Serotonin 5-HT2 receptor activation induces a long-lasting amplification of spinal reflex actions in the rat

1. C-fibre activation induces a long-term potentiation (LTP) in the spinal flexion reflex in mammals, presumably to provide enhanced reflexive protect ion of damaged tissue from further injury, Descending monoaminergic pathway s are thought to depress sensory input but may also amplify spinal reflexes ; the mechanisms of this modulation within the spinal cord 2. We used electrical stimulation of primary afferents and recordings of mo tor output, in the rat lumbar spinal cord maintained in vitro, to demonstra te that serotonin is capable of inducing a long-lasting increase in reflex strength at all ages examined (postnatal days 2-12). 3. Pharmacological analyses indicated an essential requirement for activati on of 5-HTD2C receptors, while 5-HT1A/1B, 5-HT7 and 5-HT2A receptor activat ion was not required. In addition, primary afferent-evoked synaptic potenti als recorded in a subpopulation of laminae ITT-VT spinal neurons were simil arly facilitated by 5-HT. Thus, serotonin receptor-evoked facilitatory acti ons are complex, and may involve alterations in neuronal properties at both motoneuronal and pre-motoneuronal levels. 4. This study provides the first demonstration of a descending transmitter producing a longlasting amplification in reflex strength, accomplished by a ctivating a specific serotonin receptor subtype. It is suggested that brain modulatory systems regulate reflex pathways to function within an appropri ate range of sensori-motor gain, facilitating reflexes in behavioural situa tions requiring increased sensory responsiveness.