EFFECTS OF INTRASEPTALLY INJECTED NORADRENERGIC DRUGS ON HIPPOCAMPAL SODIUM-DEPENDENT-HIGH-AFFINITY-CHOLINE-UPTAKE IN RESTING AND TRAINED MICE

It has been shown in numerous studies that memory testing can alter presynaptic cholinergic activity within the hippocampus. In the present experiments, the role of the noradrenergic input to the septal cholinergic neurons in the immediate increase in cholinergic activity induced by the first training session of a spatial reference memory task in an 8-arm radial maze was investigated. The effects of bilateral intraseptal injections of noradrenergic drugs on hippocampal sodium-dependent-high-affinity-choline-uptake (SDHACU) were studied in 'resting' animals (basal level) or in 'trained' animals injected 20 min before training and sacrificed immediately after the test. The results showed that: (1) the injection of maprotiline, a noradrenaline reuptake inhibitor (0.06 ng/site), induced an increase in hippocampal SDHACU in 'resting' animals, whereas the alpha(2)-adrenoceptor agonist UK 14304 (1.5 ng) significantly reduced the basal level of SDHACU; (2) none of the cu-adrenoceptor antagonists used (phenoxybenzamine, 10 and 100 ng; BE 2254, 100 and 500 ng; yohimbine, 0.5 and 50 ng) significantly affected the basal level of hippocampal SDHACU, and only the alpha (1)-adrenoceptor antagonist BE 2254 (500 ng) significantly reduced the testing-induced activation of SDHACU. Taken together, these findings suggest that noradrenaline may exert a bimodal regulatory influence on the activity of septo-hippocampal cholinergic neurons. The behavior-induced activation of hippocampal SDHACU could be partly mediated by the stimulation of alpha (1)-adrenoceptors, whereas postsynaptic alpha(2)-adrenoceptors may be important for the maintenance of a tonic inhibition of the steady-state cholinergic activity in the hippocampus. These results are further evidence for a regulatory noradrenergic influence on the activity of septal cholinergic neurons.