by neurons in hypothalamic nuclei and released from the posterior
pituitary. In addition to its classic role in water retention, recent studies
suggest that AVT influences reproductive and aggressive behaviors.
Anatomical evidence suggests that AVT-synthesizing neurons project to
various regions of the brain other than the posterior pituitary to modulate
neuronal functions underlying reproductive and aggressive behaviors. In
rough-skinned newts, Taricha granulosa, the concentration of AVT fluctuates seasonally in brain regions that regulate the expression of sexual
behavior (Zoeller and Moore 1986; Deviche et al. 1990). Systemic administration of AVT induces advertisement calling in males of several species
(Penna et al. 1992; Boyd 1994a; Marler et al. 1995; Propper and Dixon 1997;
Chu et al. 1998; Semsar et al. 1998). AVT-induced calling is inhibited by
application of an AVT receptor antagonist, Manning compound (Propper
and Dixon 1997), in male Great Plains toads, Bufo cognatus, suggesting a
direct role for AVT in activating calling behavior. Interestingly, AVT
enhances the rate of advertisement calling without influencing the rate of
other types of vocalizations. In gray tree frogs, Hyla versicolor, two acoustically distinct calls, advertisement and aggressive calls, are given by males
during the breeding season (Fellers 1979). Exogenous injection of AVT
increases the rate of advertisement calling, whereas the rate of aggressive
calling is unchanged (Semsar et al. 1998). In male cricket frogs, Acris
crepitans, whose advertisement and aggressive calls differ in dominant frequency (Wagner 1989), AVT changes acoustic properties of calls to resemble advertisement calling rather than aggressive calling (Marler et al. 1995;
Chu et al. 1998). Taken together, these results suggest that AVT selectively
increases rates of courtship vocalization without enhancing overall vocal
activity.
Although the robust effect of AVT in activating advertisement calling has
been fully recognized, it is becoming apparent that its potency depends on
circulating levels of T (Moore and Zoeller 1979; Moore 1987). For example,
when castrated male green tree frogs, Hyla cinerea, were treated with AVT,
only T-implanted animals were induced to call (Penna et al. 1992). T may
maintain a fundamental sexual state in males during the breeding season,
whereas AVT actually triggers vocalizations (Moore 1987; Semsar et al.
1998). The complex interplay between steroids and AVT is not yet fully
understood and is a focus of current research.
In contrast to activation of vocalizations, there is also evidence that a
hormone can inhibit call production. Injection of prostaglandin (PG) F2 a
inhibits male American toads, Bufo americanus, from advertisement calling
in response to playback of conspecific calls (Schmidt and Kemnitz 1989).
PG is an eicosanoid hormone (synthesized from cell membrane phospholipids via arachidonic acid) released from a variety of cells, and its primary
function is to control the vascular smooth muscle activity. Neural correlates
of electrically induced advertisement calling in an isolated brain preparation also disappear upon bath application of PG without affecting neural
correlates of pulmonary respiration (Schmidt and Kemnitz 1989). This
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pituitary. In addition to its classic role in water retention, recent studies
suggest that AVT influences reproductive and aggressive behaviors.
Anatomical evidence suggests that AVT-synthesizing neurons project to
various regions of the brain other than the posterior pituitary to modulate
neuronal functions underlying reproductive and aggressive behaviors. In
rough-skinned newts, Taricha granulosa, the concentration of AVT fluctuates seasonally in brain regions that regulate the expression of sexual
behavior (Zoeller and Moore 1986; Deviche et al. 1990). Systemic administration of AVT induces advertisement calling in males of several species
(Penna et al. 1992; Boyd 1994a; Marler et al. 1995; Propper and Dixon 1997;
Chu et al. 1998; Semsar et al. 1998). AVT-induced calling is inhibited by
application of an AVT receptor antagonist, Manning compound (Propper
and Dixon 1997), in male Great Plains toads, Bufo cognatus, suggesting a
direct role for AVT in activating calling behavior. Interestingly, AVT
enhances the rate of advertisement calling without influencing the rate of
other types of vocalizations. In gray tree frogs, Hyla versicolor, two acoustically distinct calls, advertisement and aggressive calls, are given by males
during the breeding season (Fellers 1979). Exogenous injection of AVT
increases the rate of advertisement calling, whereas the rate of aggressive
calling is unchanged (Semsar et al. 1998). In male cricket frogs, Acris
crepitans, whose advertisement and aggressive calls differ in dominant frequency (Wagner 1989), AVT changes acoustic properties of calls to resemble advertisement calling rather than aggressive calling (Marler et al. 1995;
Chu et al. 1998). Taken together, these results suggest that AVT selectively
increases rates of courtship vocalization without enhancing overall vocal
activity.
Although the robust effect of AVT in activating advertisement calling has
been fully recognized, it is becoming apparent that its potency depends on
circulating levels of T (Moore and Zoeller 1979; Moore 1987). For example,
when castrated male green tree frogs, Hyla cinerea, were treated with AVT,
only T-implanted animals were induced to call (Penna et al. 1992). T may
maintain a fundamental sexual state in males during the breeding season,
whereas AVT actually triggers vocalizations (Moore 1987; Semsar et al.
1998). The complex interplay between steroids and AVT is not yet fully
understood and is a focus of current research.
In contrast to activation of vocalizations, there is also evidence that a
hormone can inhibit call production. Injection of prostaglandin (PG) F2 a
inhibits male American toads, Bufo americanus, from advertisement calling
in response to playback of conspecific calls (Schmidt and Kemnitz 1989).
PG is an eicosanoid hormone (synthesized from cell membrane phospholipids via arachidonic acid) released from a variety of cells, and its primary
function is to control the vascular smooth muscle activity. Neural correlates
of electrically induced advertisement calling in an isolated brain preparation also disappear upon bath application of PG without affecting neural
correlates of pulmonary respiration (Schmidt and Kemnitz 1989). This
292
A. Yamaguchi and D.B. Kelley