Savio W.T. Cheng

Vasopressin reduces release from vasopressin-neurons and oxytocin-neurons by acting on V2-like receptors

The effects of arginine vasopressin (AVP), and of the V2-AVP receptor agonist 1-deamino[8-D-arginine] vasopressin (DDAVP) on release from the vasopressin-neurons and oxytocin-neurons of Long-Evans rats were evaluated using specific radioimmunoassays for rat neurophysins. AVP (1 microgram, 1 nmol) or DDAVP (25 ng, 25 pmol) was administered i.p. to animals 1 h before they received an i.v. infusion of 18% saline at 10 microliters/100 g b. wt./min for 60 min. Both AVP and DDAVP decreased the responsiveness (slope) but not the sensitivity threshold of vasopressin-neurons to acute changes in plasma osmolality. Since the amounts of the peptides giving comparable decreases in responsiveness were directly related to their antidiuretic potencies, it is most probable that this influence is mediated through V2-like receptors. However, while ruling out a significant contribution of V1-type receptors, the data do not exclude involvement of other vasopressin receptors (e.g. V3-type receptors). Both AVP and DDAVP also appeared to have an inhibitory effect on release from oxytocin-neurons, but in this case they significantly altered sensitivity threshold but not responsiveness to acute changes in plasma osmolality. Because AVP produced a shift in sensitivity threshold larger than that by DDAVP when the peptides were used in amounts related to their antidiuretic potencies, our results suggest that the feedback influence of AVP on oxytocin-neurons is largely, although not entirely, exercised through V2-like receptors.

Responsiveness of Oxytocin-Producing Neurons to Acute Salt Loading in Rats: Comparisons with Vasopressin-Producing Neurons

Plasma oxytocin-associated neurophysin concentration [( OT-RNP]) was used to evaluate the responsiveness of oxytocinergic neurons to an acute salt load in Long-Evans (LE) rats and Brattleboro homozygous (DI) rats. This responsiveness was compared with that of vasopressinergic neurons in LE rats as indexed by plasma vasopressin-associated neurophysin concentration [( VP-RNP]). Acute salt loading was induced by infusing 18% saline for 60 min into conscious, trained, chronically catheterized animals and plasma osmolality (Posm) and mean arterial pressure (MAP) were monitored. An increase in Posm was associated with a rise in [OT-RNP] and the relationship between delta [OT-RNP] and delta Posm was similar for both LE and DI rats over the first 40 min of infusion (21.6 and 19.7 fmol ml-1 mosm-1 kg-1, respectively). Although Posm continued to rise between 40 and 60 min infusion, [OT-RNP] actually fell slightly during this period in LE rats to a final elevation of 682 +/- 40 fmol/ml above initial values whereas [OT-RNP] in DI rats continued to rise to a final elevation of 1,927 +/- 288 fmol/ml above initial values at 60 min of infusion. The differences between these elevations at 60 min for LE and DI rats were highly significant (p less than 0.001). For LE rats, the increase of [OT-RNP] with Posm for the first 40 min of infusion was much greater than the increase in [VP-RNP] with the slope between delta [VP-RNP] and delta Posm being only 8.3 compared to 21.6 fmol ml-1 mosm-1 kg-1 in the case of delta [OT-RNP].(ABSTRACT TRUNCATED AT 250 WORDS)

Homozygous Brattleboro rats display attenuated conditioned freezing responses

In the present study we examined the influence of arginine vasopressin (AVP) on conditioned freezing behavior to aversive shock treatment by comparing the responses of Brattleboro homozygous (DI) rats, Brattleboro heterozygous (HZ) rats, and Long-Evans (LE) rats. Each animal was placed in a sound-attenuated shock chamber on the training day and given a series of 3 footshocks. On the following 4 consecutive days the rats were placed in the chambers where they had received their shock and levels of spontaneous freezing were evaluated. Levels of circulating vasopressin-associated neurophysin (NP) were subsequently determined in each rat strain. For each of the 4 test days, DI rats displayed significantly less freezing behavior when compared with LE rats and HZ rats. HZ rats displayed trends towards attenuated freezing responses when compared with LE rats. The data indicate that a relationship exists between the levels of central nervous system (CNS) and circulating AVP, and the amount of freezing displayed by each strain. These preliminary results suggest that vasopressin may be involved in appropriate autonomic and emotional responses to fearful stimuli in fear conditioning paradigms.

REPLACEMENT THERAPY WITH ARGININE VASOPRESSIN IN HOMOZYGOUS BRATTLEBORO RATS

We studied the replacement therapy of different doses of AVP in DI rats. It is surprising that relatively high levels of plasma AVP were needed to achieve significant antidiuretic effects. Measured plasma AVP concentrations are less than those predicted from the release rates of AVP by the minipumps. This difference may be due to the subcutaneous mode of release of AVP. The study also provides evidence that urinary excretion of AVP is a good indicator of the plasma AVP level.

Effects of intraperitoneal injection of lithium chloride on neurohypophyseal activity: implications for behavioral studies.

Intraperitoneal injections of lithium chloride (LiCl) were found to increase the activity of vasopressin-neurons and oxytocin-neurons as indexed by rises in plasma concentrations of vasopressin-associated neurophysin (VP-RNP) and oxytocin-associated neurophysin (OT-RNP). Plasma VP-RNP increased 12 and 4 times basal levels (greater than or equal to 20 fmol/ml) reaching values of 248 +/- 37 fmol/ml (3.0 mEq LiCl/kg body weight) and 89 +/- 10 fmol/ml (1.5 mEq LiCl/kg body weight) at 60 minutes. OT-RNP rose to 37-and 10-times basal levels (greater than or equal to 20 fmol/ml) with peak values of 749 +/- 100 fmol/ml and 188 +/- 48 fmol/ml ten minutes following injection of 3.0 or 1.5 mEq LiCl/kg body weight. Mean arterial pressure increased in response to lithium treatment by 31 +/- 6 mm Hg at 60 minutes in rats receiving 3.0 mEq LiCl/kg and by 22.5 +/- 5 mm Hg at 10 minutes in rats receiving 1.5 mEq LiCl/kg over pretreatment values (125 +/- 3 mm Hg). Heart rate decreased from a pretreatment value of 422 +/- 12 beats/min to 367 +/- 48 beats/min at 10 minutes and to 341 +/- 27 beats/min at 20 minutes for rats treated with the high and low dose of lithium, respectively. These findings suggest that the behavioral effects of LiCl could result from multiple mechanisms and involve its acute release of vasopressin and oxytocin. It is also possible that changes in cardiovascular function may act as cues when LiCl is used as an aversive stimulus.

Influence of pentobarbital and urethane on release from magnocellular neurons.

We examined the responses of vasopressin-neurons (VP-neurons) and oxytocin-neurons (OT-neurons) to acute salt-loading in a group of conscious rats (CON, n = 8) and rats under sodium pentobarbital (NEM, 50 mg/kg, i.p., n = 8) or urethane (URE, 1.6 g/kg, i.p. n = 8) anesthesia. Fifteen minutes following the induction of anesthesia, sodium pentobarbital produced an increase in basal plasma osmolality (Posm, 290 +/- 2 to 296 +/- 3 mosm/kg H2O, p less than 0.007) while urethane did not change basal Posm (287 +/- 2 to 289 +/- 2 mosm/kg H2O). Neither anesthetic agent resulted in any significant changes in basal plasma levels of vasopressin-associated neurophysin (VP-RNP) and oxytocin-associated neurophysin (OT-RNP). In response to intravenous infusion of 18% saline, all three groups of rats had similar rises in Posm. The slopes of the relationship between the rise in plasma VP-RNP and the rise in Posm were markedly reduced in both groups of anesthetized animals compared to that observed for conscious animals (CON = 2.54 +/- 0.5; NEM = 1.22 +/- 0.18; URE = 1.17 +/- 0.24 fmol.ml-1.mosm-1.kg H2O-1 p less than 0.0126). The slopes of the relationship between the rise in plasma OT-RNP and the rise in Posm were not significantly (p less than 0.4478) different between the CON group and the NEM group, while the slope for the URE group was significantly (p less than 0.05) smaller than that for the CON group (CON = 10.9 +/- 1.5; NEM = 9.3 +/- 1.5; URE = 6.3 +/- 0.7 fmol.ml-1.mosm-1.kg H2O-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute infusion of lithium chloride raises blood pressure in the conscious rat

The effects of acute infusion of lithium chloride (LiCl) were studied on mean arterial pressure (MAP) and magnocellular activity as shown by the concentrations of vasopressin-associated neurophysin ([VP-RNP]) and concentrations of oxytocin-associated neurophysin ([OT-RNP]) in plasma in conscious Long-Evans rats. Chronically-cannulated rats were infused intravenously at 10 microliter/100 g body wt/min with 13% LiCl for 20 min (total dose = 6.16 mequiv./kg body wt) or 0.65% LiCl for 60 min (total dose = 0.92 mequiv./kg body wt). Effects of 13% LiCl on mean arterial pressure were also examined in vasopressin-deficient homozygous Brattleboro rats. For Long-Evans rats, infusion of 13% LiCl produced rapid and significant (P less than 0.001) increases in mean arterial pressure, the concentration of lithium in plasma ([Li+]), plasma osmolality (Posmol), [VP-RNP], [OT-RNP] and significant decreases in heart rate and sodium concentration in plasma ([Na+]). For similar changes in plasma osmolality, lithium had a greater effect than sodium on mean arterial pressure, [VP-RNP], [OT-RNP]. For the 20 min infusion of 13% LiCl, there was a significant relationship (P less than 0.033) between delta MAP and log delta[VP-RNP] with a slope of 11.9 mmHg fmol-1 ml-1 (r = 0.5678). Unlike that of Long-Evans rats, infusion of 13% LiCl only did not produce significant changes of mean arterial pressure in Brattleboro rats. For Long-Evans rats, infusion of 0.65% LiCl resulted in more gradual and smaller elevations of blood pressure, [Li+] and smaller decreases in heart rate with no significant changes in plasma osmolality, [Na+], [VP-RNP] and [OT-RNP].(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Acute Increases in Plasma Osmolality on Plasma Vasopressin-Associated Neurophysin in Conscious Rats: Implications for Osmoregulation

We studied the responses of the hypothalamo-neurohypophysial system to intravenous infusions of 18% saline, 25% mannitol and a combination of 15% mannitol and 1.35% saline in conscious, chronically catheterized Long-Evans rats. Infusions of 18% saline and 25% mannitol produced similar increases in plasma osmolality (Posm) and plasma vasopressin-associated neurophysin concentration [( VP-RNP]). As expected, plasma sodium concentration [( Na+]) for the 18% saline-treated animals was significantly elevated while that for the 25% mannitol-treated animals was significantly reduced. Mannitol infusion caused a significantly greater loss of body weight. The slopes of the relationship between delta [VP-RNP] and delta Posm were almost identical for the two groups. Infusion of a combination of 15% mannitol and 1.35% saline produced a rise in Posm comparable to that observed for the other two infusion regimens, but caused smaller increases in [VP-RNP], the slope of the relationship between delta [VP-RNP] and delta Posm being about half of those for the infusion with 18% saline or 25% mannitol. This combination also maintained [Na+] close to normal levels and in these animals there was a loss of body weight that was significantly smaller than that for those rats receiving 25% mannitol. The results of this study imply that factors other than osmoreceptors and/or sodium receptors are involved in causing a release of neurohypophysial principles during acute infusion of hypertonic solutions.

Reduced Responsiveness to Acute Salt-Loading of Vasopressin-Neurons and Oxytocin-Neurons in the Heterozygous Brattleboro Rat

Heterozygous Brattleboro (HZ) rats exhibit a partial genetic deficiency in hypothalamic vasopressin (VP) production. The effects of this abnormality of HZ rats on the capacities of VP-neurons and oxytocin (OT)-neurons to respond to an acute salt-load were examined. Acute salt-loading was induced by intravenous infusion of 18% saline in conscious, chronically catheterized animals and the activities of VP-neurons and OT-neurons were interpreted from plasma concentrations of VP-associated neurophysin, [VP-RNP] and OT-associated neurophysin, [OT-RNP] at different time periods throughout the infusion. Plasma sodium concentration ([Na+]), plasma osmolality (Posm) and mean arterial pressure (MAP) were also monitored. Salt-loading produced significant rises in [VP-RNP] and [OT-RNP]. These rises were accompanied by increases in plasma [Na+], Posm and MAP. Releases of OT-RNP were approximately four times greater than those of VP-RNP. The responsiveness of VP-neurons to increases in Posm in the HZ rat was approximately one-half of that observed for the Long-Evans (LE) rat. Furthermore, the responsiveness of OT-neurons in these animals was approximately one-half of that for LE rats and one-third of that for homozygous Brattleboro (DI) rats. The changes in MAP during salt-loading do not appear to be different for HZ and LE rats. Hence, while VP may be involved in the rise in blood pressure during infusion of hypertonic saline, there is not a direct correlation between plasma levels of VP-RNP (and presumably VP) and rises in blood pressure.