Modulation of acetylcholine release by serotonergic 5-HT1A and 5-HT1B receptors : a microdialysis study in the awake rat

Abstract

The aim of the thesis was to investigate the effects of the 5-HT1A receptor antagonist robalzotan (NAD-299) and the 5-HT1B receptor antagonist NAS-181 ((R)-(+)-2-(3-morpholinomethyl-2Hchromen-8-yl) oxymethyl-morpholine methanesulfonate) on cholinergic, glutamatergic and GABAergic neurotransmission in the rat brain in vivo. The extracellular levels of acetylcholine (ACh), glutamate (Glu) and GABA were monitored by microdialysis in the frontal cortex (FC) and ventral hippocampus (VHipp) in separate groups of awake rats.

Robalzotan (0.3; 1 and 3 µmol/kg s.c.) caused a dose-dependent increase of extracellular ACh levels in the rat FC to a maximal value of about 210-280% of controls at the highest dose. ACh concentrations returned to the normal levels within 120 min. Similarly, in the VHipp, both robalzotan given at a dose of 3 µmol/kg s.c., and to a lesser extent another 5-HT1A receptor antagonist, WAY 100635 (1.5 µmol/kg s.c), elevated the ACh levels to about 220% and 160%, respectively. Further, the potency of robalzotan to increase cholinergic transmission in the FC was compared to that of a reversible acetylcholinesterase inhibitor donepezil (Aricept, E2020). Donepezil, given at it dose of I mg/kg s.c., increased the cortical extracellular levels of ACh to a maximum of about 350% of the control values, thereafter the ACh concentrations returned to the normal levels within 80 min. The AUCs calculated for 80 min after the administration of robalzotan (3 µmol/kg) or donepezil (1 mg/kg) indicated that cortical ACh levels increased by a similar degree, although the effective time-course of donepezil was somewhat shorter.

In the following microdialysis study, the effects of local administrations of robalzotan into the nucleus basalis magnocellularis (NBM) on the frontocortical ACh release were examined in the awake rats. Robalzotan, injected locally into the NBM at doses of 6 and 18 nmol/rat caused a dose-dependent increase of cortical extracellular ACh concentrations. The maximal ACh increase was observed between 20-40 min after the drug administration, reaching 217 % of control values at a higher dose. However, the ACh levels maintained elevated during the entire post-injection sampling period (180 min). Robalzotan (75 or 100 µM perfused locally into the FC or VHipp showed only a moderate and delayed effect, increasing the ACh levels by about 30%. These results suggest that cholinergic afferents to the FC are under tonic inhibitory control of 5-HT1A heteroreceptors in the NBM and the cholinomimetic effect of robalzotan is probably associated with increased firing rates of cortical (and possibly also hippocampal) afferents. Additionally, the effect of locally perfused robalzotan on ACh release in the frontal cortex could be indirectly mediated, for example via the blockade of inhibitory 5HT1A heteroreceptors located on the pyramidal cells projecting in a feedback loop to the NBM.

The purpose of the final study was to investigate the effects of the 5-HT1B receptor antagonist NAS181 ((R)-(+)-2(3-morpholinomethyl-2H-chromen-8-yl) oxymethyl-morpholine methanesulfonate) on cholinergic, glutamatergic and GABA-ergic neurotransmission in the rat brain in vivo. NAS- 181 (1; 5 or 10 mg/kg, s.c.) caused a dose-dependent increase in ACh levels, reaching the maximal values of 500% (FC) and 230% (VHipp) of controls at 80 min postinjection. On the contrary, NAS-181 injected at doses of 10 or even 20 mg/kg s.c. had no effect on basal extracellular levels of Glu and GABA in these areas.

In summary, the present findings indicate that ACh neurotransmission in the frontal cortex and ventral hippocampus, the brain structures strongly implicated in cognitive function, is under tonic inhibitory control of the 5-HT1A heteroreceptors in the nucleus basalis and medial septum/diagonal band areas and the 5-HT1B heteroreceptors most likely localized at the terminal areas. It is concluded that the selective or combined 5-HT1A and 5-HT1B receptor antagonists may represent a potentially new alternative in the treatment of pathologies characterized by a cholinergic deficit in the central -nervous system.