Dissociation of thirst and sodium appetite in the furo/cap model of extracellular dehydration and a role for N-methyl-D-aspartate receptors in the sensitization of sodium appetite

Abstract

Depletion of extracellular fluids motivates many animals to seek out and ingest water and sodium. Animals with a history of extracellular dehydration display enhanced sodium appetite and, in some cases, thirst. The progressive increase in sodium intake induced by repeated sodium depletions is known as sensitization of sodium appetite. Administration of the diuretic and natriuretic drug, furosemide, along with a low dose of captopril (furo/cap), elicits thirst and a rapid onset of sodium appetite. In the present studies the furo/cap model was used to explore the physiological mechanisms of sensitization of sodium appetite. However, when thirst and sodium appetite were measured concurrently in the furo/cap model, individual rats exhibited sensitization of either thirst or sodium appetite. In subsequent studies, thirst and sodium appetite were dissociated by offering either water prior to sodium or sodium before water. When water and sodium intake were dissociated in time, the furo/cap model reliably produced sensitization of sodium appetite. It is likely that neuroplasticity mediates this sensitization. Glutamatergic N-methyl-d-aspartate receptor (NMDA-R) activation is critical for the development of most forms of neuroplasticity. Therefore, we hypothesized that integrity of NMDA-R function is necessary for the sensitization of sodium appetite. Pharmacological blockade of NMDA-Rs with systemic administration of MK-801 (0.15 mg/kg) prevented the sensitization of fluid intake in general when water and sodium were offered concurrently, and prevented sensitization of sodium intake specifically when water and sodium intake were dissociated. The involvement of NMDA-Rs provides support for the possibility that sensitization of sodium appetite is mediated by neuroplasticity.

Figure 1

Dissociation of water and 1.8% saline intake in the water-first/saline-second furo/cap protocol. Non-cumulative intakes are presented on the y-axis and time is on the x-axis. Rats were offered water immediately after furo/cap treatment and allowed to drink until satiation which occurs at approximately 90 minutes. Saline access was then offered in addition to water (arrow). During this time rats drank hypertonic saline and essentially no water. All data are expressed as mean ± SEM.

Figure 2

Dissociation of water and saline intake in the saline-first/water-second protocol. Non-cumulative intakes are presented on the y-axis and time is on the x-axis. Rats were offered saline 90 minutes after furo/cap treatment. Water was provided after 90 minutes of saline access (arrow). While water was available rats drank water and miniscule amounts of saline. All data are expressed as mean ± SEM.

Figure 3

Sensitization of saline intake in the furo/cap water-first/saline-second protocol. Repeated furo/cap treatments induce a progressive elevation in saline intake (A), but no change in water intake (B) or total fluid intake (C). Over repeated depletions saline intake comprises a greater percentage of fluid intake (D), suggesting a shift in preference from water to saline intake in the water-first/sodium-second protocol (*=p<0.01 vs. depletion 1). All data are expressed as mean, and where error bars appear, ± SEM. Please note differences in ordinate across graphs.

Figure 4

Sensitization of saline intake in the water-first/saline-second protocol is not dependent on a decrease in water intake. Total 1.8% saline intake is displayed on the y-axis. When offered only 4 mls of water to drink during each test, rats displayed a progressive elevation of saline intake (*=p<0.05 vs. depletion 1, #=p<0.01 vs. depletion 1), similar to rats with unlimited water access. All data are expressed as mean ± SEM.

Figure 5

Sensitization of saline intake and total fluid intake in the furo/cap sodium-first/water-second protocol. 1.8% saline intake rapidly sensitizes upon repeated furo/cap treatments (A). Water intake does not significantly change over depletions (B). However, rats exhibit a progressive sensitization of total fluid intake (C). No significant differences in the composition of water or sodium during each depletion were observed (D). *=p<0.05 vs. depletion 1. All data are expressed as mean, and where error bars appear, ± SEM. Please note differences in ordinate across graphs.

Figure 6

Effect of MK-801 pretreatment on sensitization of fluid intake when water and sodium were offered concurrently in the furo/cap model. Vehicle pretreated rats exhibited sensitized fluid intake by the second depletion (*=p<0.005 vs vehicle, depletion 1) while MK-801 pretreated rats failed to show any increase in total fluid intake (#=p<0.05 vs vehicle, depletion 3). No significant differences were observed between vehicle and MK-801 pretreated rats during the first or second depletions. No significant differences were observed in total fluid intake during the first depletion. All data are expressed as mean ± SEM.

Figure 7

Water and saline intakes of vehicle pretreated rats from the concurrent fluid access protocol segregated by whether they exhibited sensitization of saline intake (A) or water intake (B). MK-801 pretreated rats are represented in (C). Rats were categorized as sodium appetite sensitizers if the increase in total fluid intake between the first and third depletion consisted of a 45% or greater increase in saline intake. Two vehicle pretreated rats did not exhibit sensitization of water or saline intake and are not included in this figure. In general, vehicle pretreated rats in the concurrent fluid access protocol displayed either sensitization of water intake (A) or sensitization of saline intake (B), while MK-801 pretreated rats failed to exhibit any changes in fluid intake (C). *=p<0.05 vs depletion 1, #=p<0.01 vs. depletion 2. All data are expressed as mean ± SEM.

Figure 8

Effect of MK-801 pretreatment on sensitization of saline intake in the saline-first/water-second protocol. Vehicle pretreated rats exhibit enhanced saline intake by the second depletion (*=p<0.01 vs. vehicle, depletion 1) while MK-801 pretreated rats fail to display increased intake (#=p<0.05 vs. vehicle, depletion 2). All data are expressed as mean ± SEM.