Damage to peripheral nerves can cause extensive functional reorganization of the adult mammalian nervous system. In fact, studies in non-human adult mammals have shown somatotopic reorganization as well as changes in receptive field properties throughout the somatosensory neuraxis following damage to peripheral nerves. Chronic changes in receptive fields and response properties have also been reported in the trigeminal brainstem nuclear complex (TBNC) following trigeminal nerve damage in both neonates and adults. Significant functional reorganization within subnucleus interpolaris (SpVi) was also apparent in rats subjected to infraorbital nerve transection. Several mechanisms have been proposed to account for many of the observed functional changes. They include: misdirected peripheral regrowth and changes in central arbor morphology of damaged primary afferents; peripheral and central sprouting of undamaged primary afferents; changes in morphology of second order neurons; and sprouting of central afferents to the TBNC. In addition, central monoaminergic neurons have been demonstrated to undergo considerable regeneration or collateral sprouting following direct damage or disruption of their sensory input. These monoaminergic neurons normally modulate the function of their targets. Thus, damage induced reorganization of monoaminergic projections to the TBNC could alter the response properties of cells within this region. The normal effects of monoaminergic inputs to SpVi have not yet been fully determined. We intend to use the neurotoxin 5,7-dihydroxytryptamine (specific for the destruction of serotonin and norepinephrine containing neurons) in combination with the antidepressant drug desmethylimipramine (used to protect norepinephrine containing neurons) to selectively destroy serotonergic fibers in SpVi in order to describe the normal functional role of serotonergic inputs to this region. This manipulation produced a significant change in : i) the types of peripheral receptor surfaces that activate cells in SpVi, and ii) rate of spontaneous activity. These changes reflect some of the functional alternatives observed after infraorbital nerve damage and suggest a role for serotonergic afferents in these changes. / Master of Science