Apomorphine Alters Prey-Catching Patterns in the Common Toad: Behavioral Experiments and ¹⁴C-2-Deoxyglucose Brain Mapping Studies

Abstract

Previous studies on the dopaminergic modulation of visuomotor functions in amphibians showed that the dopamine agonist apomorphine (APO) alters prey-catching strategies. After systemic administration of APO in common toads Bufo bufo, prey-oriented turning and locomotion was attenuated whereas snapping toward prey was facilitated in a dose dependent manner. With systemic APO administration, toads which had previously been hunting, that is pursuing prey, behaved in a waiting position, that is sitting motionless and waiting for prey. This suggests that APO facilitates the ingestive component and inhibits the orientational and locomotory components of prey capture. To help unravel the cerebral sites of action of APO, the present study employs the ¹⁴C-2-deoxyglucose method to compare the rate of local glucose utilization in 41 brain structures. The retinal projection fields – e.g. superficial optic tectum, pretectal nuclei, and anterior dorsal thalamic nucleus – showed an elevation in glucose utilization due to APO-induced increases in retinal output. The medial tectal layers and the ventral striatum, both involved in visuomotor functions related to prey-oriented turning and locomotion, displayed APO-induced decreases in glucose utilization. APO-induced increases in glucose utilization were observed in the medial reticular formation and the hypoglossal nucleus which participate in the motor pattern generation of snapping. APO-induced increases in glucose utilization were also detected in the nucleus accumbens and the ventral tegmentum (mesolimbic system) as well as in the ventromedial pallium (‘primordium hippocampi’) and the septum, both of which belonging to the limbic system. These structures contribute to motivational level control and may be responsible for the APO-induced elevation of the snapping rate. Various other structures revealed APO-induced increases in glucose utilization. These structures include the olfactory bulb, lateral pallium, suprachiasmatic nucleus, nucleus of the periventricular organ, and the nucleus of the solitary tract. The lateral amygdala displayed APO-induced decreases in glucose utilization. The APO-induced alterations in local cerebral glucose utilization are evaluated with reference to the distribution of dopaminergic structures, and this is compared with similar data obtained in the rat by other authors. A neural network explaining the APO-induced behavioral syndrome in the common toad is discussed.

Apomorphine effects on frog locomotor behavior

“…These studies combined provide evidence that DA may mediate both motor and reward aspects of amphibian behavior. The results of this present study are not only consistent with previous studies that provide evidence for DA-mediating motor behaviors in frogs [13,[16][17][18]34], these studies combined indicate an evolutionary conservation of DA function across vertebrate taxa. Mean±SEM of the average Trial Time relative to Day 1 for saline and APO-treated animals.…”

“…Although the systemic administration of APO in this study made it impossible to identify its location of action, we had hypothesized that APO would preferentially impair locomotion based upon two lines of evidence. First, prior studies demonstrated that APO administered at 20 mg/kg body weight in toads impaired such weight-shifting behaviors as turning and lunging [13,15]. Second, pharmacological studies of D2 receptor agonists, such as APO, in general impair locomotor abilities [29].…”

“…APO has been shown to impair orienting and turning, but facilitates snapping in a stereotyped manner. Anatomical studies using 14C-2 deoxyglucose (2-DG) mapping indicate that APO administration combined with a visual prey stimulus induced increases in 2-DG uptake in retinal projection regions, but induced decreases in 2-DG uptake in the striatum [13]. These authors suggest that prey capture consummatory behavior is facilitated by APO effects on retinal projection regions, while prey capture appetitive behavior is impaired due to APO effects on striatal function.…”

“…Glagow and Ewert have examined the effects of the DA mixed agonist, apomorphine (APO) on both appetitive and consummatory components of prey capture behavior in toads [13][14][15]. In the common toad, Bufo bufo, visual prey capture behavior consists of appetitive aspects such as orientation, approaching and turning, and consummatory aspects such as snapping.…”

Androgen Effects on Tyrosine Hydroxylase Cells in the Northern Leopard Frog, <i>Rana pipiens</i>

“…There are no studies investigating dopaminergic control of reinforcement in frogs. A few studies do show that pharmacological agents acting on dopamine systems affect locomotion [54, 55, 56, 57, 58, 59]. …”