The Frog Hypothalamus is the Source of Mate Calling Behavior
by David D. Olmsted (Copyright - 1998, 2006. Free to use for personal and
educational purposes)
Last Revised September 9, 2006
Mate Calling Behavior
Mating behavior
in the male frog consists of calling to lure a female while simultaneously
keeping away other males.
Once some other frog is within range, the male frog turns
and clasps onto the other
frog. This behavior does not have to be visually triggered. Blinded male frogs will
turn and clasp at any frog which swims close by (Noble and Aronson - 1942). If the
clasped frog is male, or a female not ready to lay eggs, then the clasped frog will
produce a release call which causes them to be released.
Robert Schmidt (1966) found
that injections of pituitary extract or injections of the pituitary hormone gonadotropin
induces mate calling in Hyla frogs within 24 hours of injection. Gonadotropin causes
the testes to release androgen and he found that this androgen (testosterone) was
the hormone which affected the brain. So the complete cycle is as follows:
- The hypothalamus releases Gonadotropin
releasing hormone (GnRH)
- This activates the pituitary which releases gonadotropin
- This activates the testes which releases testosterone
- This activates the calling centers in the brain (which turn out to be a different
area of the hypothalamus) and prepares other body centers for mating.
Robert Schmidt (1966) noticed
that mate calling is environmentally context sensitive although the sensory processes
which define this environmental context for the frog
have yet to be determined (most
likely is odor based modulated by the septum). He had this to say about mate calling
behavior in the frog Hyla cinerea:
“It seems to be a common experience that frogs
kept in the laboratory undergo a depression of reproductive activity (van Oordt
- 1960). Frogs collected locally for this study were extremely vocal while being
transported to the laboratory and often for the following day. Thereafter, the tendency
to call spontaneously or to answer a tape recorder decreased rapidly, unless the
animals were injected with pituitaries or hormones. The above data suggest that
he level of circulating androgens is, by far, the most important single factor necessary
for the production of mating calling. Hylids receiving pituitary or gonadotropin
injections were often heard to call spontaneously, without external stimulation.
As noted above, castrated animals soon became permanently mute."
"Once the mating
calling mechanisms have been activated by hormones, the next most important factor
seems to be the presence of appropriate acoustic stimulation, e.g. by conspecific
mating calls."
" It is also important that inhibiting factors be absent. Acoustic stimuli
do not seem to disturb calling frogs (Schmidt - 1964), but visual and vibratory
stimuli are likely to stop calling.”
In contrast to Hyla cinerea Robert Schmidt
(1966) has this to say about the frog
Rana pipiens:
“Attempts to induce a high and
consistent level of mating calling in Rana
have been discouraging. Although pituitary
injections increase the calling rate, the effect is quite variable and the level
of calling in never great for any length of time.”
Perhaps clue into the
nature of call recognition was reported by Robert Schmidt
who found that Rana pipiens will initiate
calling in response to the mating calls of Bufo americanus
when they are played
back on a tape recorder at half speed (Schmidt - 1968).
Mate Calling Initiated
by Hypothalamic Preoptic Area
Figure 1
Side View of the Hypothalamic Pre-Optic Region of the Frog, Rana pipiens.(Schmidt - 1984)
Optic Chiasm (OC), Posterior Nucleus (PP), Anterior Nucleus (AP)
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The source for mate calling in the frog is the
anterior nucleus of the hypothalamic preoptic area (in the past this region has
been linked with the thalamus but functionally it hypothalamic). It must be present
for normal calling to occur (Aronson and Noble - 1945, Schmidt - 1968). It
has binding sites for the hormone testosterone (Kelly et al - 1975 and Kelly
et. al. - 1978) and testosterone must be present for male frogs to initiate mate
calling. Frogs without testes do not initiate such calling (Schmidt - 1966). Also
neural responses to to mating calls has been found to occur in the anterior part
of the preoptic area by Urano and Gorbman (1981).
The preoptic area got its
name because it is the part of the hypothalamus located forward (headward
or rostal) of the optic chiasm which is the place where the optic nerves from each
eye cross over to the other side of the brain. The preoptic area is composed
of two nuclei, the anterior and posterior nuclei. Brain nuclei are clusters of
cell bodies usually identified with the nissl stain. Consequently, their dendrites
can extend for some distance away from the nuclei.
Of these two nuclei, the
anterior preoptic nucleus is the one responsible for generating the motivation
signal for mate calling for if it is stimulated with electrodes as shown in figure
1 it will produce mate calling while stimulation of the posterior nucleus will not. Notice the different diameter electrodes used. Dots below
(+) sign indicate locations where calling was produced at the indicated stimulus
strength in micro-Amps at 70 Hz. Dots below the (-) sign indicate locations where
calling was NOT produced with indicated stimulus strength. Notice that the
stimulus must be within the proper range to produce calling (stronger is not
always better). Crosses (x) indicate locations where calling was produced
by electrode penetration alone. The "K" view summarizes the locations which
produce calling with the least electrode current. The arrows show the locations
where surface stimulations by large (250 micron dia.) electrodes produced
calling.
Figure 2
Removed Regions Around the Hypothalamos of the Frog, Rana pipiens Which Did Not Affect Mate Calling.(Schmidt - 1984)
Amygdala (A), Anterior Preoptic Nucleus (AP), Cerebellum (C), Interpeduncular nucleus (IP), Isthmic Nucleus (I), Optic Chiasm (OC), Posterior Preoptic Nucleus (PP), Pretrigeminal Nucleus (PT), Stimulation Site (SS), Ventomedial Thalamic Nucleus (VM), Fifth cranial nerve (V). Site where a midline lesion cuts the rearward projecting nerve fibers as they cross over to the other side of the brain (X).
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To eliminate the possibility
that higher brain regions connected to the anterior
nucleus might actually be the source of the mate calling various regions surrounding
the hypothalamus were removed as shown in figure 2. One whole side of the brain
on the same side
as the stimulation was removed tailward (caudal) of the preoptic area leaving
the brain regions on the opposite side intact. The large dotted areas
in figure 2 show the other brain regions which were removed (the small dots are
neural cell body regions). The top row in figure 2 shows slices from a top
view of one side of the brain as depth increases from left to right. The headward
(rostal) direction is upward in these views. The bottom row gives a side cross-section
view of the middle part of the frog's brain with the headward direction toward the
right.
In this situation calling was indicated by the pattern of electrical activity in
two laryngeal muscles. This was done just anterior to the
pretrigeminal nucleus just below the cerebellum (PT in figure 2). With this done
a lesion at the "X" location in figure 4 eliminated the calling as it cut
the output fibers of the preoptic area just before they crossed over to the other
side of the brain. Horseradish peroxidase (HRP) studies have shown that preoptic
area neurons do indeed project down to the pretrigeminal nucleus in Xenopus laevis
(Wetzel and Kelly - 1981).
Lesioning the anterior preoptic nucleus prevents mate
calling while lesions more lateral and to the outside affect more of the posterior
preoptic nucleus tending to prevent mate orientation behavior (Schmidt - 1968).
The frog Rana pipiens like other frogs has a variety of calls. Most prominent for Rana pipiens are the Mating Call and the Release Call. The triggering of the
mating call has been discussed above The release call is triggered when
ever a frog is suddenly grabbed from behind when it is not a female ready to mate.
That the release call is reflexive
is supported by the fact that it is triggered not by stimulation of the hypothalamus
by a different area called the Interpeduncular Nucleus located in the trigeminal
tegmentum as shown in figure 3. Schmidt (1974a - page 325) also says this:
"It was noted that changes in stimulus rate affected the two types of calling activity
differently. The vocal-phase pulse rate of mate calling was independent of
stimulus rate over a range of at least 30-150 Hz. The frequency used most
often was 100 Hz. This usually evoked mating calling with a latency of less
than 10 seconds. Higher frequencies and the increased stimulus duration and/or
amplitude required at lower frequencies caused increased damage at the stimulation
site.On the other hand, the vocal-phase pulse rate of release calling followed
the stimulus rate. The vocal-phase most nearly resembled tactilely-evoked release
calling at a stimulation rate of about 30 Hz and therefore, this rate was
used most often. Slightly, lower stimulation rates would not evoke release
calling. As stimulation rates were increased, vocal-phase pulsing followed
the stimulus rate to about 60 Hz. Further increases resulted in increasingly
abnormal and variable pulsing of the vocal-phase."
Ending the release calling stimulus
immediately ends the release calling while ending the mate calling stimulus
in the preoptic area does NOT stop the mate calling. Instead it completes its cycle.
(Schmidt - 1974b).
Figure 3
Top View of the Frog Brain Stem Showing Locations where Stimulation Produced Calling.(Schmidt - 1974b)
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In figure 3, stimulation of the areas A and B elicit mate calling while stimulation
of area C elicits release calling. Presumably area B is on the axon path to
the calling motor centers located in a distributed fashion between the fifth (V)
sensory nuclei and the interpeduncular nucleus
HRP tracing studies in the frog Xenopus laevis by Wetzel and
Kelly (1981) show that preoptic area neurons project to the pretrigeminal nucleus
(PT in figure 2) which is located just above the interpeduncular nucleus.
The tactile inputs to the interpeduncular nucleus seems to come from the fifth (V)
sensory nuclei since
stimulation of these nuclei can also produce releasing calls.
Also lesions destroying both these nuclei and the area around them also abolished
not only release calling but also mate calling (Schmidt- 1971). So the motor generating
circuitry for calling seems to be located in a distributed fashion between
the fifth (V) sensory nuclei and the interpeduncular nucleus (Schmidt - 1976).
Both calls can be triggered in the complete absence of auditory feedback indicating
that they follow a central "program" (Schmidt - 1974a,b) and the complete
mating call can be accomplished by only one side of the brainstem although
the calls are not in phase with each other if the sides are separated and are activated
simultaneously (Schmidt-1976).
Hypothalamic Ventral Nucleus
Figure 4
Neuron Firing Rate Changes in Hypothalamic Ventral Nucleus Due to Mating Calls.(Wilczynski and Allison - 1989)
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Figure 5
(Neuron Firing Rate Changes in Hypothalamic Ventral Nucleus Due to Noise. (Wilczynski and Allison - 1989)
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The other major area
of the frog Hypothalamus forward of the optic chiasm is the
ventral nucleus. It receives inputs from the auditory processing centers
of the thalamic anterior and central nuclei and well as from the secondary
isthmal nucleus (Wilczynsky and Allison - 1989). Weaker projection from the
thalamic central nucleus along with the secondary isthmal nucleus also enervates
the hypothalamic anterior nucleus Neary (1988). This leaves the minor thalamic anterior
nucleus projection as the unique input to the hypothalamic ventral nucleus. Like
the preoptic area of the hypothalamus the ventral nucleus also has binding sites
for androgen hormones (Kelly - 1975).
The firing rate of approximately half
of all recorded neurons in the hypothalamic ventral nucleus could be influenced
by sounds (Wilczynski and Allison - 1989). Figure 4 shows the effects of mating
calls while figure 5 shows the effects of noise. Notice that the majority
of neurons increase their firing rate in response to mate calling while the
number of increases and decreases is more equal in response to noise. Wilczynski
and Allison (1989) further say:
"Units in this region had robust but highly
variable spontaneous firing rates. They could normally be held for 15-20 min, but
never more than 30 min, after which responses were no longer detectable. Many
units isolated ceased firing before a measurement sequence was completed and therefore
were not included in the study ... None of the units isolated in these
experiments showed any phase locking to the stimulus presented nor any noticeable
abrupt firing change at stimulus onset or offset."
So these neurons in the ventral nucleus of the hypothalamus
have all the characteristics of motivation defining cells which exhibit spontaneous
firing rates which change over time.
References
Aronson, L.R. and Noble,
G.K. (1945) The Sexual Behavior of Anura. 2. Neural Mechanisms Controlling
Mating in the Male Leopard Frog, Rana pipiens. Bull Am. Mus. Nat. Hist. 86:83-139
Feng, Albert S., Hall, Jim C., and Gooler, David M. (1990) Neural Basis of
Sound Pattern Recognition in Anurans. Progress in Neurobiology, 34:313-329
Kelly, D.B. Morrel, J.I., Pfaff, D.W. (1975) Autoradiographic Localization
of Hormone Concentrating Cells in the Brain of an Amphibian, Xenopus laevis. 1.
Testosterone. J Comp. Neurol. 164:47-62
Kelly, D.B. Lieberburg, I, McEwen, B.S.,
Pfaff, D.W. (1978) Autoradiographic and Biochemical Studies of Steroid Hormone
Concentrating Cells in the Brain of Rana pipiens. Brain Res. 140:287-305
Neary,
T.J. (1988) Forebrain Auditory Pathways in Ranid Frogs in The Evolution of
the Amphibian Auditory System - edited by B. Fritzsch, M.J. Ryan, and
W. Wilczynski. Wiley, New York
Noble, G.K. and Aronson, L.R. (1942). The Sexual
Behavior of Anura. 1. The Normal Mating Pattern of Rana pipiens. Bull. Amer. Mus.
Nat. Hist. 80:127-142
Schmidt, Robert S. (1964) Hearing and Responses to Calls in
Anurans. Behaviour 23:280-293
Schmidt, Robert S. (1966) Hormonal Mechanisms of
Frog Calling. Copeia 1966:637-644
Schmidt, Robert S. (1968) Preoptic Activation
of Frog Mating Behavior. Behaviour 30:239-257
Schmidt, Robert S. (1971) A
Model of the Central Mechanisms of Male Anuran Acoustic Behavior. Behaviour
39:288:317
Schmidt, Robert S. (1974a) Neural Correlates of Frog Calling: Independence
from Peripheral Feedback. J. Comp. Physiol. 88:321-333
Schmidt, Robert S.
(1974b) Neural Correlates of Frog Calling: Trigeminal Tegmentum. J. Comp.
Physiol. 92:229-254
Schmidt, Robert S. (1976) Neural Correlates of Frog Calling:
Isolated Brainstem. J. Comp. Physiol. 108:99-113
Schmidt, Robert S. (1984) Neural
Correlates of Frog Calling: Preoptic Area Trigger of 'Mating Calling'. J.
Comp. Physiol. A 154:847-853
Wetzel, D.M. and Kelly, D.B. (1981) Neural circuitry
for Mate Calling in Male south African Clawed Frogs, Xenopus laevis. Soc. Neurosci.
Abstr. 7:269
Urano, A. and Gorbman, A. (1981) Effects of Pituitary Hormonal
Treatment on Responsiveness of Anterior Preoptic Neurons in Male Leopard Frogs, Rana pipiens. J. Comp. Physiol. 141:163-171
Wetzel, D.M. and Kelly, D.B. (1981)
Neural Circuitry for Mate Calling in Male South African Clawed Frogs, Xenopus laevis. Soc. Neurosci. Abstr. 7:269
Wilczynski, Walter and Allison, John,
D. (1989) Acoustic Modulation of Neural Activity in the Hypothalamus of the
Leopard Frog. Brain, Behavior, and Evolution 33:317-324