In Vivo Measurement of Extracellular 5‐Hydroxytryptamine in Hippocampus of the Anaesthetized Rat Using Microdialysis: Changes in Relation to 5‐Hydroxytryptaminergic Neuronal Activity

The effect of manipulating the activity of central 5‐hydroxytryptamine (5‐HT) neurones on extracellular 5‐HT in ventral hippocampus of the chloral hydrate‐anaesthetized rat was studied using the brain perfusion method, microdialysis. Basal levels of 5‐HT in the dialysates were close to the detection limits of our assay using HPLC with electrochemical detection. However, addition of the selective 5‐HT reuptake inhibitor citalopram (10 −6 M ) to the perfusion medium produced readily measurable amounts of dialysate 5‐HT. Citalopram, therefore, was used throughout our experiments. Hippocampal dialysate levels of 5‐HT sharply declined over the first hour after dialysis probe implantation, but then became constant. This stable output of 5‐HT was reduced by 57% in rats treated 14 days previously with intracerebroven‐tricular injections of the 5‐HT neurotoxin 5,7‐dihydroxy‐tryptamine. Electrical stimulation (1‐ms pulse width, 300 μA, 2–20 Hz) of the dorsal raphe nucleus for 20 min caused a rapid rise in hippocampal 5‐HT output, which immediately declined on cessation of the stimulus and was frequency‐dependent. Addition of tetrodotoxin (10 −6 M ) to the perfusion medium reduced 5‐HT levels to 75% of predrug values. Injection of the 5‐HT 1A agonist 8‐hydroxy‐2‐(di‐ n ‐propylami‐no)tetralin (0.5 and 2.5 μg) into the dorsal raphe nucleus caused a dose‐related fall in hippocampal output of 5‐HT compared to saline‐injected controls. We conclude from these data that the spontaneous output of endogenous 5‐HT into hippocampal dialysates, measured under our experimental conditions, predominantly originates from central 5‐HT neurones and changes in accordance with their electrical activity.