Effects of 5-HT5A receptor blockade on amnesia or forgetting.

Previously the effects (0.01-3.0 mg/kg) of post-training SB-699551 (a 5-HT5A receptor antagonist) were reported in the associative learning task of autoshaping, showing that SB-699551 (0.1 mg/kg) decreased lever-press conditioned responses (CR) during short-term (STM; 1.5-h) or (3.0 mg/kg) long-term memory (LTM; 24-h); relative to the vehicle animals. Moreover, as pro-cognitive efficacy of SB-699551 was reported in the ketamine-model of schizophrenia. Hence, firstly aiming improving performance (conditioned response, CR), in this work autoshaping lever-press vs. nose-poke response was compared; secondly, new set of animals were randomly assigned to SB-699551 plus forgetting or amnesia protocols. Results show that the nose-poke operandum reduced inter-individual variance, increased CR and produced a progressive CR until 48-h. After one week of no training/testing sessions (i.e., interruption of 216 h), the forgetting was observed; i.e., the CR% of control-saline group significantly decreased. In contrast, SB-699551 at 0.3 and 3.0 mg/kg prevents forgetting. Additionally, as previously reported the non-competitive NMDA receptor antagonist dizocilpine (0.2 mg/kg) or the non-selective cholinergic antagonist scopolamine (0.3 mg/kg) decreased CR in STM. SB-699551 (0.3 mg/kg) alone also produced amnesia-like effect. Co-administration of SB-699551-dizocilpine or SB-699551-scopolamine reversed the SB-699551 induced-amnesic effects in LTM (24-h). Nose-poke seems to be a reliable operandum. The anti-amnesic and anti-forgetting mechanisms of amnesic SB-699551-dose remain unclear. The present findings are consistent with the notion that low doses of 5-HT5A receptor antagonists might be useful for reversing memory deficits associated to forgetting and amnesia. Of course, further experiments are necessary.

Intrahippocampal administration of 5-HT6 receptor drugs on memory consolidation and amnesia protocols.

To our knowledge the intrahippocampal serotonergic 5-HT6 receptor tone on memory and amnesia models remains unexplored. Hence, in the present work we tested intrahippocampal administration of serotonin or 5-hydroxytryptamine (5-HT)6 receptor experimental molecules with differential intrinsic activity. Methods: In the present study, Automatized Autoshaping memory task was used, useful measuring memory, neural markers, and pharmacological effects. We are hypothesizing that experimental molecules with differential intrinsic activity might reveal serotonergic tone. Particularly, intrahippocampal administration of 5-HT6 receptor compounds with differential intrinsic activity (i.e., agonistic and antagonistic) might evidencing a serotonergic tone via this receptor. Bilateral intrahippocampal dose-response curves show that administration of EMD386088 (10 and 100 µg) had no effect or (50 µg) decreased conditioned responses (CR) in short- and long-term memory (STM and LTM, respectively); while SB-399885 (10 or 100 µg) significantly decreased CR in STM and LTM (24 and 48-h) or (50 µg) had no effect; thus suggesting that there is a 5-HT6 receptor tone regulating both STM and LTM. Moreover, intrahippocampal inactive doses of EMD386088 (5 µg) plus SB-399885 (0.5 µg) did not affect STM and LTM; however, partially or completely prevented the scopolamine or dizocilpine-induced amnesia. Thus confirming that both drugs exerted their effects through 5-HT6 receptor and that there is a hippocampal serotonergic tone under amnesic states, similar to that striatal.