Effects of Lesions of the Ventral Ventral Median ...

Behavioral Neuroscience 1991, Vol. 105, No. 5, 721-726

Copyright 1991 by the American Psychological Association, Inc. 0735-7044/91/S3.00

Effects of Lesions of the Ventral Ventral Median Preoptic Nucleus or Subfornical Organ on Drinking and Salt Appetite After Deoxycorticosterone Acetate or Yohimbine Douglas A. Fitts University of Washington Lesions of the ventral ventral median preoptic nucleus (VVMnPO) enhanced daily salt appetite induced by subcutaneous (sc) injections of deoxycorticosterone acetate (DOCA) but did not affect acute salt appetite or water intake after sc injections of 5 mg/kg of the a-2-adrenoreceptor blocker yohimbine. Lesions of the subforaical organ (SFO) or its rostral fiber pathways had no effect on fluid intakes during DOCA treatments but significantly reduced water intake after yohimbine. These findings extend those of a previous report (Fitts, Tjepkes, & Bright, 1990) of enhanced DOCA-induced saline intake in WMnPO-lesioned rats and demonstrate that the effect is specific to lesions of the VVMnPO. The mechanism of the thirst and salt intake elicited by yohimbine is still unclear, but the SFO and its fiber pathways appear to be important for the expression of the water drinking component. Neither lesion reliably affected yohimbineinduced salt appetite.

Recently my colleagues and I proposed that the ventral part of the ventral median preoptic nucleus (WMnPO) was important for the elicitation of salt appetite by some treatments in the rat (Fitts & Masson, 1990; Fitts, Tjepkes, & Bright, 1990). Angiotensin infusions into this tissue produced consumption of a hypertonic saline solution (Fitts & Masson, 1990), and electrolytic ablation of the region reduced, but did not always abolish, saline intake in response to treatments that are presumed to be related to angiotensin, such as peripheral captopril injections and sodium depletion (Fitts et al., 1990). The lesion did not affect water intake after these treatments or after subcutaneous isoproterenol or hypertonic saline and affected neither water nor saline intake after water deprivation or combined food and water deprivation. Surprisingly, the lesion appeared actually to augment saline intake during deoxycorticosterone acetate (DOCA) treatments. In contrast, lesions of the subfornical organ (SFO), another forebrain region that is rich in angiotensin receptors (Saavedra et al., 1986), reduced water intake but not salt appetite during peripheral captopril administration (Thunhorst, Fitts, & Simpson, 1987, 1989) and only transiently reduced salt appetite after sodium depletion (Thunhorst, Ehrlich, & Simpson, 1990). The effects of SFO lesions on DOCA-induced water or saline intakes have not been examined. The purpose of the present study was to compare the effects of VVMnPO and SFO lesions on drinking and salt appetite during peripheral DOCA or yohimbine treatments. The DOCA experiments were intended to extend the original unexpected finding of enhanced saline drinking in WMnPO

lesioned rats by examining the specificity of the effect to the VVMnPO region (Fitts et al., 1990). The yohimbine experiments were designed to determine whether either of these forebrain structures is involved in the rapidly induced thirst and salt appetite after a-2-adrenoceptor blockade. The mechanisms for the yohimbine-induced effects are currently unclear (Johnson, Beltz, & Edwards, 1989,1990) but may result from a reduction of tonic a-2-adrenergic inhibition. General Method Animals Subjects were male Long-Evans rats that weighed 350-550 g at the start of the experiments. The animals were housed in hanging wire-mesh cages with Wayne lab chow, tap water, and 0.3 M NaCl solution freely available when experiments were not being conducted. Saline solutions were provided at least 1 day before lesion surgeries, so deficits in drinking induced by various stimuli were specific to the lesions and not to a lack of preoperative access (Schulkin, 1988). Temperature was controlled at 23 °C, and the room lights were on 12 hr per day.

Fluid Intake Measurements Tap water and 0.3 M NaCl in tap water were provided in graduated cylinders that were fitted with drinking tubes. Intakes were recorded to the nearest milliliter. Fresh water and saline solutions were given daily. The positions of the drinking tubes were alternated daily to control for position preferences.

Lesions This research was supported by National Institutes of Health Research Grant NS22274 to Douglas A. Fitts. I acknowledge the technical assistance of Michel Goodin and Barbara Hoffer. Correspondence concerning this article should be addressed to Douglas A. Fitts, Department of Psychology, NI-25, University of Washington, Seattle, Washington 98195.

Rats were anesthetized with Equi-Thesin (chloral hydrate, pentobarbital, and magnesium sulfate in propylene glycol and ethanol; 0.35 ml/100 g ip) for stereotaxic lesions. The skull was leveled between bregma and lambda, and a 3-mm hole was drilled in the skull near bregma. The 33-gauge tungsten wire electrode was insulated with Teflon for the entire length except for the cross-sectional area 721

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DOUGLAS A. F1TTS

at the tip. It was lowered on the midline without retracting the midsagittal sinus. VVMnPO lesions were made using a single penetration of the electrode 1 . 2 m m rostral to bregma and 7 . 4 m m ventral to the surface of the sinus; electric current was passed at 1 mA for 10 s. SFO lesions were made using three penetrations of the electrode 0.1, 0.4, and 0.7 mm caudal to bregma (and 4.6, 4.4, and 4.2 mm ventral to the sinus, respectively; electric current was passed at 1 mA for 10 s on each penetration. Sham lesions were made by lowering the electrode through the sinus and not passing current. Bone wax was used to seal the trephine hole before closing the wound. A prophylactic dose of 0.2 ml gentamicin was given intramuscularly after the surgery and Betadinc (povidone-iodine) was applied topically to the wound.

Drugs Yohimbine hydrochloride and deoxycorticosterone acetate were purchased from Sigma Chemical Company (St. Louis, MO). The vehicles were polyethylene glycol (molecular weight = 200) for yohimbine and sesame oil for DOCA (both also from Sigma). All drugs were mixed immediately before each experiment.

remained at the rostroventral pole where it could possibly have retained some reciprocal connectivity with the MnPO and OVLT, it was not considered a good SFO lesion.

Statistics Saline and water intake data were analyzed using analyses of variance (ANOVAs). Planned comparisons used Fisher's least signi ficant difference test if the F ratio was significant and the Bonferroni correction if it was not. A probability of less than .05 was required for significance.

Experiment 1: Effects of Forebrain Lesions on DOCA-Induced Salt Appetite The purpose of this experiment was to compare DOCAinduced saline intakes in rats with either VVMnPO or SFO lesions. In particular, we wished to extend a previous finding (Fitts et al., 1990) of enhanced responding in VVMnPO lesioned rats to various doses of DOCA and to examine the effects of these treatments on plasma protein and osmolality.

Blood Sampling

The effect of SFO lesions on DOCA-indueed salt appetite was studied in a separate group of rats. Both lesion treatments

At the end of one experiment, selected rats were rapidly sampled for 3 ml of arterial blood by cardiac puncture under halothane anesthesia. Blood was collected using heparinized needles within 2 min of anesthesia. The blood was centrifuged for 5 min to extract plasma. Plasma osmolality and prolein concentrations were determined immediately by freezing point depression and refractometry methods, respectively.

Histology Lesioned rats were given an overdose of sodium pentobarbital and then perfused through the heart with saline to clear blood, which was followed by 10% formal (methylal) saline for fixation. Brains were removed and stored in formal saline before being cut on a freezing microtome at 50 pm in sagittal sections. Sections were mounted on glass slides and stained with thionine, coverslipped, and examined using a microprojector or light microscope. The animals' responses on the drinking tests were not known to the experimenters who evaluated the slides. Detailed criteria for evaluating these lesions have been previously described (e.g., Fitts et al., 1990; Thunhorst etal., 1987). Briefly, to qualify as a good VVMnPO lesion, the most ventral 0.3 mm (approximately the bottom one-third) of the ventral MnPO and the dorsal pole of the organum vasculosum laminae terminalis (OVLT) had to be completely destroyed. This operational definition coincides functionally with a region of the ventral MnPO that was sensitive to angiotcnsin in producing salt appetite (Fitts & Masson, 1990). Remnants of the ventral pole or main body of the OVLT lost all dorsal connectivity with the MnPO along the rostral wall of the third ventricle; otherwise the lesions were considered partial or "missed." For this reason, the VVMnPO lesion was probably a functional lesion of Ihc OVLT as well. To qualify as a good SFO lesion, at least 90% of the SFO must have been destroyed. The SFO is a discrete nucleus that stains darkly with thionine against the overlying hippocampal commissure, making its relative degree of destruction easier to determine. It is small but elongated along the rostral wall of the dorsal third ventricle; therefore, three graded penetrations of the electrode were necessary to destroy it completely without greatly increasing the amperage and causing much accessory damage. If a fragment of any size of the SFO

were compared with appropriate sham-lesioned controls.

Method VVMnPO lesions. Rats were given lesions that were targeted at the VVMnPO (« = 70) or sham lesions (n = 33) and allowed 10 days to recover. Baseline water and 0.3 M NaCl solution intakes were measured for 4 days, which was followed by 3 days of intakes enhanced by once daily subcutaneous injections of DOCA at doses of 0.5, 1.0, 3.0, or 5.0 mg/kg in 1.0 ml/kg oil in randomly assigned groups. Blood was sampled from rats in the 3 mg/kg group 24 hr after the third daily DOCA injection for measurement of plasma protein and osmolality. SFO lesions. Seventeen rats received lesions that were targeted at the SFO, and 8 rats received sham lesions in randomly assigned groups. After 11 days of recovery, all rats received 3 mg/kg DOCA for 3 days as described for the WMnPO lesions.

Results VVMnPO lesions.

Histological analysis of the brains of

the rats in this experiment showed that 21 rats had complete lesions of the VVMnPO. Forty-five rats had lesions that either missed the VVMnPO rostrally or caudally or did only partial damage to the VVMnPO, and these were combined into a separate group for analysis. Four adipsic lesioned rats were excluded before the treatments began. The extent of damage to the entire ventral MnPO below the anterior commissure was variable, but no rat had a complete lesion of this entire nucleus. Rostral lesions often damaged the ventral diagonal band, and caudal lesions were always placed in the third ventricle and did small amounts of damage to the periventricular nucleus and medial preoptic nuclei bilaterally. The supraoptic, paraventricular, and suprachiasmatic nuclei always remained intact. Mean daily saline intakes during the DOCA treatments are presented in Figure 1. The saline intake data were analyzed using a mixed-model ANOVA with the average daily

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intakes during the baseline and DOCA treatment periods as a within-subjects factor and the three lesion groups and four doses as between-subjects factors. The triple interaction was significant, F(6, 87) = 2.41, p < .05, which indicated that the lesions caused increasingly enhanced saline intake with increasing doses but only in the DOCA conditions and not in the baseline condition. This interpretation was confirmed by two-tailed planned comparisons between the lesioned groups at each dose and condition. No significant differences existed among the groups during the baseline condition before any DOCA treatments. During the DOCA treatments, the lesion groups did not differ from one another at the lowest dose, 0.5 mg/kg; however, the mean saline intakes of the VVMnPO-lesioned groups were significantly greater than the other lesion groups at 1, 3, and 5 mg/kg. The missed lesion group drank significantly more saline than the shams at the two highest doses, 3 and 5 mg/kg. Water intake during the experiment was analyzed using a design identical to that described for saline intake. The only significant effect involving the lesion group factor was the main effect, F(2, 87) = 3.93, p < .05; that is, VVMnPOlesioned rats drank an average of about 9 ml more water than sham-lesioned rats across all conditions of the experiment. This moderate daily polydipsia under both basal maintenance and stimulated conditions is common among VVMnPO-lesioned rats (Fitts et al., 1990). The other significant effect for water intake in this experiment was the expected interaction of the dose of DOCA with the withinsubjects factor of baseline versus DOCA, F(3, 87) = 18.10, p < .001. Across all lesion conditions, the dose groups drank similar amounts of water before any DOCA treatments but increased water intake according to the dose of DOCA after the treatments began. Plasma protein and osmolality were measured 24 hr after the third DOCA injection in the 3 mg/kg groups, and Bonferroni tests failed to reveal significant differences on either measure. Means and standard errors for osmolality (mosm/ kg) and protein (g/dl), respectively, for the different groups were: sham lesions (n = 7), 309 ± 1 and 5.5 ± 0.1; VVMnPO lesions (n = 5), 312 ± 1 and 5.5 ± 0.2; and missed lesions (n = 9), 309 ± 3 and 5.4 ± 0.1. These findings agree with two separate experiments in a previous study of VVMnPO lesions (Fitts et al., 1990), which also failed to detect the hyperosmolality that is commonly associated with larger lesions in the anteroventral third ventricle (AV3V) area. The mean osmolalities of the present groups were well within the range of means of the nine groups of rats with sham, VVMnPO, or missed lesions under basal maintenance conditions in the previous study (Fitts et al., 1990), 305-314 mosm/kg. The means of plasma protein concentrations in Fitts et al. (1990) ranged from 5.5 to 5.9 g/dl, and the consistently low values that were observed in the present experiment probably reflect a continuing positive sodium balance and volume expansion 24 hr after the last DOCA treatment. SFO lesions. Histological analysis of the brains of the rats revealed that 5 rats had at least 90% destruction of the SFO including part of the rostral stalk (SFO lesioned group). Of the other 12 rats, 6 had lesions that were centered on the

45 40 35

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ml/day 15 10 5 0 0 0.5

1

3

5

Dose of DOCA (mg/kg) Figure 1. Daily saline intake (M + SE) during 3-day treatment with once-daily subcutaneous injections of deoxycorticosterone acetate (DOCA) in groups of rats that had sham lesions, lesions of the ventral ventral median preoptic nucleus (VVMnPOx), or histologically identified missed lesions (Miss) in Experiment 1. (Baseline intakes did not differ before DOCA treatments, hut intakes were significantly elevated during DOCA in both lesion groups. *p < .05 compared with sham lesion; fp < .05 compared with missed lesion. Water intake was not affected by the lesion treatments.)

rostroventral stalk of the SFO without completely destroying the SFO (stalk lesioned group). These rats were considered separately from the other missed-lesion rats because of numerous reports showing that damage to the efferent and afferent axons traversing the rostroventral stalk of the SFO produce nearly identical effects to complete SFO lesions on drinking behavior (e.g., Eng & Miselis, 1981; Lind & Johnson, 1982; Lind, Thunhorst, & Johnson, 1984; Masson & Fitts, 1989). The remaining 6 had lesions that either did not damage the SFO or damaged only the caudal part of the SFO (missed lesion group). Water intake was characteristically reduced on the 1 st day after the lesion in both the SFO-lesioned and the stalk-lesioned groups compared with the sham-lesioned group (Bonferroni test, one-tailed, p < .05). Saline intake was not affected. Means and standard errors for the water and saline intakes were: sham lesion, 12 ± 8 and 6 ± 5; SFO lesion, 6 ± 5 and 6 ± 4; stalk lesion, 5 + 5 and 3 ± 5; and missed lesion, 12 ± 5 and 5 ± 4. Water and saline intakes were measured for 4 days of baseline, 3 days of DOCA treatments, and 1 day of recovery after the cessation of DOCA treatments. Mean saline intakes are shown in Figure 2. There was no main effect of the lesion treatment. Both water intake, F(2, 42) = 20.15, p < .001, and saline intake, F(2, 42) = 34.80, p < .001, were significantly greater during DOCA treatment than in the baseline condition in all groups. The interaction of lesion groups and treatment periods was not significant in either analysis, and

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DOUGLAS A. FITTS excluded before the experiment began. The rats were allowed to recover for 2 weeks with water and saline solution freely available. The SFO experiment used the same rats that had previously been used in the DOCA experiment (see Experiment 1). They were allowed 1 week to recover from the DOCA treatments, and yohimbine was administered 3 weeks after the lesion. Water and saline solution were freely available during recovery. In both VVMnPO and SFO experiments, yohimbine was administered subcutaneously at a dose of 5 mg/kg in a 1 ml/kg volume of polyethylene glycol (molecular weight = 200). Water and 0.3 MNaCl solution intakes were measured hourly for 4 hr. Food was available during the tests.

15

Saline

Intake ml/day

Results

Baseline

DOCA

Recovery

Figure 2. Daily saline intake (M ± SE) during 3-day treatment with once-daily subcutaneous injections of deoxycorticosterone acetate (DOCA) in groups of rats that had sham lesions, lesions of the subfornical organ (SFO), lesions of the efferent fibers from the rostral stalk of the SFO without complete damage to the SFO (Stalk), or histologically identified missed lesions that did not damage the rostral stalk (Miss) in Experiment 1. (No significant differences were found according to lesion group. Water intake was also not affected by the lesion treatments.)

A histological analysis revealed that 8 rats had complete VVMnPO lesions, and the other 5 rats in the VVMnPOtargeted group had lesions that missed in the rostral direction. The sample sizes and groups for the SFO experiment were as given in the results for Experiment 1. The SFO and VVMnPO experiments were analyzed separately because of the different histories of the groups of rats. However, the data for the control groups were very similar, and the two experiments have been combined for convenience of presentation in Figure 3. Intakes were analyzed using one-way ANOVAs. The VVMnPO lesion affected neither water nor saline intake that was evoked by yohimbine. Both F ratios were less than 1.0, and the means were close

the mean saline intakes of both the SFO- and stalk-lesioned groups were slightly smaller than those of the sham group. Discussion

JL, This experiment replicates a previous unexpected finding of enhanced saline intake in response to DOCA treatments in VVMnPO-lesioned rats (Fitts et al., 1990) and extends it to several doses of DOCA. The effect is specific to tissues in or near the WMnPO because rats with SFO lesions or missed VVMnPO lesions showed either lesser increases or no increase of saline intake during DOCA. The smaller effects that were seen in the VVMnPO missed-lesion rats may have resulted from partial damage to the VVMnPO.

JL

Fluid

JL. ml/4 hr

1

Experiment 2: Effects of Forebrain Lesions on

Yohimbine-Induced Thirst and Salt Appetite The purpose of this experiment was to examine the effects of lesions of the SFO or VVMnPO on water and saline intakes induced by subcutaneous injections of yohimbine. This «-2adrenoceptor blocker causes thirst and a small but rapid increase of saline intake for approximately 2-4 hr (Johnson et al., 1989, 1990). The mechanism is unknown.

Method Fifteen rats received lesions that were targeted for the VVMnPO, and 7 rats received sham lesions. Two adipsic lesioned rats were

Sham WMnPO

SFO

Stalk

Miss

Lesion Treatment Before Yohimbine

Figure 3. Water and saline intakes (M + SE) 4 hr after subcutaneous injection with 5 mg/kg yohimbine in groups that had sham lesions (Sham), lesions of the ventral ventral median preoptic nucleus (VVMnPO), lesions of the subfornical organ (SFO), lesions of the efferent fibers from the ventral stalk of the SFO (Stalk), or histologically identified missed lesions (Miss) in Experiment 2. (Saline intake was not significantly different from the sham group in any lesioned groups. Water intake was significantly reduced from the Sham-group level in both the SFO and Stalk groups.)

FOREBRAIN LESIONS AND SALT APPETITE to the control values. Saline intake was also not affected by the SFO lesion, but both the SFO-lesioned and stalk-lesioncd rats drank significantly less water than the sham-lesioned rats after yohimbine, F(3, 21) = 5.38,p < .05. The SFO-lesioned group also happened to have the smallest absolute intake of saline, so a general effect on drinking cannot be ruled out. Overall, the saline intakes that were observed in this experiment were smaller than those reported by others using yohimbine at similar doses (Johnson et al., 1989). Nevertheless, the intakes were reliable and similar to other findings in this laboratory using unoperated rats of the same stock, sex, and approximate weight (Fitts, 1990). Discussion The data from this experiment are consistent with, but do not prove, the interpretation that water intake, but not saline intake, that is induced by yohimbine results in part from angiotensin synthesis or synaptic release in the brain. Lesions of the SFO or its rostral stalk are known to abolish water drinking that is induced by treatments associated with peripheral angiotensin (Simpson, 1981), and these lesions in the present experiment reduced the water-drinking response to yohimbine. Lesions of the VVMnPO reduce salt appetite after some treatments that appear to be related to peripheral or central angiotensin (Fitts & Masson, 1990; Fitts et al., 1990), but these lesions failed to affect the salt appetite after yohimbine injections. Together, these findings could suggest that yohimbine-induced water intake depends on angiotensin, whereas salt appetite does not. Clearly, alternative interpretations are possible if not probable. The complexity of the receptor pharmacology and connectivity of both the SFO and the MnPO allows many possibilities for potential mechanisms for the demonstrated effects on water intake and for the lack of effects on salt appetite.

General Discussion Many investigators have speculated that structures in the AV3V region, including the periventricular and medial preoptic nuclei, the OVLT, and the ventral MnPO, might participate in the arousal of salt appetite and particularly in angiotensin-induced salt appetite (for a review see Rowland & Fregley, 1988). Experimental verification of this notion has proven difficult because lesions of the entire AV3V, the medial preoptic nuclei bilaterally, or the complete ventral MnPO can cause problems of body fluid homeostasis including acute adipsia and natriuresis, chronic hypematremia, volume disturbances, weight loss, and cardiovascular and endocrine effects. Furthermore, these lesions tend to reduce responsiveness to a wide range of dipsogenic treatments and often produce unstable baseline water and saline intakes (Buggy & Johnson, 1977; Gardiner, Jolley, Vagnucci, & Strieker, 1986; Gardiner & Strieker, 1985a, 1985b; Lind & Johnson, 1980; McGowan, Brown, & Grossman, 1988). Given these enormous differences in the initial hydrational conditions and spontaneous fluid intakes between lesioned and sham-lesioned rats, one must be particularly careful in in-

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terpreting the results of experiments using treatments that cause further hydrational stress, such as sodium depletion (e.g., Bealer & Johnson, 1979). The WMnPO lesion is considerably smaller than the lesions just cited and appears to have either no effect or much smaller effects on many of the confounding variables just mentioned (Fitts et al., 1990). Prolonged postsurgical adipsia occurs in only a small proportion of rats, and extraordinary measures are generally not necessary to restore hydration and promote survival after the lesion. We have found no chronic effects on sodium excretion or on osmotic or volume controls as determined by plasma osmolality, electrolytes, protein, or hematocrit. A moderate increase in daily water intake is usually present. However, only 1 or 2 rats per experiment show moderately elevated saline intakes, and the group baseline saline intakes are not significantly different from controls. Body weights stabilize in the low normal range compared with sham-lesioned animals, and the lesioned rats show no deficits in drinking responses to dipsogenic treatments such as isoproterenol, water deprivation, or hypertonic saline. Salt appetite is reduced, although not always abolished, after treatments with peripheral captopril or sodium depletion. The effect does not generalize to all mechanisms of salt appetite because the response to DOCA was enhanced rather than suppressed (Fitts et al., 1990). Compared with the AV3V or complete ventral MnPO lesions, the VVMnPO lesion produces effects on hydrationrelated variables that are relatively selective for stimulated saline intake. Other areas in the AV3V probably also contribute, but in previous studies (Fitts et al., 1990) similar lesions either rostral or caudal to the VVMnPO by as little as 0.2 mm tend to be less effective (e.g., see Figure 1). The SFO lesion also does not produce radical alterations of body fluid homeostasis under normal maintenance conditions. The lesion is well-known to produce deficits in drinking and blood pressure in response to circulating angiotensin II (for a review see Simpson, 1981) and also reduces osmotic drinking (Lind et al., 1984). The lesion reduces acutely both water and saline drinking after sodium depletion (Thunhorst et al., 1990), although the effects do not persist for 24 hr, and it does not affect captopril-induced salt appetite (Thunhorst etal., 1987). In the present experiments, lesions in or near the VVMnPO region exaggerated DOCA-enhanced saline intake, but lesions in or near the SFO did not. Therefore, this enhancing effect of forebrain lesions on DOCA-induced saline intake is specific to the most ventral pole of the lamina terminalis. There is no data at present to indicate whether this effect results from damage to cell bodies or to fibers of passage. In contrast, lesions of the medial nucleus of the amygdala abolish salt appetite to mineralocorticoicls without affecting salt appetite to sodium depletion (Schulkin, Marini, & Epstein, 1989). The effects of these SFO and VVMnPO lesions on water and saline intakes that are elicited by DOCA or yohimbine are open to a wide range of interpretations, and other types of experiments will be necessary to determine their mechanisms. However, an interesting unifying feature of these dif-

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ferent treatmenls is that they both cither directly or indirectly generate inhibitory and counterregulatory events that can affect the central nervous system. For instance, it has been suggested that central a-2-adrcnoceptor activity tonically inhibits drinking and salt appetite and that a blockade of this system with yohimbine releases this inhibition and accounts for the observed behaviors (Johnson et al., 1989, 1990). The data could then indicate that the SFO is one source of a tonic stimulation of drinking that is held in check by a-2-adrcncrgic inhibition; blockade of a-2-adrenoceptors then releases the behavior only to the extent that the SFO and other critical sites are present. The SFO is well-known to be a target for both humoral and central agents that stimulate water intake; however, we have recentlyprovided other evidence that the SFO may also be a target for inhibitory' influences on drinking (Ehrlich & Fitts, 1990). Peripheral treatments with high doses of DOCA greatly expand extracellular volume, thereby provoking numerous inhibitory or counterregulatory mechanisms that eventuallylead to an "escape" from its sodium-retentive effects. These mechanisms probably also influence the brain to inhibit salt appetite, which may account for the high pharmacological doses of mineralocorticoids that are necessary to generate this behavior even though the steroids are known to participate in salt appetite under physiological conditions (Sakai, Nicolai'dis, & Epstein, 1986). Aside from a direct neural input from volume and baroreceptors, these inhibitory mechanisms on salt appetite could include either circulating or central agents such as the atrial and brain natriuretic peptides (Fitts, Thunhorsl, & Simpson, 1985) or a putative natriuretic hormone. Interference with a central receptive site or fibers of passage necessary for these mechanisms could release the mineralocorticoid-induced salt appetite that would otherwise be expressed in the absence of extracellular volume expansion. The enhanced responding to DOCA by VVMnPO-lesioned rats could be explained as a release from this inhibitory input.

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and lesions of the ventral part of the ventral median preoptic nucleus. Behavioral Neuroscience, 104, 818-827. Gardiner, T. W., Jolley, J. R., Vagnucci, A. H., & Strieker, E. M. (1986). Enhanced sodium appetite in rats with lesions centered on nucleus medianus. Behavioral Neuroscience, 100, 531—535. Gardiner, T. W., & Strieker, E. M. (1985a). Hyperdipsia in rats after electrolytic lesions of nucleus medianus. American Journal of Physiology, 248, R214-R223. Gardiner, T. W., & Strieker, E. M. (1985b). Impaired drinking responses of rats with lesions of nucleus medianus: Circadian dependence. American Journal of Physiology, 248, R224-R230. Johnson, A. K., Beltz, T. G., & Edwards, G. L. (1989). A rapid onset sodium appetite induced by yohimbine. Society for Neuroscience Abstracts, 15, 964. Johnson, A. K., Beltz, T. G., & Edwards, G. L. (1990). The nature of sodium appetite induced by yohimbine. FASEB Journal, 4, A1195. Lind, R. W., & Johnson, A. K. (1980). A further characterization of the effects of AV3 V lesions on ingestive behavior. American Journal of Physiology, 245, R83-R90. Lind, R. W., & Johnson, A. K. (1982). Subfornical organ-median preoptic connections and drinking and pressor responses to angiotensin 11. Journal of Neuroscience, 2, 1043-1051. Lind, R. W., Thunhorst, R. L., & Johnson, A. K. (1984). The subfomical organ and the integration of multiple factors in thirst. Physiology & Behavior, 32, 69-74. Masson, D. B., & Fitts, D. A. (1989). Subfornical organ connectivity and drinking to captopril or carbachol in rats. Behavioral Neuroscience, 103, 873-880. McGowan, M. K., Brown, B., & Grossman, S. P. (1988). Lesions of the MPO or AV3V: Influences of fluid intake. Physiology & Behavior, 42, 331-342. Rowland, N. E., & Fregly, M. J. (1988). Sodium appetite: Species and strain differences and role of renin-angiotensin-aldosterone system. Appetite, 11, 143-178. Saavedra, J. M., Israel, A., Plunkett, L. M., Kurihara, M., Shigematsu, K., & Correa, F. M. A. (1986). Quantitative distribution of angiotensin II binding sites in rat brain by autoradiography. Peptides, 7, 679-687. Sakai, R. R., Nicolai'dis, S., & Epstein, A. N. (1986). Salt appetite is suppressed by interference with angiotensin II and aldosterone. American Journal of Physiology, 251, R762-R768. Schulkin. J. (1988). The effects of preoperative ingestive events on feeding and drinking behavior following brain damage. Psychobiology, 16, 185-195. Schulkin, J., Marini, J., & Epstein, A. N. (1989). A role for the medial region of the amygdala in mineralocorticoid-induced salt hunger. Behavioral Neuroscience, 103, 178-185. Simpson, J. B. (1981). The circumventricular organs and the central actions of angiotensin. Neuroendocrinology, 32, 248-256. Thunhorst, R. L., Ehrlich, K. J., & Simpson, J. B. (1990). Subfornical organ participates in salt appetite. Behavioral Neuroscience, 104, 637-642. Thunhorst, R. L., Fitts, D. A., & Simpson, J. B. (1987). Separation of captopril effects on salt and water intake by subfornical organ \esions.AmericanJournalofPhysiology, 252, R409-R418. Thunhorst, R. L., Fitts, D. A., & Simpson, J. B. (1989). Angiotensinconverting enzyme in subfornical organ mediates captopril-induced drinking. Behavioral Neuroscience, 103, 1302-1310.

Received January 22, 1991 Revision received March 20, 1991 Accepted March 20, 1991 •