Physiology and Behavior, Vol. 14, pp. 477-481. Brain Research Publications Inc., 1975. Printed in the U.S.A.
Polyethylene Glycol Induced Thirst: A Dual Stimulatory Mechanism? CHARLES S. WEISS AND C. ROBERT ALMLI 2
Department o f Psychology, Ohio University, Athens, Ohio 45701
(Received 29 August 1974) WEISS, C. S. AND C. R. ALML1. Polyethylene glycol induced thirst: a dual stimulatory mechanism? PHYSIOL. BEHAV. 14(4) 477-481, 1975.- Hyperoncotic colloid dialysis with polyethylene glycol (PG) resulted in significant water intakes for rats within 1 hr post-treatment, when water was made immediately available. Associated with reliable drinking were azotemia (52%) and hypernatremia (6%), but not hypovolemia. When PG treated rats were delayed without food-water for 6 hr post-PG treatment, the greatest volumes of water were consumed and the rats were both azotemic (111%) and hypovolemic (11-25%). The dipsogenic properties of urea were tested, and demonstrated. The combined data suggest that PG treatment may stimulate thirst via cellular (azotemia and/or hypernatremia) and extracellular (hypovolemia) mechanisms. Albino rats Azotemia
Polyethylene glycol Waterintake UREA-NaC1 Cellular thirst Hypovolemia Hypematremia Extracellular thirst
HYPERONCOTIC colloid dialysis with polyethylene glycol (PG) solution has been considered by investigators to be solely an extracellular stimulus for thirst [8,19], and even has been used in neurological investigations as a dipsogenic control for assessing deficits in cellular thirst [3]. However, data are available which question the purity of PG as a pure extracellular stimulus. Almli [2] measured vascular conditions of rats at the behavioral onset of drinking and 1 hr post-PG treatment, and found that the onset of drinking was associated with hypervolemia (not hypovolemia) and after 1 hr, plasma osmolality was elevated. In addition, Stricker [18] has reported elevated blood urea in response to PG treatment. While modest, the dipsogenic properties of urea have been demonstrated [1, 6, 7, 9, 10, 14]. These data raise questions as to the ultimate dipsogenic stimulatory mechanism of PG treatment. T h e present investigation evaluates the dipsogenic properties of PG treatment, by measuring drinking behaviors (Experiment 1) and serum conditions (Experiment 2) following PG treatment. In Experiment 3, the dipsogenic properties of urea are evaluated.
Latency to drink
25°C) room, under continuous illumination. Purina rat chow and tap water were freely available in the home-cage unless otherwise specified. Apparatus and procedure. Four groups (N = 4) of rats had water intakes measured following polyethylene glycol (PG) or sham (AL) injections. The PG injections were 5 cm 3 of 25% solution (PG = 20,000 MW, 0.15 M NaC1 as solvent) and the AL injections consisted of placing the needle under the skin. All injections were subcutaneous in the hind quarters region. All rats were adapted to individual drinking boxes (equip~ped with water tubes calibrated at 0.2 ml) for five days at one hour per day prior to treatments. Eight rats each received either PG (N = 4) or AL (N = 4) injection and were immediately given access to water in the drinking boxes for 12 hr with intake to nearest 0.2 ml measured each hour, i.e., groups P G - 1 2 and A L - 1 2 , respectively. Eight more rats received either PG (N = 4) or AL (N = 4) injection, were delayed in the home cage without food or water for 6 hr, then were given access to water for 6 hr with intake measured each hour, i.e., groups P G - D 6 and A L - D 6 , respectively. Food was never present in the drinking boxes.
EXPERIMENT 1
Me th od
Results
Animals. Sixteen male albino rats (Holtzman strain) at
All results of these experiments were analyzed with analysis of variance and two-tailed t-tests. Presented in Table 1 are mean (_+ estimated standard
9 0 - 1 1 0 days of age were housed in individual rackmounted wire cages in a temperature controlled (22 ° -
1This research was supported by NIMH Grant No. 22482-01 and NICHHD Grant No. HD 08504-01 to C. Robert Almli. We thank James Class and O'Bleness Memorial Hospital for vascular analysis, and R. T. Lemer for assistance in collection of data. 2Address reprint requests to: C. R. Almli, Department of Psychology, Ohio University, Athens, Ohio 45701. 477
478
WEISS AND ALMLI TABLE 1 WATER INTAKES (X -+ S.E.) IN ML FOR RATS FOLLOWING POLYETHYLENE GLYCOL AND SHAM INJECTIONS
0-1 Hr
Water Intake by Intervals 0-6 Hr 7-12 Hr
Total Intake 6 Hr
12 Hr
AL-12 N=4
0.35 +- 0.24
1.40 +- 0.62
0.90 _+0.47
2.30 +- 0.61
PG-12 N=4
1.20 +- 0.36
5.10 ± 0.51
3.00 + 0.44
8.10 ± 0.40
AL-D6 N=4
1.65 -+ 0.51
3.92 ± 0.27
3.92 -+ 0.27
PG-D6 N=4
12.35 -+ 2.10
17.70 +- 2.37
17.70 ± 2.37
error of the mean, S.E.) water intake data (ml) of rats following the PG and AL treatments. The P G - D 6 treatment resulted in the greatest volume of water drunk at each measurement interval, and regardless of whether the total access to water was 6 or 12 hr (p< 0.01). The P G - 1 2 treatment was the second most potent dipsogenic treatment, resulting in 6 hr and total water intake greater than either A L - 1 2 or A L - D 6 (p0.05), and both treatments resulted in greater drinking than did A L - 12 (p< 0.05). Thus, when compared to the control groups ( A L - D 6 and A L - 1 2 ) the PG treated groups (with or without a delay between injection and access to water) drank reliably greater volumes of water. However, the dipsogenic properties of PG injection were potentiated with the addition of a delay, in spite of the fact that the nondelayed group had twice as much time to drink. The AL treatments resulted in a similar delay effect, the A L - D 6 group drank reliably more than the A L - 1 2 group even though the latter group had 12 hr as opposed to 6 hr of access to water. EXPERIMENT 2 The purpose of this experiment was to measure serum conditions of PG and AL treated rats in an attempt to determine serum changes associated with (a) reliable drinking following PG injection and (b) the potentiating effect upon drinking of the 6 hr delay for both PG and AL treatments. Method Animals. Sixty-six male albino rats (N = 12 per group) received PG injection and were delayed in the home cage without food or water for various lengths of time prior to blood sampling. Blood samples were withdrawn at 1, 2, 4, and 6 hr post-injection. Two additional groups of rats were tested; one group (N = 6) received PG injection and had access to water for 6 hr, while the other group (N = 6) received sham-injection (needle under skin) and were delayed in the home cage for 6 hr where food and water were absent. Both groups had blood sampled at the end
of 6 hr. A final ad lib group (N = 6) had blood sampled immediately following sham-injection. The PG injections were identical in concentration, volume, and route to those used in Experiment 1. Blood samples were withdrawn under light ether anesthesia via heart puncture using 5 cm 3 syringes. The whole blood was centrifuged and analyzed for hematocrit (Guest card), and the serum was analyzed for protein concentration (refractometer), sodium, potassium (flame photometer) and urea-nitrogen (diacetyl monoximespectrophotometer method [5 ] ). Results
Presented in Table 2 are mean (+- S. E.) serum data of rats following: (1) sham-injection, (2) sham-injection plus 6 hr delay (food and water absent during the delay), (3) PG injections with delays of 1, 2, 4, and 6 hr (no food or water present), and (4) PG injections with 6 hr of drinking. In Experiment 1, reliable drinking was found to occur within the first hour for rats injected with PG and given immediate access to water (i.e., P G - 12 treatment ( 1). Of the 5 blood conditions measured at 1 hr p o s t - P G injection, only sodium and urea were reliably elevated over ad lib control levels. During the first hour, sodium had reliably increased by 6% over control levels and urea had increased by 52% (p 0.05, respectively). The remaining serum variables, however, did not show reliable elevations until 3 - 5 hr after the first measurement of reliable drinking, i.e., at the fourth and sixth hours of the delay. Potassium was reliably elevated over control levels by 44% at the end of the 6 hr delay (p
Polyethylene Glycol Induced Thirst: A Dual Stimulatory Mechanism? CHARLES S. WEISS AND C. ROBERT ALMLI 2
Department o f Psychology, Ohio University, Athens, Ohio 45701
(Received 29 August 1974) WEISS, C. S. AND C. R. ALML1. Polyethylene glycol induced thirst: a dual stimulatory mechanism? PHYSIOL. BEHAV. 14(4) 477-481, 1975.- Hyperoncotic colloid dialysis with polyethylene glycol (PG) resulted in significant water intakes for rats within 1 hr post-treatment, when water was made immediately available. Associated with reliable drinking were azotemia (52%) and hypernatremia (6%), but not hypovolemia. When PG treated rats were delayed without food-water for 6 hr post-PG treatment, the greatest volumes of water were consumed and the rats were both azotemic (111%) and hypovolemic (11-25%). The dipsogenic properties of urea were tested, and demonstrated. The combined data suggest that PG treatment may stimulate thirst via cellular (azotemia and/or hypernatremia) and extracellular (hypovolemia) mechanisms. Albino rats Azotemia
Polyethylene glycol Waterintake UREA-NaC1 Cellular thirst Hypovolemia Hypematremia Extracellular thirst
HYPERONCOTIC colloid dialysis with polyethylene glycol (PG) solution has been considered by investigators to be solely an extracellular stimulus for thirst [8,19], and even has been used in neurological investigations as a dipsogenic control for assessing deficits in cellular thirst [3]. However, data are available which question the purity of PG as a pure extracellular stimulus. Almli [2] measured vascular conditions of rats at the behavioral onset of drinking and 1 hr post-PG treatment, and found that the onset of drinking was associated with hypervolemia (not hypovolemia) and after 1 hr, plasma osmolality was elevated. In addition, Stricker [18] has reported elevated blood urea in response to PG treatment. While modest, the dipsogenic properties of urea have been demonstrated [1, 6, 7, 9, 10, 14]. These data raise questions as to the ultimate dipsogenic stimulatory mechanism of PG treatment. T h e present investigation evaluates the dipsogenic properties of PG treatment, by measuring drinking behaviors (Experiment 1) and serum conditions (Experiment 2) following PG treatment. In Experiment 3, the dipsogenic properties of urea are evaluated.
Latency to drink
25°C) room, under continuous illumination. Purina rat chow and tap water were freely available in the home-cage unless otherwise specified. Apparatus and procedure. Four groups (N = 4) of rats had water intakes measured following polyethylene glycol (PG) or sham (AL) injections. The PG injections were 5 cm 3 of 25% solution (PG = 20,000 MW, 0.15 M NaC1 as solvent) and the AL injections consisted of placing the needle under the skin. All injections were subcutaneous in the hind quarters region. All rats were adapted to individual drinking boxes (equip~ped with water tubes calibrated at 0.2 ml) for five days at one hour per day prior to treatments. Eight rats each received either PG (N = 4) or AL (N = 4) injection and were immediately given access to water in the drinking boxes for 12 hr with intake to nearest 0.2 ml measured each hour, i.e., groups P G - 1 2 and A L - 1 2 , respectively. Eight more rats received either PG (N = 4) or AL (N = 4) injection, were delayed in the home cage without food or water for 6 hr, then were given access to water for 6 hr with intake measured each hour, i.e., groups P G - D 6 and A L - D 6 , respectively. Food was never present in the drinking boxes.
EXPERIMENT 1
Me th od
Results
Animals. Sixteen male albino rats (Holtzman strain) at
All results of these experiments were analyzed with analysis of variance and two-tailed t-tests. Presented in Table 1 are mean (_+ estimated standard
9 0 - 1 1 0 days of age were housed in individual rackmounted wire cages in a temperature controlled (22 ° -
1This research was supported by NIMH Grant No. 22482-01 and NICHHD Grant No. HD 08504-01 to C. Robert Almli. We thank James Class and O'Bleness Memorial Hospital for vascular analysis, and R. T. Lemer for assistance in collection of data. 2Address reprint requests to: C. R. Almli, Department of Psychology, Ohio University, Athens, Ohio 45701. 477
478
WEISS AND ALMLI TABLE 1 WATER INTAKES (X -+ S.E.) IN ML FOR RATS FOLLOWING POLYETHYLENE GLYCOL AND SHAM INJECTIONS
0-1 Hr
Water Intake by Intervals 0-6 Hr 7-12 Hr
Total Intake 6 Hr
12 Hr
AL-12 N=4
0.35 +- 0.24
1.40 +- 0.62
0.90 _+0.47
2.30 +- 0.61
PG-12 N=4
1.20 +- 0.36
5.10 ± 0.51
3.00 + 0.44
8.10 ± 0.40
AL-D6 N=4
1.65 -+ 0.51
3.92 ± 0.27
3.92 -+ 0.27
PG-D6 N=4
12.35 -+ 2.10
17.70 +- 2.37
17.70 ± 2.37
error of the mean, S.E.) water intake data (ml) of rats following the PG and AL treatments. The P G - D 6 treatment resulted in the greatest volume of water drunk at each measurement interval, and regardless of whether the total access to water was 6 or 12 hr (p< 0.01). The P G - 1 2 treatment was the second most potent dipsogenic treatment, resulting in 6 hr and total water intake greater than either A L - 1 2 or A L - D 6 (p0.05), and both treatments resulted in greater drinking than did A L - 12 (p< 0.05). Thus, when compared to the control groups ( A L - D 6 and A L - 1 2 ) the PG treated groups (with or without a delay between injection and access to water) drank reliably greater volumes of water. However, the dipsogenic properties of PG injection were potentiated with the addition of a delay, in spite of the fact that the nondelayed group had twice as much time to drink. The AL treatments resulted in a similar delay effect, the A L - D 6 group drank reliably more than the A L - 1 2 group even though the latter group had 12 hr as opposed to 6 hr of access to water. EXPERIMENT 2 The purpose of this experiment was to measure serum conditions of PG and AL treated rats in an attempt to determine serum changes associated with (a) reliable drinking following PG injection and (b) the potentiating effect upon drinking of the 6 hr delay for both PG and AL treatments. Method Animals. Sixty-six male albino rats (N = 12 per group) received PG injection and were delayed in the home cage without food or water for various lengths of time prior to blood sampling. Blood samples were withdrawn at 1, 2, 4, and 6 hr post-injection. Two additional groups of rats were tested; one group (N = 6) received PG injection and had access to water for 6 hr, while the other group (N = 6) received sham-injection (needle under skin) and were delayed in the home cage for 6 hr where food and water were absent. Both groups had blood sampled at the end
of 6 hr. A final ad lib group (N = 6) had blood sampled immediately following sham-injection. The PG injections were identical in concentration, volume, and route to those used in Experiment 1. Blood samples were withdrawn under light ether anesthesia via heart puncture using 5 cm 3 syringes. The whole blood was centrifuged and analyzed for hematocrit (Guest card), and the serum was analyzed for protein concentration (refractometer), sodium, potassium (flame photometer) and urea-nitrogen (diacetyl monoximespectrophotometer method [5 ] ). Results
Presented in Table 2 are mean (+- S. E.) serum data of rats following: (1) sham-injection, (2) sham-injection plus 6 hr delay (food and water absent during the delay), (3) PG injections with delays of 1, 2, 4, and 6 hr (no food or water present), and (4) PG injections with 6 hr of drinking. In Experiment 1, reliable drinking was found to occur within the first hour for rats injected with PG and given immediate access to water (i.e., P G - 12 treatment ( 1). Of the 5 blood conditions measured at 1 hr p o s t - P G injection, only sodium and urea were reliably elevated over ad lib control levels. During the first hour, sodium had reliably increased by 6% over control levels and urea had increased by 52% (p 0.05, respectively). The remaining serum variables, however, did not show reliable elevations until 3 - 5 hr after the first measurement of reliable drinking, i.e., at the fourth and sixth hours of the delay. Potassium was reliably elevated over control levels by 44% at the end of the 6 hr delay (p
