Anuran calling circuits: inhibition of pretrigeminal nucleus by prostaglandin

Neural correlates of mating calling and pulmonary respiration were recorded from isolated brain stems of male Northern leopard frogs (Rana p. pipiens) before and after exposure of the brain stems to prostaglandin F2 alpha (PG) or saline. Diffusion of PG (but not saline) from a pipette directly over the pretrigeminal nuclei abolished "calling" temporarily. Similar application of PG nearby had no effect. Exposure of only the anterior 1/2 of the brain stem, containing the pretrigeminal nuclei but not the pulmonary respiration generator, to PG (but not saline) abolished generation of slow waves by the pretrigeminal nucleus portion of the mating calling pattern generator. Exposure of only the posterior 1/2 of the brain stem, containing the pulmonary respiration generator but not the pretrigeminal nuclei, to PG had no effect on the correlates of pulmonary respiration. These results are consistent with the hypothesis that the inhibition of calling by PG is through an effect largely, perhaps exclusively, on the pretrigeminal nuclei.

Neural correlates of frog calling: production by two semi-independent generators

The anterior preoptic nuclei of the isolated brainstem of male, Northern leopard frogs (Rana p. pipiens) were stimulated electrically and neural correlates of mating calling recorded from the rhombencephalic mating calling pattern generator. Lesions of discrete areas of the brainstem showed that the mating calling generator is separable into two generators, the pretrigeminal nucleus and the classical pulmonary respiration generator (which is approximately co-extensive with the motor nuclei IX-X). Each of these still can produce pulses when isolated from the other. Their interaction changes the expiratory phase of breathing into the vocal phase of calling. All stages of intermediates between these phases could be seen. An updated and simplified model of call production and evolution is presented.

Origin of anuran calling: description of toad releasing

Electromyographic electrodes were implanted in forelimb muscles of male American toads (Bufo americanus). Toads then were allowed to clasp an artificial clasp object incorporating a pair of solenoids that, when activated, produced a stimulus simulating the side vibrations normally associated with release signalling by conspecific males. Electromyographic activity, in response to the release stimulus, was recorded. The adductor muscle contracted immediately after stimulus onset (causing intensified clasping), responding to the stimulus as a displacement threat. Then the abductor muscle contracted (causing rapid unclasping, or releasing), responding to the stimulus as a release signal. These techniques have provided a better understanding of releasing than has been possible with natural objects (i.e., living males).

Releasing (unclasping) in male American toads: a neural substrate in the lateral subtoral tegmentum

Release signals (chirps and side vibrations) are given by a male toad being clasped by another toad. These signals trigger immediate releasing (i.e. unclasping) by the clasping toad. First, small electrolytic lesions in the lateral subtoral tegmentum of male American toads abolished releasing, i.e. these toads persisted in clasping release-signaling males. Second, stimulation of this area caused bilateral abduction of the forelimbs, as occurs in normal releasing. Third, neural responses, resulting from tactile stimulation of either thumbpad, could be recorded from this region. Supposedly, similar stimuli would be generated by the release vibrations of the sides given by a male being clasped. These data are consistent with the participation of the lateral subtoral tegmentum in the detection of release signals and in the triggering of releasing. Release signaling was probably an important step in the evolution of anuran vocalizations. The midbrain tegmentum may contain primitive circuits for the detection of stimuli eliciting anuran acoustic behaviors.

Anuran mating calling circuits: inhibition by prostaglandin

Male American toads (Bufo americanus) were induced to mating call in response to electronically simulated, conspecific mating calls. The injection of prostaglandin (PG) F2 alpha caused suppression of mating call answering. Neural correlates of mating calling were triggered by electrical stimulation of the anterior preoptic nucleus in the isolated brainstem of male, Northern leopard frogs (Rana p. pipiens). The addition of PGF2 alpha to the bath completely abolished the correlates of mating calling without changing the correlates of pulmonary respiration. The suppression of mating calling shown here, along with the suppression of release signalling described by Diakow and Nemiroff (1981), supports the hypothesis of a close interrelation between the neural circuits of these two calls. The suppression of the neural correlates of mating calling in an isolated preparation shows a central site of action of the PG. The retention of normal correlates of pulmonary respiration, even after suppression of mating calling correlates, suggests that the generation of mating calling patterns involves the extension and pulsing of the expiratory phase of breathing.

Mating call phonotaxis in female American toad: lesions of anterior preoptic nucleus

Lesions of the preoptic area and immediately adjacent septal area were made in female American toads, Bufo americanus. The ability to show mating call phonotaxis (MCP) was then tested for. The intensity of MCP was found to decrease as the degree of damage to the anterior tip of the anterior preoptic nucleus (APON) increased. The APON is known to concentrate androgens and to be essential for mating calling by male anurans. Perhaps the APON is involved also in the detection of the elevated levels of prostaglandin associated with MCP. Even in lesioned toads in which phonotaxis was nearly absent, there was an occasional MCP response that seemed entirely normal. Therefore, the APON is not involved in generating or guiding the motor mechanisms of phonotaxis.

Mating call phonotaxis in female American toads: lesions of central auditory system

Female American toads were tested for mating call phonotaxis (prostaglandin-induced) after brain lesions. The pretrigeminal nucleus, all of the telencephalon (except the preoptic area and immediately adjacent septal area), the dorsal thalamus (including all of the anterior nuclei and most of the central nuclei), the optic tectum, and the dorsal and medial parts of the torus were removed without interferring with the hormonal sensitization, acoustic triggering, or production of phonotaxis. These lesions removed most of the forebrain and much of the midbrain auditory areas. These data are compared with similar lesion data on the triggering mechanisms for three other anuran acoustic behaviors. The lateral subtoral tegmentum may have a basic, special importance in the triggering of all four behaviours.

Mating call phonotaxis in the female American toad: induction by hormones

Gravid, female American toads are known to move toward conspecific mating calls. This behavior, as well as ovulation, often can be induced by the injection of human chorionic gonadotropin (HCG). It was found that HCG-induced phonotaxis could be terminated by the injection of indomethacin (an inhibitor of prostaglandin synthesis) and then reinstated by the injection of prostaglandin F2 alpha. A drug regimen was devised that allowed elicitation of phonotaxis, indistinguishable from that induced by HCG and in the absence of ovulation. This involved the administration of prostaglandin F2 alpha and arginine vasotocin (or mesotocin) following progesterone priming. It has not been determined whether the peptide is really essential.

Neural correlates of frog calling. Masculinization by androgens

Neural correlates of mating calling can be recorded from isolated brainstems of male, Northern leopard frogs after the circuits for this behavior have been triggered by electrical stimulation of the preoptic area. Correlates can be evoked reliably and by a stimulus of low amplitude. However, such correlates can be evoked only rarely from female brainstems, and then only by a much larger stimulus. The sensitivity to triggering in female brainstems can be masculinized by previous treatment of the intact frog with testosterone propionate or dihydrotestosterone, but not by estradiol benzoate. This suggests that the action of the androgens is direct and does not require aromatization to estrogens. Comparisons with other studies suggest that the androgen effect may be mainly on posterior parts of the calling circuits (i.e., call pattern generator or motoneurons), rather than on the preoptic area trigger of the generator.

Sexual dimorphism in succinic dehydrogenase staining of toad pretrigeminal nucleus

The anuran (toad and frog) pretrigeminal nucleus is probably involved in the production of vocalizations, including the male mating call. This nucleus is especially obvious when stained by the succinic dehydrogenase method. In the American toad, a striking sexual dimorphism is shown by such staining. The male nucleus contains numerous, large cells that stain intensely. Such cells are smaller and infrequent in females.

Masculinization of toad pretrigeminal nucleus by androgens

The magnocellular portion of the pretrigeminal nucleus (a structure involved in vocalization) of male toads contains large cells than does the female nucleus. Testosterone propionate injections caused essentially complete, and dihydrotestosterone injections incomplete, mascullinization of the female nucleus. Estradiol benzoate injections had little or no effect. The androgen effects are therefore largely direct rather than involving extensive aromatization of androgen to estrogen.

Pretrigeminal nucleus of leopard frog concentrates 2-deoxy-d-glucose during release calling

After injection of 14C-2-deoxy-D-glucose, some leopard frogs were induced to release call repeatedly, while others were used as silent controls. Autoradiograms of brains of many of the calling frogs, but none of the silent ones, showed heavy labeling of the pretrigeminal nucleus. This supports the hypothesis that this nucleus is involved in release calling.

Catecholaminergic neurons of treefrog isthmotrigeminal tegmentum

Neurons that take up 3H-noradrenaline were detected, by autoradiography, around the ventral tip of the isthmic nucleus in treefrogs. No uptake was detected in the adjacent pretrigeminal nucleus, suggesting that this is not homologous to the locus coeruleus.

A minimum-volume, flow-through system for rearing frog tadpoles: aquarium

A flow-through system requiring a low flow rate (4-10 ml/minute/aquarium) was developed for rearing tadpoles. This permitted tadpoles to be raised in tap water that had been dechlorinated and acidified. The aquarium consisted of a 7-liter plastic outer shell. A screen-bottomed basket, inserted into the shell, facilitated handling of the tadpoles. A bottom-scraper and flushing system permitted periodic removal of solid wastes.

Neural mechanisms of releasing (unclasping) in American toad

Abstract

The tendency to clasp is increased greatly, in the toad Bufo cognatus, by injections of gonadotrophin. In B. americanus, the forebrain and medial parts of the inferior colliculi are not necessary for releasing (i.e. unclasping) behavior. Small lesions at the lateral edge of the anterio-dorsal nucleus of the inferior colliculi abolish releasing. After removal of the clasp-inhibition mechanisms of the trigemino-isthmic tegmentum, toads show strong clasping (foreleg adduction) and strong releasing (hindleg kicking) movements simultaneously. This suggests that releasing is a distinct behavior pattern, rather than merely an inhibition of clasping Normal releasing can be evoked in B. americanus after complete bilateral labyrinthectomy, and the presence of a nearby, release-calling male does not evoke releasing by a clasping male. Therefore, it must be mainly the tactile effects, rather than the auditory or vestibular effects, of the release signals that evoke releasing. The effects of section of the dorsal roots of various spinal nerves in B. americanus confirm that the foreleg is the most important area for reception of the tactile stimuli evoking clasping and releasing. Gross lesions of the forefoot and sections of various nerves to the forefoot show that receptors involved in eliciting these behaviors are widely distributed in this area. The forefoot sensory field of the deep radial nerve is especially involved in clasping.

Preoptic activation of mating call orientation in female anurans

Abstract

Techniques are described for inducing ovulation in tree frogs (Hyla cinerea) and toads (Bufo woodhousei fowleri) and for testing the ability of these gravid females to orient to homospecific mating calls. A few preliminary tests were also made with heterospecific mating calls. Several responses to calls are described. Of special interest is an escape response made by Bufo to heterospecific calls. This may serve as an isolating mechanism by effecting avoidance of heterospecific calling males. Males of the same species could not be induced to move toward mating calls. Testing of females with forebrain lesions showed that the telencephalon and the dorsal part of the preoptic area are not necessary for orientation behavior, but that the region of the ventral magnocellular preoptic nucleus is essential.