Ethanol-stimulated serotonin release in the ventral hippocampus: an absence of rapid tolerance for the alcohol-preferring P rat and insensitivity in the alcohol-nonpreferring NP rat

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Abstract

This study examined the acute effects of intraperitoneal administration of ethanol on the extracellular levels of serotonin (5-HT) in the ventral hippocampus (vHIP) of adult, male alcohol-preferring P and -nonpreferring NP rats. Using in vivo microdialysis coupled with HPLC and electrochemical detection, the effects of acute administration of saline or 1.0, 1.75, or 2.5 g/kg ethanol on the extracellular levels of 5-HT in the vHIP were examined. Saline and 1.0 g/kg ethanol did not alter the extracellular levels of 5-HT. However, the 1.75-g/kg dose resulted in a transient increase in 5-HT levels in the vHIP of P rats only. Administration of 2.5 g/kg ethanol increased 5-HT levels to 180% of baseline in P rats (P<.05), but was without effect on NP rats. The 2.5-g/kg dose also significantly increased the extracellular levels of 5-HT in the vHIP of P rats, which had been pretreated with the same dose of ethanol 18–24 h earlier (P<.05). Comparison of the response of ethanol pretreated P rats with animals that had been pretreated with saline 24 h earlier did not reveal any significant differences in ethanol-stimulated increases in 5-HT levels between the groups. These data suggest that ethanol may activate terminals of the median raphe 5-HT system in P rats because the vHIP receives its 5-HT inputs primarily from the median raphe nucleus (MRN). Rapid tolerance does not develop to this activation of the system in the vHIP of P rats. In addition, the data suggest that the 5-HT system in the vHIP of NP rats may be relatively insensitive to the stimulating effect of acute ethanol of 5-HT release.

Introduction

In vivo microdialysis studies have shown that acute intraperitoneal administration of ethanol increases the extracellular levels of serotonin (5-HT) in the nucleus accumbens of Wistar (Yoshimoto et al., 1992a) and the selectively bred high alcohol-drinking (HAD) and low alcohol-drinking (LAD) lines of rats (Yoshimoto et al., 1992b), and in the ventral hippocampus (vHIP) of Wistar rats (Bare et al., 1998). Additionally, in Wistar rats, rapid tolerance develops to the ethanol-stimulated increase in 5-HT levels in the vHIP (Bare et al., 1998), an area that receives 5-HT innervations primarily from the median raphe nucleus (MRN) (Azmitia and Segal, 1978).

Several studies have suggested a role for 5-HT in the development of tolerance to the motor-impairing and hypothermic effects of ethanol. Depletion of brain 5-HT Frankel et al., 1978a, Frankel et al., 1978b or lesions of the 5-HT system (Le et al., 1980) reduce the rate of tolerance development to hypothermic and motor-impairing effects of ethanol. Additionally, administration of the 5-HT precursor l-tryptophan increases the rate of tolerance development to these effects of ethanol (Le et al., 1979). The 5-HT projections from the MRN appear to be involved in mediating tolerance development to ethanol, in that electrolytic lesions of the MRN, but not the dorsal raphe nucleus (DRN), reduces the rate of tolerance development to the hypothermic and motor-impairing effects of ethanol (Le et al., 1981).

Alcohol-preferring P and -nonpreferring NP rats have been selectively bred from an outbred Wistar stock for high and low alcohol-seeking behavior, respectively (Lumeng et al., 1977). In addition to high alcohol intake, P rats differ from alcohol-nonpreferring NP rats and Wistar rats in some behavioral responses to ethanol. P rats readily develop rapid tolerance to the high dose motor-impairing effects of ethanol and this tolerance persists for three to four times longer in P rats compared to NP and Wistar rats (Gatto et al., 1987). In addition, P rats demonstrate a number of differences in serotonergic measures compared to NP rats, including lower tissue levels of 5-HT in the hippocampus and several other regions McBride et al., 1993, Murphy et al., 1982, Murphy et al., 1987. The reduced levels of 5-HT in P rats appear to be a result of reduced serotonergic innervation Zhou et al., 1991, Zhou et al., 1994a and fewer 5-HT neurons in the MRN and DRN (Zhou et al., 1994b). The reduced number of 5-HT neurons in the DRN did not result in a compensatory increase in firing rates of the remaining neurons because there was no difference in the firing rates of 5-HT neurons in the DRN between P rats and NP or Wistar rats (Morzorati and Johnson, 1999). However, Smith and Weiss (1999) have shown that P rats, compared to NP and Wistar rats, have higher basal extracellular levels of 5-HT in the nucleus accumbens, an area that receives 5-HT innervation primarily from the DRN Azmitia and Segal, 1978, Steinbusch and Nieuwenhuys, 1983.

Because of the reduced 5-HT innervation in the hippocampus of the P rat Zhou et al., 1991, Zhou et al., 1994a, and the difference in the persistence of tolerance to the motor-impairing effects of ethanol observed for the P line compared to NP and Wistar rats (Gatto et al., 1987), the present study was undertaken to examine the effects of acute intraperitoneal ethanol administration on the response of MRN 5-HT terminals in the vHIP of P and NP rats. Currently, there are no data available on the effects of ethanol administration on components of the MRN 5-HT system of P and NP rats. Therefore, the present study was undertaken to examine the effects of acute ethanol administration on the extracellular levels of 5-HT in the vHIP of P and NP rats, and to determine if prior exposure to a high dose of ethanol alters the response of this 5-HT system to a subsequent ethanol challenge given 24 h later. The hypothesis to be tested is that the vHIP 5-HT systems of P and NP rats are sensitive to the acute effects of ethanol and that rapid tolerance will develop to the effects of ethanol on this 5-HT system in P but not NP rats.

Section snippets

Animals

Adult, male alcohol-preferring P and -nonpreferring NP rats from generations 43–48, weighing 250–350 g at the time of surgery, were housed individually in temperature- and humidity-controlled rooms on a normal 12-h light/dark cycle (lights on at 7:00 a.m.). Food and water were available ad libitum. The animals used in this experiment were maintained in facilities fully accredited by the Association for the Assessment and Accreditation of Laboratory Animal Care (AAALAC). All research protocols

Results

Representative probe placements for the animals used in this study are shown in Fig. 1. Only animals with verified probe placements in the vHIP were included in the data analysis. All were in the vHIP within approximately 1.5 mm of each other. Probes were placed between 4.52 and 6.04 mm caudal of bregma according to the atlas of Paxinos and Watson (1998).

Basal extracellular 5-HT levels did not differ between P (n=27) and NP rats (n=28) pretreated with saline (164±24 and 119±18 pM, respectively,

Discussion

Results from this study suggest that ethanol activates the MRN terminals in the vHIP when moderate to high doses are administered to alcohol-preferring P rats but not to alcohol-nonpreferring NP rats, and that rapid tolerance does not develop to the ethanol-stimulated increase in extracellular 5-HT levels in the vHIP of P rats. These conclusions are supported by the findings that ethanol increased the extracellular 5-HT levels in the vHIP of P but not NP rats in a dose-related manner (Fig. 2).

Acknowledgements

We gratefully acknowledge the technical assistance of Thomas Zinski, Allison R. Jones, Tiffany E. Hill, and Carol Kulesavage. This study was supported in part by NIAAA Grant Nos. AA07611 and AA10721.

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