Excitatory action of the bird antidiuretic hormone vasotocin on neurons in the subfornical organ

Abstract

The responsiveness of spontaneously active neurons in the subfornical organ (SFO) of adult ducks to angiotensin II (ANGII) and the bird specific antidiuretic hormone, arginine vasotocin (AVT), the analog of the mammalian arginine vasopressin (AVP), were investigated in brain slices with extracellular recording technique. 65% (n = 66) of the neurons increased their activity after superfusion with ANGII, the rest were unresponsive. Application of AVT activated 52% (n = 68) of the investigated neurons and like ANGII never caused an inhibition of the spontaneously active SFO neurons. A close correlation exists between the ANGII and AVT sensitivity of duck SFO neurons, because 29 out of 33 neurons were excited by AVT as well as ANGII. The relatively weak antagonistic effect of the V1-type receptor antagonist Pmp-Tyr (Me)-Arg8-vasopressin on the AVT induced excitation suggests a different pharmacology of the bird AVT receptor as compared to the mammalian AVP receptor. The excitatory response of ANGII and AVT on the very same neurons suggest a similar function of both peptides on SFO mediated effects in vivo, such as an increase in water intake. However, peripheral AVT concentrations, unlike ANGII concentrations in the blood are not high enough to activate SFO neurons from the blood side of the blood brain barrier. Therefore AVT is presumably released from synapses of neurons originating within or projecting to the SFO. The identity of the ANGII and AVT reactive neurons suggests that synaptically released AVT should facilitate SFO mediated drinking.

Distribution of the vasotocin type 4 receptor throughout the brain of the chicken, Gallus gallus

“…Neurons within the avian SFO show excitatory responses not only to angiotensin II but also to arginine vasotocin (AVT). The peak excitatory effect on SFO neurons from a combination of AVT and angiotensin II administration was greater than that with the angiotensin II stimulus alone (Schmid et al, ). A recent study in mammals has suggested that the cell bodies of tanycytes located in the OVLT, SFO, and APa have tight junctions along their cerebroventricular side serving as a displaced barrier.…”

Selective potentiation of N‐type calcium channels by angiotensin II in rat subfornical organ neurones

“…Although the direct excitatory actions of ANG on SFO neurones have been described extensively, both in vivo (Felix & Akert, 1974; Felix & Schlegel, 1978; Ferguson & Renaud, 1986) and in vitro (Buranarugsa & Hubbard, 1979; Schmid & Simon, 1992; Schmid et al 1995; Ferguson et al 1997), the mechanisms underlying the stimulatory effects have not been explored in depth. Studies from our laboratory have shown inhibition of a transient K + conductance by ANG (Ferguson & Li, 1996), although this observation is not likely to explain completely the mechanism of ANG excitation, as we have also observed a decrease in input resistance induced by ANG (Ferguson et al 1997).…”