6‐hydroxydopamine lesions in anuran amphibians: A new model system for Parkinson's disease?

Abstract

We investigated the effects of dopamine depletion on acoustically guided behavior of anurans by conducting phonotaxis experiments with female gray treefrogs (Hyla versicolor) before and 90 min after bilateral injections of 3, 6, or 12 microg 6-hydroxydopamine (6-OHDA) into the telencephalic ventricles. In experiments with one loudspeaker playing back a standard artificial mating call, we analyzed the effects of 6-OHDA on phonotactic response time. In choice tests we measured the degree of distraction from the standard call (20 pulses/s) by three different variants with altered pulse-rate (30/s, 40/s, 60/s). Five days after experiments, brains were immunostained for tyrosine hydroxylase. Labeled neurons were counted in the suprachiasmatic nucleus, posterior tuberculum, interpeduncular nucleus, and locus coeruleus, and correlation between neuronal numbers and behavioral scores was tested. Response times increased together with 6-OHDA concentrations, which was mainly due to longer immobile periods before the animals started movement. In choice tests the most irrelevant stimulus (60/s) distracted 6-OHDA injected females from the standard stimulus, while sham injected controls were undistracted. The number of catecholaminergic neurons decreased with increasing 6-OHDA concentration in the suprachiasmatic nucleus, posterior tuberculum, and interpeduncular nucleus. The normalized number of immunoreactive neurons in the posterior tuberculum was positively correlated with phonotaxis scores in the one-speaker test, demonstrating that motor deficits are a function of tubercular cell loss. We conclude that bilateral 6-OHDA lesions in anuran amphibians cause motor (difficulty to start movements) as well as cognitive symptoms (higher distraction by irrelevant stimuli) that have also been described for human Parkinson patients.

Functional coupling between substantia nigra and basal ganglia homologues in amphibians.

“…We showed that putative striatal and pallidal homologues have distinct patterns of functional activation and argued that the striatopallidal transition zone identified by Endepols, Schul, et al (2004) is functionally related to the pallidum. We furthermore demonstrated a functional relationship between neural activation in the basal ganglia and dopaminergic projections from the PT, which further bolsters the hypothesis that homologous basal ganglia circuits regulate motor behaviors in tetrapods.Previous researchers have used regional variation in anatomical connectivity and neurochemical markers to propose amphibian homologues of the striatum and dorsal pallidum, with a transition zone sharing some characteristics of striatal and pallidal regions (Endepols, Schul, et al, 2004; Mülhenbrock-Lenter et al, 2005). We argue, on the basis of neural activation patterns, that the transition zone is functionally related to the caudal/pallidal rather than to the rostral/striatal portion of the basal ganglia.…”

“…For each male, we measured silver grain density in four sections through the basal ganglia spaced by at least 32 μm, sampling one dorsal and one ventral photomicrograph in each of the four sections as described (Hoke et al, 2007). The two rostral sections corresponded to striatum as identified by Endepols, Schul, et al (2004), and the two caudal sections spanned the striatopallidal transition area (Endepols, Schul, et al, 2004). For the females, we carried out more extensive sampling throughout the basal ganglia.…”

“…anuran; basal ganglia; tyrosine hydroxylase; immediate-early gene expression; egr-1 Previous research has proposed that homologous basal ganglia circuits may exist in both amphibians and mammals (reviewed in Endepols, Schul, Gerhardt, & Walkowiak, 2004;Marín, Smeets, & González, 1998). The "direct" basal ganglia pathway in mammals involves a dopaminergic projection from the substantia nigra pars compacta (SNc) to the striatum.…”

“…Catecholaminergic cells in the PT contain dopamine (González & Smeets, 1991) and project to the striatum (Marín, González, & Smeets, 1997b;Marín, Smeets, & González, 1997 analyses, these cells are proposed to be homologous to the dopaminergic SNc neurons in mammals (reviewed in Marín et al, 1998;Smeets & González, 2000). Chemoarchitecture and anatomical connections suggest that the amphibian basal ganglia consist of the homologues of the mammalian striatum (rostrally) and dorsal pallidum (caudally), with an intermediate portion that shares some characteristics with the striatum and other characteristics with the pallidum (Endepols, Schul, et al, 2004; Mülhenbrock-Lenter, Endepols, Roth, & Walkowiak, 2005; see also Marín, González, & Smeets, 1997c). Major outputs of the basal ganglia include direct and indirect paths to motor areas of the brainstem (Marín, González, & Smeets, 1997a;Wilczynski & Northcutt, 1983b).…”

“…The Rostrocaudal × Dorsoventral interaction effect arose because the ventral portion of the secondmost rostral section was more similar to its caudal neighbor than to its rostral neighbor, in contrast to the opposite finding in the dorsal position (see Table 3). Thus, we concluded that the intermediate portion of the P. pustulosus basal ganglia (pallidal/striatal transition zone) is functionally distinct from the rostral portion (striatum proper, based on Endepols, Schul, et al, 2004).We used principal-components analysis as a second statistical approach to analyze regional variation in functional activation of the basal ganglia. As we had found no differences based on experiment in the repeated-measures ANOVA analysis discussed above, we combined egr-1 mRNA measures from the two experiments in a principal-components analysis.…”