Chronic water restriction reduces sensitivity to brain stimulation reward in male and female rats

Abstract

States of physiological need motivate individuals to seek and consume stimuli that restore homeostatic balance. This goal-directed behavior is driven, in part, by pathways that process reward and are sensitive to changes in physiological state, including the mesolimbic dopamine system. The effects of hunger and its physiological markers have been more widely studied for their role in modulating reward signaling pathways. However, fluid need produces robust goal-directed behavior and has also been shown to affect neural substrates of reward processing. To test how acute and chronic states of thirst might alter reward sensitivity, we used the intracranial self-stimulation (ICSS) rate-frequency paradigm (Carlezon & Chartoff, 2007) with male and female Long Evans rats. We hypothesized that sensitivity to ICSS would increase under an acute need state for water and would decrease under chronic deprivation. We found that acute water deprivation for 22-hours prior to the ICSS session did not alter any parameters of reward sensitivity. To elicit motivated behavior toward water in the absence of physiological need, we chemogenetically activated glutamatergic neurons of the subfornical organ (SFO). Despite eliciting more water consumption than acute deprivation, acute chemogenetic activation of SFO neurons also did not alter reward sensitivity. Finally, subjects underwent a five-day chronic water restriction protocol with daily ICSS sessions to determine the effects of sustained physiological need. Chronic water restriction resulted in reduced sensitivity to ICSS. Together, these results indicate that persistent changes in physiological state alter the responsiveness of reward circuitry that could potentially exacerbate maladaptive reward-seeking behaviors.

Figure 1.

Methods. A) Stimulation frequencies for each block within one pass of rate-frequency measurement. On each trial one stimulation frequency is available. The trial begins with 5 500-ms stimulation train (1 per second), followed by 50s access to a lever to deliver stimulation on an FR1 schedule. At the conclusion of the 50s, the lever retracts for a 5s inter-trial interval. The frequency on the next trial decreases on step in a logarithmic function from 141 Hz to 28 Hz until all 15 frequencies are tested (15 min per pass). Five passes are available in a single test session. B) Experimental timeline for acute water deprivation and chemogenetic activation of SFO neuron sessions. C) Experimental timeline for chronic water restriction. D) Example rate-frequency curve from one pass. Rate-frequency curves are described by Theta, the minimum frequency to support pressing; M-50, the frequency that supports the mid-level of pressing from the linear portion of the curve; and Alpha, the minimum frequency that supports maximal pressing rate.

Figure 2.

Histological verification of electrode placements targeting the medial forebrain bundle at the level of the lateral hypothalamus (−2.8 AP, +/− 1.7 ML, −7.8 DV from dura). Male subject placements are displayed as black circles, while female subject placements are displayed as grey circles.

Figure 3.

Subject body weights for A) Acute water deprivation baseline and treatment, B) SFO DREADD and CNO Control baseline and treatment, and C) Chronic water restriction baseline and treatment days.

Figure 4.

Acute water deprivation (A), SFO DREADD activation (B), and CNO control (C) change from baseline for theta, M-50, alpha, max lever pressing per trial, and total lever pressing per pass. No effect on rate-frequency or lever pressing values was observed. Water deprivation and SFO activation resulted in an increase in post ICSS 1-hour water consumption (D). SFO DREADD activation water consumption was significantly greater than 22-hour water deprivation water consumption. No significant relationship between 1-hr water intake and theta values was observed for any acute conditions (E). * p <.05, ** p <.01, *** p <.001.

Figure 5.

SFO DREADD expression co-labeled with cFos. A) Representative image of SFO CaMKIIa-hM3Dq-mCherry expression (red). B) Representative image of cFos immunoreactivity (green). C) Co-labeled SFO CaMKIIa-hM3Dq-mCherry and cFos expression. D) Quantification of cFos expression in SFO DREADD subjects. CNO infusion 90 minutes prior to perfusion produced significant expression of cFos in the SFO compared to vehicle. * p <.05.

Figure 6.

Chronic water restriction daily change compared to baseline for A) theta, B) M-50, C) alpha, D) max lever pressing per trial, E) total lever pressing per pass, and F) water consumption. Theta was significantly increased on days two, three, and four of restriction. Water consumption during ad lib water access increased significantly following day 5 of restriction compared to baseline. * p <.05, ** p <.01, *** p <.001.