Summary The dorsal raphe nucleus (DRN) contains the largest group of serotonin-producing neurons in the brain and projects to regions controlling reward. Although pharmacological studies suggest that serotonin inhibits reward-seeking, electrical stimulation of the DRN strongly reinforces instrumental behavior. Here, we provide a targeted assessment of the behavioral, anatomical, and electrophysiological contributions of serotonergic and non-serotonergic DRN neurons to reward processes. To explore DRN heterogeneity, we used a simultaneous two-vector knockout/optogenetic stimulation strategy, as well as cre-induced and cre-silenced vectors in several cre-expressing transgenic mouse lines. We found that the DRN is capable of reinforcing behavior primarily via non-serotonergic neurons, whose main projection target is the ventral tegmental area (VTA). Furthermore, these non-serotonergic projections provide glutamatergic excitation of VTA dopamine neurons and account for a large majority of the DRN-VTA pathway. These findings help to resolve apparent discrepancies between the roles of serotonin versus the DRN in behavioral reinforcement.

These experiments investigated the involvement of several temporal lobe regions in consolidation of recognition memory. Anisomycin, a protein synthesis inhibitor, was infused into the hippocampus, perirhinal cortex, insular cortex, or basolateral amygdala of rats immediately after the sample phase of object or object-in-context recognition memory training. Anisomycin infused into perirhinal or insular cortices blocked long-term (24 h), but not short-term (90 min) object recognition memory. Infusions into the hippocampus or amygdala did not impair object recognition memory. Anisomycin infused into the hippocampus blocked long-term, but not short-term object-in-context recognition memory, whereas infusions administered into the perirhinal cortex, insular cortex, or amygdala did not affect object-in-context recognition memory. These results clearly indicate that distinct regions of the temporal lobe are differentially involved in long-term object and object-in-context recognition memory. Whereas perirhinal and insular cortices are required for consolidation of familiar objects, the hippocampus is necessary for consolidation of contextual information of recognition memory. Altogether, these results suggest that temporal lobe structures are differentially involved in recognition memory consolidation.

Structural synaptic changes have been suggested to underlie long-term memory formation. In this work, we investigate if hippocampal mossy fiber synaptogenesis induced by water maze overtraining can be related with long-term spatial memory performance. Rats were trained in a Morris water maze for one to five identical daily sessions and tested for memory retrieval 1 week and 1 month after training. After the last test session, the rat brains were obtained and processed for Timm's staining to analyze mossy fiber projection. The behavioral results showed that with more training, animals showed a better performance in the memory tests, and this performance positively correlates with Timm's staining in the stratum oriens. Furthermore, with the use of the NMDA antagonist MK801 before, but not after acquisition, water maze spatial memory was impaired. Increased Timm's staining in the stratum oriens was observed in the animals treated with MK801 after acquisition but not in those treated before. Finally, we observed that mossy fiber synaptogenesis occurs mainly in the septal region of the dorsal hippocampus, supporting the idea that this anterior region is important for spatial memory. Altogether, these results suggest that mossy fiber synaptogenesis can be related with spatial long-term memory formation.

Glucocorticoids and corticotropin-releasing hormone (CRH) are key regulators of stress responses. Different types of stress activate the CRH system; in hypothalamus, CRH expression and release are increased by physical or psychological stressors while in amygdala, preferentially by psychological stress. Learning and memory processes are modulated by glucocorticoids and stress at different levels. To characterize the kind of stress provoked by a hippocampal-dependent task such as spatial learning, we compared the expression profile of glucocorticoid receptor (GR), pro-CRH and CRH-R1 mRNAs (analyzed by RT-PCR), in amygdala, hippocampus and hypothalamus and quantified serum corticosterone levels by radioimmunoassay at different stages of training. mRNA levels of brain-derived neurotrophic factor (BDNF) were also quantified due to its prominent role in learning and memory processes. Male Wistar rats trained for 1, 3 or 5 days in the Morris water-maze (10 trials/day) were sacrificed 5–60 min the after last trial. A strong stress response occurred at day one in both yoked and trained animals (increased corticosterone and hypothalamic pro-CRH and CRH-R1 mRNA levels); changes gradually diminished as the test progressed. In amygdala, pro-CRH mRNA levels decreased while those of BDNF augmented when stress was highest, in yoked and trained animals. Hippocampi, of both yoked and trained groups, had decreased levels of GR mRNA on days 1 and 3, normalizing by day 5, while those of pro-CRH and CRH-R1 increased after the 3rd day. Increased gene expression, specifically due to spatial learning, occurred only for hippocampal BDNF since day 3. These results show that the Morris water-maze paradigm induces a strong stress response that is gradually attenuated. Inhibition of CRH expression in amygdala suggests that the stress inflicted is of physical but not of psychological nature and could lead to reduced fear or anxiety.

The detection and processing of novel information encountered in our environment is crucial for proper adaptive behavior and learning. Hippocampus is a prime structure for novelty detection that receives high-level inputs including context information. It is of our interest to understand the mechanisms by which the hippocampus processes contextual information. For this, we performed in vivo microdyalisis in order to monitor extracellular changes in neurotransmitter levels during Object Location Memory (OLM), a behavioral protocol developed to evaluate contextual information processing in recognition memory. Neurotransmitter release was evaluated in the dorsal hippocampus and insular cortex during OLM in 3-month-old B6129SF2/J mice. We found a simultaneous release of dopamine and norepinephrine in hippocampus during OLM, while neurochemical activity remained unaltered in the cortex. Additionally, we administered 6-hydroxy-dopamine (6-OHDA), a neurotoxic compound selective to dopaminergic and noradrenergic neurons, in the dorsal hippocampus in a different group of mice. Depletion of catecholaminergic terminals in the hippocampus by 6-OHDA impaired OLM but did not affect novel object recognition. Our results support the relevance of hippocampal catecholaminergic neurotransmission in recognition memory. The significance of catecholaminergic function may be extended to the clinical field as it has been reported that innervation of hippocampus by the noradrenergic and dopaminergic system is reduced and atrophied in aging and Alzheimer's disease brain. © 2017 Wiley Periodicals, Inc.