collapse or loss of bladder functions can result.
Nerves of the spinal cord must be able to carry messages both to and from the animal's brain. The extruded disc material puts pressure on the nerves and interferes with nerve transmission.
Urgent medical care is required. Your veterinarian can judge the degree of injury to the nerves during the physical exam. X-rays probably will be necessary.
If there is any loss of the feeling in a foot or leg in addition to back pain when your pet moves, the spinal cord injury probably is severe.
Surgery can relieve the pressure on the spinal cord if the damage is not extensive. A veterinary neurologist should be consulted when possible.
If the disc does not require surgery, restricted activity is advised and appropriate doses of Glucosamine, Cetyl Myristoleate and anti-inflammatory medication may also be necessary to control the discomfort, especially at first."
There's more information here, although it's filled with a lot of medical terms:
Ask your vet for a referral to a veterinary neurologist or orthopaedist who treats degenerative disc disease in cats.