Site-specific microinjection of baclofen into the anterior ventral tegmental area reduces binge-like ethanol intake in male C57BL/6J mice

Abstract

The GABAB agonist baclofen has been shown to alter ethanol intake in human and animal studies (E. M. Moore et al., 2007). GABA-subB receptors are located within the ventral tegmental area (VTA; A. Imperato & G. DiChiara, 1986) and therefore may be involved in modulating voluntary ethanol intake. The present study assessed the effects of baclofen in a variation on a new mouse model of binge-like ethanol intake that takes advantage of the nocturnal nature of this species (J. S. Rhodes, K. Best, J. K. Belknap, D. A. Finn, & J. C. Crabbe, 2005; J. S. Rhodes et al., 2007). Baclofen or saline was microinjected into the anterior or posterior VTA of male C57BL/6J mice. Immediately afterward, mice were presented with ethanol, water, or sugar water using the Drinking in the Dark model, a procedure of fluid administration for 2 hr, 3 hr into the dark cycle). Fluid intake was recorded at 30, 60, 90, and 120 min; retro-orbital sinus bloods were sampled upon termination of the 120-min ethanol access period. Baclofen reduced binge-like ethanol intake when microinjected into the anterior VTA, whereas posterior VTA microinjections did not alter ethanol intake. Baclofen had no effect on water or sugar water intake when administered to anterior or posterior VTA. These results add to the growing literature suggesting that GABA-subB receptor systems are important in the modulation of binge-like ethanol intake and suggest that the GABA-subB receptor system may have different roles in anterior versus posterior VTA.

Figure 1

Drinking in the Dark microinjection procedures. Animals were given DID access to ethanol or water for 7 days before being surgically implanted with bilateral guide cannulae. DID access resumed on experimental day 10, and habituation to the microinjection procedure was gradually increased immediately prior to DID access over the next 6 days. Microinjections were administered on day 16.

Figure 2

Cannulae placement. A. Location of microinjection tracts in the posterior VTA. B. Location of microinjection tracts in the anterior VTA. Black circles represent microinjection tract placements in animals given DID access to ethanol prior to drug delivery; grey squares represent microinjection tract placements in animals with DID access to water. Figures were adapted from Franklin & Paxinos (1997).

Figure 3

Daily pattern of ethanol intake in male C57BL/6J mice. A. The pattern of ethanol intake for male mice was relatively stable over days 1–7 of the procedure (n=37). B. The pattern of ethanol intake over days 9–14 was somewhat more variable (n=18–19).

Figure 4

Pattern of ethanol intake following intra-VTA microinjection of baclofen (30 min time bins). A. Ethanol intakes following anterior VTA baclofen microinjection at both concentrations are significantly reduced compared to saline microinjection at all time points (n=6). B. Ethanol intakes following posterior VTA baclofen microinjections do not differ from saline controls at any time point (n=6–7).

Figure 5

Effect of baclofen microinjected into either the anterior or posterior VTA on 2 h binge-like ethanol intake and corresponding BECs. A. Baclofen reduced binge-like ethanol intake in the anterior VTA but had no effect in the posterior VTA (n=6–7). B. Blood ethanol concentrations followed the same pattern of ethanol intake following baclofen microinjections (n=4–5).

Figure 6

Daily pattern of water intake in male C57BL/6J mice. A. The pattern of water intake prior to surgery for male mice was relatively stable over days 1–7 of the procedure (n=27). B. The pattern of water intake over days 9–14 was more variable (n=13–14).

Figure 7

Pattern of water intake following intra-VTA microinjection of baclofen (30 min time bins). A. Anterior intra-VTA baclofen microinjection did not alter water intake (n=6–7). B. Posterior intra-VTA baclofen microinjections also did not alter water intake (n=7).

Figure 8

Effect of baclofen microinjected into either the anterior or posterior VTA on water intake. Baclofen had no effect on water intake in either the anterior or posterior VTA, suggesting that the effects of baclofen do not extend to general fluid intake (n=6–7).