the firing rate of neurons in the nucleus cuneiformis in response to formalin in male rat

introduction: although formalin-induced activity in primary afferent fibers and spinal dorsal ‎horn is well described, the midbrain neural basis underlying each phase of behavior in ‎formalin test has not been clarified. the present study was designed to investigate the nucleus ‎cuneiformis (cnf)‎‏ ‏neuronal responses during two phases after subcutaneous injection of ‎formalin into the hind paw of rat.‎ materials & methods: in this study, seventy six male nmri adult rats, weighing 230-320 g ‎were used. control group (n=24), which was tested merely for determining spontaneous firing ‎rate of cnf neurons. saline group (n=15) which received saline (50µl s.c.) instead of ‎formalin into the plantar surface of hind paw after 15 min baseline recording. formalin group ‎that formalin-induced neural activity of 37 cells simultaneously recorded from the cnf during ‎first phase (0-5 min) and second phase (15-60 min) of formalin test in 5-min intervals, using ‎an extracellular single unit recording technique.‎ results: the‏ ‏baseline firing rate of neurons in the cnf varied between 1.2 and 39.2 spikes/sec ‎and the average frequency of spontaneous activity over 1 h was 11.8 ± 1.1 spikes/sec. there ‎were three neural clusters after formalin injection. neurons in cluster 1 (46%) exhibited ‎severe, transient excitatory response in the first (acute) phase while neurons in cluster 2 (35%) ‎exhibited tonic but long-lasting excitatory response in the second (chronic) phase. cluster 3, a ‎small portion of neurons (about one fifth) which failed to show any evident responses to ‎formalin test. ‎ conclusion: our findings suggest that alteration of neural activity and pattern in the ‎spontaneous background of cnf neurons can be mediated a role in the transmission of ‎nociceptive information induced by the peripheral injection of formalin and can be discussed ‎in light of the role of these neurons in nociceptive information processing following ‎peripheral stimuli.‎