Behavior-associated and post-consumption glucose entry into the nucleus accumbens extracellular space during glucose free-drinking in trained rats

Abstract

Glucose is the primary energetic substrate for the metabolic activity of brain cells and its proper delivery from the arterial blood is essential for neural activity and normal brain functions. Glucose is also a unique natural reinforcer, supporting glucose-drinking behavior without food or water deprivation. While it is known that glucose enters brain tissue via gradient-dependent facilitated diffusion, it remains unclear how glucose levels are changed during natural behavior and whether the direct central action of ingested glucose can be involved in regulating glucose-drinking behavior. Here, we used glucose biosensors with high-speed amperometry to examine the pattern of phasic and tonic changes in extracellular glucose in the nucleus accumbens (NAc) during unrestricted glucose-drinking in well-trained rats. We found that the drinking behavior is highly cyclic and is associated with relatively large and prolonged increases in extracellular glucose levels. These increases had two distinct components: a highly phasic but relatively small behavior-related rise and a larger tonic elevation that results from the arrival of consumed glucose into the brain's extracellular space. The large post-ingestion increases in NAc glucose began minutes after the cessation of drinking and were consistently associated with periods of non-drinking, suggesting that the central action of ingested glucose could inhibit drinking behavior by inducing a pause in activity between repeated drinking bouts. Finally, the difference in NAc glucose responses found between active, behavior-mediated and passive glucose delivery via an intra-gastric catheter confirms that motivated behavior is also associated with metabolic glucose use by brain cells.

Keywords: cerebral blood flow; drinking behavior; enzyme-based glucose sensors; glucose; high-speed amperometry; metabolism; neuronal activity.

Figure 1

Dynamic changes in nucleus accumbens (NAc) extracellular glucose during the cue-initiated drinking bout. Panel (A) shows overall changes in glucose for 60 min after the cue-initated bout (30-s bins). (B) shows rapid changes in glucose (2-s bins) after the presentation of the drinking tube, while (C,D) show changes in glucose (4-s bins) at the start of drinking and end of drinking, respectively. There was a significant increase in [glucose] for 67 s after the tube presentation (A, F_(6,204) = 1.51, p < 0.05), prior to the onset of drinking (horizontal lines in B–D). At the onset of drinking, there was a significant decrease in glucose for 138 s (C, F_(6,210) = 1.51, p < 0.05), when the majority of rats were engaged in drinking. At the end of drinking, there was a significant increase in glucose levels (D, F_(6,540)) = 13.94, p < 0.05). Shaded areas represent the duration of the main effect in (B–D), while filled symbols show individual points that were significant when compared to baseline using a Fisher test. (E) shows the NAc glucose response during 1-min presentation of a novel object.

Figure 2

Dynamic changes in NAc extracellular glucose during the self-initiated drinking bout. Panel (A) shows overall changes for 30 min before and after the entire second, self-initiated bout (30-s bins), where there was a significant change in [glucose] for the entire analysis interval (F_(5,450) = 8.44, p < 0.05). Individual points significantly different from the [glucose] nadir (7 min before the onset of drinking, −25.3 μM), are shown as filled symbols. (B,C) show rapid changes in glucose (4-s bins) after the start and end of self-initiated drinking, respectively. While there was no significant change in [glucose] after the start of drinking, there was a significant increase overall after the end of drinking (F_(5,450) = 5.50, p < 0.05). No individual point in (C) was significantly different from baseline. Durations of drinking in each subject are shown as horizontal lines in (B,C), shaded areas represent the duration of the main effect on each graph. Due to technical complications, one subject was removed from this analysis (n = 6).

Figure 3

Fluctuations in NAc [glucose] during both drinking bouts relative to the initial drinking bout’s baseline. Panel (A) shows overall changes in glucose concentrations during periods of drinking (8-s bins) and no drinking (30-s bins), with respect to the first bout’s initial baseline (=0), and are combined into one common time-scale. Data shown for the median time periods for each behavior (pre-drinking 1: −59 s, duration of drinking 1: 266 s, time between end of drinking 1 and start of drinking 2: 4253 s duration of drinking 2: 177 s). (B) shows the glucose concentration from all individual subjects (4 min bins) during drinking and no drinking periods in (A). Bout 1 is defined as the period from the onset of drinking 1 to the first peak in glucose concentration (1180 s), bout 2 is defined as the time between the first peak in glucose concentration 2, through the second drinking episode, to the peak in glucose concentration after the second bout (1210 s to 5494 s). Lines in (B) show mean ± SEM glucose concentration relative to the start of Drinking 1 (Drinking 1: −19.3 ± 11.74 μM, No Drinking 1: 116.5 ± 13.96 μM, Pre-Drinking 2: 49.69 ± 12.07 μM, Drinking 2: 23.07 ± 17.53 μM, No Drinking 2: 95.14 ± 17.17 μM).

Figure 4

Changes in NAc [glucose] induced by intra-gastric glucose injections. Panel (A) shows overall changes in NAc glucose for 60 min after intra-gastric glucose injections (4 and 8 ml) and after drinking (mean 5.17 ml) in the same rats (30-s bins). (B) compares the duration and magnitude of the response (as area under the curve) for each condition in (A), where there was an overall main effect (One-Way RM ANOVA, F_(2,4) = 9.45, p < 0.05), and the difference between 8 mg and drinking approached significance p = 0.051. (C) compares the initial response between all rats when they began drinking as a result of the tube presentation, and when a subset of rats received an intragastric injection of glucose (4 and 8 mg), relative to the pre-presentation and pre-injection baseline, respectively. During this time interval the response to a 4 and 8 ml intragastric injection was not significantly different, and was combined. For clarity, error bars not shown.