Pharmacological characterization of the D1- and D2-like dopamine receptors from the brain of the leopard frog, Rana pipiens

Ecology, Cognition, and the Hippocampus: A Tale of Two Frogs

The underlying hypothesis that motivates research into the relationship between ecology, cognition, and the hippocampus is that selection to solve problems in nature shapes cognition through changes in the hippocampus. This hypothesis has been explored almost exclusively in mammals and birds. However, if one is interested in the principles that shape the evolution of vertebrate cognition, work in amphibians is essential. To address this gap, we have developed a research program contrasting cognitive abilities and hippocampal neurobiology in two species of frog with distinct social and spatial ecologies. We have found that the poison frog Dendrobates auratus, a diurnal species whose interactions with the physical and social environment are complex, is more adept and flexible at spatial learning and learned inhibition than the túngara frog, a nocturnal species that lacks complex interactions with the spatial and social environment. Because spatial learning and learned inhibition are closely associated with hippocampal function in other vertebrates, we used RNA sequencing to characterize molecular differences in the hippocampus of the two species. We have found that D. auratus has greater levels of expression of genes associated with neurogenesis, synaptic plasticity, and cellular activity, and lower levels of expression of genes associated with apoptosis, compared to the túngara frog. Our studies are consistent with the idea that D. auratus, with their more complex social and spatial ecology, have enhanced hippocampally dependent cognitive abilities compared to túngara frogs. Further characterization of the features of hippocampal neurobiology that confer distinctive cognitive abilities will help elucidate the neural features that are necessary for the evolution of enhanced hippocampally dependent cognition.

Dopamine differentially modulates the excitability of striatal neurons of the direct and indirect pathways in lamprey

The functions of the basal ganglia are critically dependent on dopamine. In mammals, dopamine differentially modulates the excitability of the direct and indirect striatal projection neurons, and these populations selectively express dopamine D1 and D2 receptors, respectively. Although the detailed organization of the basal ganglia is conserved throughout the vertebrate phylum, it was unknown whether the differential dopamine modulation of the direct and indirect pathways is present in non-mammalian species. We aim here to determine whether the receptor expression and opposing dopaminergic modulation of the direct and indirect pathways is present in one of the phylogenetically oldest vertebrates, the river lamprey. Using in situ hybridization and patch-clamp recordings, we show that D1 receptors are almost exclusively expressed in the striatal neurons projecting directly to the homolog of the substantia nigra pars reticulata. In addition, the majority of striatal neurons projecting to the homolog of the globus pallidus interna/globus pallidus externa express D1 or D2 receptors. As in mammals, application of dopamine receptor agonists differentially modulates the excitability of these neurons, increasing the excitability of the D1-expressing neurons and decreasing the excitability of D2-expressing neurons. Our results suggest that the segregated expression of the D1 and D2 receptors in the direct and indirect striatal projection neurons has been conserved across the vertebrate phylum. Because dopamine receptor agonists differentially modulate these pathways, increasing the excitability of the direct pathway and decreasing the excitability of the indirect pathway, this organization may be conserved as a mechanism that biases the networks toward action selection.

Effects of amphetamine on conditioned place preference and locomotion in the male green tree frog, Hyla cinerea

Neural systems mediating motivation and reward have been well described in mammalian model systems, especially with reference to reward properties of drugs of abuse. Far less is known of the neural mechanisms underlying motivation and reward in non-mammals. The behavioral procedure conditioned place preference (CPP) is often used to quantify reward properties of psychoactive drugs. The indirect dopamine agonist d-amphetamine (AMPH) is known for its properties for inducing CPP in mammals and for inducing dose-related stereotypic movements. We used the green tree frog, Hyla cinerea, to examine whether AMPH could induce both CPP and a dose response change in motor behaviors. We demonstrated that H. cinerea can show place conditioning to AMPH following 14 days of training and that AMPH can cause reversal of a strong baseline place preference. Amphetamine-treated animals (20 mg/kg b.w.) received the drug paired with the previously non-preferred context, and vehicle paired with the preferred context. Control animals received vehicle in both preferred and non-preferred contexts. Amphetamine-treated animals switched context preference following conditioning, whereas control animals did not. We also demonstrated in an open-field experiment that AMPH did not cause any noticeable changes in motor movement or behaviors across a range of doses (0, 10, 20 mg/kg b.w.). This study represents the first examination of the behavioral effects of AMPH in amphibians. These results may contribute to a better understanding of the function and pharmacology of a reward system that may mediate natural behaviors in frogs and other vertebrates.

Species differences in the relative densities of D1- and D2-like dopamine receptor subtypes in the Japanese quail and rats: an in vitro quantitative receptor autoradiography study

Evidence has accumulated that the regulation of male sexual behavior by dopamine might not be the same in Japanese quail (and perhaps all birds) as it is in mammals. For example, the non-selective dopamine receptor agonist, apomorphine (APO), facilitates male sexual behavior in rats but inhibits it in quail. Although the general organization of the dopamine system is similar in birds and mammals, it is possible that the relative distribution and/or density of binding sites are different. We therefore compared the relative densities of D1-like and D2-like receptor subtypes in Japanese quail and rats, with the use of in vitro quantitative receptor autoradiography. Brain sections from 8 male rats and 8 male quail were labeled with [(3)H]SCH-23390 and [(3)H]Spiperone. In general we found a systematic species difference in the relative density of D1- vs. D2-like receptors such that the D2/D1 ratio is higher in quail than in rats in areas, known to be important target sites for dopamine action such as striatal regions or the preoptic area, which is also associated with activation of sexual behavior. This difference might explain the variation in the behavioral effectiveness of APO in rats as compared to quail; with a higher relative density of D2-like receptors in quail, a similar dose of APO would be more likely to activate inhibitory processes in quail than in rats.

Modulation of adenylyl cyclase activity in rat striatal homogenate by dopaminergic receptors

We have characterized the modulation of adenylyl cyclase (AC) activity by ligands of dopaminergic receptors in rat striatal homogenate and compared the results with receptor-ligand binding affinities. Despite the fact that rat striatum contains high level of both dopamine D(1) and D(2) receptors, only the D(1)-specific AC activation by agonists could be determined. All D(1)-receptor agonists (dopamine, dihydrexidine, and A 77636) used were able to increase cAMP accumulation in a concentration-dependent manner, while D(1)-receptor antagonists (SCH23390, SKF83566, and butaclamol) blocked the effects induced by the aforementioned agonists. At the same time, the D(2)-receptor agonist quinpirole and antagonist sulpiride had no effect on cAMP accumulation in striatal homogenate neither on the basal level nor on the activated level of AC, while inhibited [(3)H]raclopride binding to these membranes. Comparing the ligands of the D(1) receptor in modulating the activity of AC and displacing D(1)-receptor-specific radioligand [(3)H]SCH23390 binding revealed that the ligands modulate both of these processes with similar affinities. It indicates that under given experimental conditions, only dopamine D(1)-receptor-mediated stimulation of AC activity can be measured in membrane homogenate of rat striatum, while dopamine D(2)-receptor effects remain fully hidden.

Altered mesoaccumbens and nigro-striatal dopamine physiology is associated with stereotypy development in a non-rodent species

Stress-induced changes in mesoaccumbens dopamine neurophysiology have been associated with the development of stereotypic behaviour in in-bred strains of laboratory rodents. This experiment evaluated whether similar changes are associated with environmentally-induced stereotypic behaviour in a higher-vertebrate species, the horse. D1- and D2-like dopamine receptor densities (B(max)) and dissociation constants (K(d)) were measured in control (n=9) and stereotypy (n=9) horses in the nucleus accumbens, caudate nucleus, putamen, substantia nigra and ventral tegmentum brain regions. Results revealed that stereotypy horses had significantly higher (P<0.05) dopamine D1 and D2 receptor densities (B(max)) in the nucleus accumbens compared to non-stereotypy controls. D1 receptor densities (B(max)) and D2 receptor affinity (K(d)) were also significantly lower in the caudate nucleus brain region of stereotypy horses (P<0.05). No other significant results were observed. These results demonstrate that stereotypy horses have increased activity within the mesoaccumbens dopamine pathway and, thus, that the development of environmentally-induced stereotypy may be associated with changes in motivational systems within the animal.

7alpha-Hydroxypregnenolone acts as a neuronal activator to stimulate locomotor activity of breeding newts by means of the dopaminergic system

It is becoming clear that steroids can be synthesized de novo by the brain and other nervous systems. Such steroids are called neurosteroids, and de novo neurosteroidogenesis from cholesterol is a conserved property of vertebrate brains. In this study, we show that the newt brain actively produces 7alpha-hydroxypregnenolone, a previously undescribed amphibian neurosteroid that stimulates locomotor activity. 7alpha-hydroxypregnenolone was identified as a most abundant amphibian neurosteroid in the newt brain by using biochemical techniques combined with HPLC, TLC, and GC-MS analyses. The production of 7alpha-hydroxypregnenolone in the diencephalon and rhombencephalon was higher than that in the telencephalon and peripheral steroidogenic glands. In addition, 7alpha-hydroxypregnenolone synthesis in the brain showed marked changes during the annual breeding cycle, with a maximal level in the spring breeding period when locomotor activity of the newt increases. Behavioral analysis of newts in the nonbreeding period demonstrated that administration of this previously undescribed amphibian neurosteroid acutely increased locomotor activity. In vitro analysis further revealed that 7alpha-hydroxypregnenolone treatment resulted in a dose-dependent increase in the release of dopamine from cultured brain tissue of nonbreeding newts. The effect of this neurosteroid on locomotion also was abolished by dopamine D(2)-like receptor antagonists. These results indicate that 7alpha-hydroxypregnenolone acts as a neuronal activator to stimulate locomotor activity of breeding newts through the dopaminergic system. This study demonstrates a physiological function of 7alpha-hydroxypregnenolone that has not been described previously in any vertebrate class. This study also provides findings on the regulatory mechanism of locomotor activity from a unique standpoint.

Sex differences and hormone influences on tyrosine hydroxylase immunoreactive cells in the leopard frog

We examined sex differences in tyrosine hydroxylase immunoreactive (TH-ir) cell populations in the preoptic area (POA), suprachiasmatic nucleus (SCN), posterior tuberculum (TP), and caudal hypothalamus (Hy) in the leopard frog (Rana pipiens), in addition to the effects of natural variation in sex steroid hormones on these same populations in both sexes. All four of these populations have been shown to be dopaminergic. Gonadal sex, androgens, and estrogen all influenced TH-ir cell numbers, but in a complicated pattern of interactions. After factoring out the effects of sex steroids by multiple regression, TH-ir cell numbers in all four areas differed between the sexes, with males having a greater number of TH-ir cells. The influence of androgens and estrogen differed by region and sex of the animals. Androgens were the main influence on TH-ir cell numbers in the POA and SCN. Plasma androgen concentrations were positively correlated with TH-ir cell numbers in both areas in males. In females, androgen concentration was negatively correlated with TH-ir cell numbers in the POA; there was no significant relationship in the SCN in females. In the more caudal populations, estrogen (E2) levels were positively correlated with TH-ir cell numbers in the TP of both males and females. In the caudal hypothalamus, E2 levels were positively correlated with TH-ir cell numbers in females, but there was no significant correlation in males. The results indicate that gonadal sex imposes a baseline sex difference in the four TH-ir (dopamine) populations, resulting in a higher number of such cells in males. Individual and sex-linked differences in gonadal steroid hormones lead to variation around this baseline condition, with androgens having a greater influence on rostral populations and estrogen on caudal populations. Last, an individual's gonadal sex determines the effect that androgens and estrogen have on each population.