Ethanol consumption in the Sprague-Dawley rat increases sensitivity of the dorsal raphe nucleus to 5,7-dihydroxytryptamine.

Alcoholism afflicts 1 in 13 US adults, and comorbidity with depression is common. Levels of serotonin (5-HT) metabolites in alcoholic or depressed humans and rat strains are lower compared to healthy counterparts. Rats bred for ethanol (EtOH) preference are common in EtOH studies, however out-bred strains better model the range of EtOH consumption in humans. We examined voluntary EtOH consumption in out-bred Sprague-Dawley (SD) rats placed in the 20% EtOH intermittent access drinking paradigm (IA). Acquisition of 20% EtOH consumption (g EtOH/kg/24h) was assessed during the first 6-8 weeks of IA. Rats naturally separated into two groups (Drinkers or Non-drinkers) based on EtOH intake above or below 0.5 g/kg/24h prior to treatment intervention. We examined the effect of central 5-HT depletion on EtOH consumption by infusing 5,7-dihyroxytryptamine (5,7-DHT; i.c.v., 200-300 µg) or vehicle and measured EtOH consumption for 4 weeks post-operatively in IA. Compared to baseline, there was no effect of vehicle or 5,7-DHT on EtOH consumption during the post-operative period. Quantification of 5-HT depletion in the dorsal raphe nucleus (DRN) using tryptophan hydroxylase-2 (TPH2) immunohistochemistry resulted in a 76% decrease in staining with 5,7-DHT treatment. Interestingly, preservation of the ventromedial (VM) sub-regions was evident in all animals treated with 5,7-DHT, regardless of drinking behavior. In addition, Drinkers treated with 5,7-DHT had significantly more TPH2 depletion in the DRN compared to Non-drinkers. Our findings indicate that out-bred SD rats exhibit a natural EtOH consumption behavior (Drinker or Non-drinker) that is stable across time and independent of 5-HT depletion in the CNS. In addition, rats that regularly consumed >0.5 g EtOH/kg had greater sensitivity to 5,7-DHT in the DRN, indicating an interaction between EtOH and sensitivity of DRN 5-HT cells to neurotoxic substances. This may contribute to the dysfunctionality of the 5-HT system in alcoholic humans and lead to a better understanding of current pharmacological treatments for this addiction.

Cellular profile of the dorsal raphe lateral wing sub-region: relationship to the lateral dorsal tegmental nucleus.

As one of the main serotonergic (5HT) projections to the forebrain, the dorsal raphe nucleus (DRN) has been implicated in disorders of anxiety and depression. Although the nucleus contains the densest population of 5HT neurons in the brain, at least 50% of cells within this structure are non-serotonergic, including a large population of nitric oxide synthase (NOS) containing neurons. The DRN has a unique topographical efferent organization and can also be divided into sub-regions based on rostro-caudal and medio-lateral dimensions. NOS is co-localized with 5HT in the midline DRN but NOS-positive cells in the lateral wing (LW) of the nucleus do not express 5HT. Interestingly, the NOS LW neuronal population is immediately rostral to and in line with the cholinergic lateral dorsal tegmental nucleus (LDT). We used immunohistochemical methods to investigate the potential serotonergic regulation of NOS LW neurons and also the association of this cell grouping to the LDT. Our results indicate that >75% of NOS LW neurons express the inhibitory 5HT1A receptor and are cholinergic (>90%). The findings suggest this assembly of cells is a rostral extension of the LDT, one that it is subject to regulation by 5HT release. As such the present study suggests a link between 5HT signaling, activation of cholinergic/NOS neurons, and the stress response including the pathophysiology underlying anxiety and depression.

Functional organization of the dorsal raphe efferent system with special consideration of nitrergic cell groups.

The serotonin (5HT) system of the brain is involved in many CNS functions including sensory perception, stress responses and psychological disorders such as anxiety and depression. Of the nine 5HT nuclei located in the mammalian brain, the dorsal raphe nucleus (DRN) has the most extensive forebrain connectivity and is implicated in the manifestation of stress-related psychological disturbances. Initial investigations of DRN efferent connections failed to acknowledge the rostrocaudal and mediolateral organization of the nucleus or its neurochemical heterogeneity. More recent studies have focused on the non-5HT contingent of DRN cells and have revealed an intrinsic intranuclear organization of the DRN which has specific implications for sensory signal processing and stress responses. Of particular interest are spatially segregated subsets of nitric oxide producing neurons that are activated by stressors and that have unique efferent projection fields. In this regard, both the midline and lateral wing subregions of the DRN have emerged as prominent loci for future investigation of nitric oxide function and modulation of sensory- and stressor-related signals in the DRN and coinciding terminal fields.