Animals, Brain-Derived Neurotrophic Factor, Excitatory Postsynaptic Potentials, Freund's Adjuvant, Ganglia, Spinal, Hyperalgesia, Immunization, Passive, In Vitro Techniques, Male, Myelitis, Nerve Growth Factors, Neurons, Afferent, Neurotrophin 3, Pain, Rats, Rats, Sprague-Dawley, Receptor, trkB, Sodium Channels, Spinal Cord, Substantia Gelatinosa, Synaptic Transmission, Time Factors