Basic characteristics of noradrenaline release in the hippocampus of intact and 6-hydroxydopamine-lesioned rats as studied by in vivo microdialysis.

Extracellular levels of noradrenaline (NA) were measured in the hippocampus of awake and halothane-anaesthetized rats, using intracerebral dialysis coupled to a highly sensitive radioenzymatic assay. In awake animals, steady state NA output after 1 h of perfusion was 69 +/- 21 fmol/30 microliter (15 min). Non-noxious sensory stimulation increased the steady state NA output by 35%, while halothane anaesthesia reduced the levels by 63-72%. During halothane anaesthesia KCl (100 mM) or desipramine (5 microM) added to the perfusion medium increased the NA levels 27-fold and 8-fold, respectively. Tetrodotoxin added in the presence of desipramine reduced the extracellular NA levels by 85%. In 6-OHDA lesioned animals the NA levels were below the limit of detection in virtually all samples (i.e. less than 3 fmol/30 microliter) at 10-16 days after lesion, but there was a partial recovery at longer postoperative time points. These results support the conclusion that the extracellular NA levels recovered in the dialysis perfusates are neuronally derived and can be used as an index of noradrenergic synaptic activity in the brain.