Endogenous dopaminergic modulation of the lamprey spinal locomotor network

The lamprey spinal cord contains three dopaminergic systems. The most extensive is the ventromedial plexus in which dopamine is co-localized with 5-HT and tachykinins. In this study we have investigated the effects of endogenously released dopamine on NMDA-induced spinal activity, and for comparison applied dopamine exogenously. The dopamine reuptake blocker bupropion increases the levels of extracellular dopamine in the spinal cord. Bath application of bupropion during ongoing NMDA-induced network activity (around 2 Hz) resulted in an initial increase of the burst rate followed by a transitional phase with the fast rhythm superimposed on a much slower ventral root burst activity (below 0.25 Hz). Finally only the slow rhythm was observed. The same response pattern with regard to the fast and slow rhythms was observed when dopamine was slowly perfused over the spinal cord, resulting in a gradual build-up of dopamine concentration. At low constant dopamine concentrations, however, an increased burst frequency was maintained, but at somewhat higher concentrations the fast burst rate instead was decreased. The degree of modulation of fictive locomotion by dopamine was also tested at low and high NMDA concentrations. Dopamine was found to exert stronger effects at low NMDA concentrations. With high NMDA concentrations dopamine did not induce the transition phase or the slow ventral root bursting. The slow alternating ventral root bursts, induced by bupropion, shifted to synchronized activity when glycinergic synaptic transmission was blocked with strychnine, testifying that the alternation depended on a crossed glycinergic action as previously shown for the fast rhythm.