Dynamics of odor-source localization: Insights from real-time odor plume recordings and head-motion tracking in freely moving mice

Animals navigating turbulent odor plumes exhibit a rich variety of behaviors, and employ efficient strategies to locate odor sources. A growing body of literature has started to probe this complex task of localizing airborne odor sources in walking mammals to further our understanding of neural encoding and decoding of naturalistic sensory stimuli. However, correlating the intermittent olfactory information with behavior has remained a long-standing challenge due to the stochastic nature of the odor stimulus. We recently reported a method to record real-time olfactory information available to freely moving mice during odor-guided navigation, hence overcoming that challenge. Here we combine our odor-recording method with head-motion tracking to establish correlations between plume encounters and head movements. We show that mice exhibit robust head-pitch motions in the 5-14Hz range during an odor-guided navigation task, and that these head motions are modulated by plume encounters. Furthermore, mice orient towards the odor source upon plume contact. Head motions may thus be an important part of the sensorimotor behavioral repertoire during naturalistic odor-source localization.

Mechanistic Hypotheses for Proprioceptive Sensing Within the Avian Lumbosacral Spinal Cord

Animals need to accurately sense changes in their body position to perform complex movements. It is increasingly clear that the vertebrate central nervous system contains a variety of cells capable of detecting body motion, in addition to the comparatively well-understood mechanosensory cells of the vestibular system and the peripheral proprioceptors. One such intriguing system is the lower spinal cord and column in birds, also known as the avian lumbosacral organ (LSO), which is thought to act as a set of balance sensors that allow birds to detect body movements separately from head movements detected by the vestibular system. Here, we take what is known about proprioceptive, mechanosensory spinal neurons in other vertebrates to explore hypotheses for how the LSO might sense mechanical information related to movement. Although the LSO is found only in birds, recent immunohistochemical studies of the avian LSO have hinted at similarities between cells in the LSO and the known spinal proprioceptors in other vertebrates. In addition to describing possible connections between avian spinal anatomy and recent findings on spinal proprioception as well as sensory and sensorimotor spinal networks, we also present some new data that suggest a role for sensory afferent peptides in LSO function. Thus, this perspective articulates a set of testable ideas on mechanisms of LSO function grounded in the emerging spinal proprioception scientific literature.

Differential development of myelin in zebra finch song nuclei

Songbirds learn vocalizations by hearing and practicing songs. As song develops, the tempo becomes faster and more precise. In the songbird brain, discrete nuclei form interconnected myelinated circuits that control song acquisition and production. The myelin sheath increases the speed of action potential propagation by insulating the axons of neurons and by reducing membrane capacitance. As the brain develops, myelin increases in density, but the time course of myelin development across discrete song nuclei has not been systematically studied in a quantitative fashion. We tested the hypothesis that myelination develops differentially across time and song nuclei. We examined myelin development in the brains of the zebra finch (Taeniopygia guttata) from chick at posthatch day (d) 8 to adult (up to 147 d) in five major song nuclei: HVC (proper name), robust nucleus of the arcopallium (RA), Area X, lateral magnocellular nucleus of the anterior nidopallium, and medial portion of the dorsolateral thalamic nucleus (DLM). All of these nuclei showed an increase in the density of myelination during development but at different rates and to different final degrees. Exponential curve fits revealed that DLM showed earlier myelination than other nuclei, and HVC showed the slowest myelination of song nuclei. Together, these data show differential maturation of myelination in different portions of the song system. Such differential maturation would be well placed to play a role in regulating the development of learned song.

Distribution and co-expression of adrenergic receptor-encoding mRNA in the mouse inferior colliculus

Adrenergic receptors are mediators of adrenergic and noradrenergic modulation throughout the brain. Previous studies have provided evidence for the expression of adrenergic receptors in the midbrain auditory nucleus, the inferior colliculus (IC), but have not examined the cellular patterns of expression in detail. Here, we utilize multichannel fluorescent in situ hybridization to detect the expression of adrenergic receptor-encoding mRNA in the inferior colliculus of male and female mice. We found expression of α₁ , α_2A , and β₂ receptor-encoding mRNA throughout all areas of the IC. While we observed similar levels of expression of α₁ receptor-encoding mRNA across the subregions of the IC, α_2A and β₂ receptor-encoding mRNA was expressed differentially. To account for developmental changes in noradrenergic receptor expression, we measured expression levels in mice aged P15, P20, and P60. We observed little change in levels of expression across these ages. To ascertain the modulatory potential of multiple adrenergic receptor subtypes in a single IC cell, we measured co-expression of α₁ , α_2A , and β₂ receptor-encoding mRNA. We found greater proportions of cells in the IC that expressed no adrenergic receptor-encoding mRNA, α1 and α2A adrenergic receptor-encoding mRNA, and α1, α2A, and β2 receptor-encoding mRNA than would be predicted by independent expression of each receptor subtype. These data suggest a coordinated pattern of adrenergic receptor expression in the IC and provide the first evidence for adrenergic receptor expression and co-expression in the subregions of the mouse auditory midbrain.

The Balance Hypothesis for the Avian Lumbosacral Organ and an Exploration of Its Morphological Variation

Birds (Aves) exhibit exceptional and diverse locomotor behaviors, including the exquisite ability to balance on two feet. How birds so precisely control their movements may be partly explained by a set of intriguing modifications in their lower spine. These modifications are collectively known as the lumbosacral organ (LSO) and are found in the fused lumbosacral vertebrae called the synsacrum. They include a set of transverse canal-like recesses in the synsacrum that align with lateral lobes of the spinal cord, as well as a dorsal groove in the spinal cord that houses an egg-shaped glycogen body. Based on compelling but primarily observational data, the most recent functional hypotheses for the LSO consider it to be a secondary balance organ, in which the transverse canals are analogous to the semicircular canals of the inner ear. If correct, this hypothesis would reshape our understanding of avian locomotion, yet the LSO has been largely overlooked in the recent literature. Here, we review the current evidence for this hypothesis and then explore a possible relationship between the LSO and balance-intensive locomotor ecologies. Our comparative morphological dataset consists of micro-computed tomography (μ-CT) scans of synsacra from ecologically diverse species. We find that birds that perch tend to have more prominent transverse canals, suggesting that the LSO is useful for balance-intensive behaviors. We then identify the crucial outstanding questions about LSO structure and function. The LSO may be a key innovation that allows independent but coordinated motion of the head and the body, and a full understanding of its function and evolution will require multiple interdisciplinary research efforts.

Expression of the potassium-chloride co-transporter, KCC2, within the avian song system

Songbirds learn to produce vocalizations early in life by listening to, then copying the songs of conspecific males. The anterior forebrain pathway, homologous to a basal ganglia-forebrain circuit, is essential for song learning. The projection between the striato-pallidal structure, Area X, and the medial portion of the dorsolateral thalamic nucleus (DLM) is strongly hyperpolarizing in adults, due to a very negative chloride reversal potential (Person & Perkel, Neuron 46:129-140, 2005). The chloride reversal potential is determined, in part, by the expression level of a neuron-specific potassium-chloride cotransporter, KCC2, which is developmentally upregulated in mammals. To determine whether a similar upregulation in KCC2 expression occurs at the Area X to DLM synapse during development, we examined the expression level of KCC2 in adult zebra finches across the song system as well as during development in the Area X - DLM synapse. We demonstrate that KCC2 is expressed in a subset of neurons throughout the song system, including HVC (used as a proper name), robust nucleus of the arcopallium (RA), lateral magnocellular nucleus of the anterior nidopallium (LMAN), Area X, and DLM. The majority of pallidal-like projection neurons in Area X showed KCC2 immunoreactivity. In adults, KCC2 expression was robust within DLM, and was upregulated between 14 and 24 days post hatching, before the onset of song learning. Light and electron microscopic analysis indicated that KCC2 immunoreactivity is strongly associated with the plasma membrane. Thus, in the song system as in the mammalian brain, KCC2 expression is well placed to modulate the GABAA reversal potential.

Dopaminergic Input to the Inferior Colliculus in Mice

The response of sensory neurons to stimuli can be modulated by a variety of factors including attention, emotion, behavioral context, and disorders involving neuromodulatory systems. For example, patients with Parkinson’s disease (PD) have disordered speech processing, suggesting that dopamine alters normal representation of these salient sounds. Understanding the mechanisms by which dopamine modulates auditory processing is thus an important goal. The principal auditory midbrain nucleus, the inferior colliculus (IC), is a likely location for dopaminergic modulation of auditory processing because it contains dopamine receptors and nerve terminals immunoreactive for tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis. However, the sources of dopaminergic input to the IC are unknown. In this study, we iontophoretically injected a retrograde tracer into the IC of mice and then stained the tissue for TH. We also immunostained for dopamine beta-hydroxylase (DBH), an enzyme critical for the conversion of dopamine to norepinephrine, to differentiate between dopaminergic and noradrenergic inputs. Retrogradely labeled neurons that were positive for TH were seen bilaterally, with strong ipsilateral dominance, in the subparafascicular thalamic nucleus (SPF). All retrogradely labeled neurons that we observed in other brain regions were TH-negative. Projections from the SPF were confirmed using an anterograde tracer, revealing TH-positive and DBH-negative anterogradely labeled fibers and terminals in the IC. While the functional role of this dopaminergic input to the IC is not yet known, it provides a potential mechanism for context dependent modulation of auditory processing.

Network analysis of microRNA and mRNA seasonal dynamics in a highly plastic sensorimotor neural circuit

Background

Adult neurogenesis and the incorporation of adult-born neurons into functional circuits requires precise spatiotemporal coordination across molecular networks regulating a wide array of processes, including cell proliferation, apoptosis, neurotrophin signaling, and electrical activity. MicroRNAs (miRs) - short, non-coding RNA sequences that alter gene expression by post-transcriptional inhibition or degradation of mRNA sequences - may be involved in the global coordination of such diverse biological processes. To test the hypothesis that miRs related to adult neurogenesis and related cellular processes are functionally regulated in the nuclei of the avian song control circuit, we used microarray analyses to quantify changes in expression of miRs and predicted target mRNAs in the telencephalic nuclei HVC, the robust nucleus of arcopallium (RA), and the basal ganglia homologue Area X in breeding and nonbreeding Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelli).

Results

We identified 46 different miRs that were differentially expressed across seasons in the song nuclei. miR-132 and miR-210 showed the highest differential expression in HVC and Area X, respectively. Analyzing predicted mRNA targets of miR-132 identified 33 candidate target genes that regulate processes including cell cycle control, calcium signaling, and neuregulin signaling in HVC. Likewise, miR-210 was predicted to target 14 mRNAs differentially expressed across seasons that regulate serotonin, GABA, and dopamine receptor signaling and inflammation.

Conclusions

Our results identify potential miR–mRNA regulatory networks related to adult neurogenesis and provide opportunities to discover novel genetic control of the diverse biological processes and factors related to the functional incorporation of new neurons to the adult brain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2175-z) contains supplementary material, which is available to authorized users.

Two tales of how expectation of reward modulates behavior

Expectation of reward modulates many types of behaviors. Here we highlight two lines of research on reward-modulated perceptual decision making in primates and social context-modulated singing in songbirds, respectively. These two seemingly distinct behaviors are both known to involve cortico-basal ganglia-thalamic circuits. The underlying computations may be conceptualized using a simple, common framework. We summarize and compare our current knowledge of the two fields to motivate new experiments for each field, with the goal of finding general principles for how the brain implements reward-modulated behavior.

The role of ultrasonic vocalizations in mouse communication

Human speech and language underlie many aspects of social behavior and thus understanding their ultimate evolutionary function and proximate genetic and neural mechanisms is a fundamental goal in neuroscience. Mouse ultrasonic vocalizations have recently received enormous attention as possible models for human speech. This attention has raised the question of whether these vocalizations are learned and what roles they play in communication. In this review, we first discuss recent evidence that ultrasonic vocalizations are not learned. We then review current evidence addressing how adult vocalizations may communicate courtship, territorial and/or other information. While there is growing evidence that these signals play key roles in communication, many important questions remain unanswered.

Postsynaptic neural activity regulates neuronal addition in the adult avian song control system

A striking feature of the nervous system is that it shows extensive plasticity of structure and function that allows animals to adjust to changes in their environment. Neural activity plays a key role in mediating experience-dependent neural plasticity and, thus, creates a link between the external environment, the nervous system, and behavior. One dramatic example of neural plasticity is ongoing neurogenesis in the adult brain. The role of neural activity in modulating neuronal addition, however, has not been well studied at the level of neural circuits. The avian song control system allows us to investigate how activity influences neuronal addition to a neural circuit that regulates song, a learned sensorimotor social behavior. In adult white-crowned sparrows, new neurons are added continually to the song nucleus HVC (proper name) and project their axons to its target nucleus, the robust nucleus of the arcopallium (RA). We report here that electrical activity in RA regulates neuronal addition to HVC. Decreasing neural activity in RA by intracerebral infusion of the GABAA receptor agonist muscimol decreased the number of new HVC neurons by 56%. Our results suggest that postsynaptic electrical activity influences the addition of new neurons into a functional neural circuit in adult birds.

Dopamine modulates auditory responses in the inferior colliculus in a heterogeneous manner

Perception of complex sounds such as speech is affected by a variety of factors, including attention, expectation of reward, physiological state, and/or disorders, yet the mechanisms underlying this modulation are not well understood. Although dopamine is commonly studied for its role in reward-based learning and in disorders, multiple lines of evidence suggest that dopamine is also involved in modulating auditory processing. In this study, we examined the effects of dopamine application on neuronal response properties in the inferior colliculus (IC) of awake mice. Because the IC contains dopamine receptors and nerve terminals immunoreactive for tyrosine hydroxylase, we predicted that dopamine would modulate auditory responses in the IC. We recorded single-unit responses before, during, and after the iontophoretic application of dopamine using piggyback electrodes. We examined the effects of dopamine on firing rate, timing, and probability of bursting. We found that application of dopamine affected neural responses in a heterogeneous manner. In more than 80 % of the neurons, dopamine either increased (32 %) or decreased (50 %) firing rate, and the effects were similar on spontaneous and sound-evoked activity. Dopamine also either increased or decreased first spike latency and jitter in almost half of the neurons. In 3/28 neurons (11 %), dopamine significantly altered the probability of bursting. The heterogeneous effects of dopamine observed in the IC of awake mice were similar to effects observed in other brain areas. Our findings indicate that dopamine differentially modulates neural activity in the IC and thus may play an important role in auditory processing.

Striatal dopamine modulates song spectral but not temporal features through D1 receptors

The activity of midbrain dopaminergic neurons and their projection to the basal ganglia (BG) are thought to play a critical role in the acquisition of motor skills through reinforcement learning, as well as in the expression of learned motor behaviors. The precise role of BG dopamine (DA) in mediating and modulating motor performance and learning, however, remains unclear. In songbirds, a specialized portion of the BG is responsible for song learning and plasticity. Previously we found that DA acts on D1 receptors in Area X to modulate the BG output signal and thereby trigger changes in song variability. Here, we investigate the effect of D1 receptor blockade in the BG on song behavior in the zebra finch. We report that this manipulation abolishes social context-dependent changes in variability not only in harmonic stacks, but also in other types of syllables. However, song timing seems not to be modulated by this BG DA signal. Indeed, injections of a D1 antagonist in the BG altered neither song duration nor the change of song duration with social context. Finally, D1 receptor activation in the BG was not necessary for the modulation of other features of song, such as the number of introductory notes or motif repetitions. Together, our results suggest that activation of D1 receptors in the BG is necessary for the modulation of fine acoustic features of song with social context, while it is not involved in the regulation of song timing and structure at a larger time scale.

The avian subpallium: new insights into structural and functional subdivisions occupying the lateral subpallial wall and their embryological origins

The subpallial region of the avian telencephalon contains neural systems whose functions are critical to the survival of individual vertebrates and their species. The subpallial neural structures can be grouped into five major functional systems, namely the dorsal somatomotor basal ganglia; ventral viscerolimbic basal ganglia; subpallial extended amygdala including the central and medial extended amygdala and bed nuclei of the stria terminalis; basal telencephalic cholinergic and non-cholinergic corticopetal systems; and septum. The paper provides an overview of the major developmental, neuroanatomical and functional characteristics of the first four of these neural systems, all of which belong to the lateral telencephalic wall. The review particularly focuses on new findings that have emerged since the identity, extent and terminology for the regions were considered by the Avian Brain Nomenclature Forum. New terminology is introduced as appropriate based on the new findings. The paper also addresses regional similarities and differences between birds and mammals, and notes areas where gaps in knowledge occur for birds.

Endocannabinoids mediate synaptic plasticity at glutamatergic synapses on spiny neurons within a basal ganglia nucleus necessary for song learning

Activation of type 1 cannabinoid receptors (CB(1)R) in many central nervous system structures induces both short- and long-term changes in synaptic transmission. Within mammalian striatum, endocannabinoids (eCB) are one of several mechanisms that induce synaptic plasticity at glutamatergic terminals onto medium spiny neurons. Striatal synaptic plasticity may contribute a critical component of adaptive motor coordination and procedural learning. Songbirds are advantageous for studying the neural mechanisms of motor learning because they possess a neural pathway necessary for song learning and adult song plasticity that includes a striato-pallidal nucleus, area X (homologous to a portion of mammalian basal ganglia). Recent findings suggest that eCBs contribute to vocal development. For example, dense CB(1)R expression in song control nuclei peaks around the closure of the sensori-motor integration phase of song development. Also, systemic administration of a CB(1)R agonist during vocal development impairs song learning. Here we test whether activation of CB(1)R alters excitatory synaptic input on spiny neurons in area X of adult male zebra finches. Application of the CB(1)R agonist WIN55212-2 decreased excitatory postsynaptic current (EPSC) amplitude; that decrease was blocked by the CB(1)R antagonist AM251. Guided by eCB experiments in mammalian striatum, we tested and verified that at least two mechanisms indirectly activate CB(1)Rs through eCBs in area X. First, activation of group I metabotropic glutamate receptors with the agonist 3,5-dihydroxyphenylglycine (DHPG) induced a CB(1)R-mediated reduction in EPSC amplitude. Second, we observed that a 10 s postsynaptic depolarization induced a calcium-mediated, eCB-dependent decrease in synaptic strength that resisted rescue with late CB(1)R blockade. Together, these results show that eCB modulation occurs at inputs to area X spiny neurons and could influence motor learning and production.

A novel model for examining recovery of phonation after vocal nerve damage

OBJECTIVES/HYPOTHESIS

Recurrent laryngeal nerve injury remains a dominant clinical issue in laryngology. To date, no animal model of laryngeal reinnervation has offered an outcome measure that can reflect the degree of recovery based on vocal function. We present an avian model system for studying recovery of learned vocalizations after nerve injury.

STUDY DESIGN

Prospective animal study.

METHODS

Digital recordings of bird song were made from 11 adult male zebra finches; nine birds underwent bilateral crushing of the nerve supplying the vocal organ, and two birds underwent sham surgery. Songs from all the birds were then recorded regularly and analyzed based on temporal and spectral characteristics using computer software. Indices were calculated to indicate the degree of similarity between preoperative and postoperative song.

RESULTS

Nerve crush caused audible differences in song quality and significant drops (P<0.05) in measured spectral and, to a lesser degree, temporal indices. Spectral indices recovered significantly (mean=43.0%; standard deviation [SD]=40.7; P<0.02), and there was an insignificant trend toward recovery of temporal index (mean=28.0%; SD=41.4; P=0.0771). In five of the nine (56%) birds, there was a greater than 50% recovery of spectral indices within a 4-week period. Two birds exhibited substantially less recovery of spectral indices and two birds had a persistent decline in spectral indices. Recovery of temporal index was highly variable as well, ranging from persistent further declines of 45.1% to recovery of 87%. Neither sham bird exhibited significant (P>0.05) differences in song after nerve crush.

CONCLUSION

The songbird model system allows functional analysis of learned vocalization after surgical damage to vocal nerves.

Anatomy of a songbird basal ganglia circuit essential for vocal learning and plasticity

Vocal learning in songbirds requires an anatomically discrete and functionally dedicated circuit called the anterior forebrain pathway (AFP). The AFP is homologous to cortico-basal ganglia-thalamo-cortical loops in mammals. The basal ganglia portion of this pathway, Area X, shares many features characteristic of the mammalian striatum and pallidum, including cell types and connectivity. The AFP also deviates from mammalian basal ganglia circuits in fundamental ways. In addition, the microcircuitry, role of neuromodulators, and function of Area X are still unclear. Elucidating the mechanisms by which both mammalian-like and unique features of the AFP contribute to vocal learning may help lead to a broad understanding of the sensorimotor functions of basal ganglia circuits.

Plastic and stable electrophysiological properties of adult avian forebrain song-control neurons across changing breeding conditions

Steroid sex hormones drive changes in the nervous system and behavior in many animal taxa, but integrating the former with the latter remains challenging. One useful model system for meeting this challenge is seasonally breeding songbirds. In these species, plasma testosterone levels rise and fall across the seasons, altering song behavior and causing dramatic growth and regression of the song-control system, a discrete set of nuclei that control song behavior. Whereas the cellular mechanisms underlying changes in nucleus volume have been studied as a model for neural growth and degeneration, it is unknown whether these changes in neural structure are accompanied by changes in electrophysiological properties other than spontaneous firing rate. Here we test the hypothesis that passive and active neuronal properties in the forebrain song-control nuclei HVC and RA change across breeding conditions. We exposed adult male Gambel's white-crowned sparrows to either short-day photoperiod or long-day photoperiod and systemic testosterone to simulate nonbreeding and breeding conditions, respectively. We made whole-cell recordings from RA and HVC neurons in acute brain slices. We found that RA projection neuron membrane time constant, capacitance, and evoked and spontaneous firing rates were all increased in the breeding condition; the measured electrophysiological properties of HVC interneurons and projection neurons were stable across breeding conditions. This combination of plastic and stable intrinsic properties could directly impact the song-control system's motor control across seasons, underlying changes in song stereotypy. These results provide a valuable framework for integrating how steroid hormones modulate cellular physiology to change behavior.

Time course of changes in Gambel's white-crowned sparrow song behavior following transitions in breeding condition

Seasonal changes in behavior and in its underlying neural substrate are common across animal taxa. These changes are often triggered by steroid sex hormones. Song in seasonally breeding songbirds provides an excellent example of this phenomenon. In these species, dramatic seasonal changes mediated by testosterone and its metabolites occur in adult song behavior and in the neural circuitry controlling song. While song rate can quickly change in response to seasonal breeding cues, it is unknown how quickly other aspects of song change, particularly the stereotypy of song phonology and syntax. In this study we determined whether and how quickly song rate, phonology, and syntax change in response to breeding and non-breeding physiological cues. We asked these questions using Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), a closed-ended learner with well-characterized changes in the neural circuitry controlling song behavior. We exposed ten photosensitive sparrows to long-day photoperiod and implanted them with subcutaneous testosterone pellets (day 0) to simulate breeding conditions. We continuously recorded song and found that song rate increased quickly, reaching maximum around day 6. The stereotypy of song phonology changed more slowly, reaching maximum by day 10 or later. Song syntax changed minimally after day 6, the earliest time point examined. After 21 days, we transitioned five birds from breeding to non-breeding condition. Song rate declined precipitously. These results suggest that while song rate changes quickly, song phonology changes more slowly, generally following or in parallel with previously investigated changes in the neural substrate.

Organization of the songbird basal ganglia, including area X

Area X is a songbird basal ganglia nucleus that is required for vocal learning. Both Area X and its immediate surround, the medial striatum (MSt), contain cells displaying either striatal or pallidal characteristics. We used pathway-tracing techniques to compare directly the targets of Area X and MSt with those of the lateral striatum (LSt) and globus pallidus (GP). We found that the zebra finch LSt projects to the GP, substantia nigra pars reticulata (SNr) and pars compacta (SNc), but not the thalamus. The GP is reciprocally connected with the subthalamic nucleus (STN) and projects to the SNr and motor thalamus analog, the ventral intermediate area (VIA). In contrast to the LSt, Area X and surrounding MSt project to the ventral pallidum (VP) and dorsal thalamus via pallidal-like neurons. A dorsal strip of the MSt contains spiny neurons that project to the VP. The MSt, but not Area X, projects to the ventral tegmental area (VTA) and SNc, but neither MSt nor Area X projects to the SNr. Largely distinct populations of SNc and VTA dopaminergic neurons innervate Area X and surrounding the MSt. Finally, we provide evidence consistent with an indirect pathway from the cerebellum to the basal ganglia, including Area X. Area X projections thus differ from those of the GP and LSt, but are similar to those of the MSt. These data clarify the relationships among different portions of the oscine basal ganglia as well as among the basal ganglia of birds and mammals.

A novel basal ganglia pathway forms a loop linking a vocal learning circuit with its dopaminergic input

Dopamine has been implicated in mediating contextual modulation of motor behaviors and learning in many species. In songbirds, dopamine may act on the basal ganglia nucleus Area X to influence the neural activity that contributes to vocal learning and contextual changes in song variability. Neurons in midbrain dopamine centers, the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA), densely innervate Area X and show singing-related changes in firing rate. In addition, dopamine levels in Area X change during singing. It is unknown, however, how song-related information could reach dopaminergic neurons. Here we report an anatomical pathway that could provide song-related information to the SNc and VTA. By using injections of bidirectionally transported fluorescent tracers in adult male zebra finches, we show that Area X and other song control nuclei do not project directly to the SNc or VTA. Instead, we describe an indirect pathway from Area X to midbrain dopaminergic neurons via a connection in the ventral pallidum (VP). Specifically, Area X projects to the VP via axon collaterals of Area X output neurons that also project to the thalamus. Dual injections revealed that the area of VP receiving input from Area X projects to the SNc and VTA. Furthermore, VP terminals in the SNc and VTA overlap with cells that project back to Area X. A portion of the arcopallium also projects to the SNc and VTA and could carry auditory information. These data demonstrate an anatomical loop through which Area X activity could influence its dopaminergic input.

Noradrenergic and GABA B receptor activation differentially modulate inputs to the premotor nucleus RA in zebra finches

Neuromodulators can rapidly modify neural circuits, altering behavior. Songbirds provide an excellent system for studying the role of neuromodulation in modifying circuits that underlie behavior because song learning and production are mediated by a discrete set of interconnected nuclei. We examined the neuromodulatory effects of noradrenergic and GABA B receptor activation on synaptic inputs to the premotor robust nucleus of the arcopallium (RA) in zebra finches using whole cell voltage-clamp recording in vitro. In adults, norepinephrine strongly reduced input from the lateral magnocellular nucleus of the anterior nidopallium (LMAN) but only slightly reduced the input from nucleus HVC (proper name), the excitatory input from axon collaterals of other RA neurons, and input from GABAergic interneurons. The effect of norepinephrine was mimicked by the alpha2 adrenoceptor agonist UK14,304 and blocked by the alpha2 antagonist yohimbine. Conversely, the GABA B receptor agonist baclofen strongly decreased HVC, collateral, and GABAergic inputs to RA neurons while causing little reduction in the LMAN input. In juveniles undergoing song learning, norepinephrine reduced the LMAN input, caused only a small reduction in the HVC input, and greatly reduced the collateral and GABAergic inputs. Baclofen caused similar results in juvenile and adult birds, reducing HVC, collateral, and GABAergic inputs significantly more than the LMAN input. Significant increases in paired-pulse ratio accompanied all reductions in synaptic transmission, suggesting a presynaptic locus. The reduction in the LMAN input by norepinephrine may be important for mediating changes in song elicited by different social contexts and is well-placed to play a role in song learning.

Pallidal neuron activity increases during sensory relay through thalamus in a songbird circuit essential for learning

Disinhibition of the thalamus remains the primary model of information transfer between the basal ganglia and the cortex. Yet in apparent conflict with this model, the globus pallidus, a GABAergic basal ganglia output structure, often exhibits marked increases in firing rate during movement. To investigate the translation of pallidal activity and its relay through the thalamus, we explored a basal ganglia-thalamic pathway essential for song learning in songbirds. We found that single units in the thalamic nucleus DLM of urethane-anesthetized adult male zebra finches responded selectively to playback of the bird's own song, like neurons in its upstream and downstream nuclei. Because the pallidal input to these neurons forms giant calyx-like synapses, we were able to record extracellular signals from these presynaptic terminals as well. Pallidal units were distinctly excited by song playback, suggesting an increase in GABAergic transmission in the thalamus during sensory processing. However, this overall increased firing rate was phasic, punctuated by rapid decelerations in firing rate. In several cases, we were able to record presynaptic and postsynaptic units simultaneously. Correlating the presynaptic and postsynaptic activity, we found that disinhibition of thalamus may entail pallidal firing rate decelerations rather than simple long pauses in spontaneous activity, as has long been assumed.

Seasonal changes in intrinsic electrophysiological activity of song control neurons in wild song sparrows

Song behavior and its underlying neural substrate can change seasonally in adult songbirds. To test whether environmental cues induce seasonal changes in electrophysiological characteristics of song control neurons, we measured in vitro intrinsic neuronal activity in the song control nucleus RA of adult male song sparrows (Melospiza melodia) in both the fall non-breeding and spring breeding seasons. We found that RA neurons in spring-captured birds show a more than threefold increase in spontaneous firing rate compared to those from fall-captured birds. We conclude that environmental cues are sufficient to induce seasonal changes in electrophysiological properties of song control neurons, and that changes in these properties may underlie seasonal changes in song behavior.

Physiological properties of zebra finch ventral tegmental area and substantia nigra pars compacta neurons

The neurotransmitter dopamine plays important roles in motor control, learning, and motivation in mammals and probably other animals as well. The strong dopaminergic projection to striatal regions and more moderate dopaminergic projections to other regions of the telencephalon predominantly arise from midbrain dopaminergic neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). Homologous dopaminergic cell groups in songbirds project anatomically in a manner that may allow dopamine to influence song learning or song production. The electrophysiological properties of SNc and VTA neurons have not previously been studied in birds. Here we used whole cell recordings in brain slices in combination with tyrosine-hydroxylase immunolabeling as a marker of dopaminergic neurons to determine electrophysiological and pharmacological properties of dopaminergic and nondopaminergic neurons in the zebra finch SNc and VTA. Our results show that zebra finch dopaminergic neurons possess physiological properties very similar to those of mammalian dopaminergic neurons, including broad action potentials, calcium- and apamin-sensitive membrane-potential oscillations underlying pacemaker firing, powerful spike-frequency adaptation, and autoinhibition via D2 dopamine receptors. Moreover, the zebra finch SNc and VTA also contain nondopaminergic neurons with similarities (fast-firing, inhibition by the mu-opioid receptor agonist [d-Ala(2), N-Me-Phe(4), Gly-ol(5)]-enkephalin (DAMGO)) and differences (strong h-current that contributes to spontaneous firing) compared with GABAergic neurons in the mammalian SNc and VTA. Our results provide insight into the intrinsic membrane properties that regulate the activity of dopaminergic neurons in songbirds and add to strong evidence for anatomical, physiological, and functional similarities between the dopaminergic systems of mammals and birds.

Rhythmic activity in a forebrain vocal control nucleus in vitro

The learned vocalizations of songbirds constitute a rhythmic behavior that is thought to be governed by a central pattern generator and that is accompanied by highly patterned neural activity. Phasic premotor activity is observed during singing in HVC [used as a proper name following the nomenclature of Reiner et al. (2004)], a telencephalic song system nucleus that is essential for song production. Moreover, HVC neurons display phasic patterns of auditory activity in response to song stimulation. To address the cellular basis of pattern generation in HVC, we investigated its rhythm-generating abilities. We report here the induction of sustained, rhythmic activity patterns in HVC when isolated in vitro. Brief, high-frequency stimulation evoked repetitive postsynaptic potentials (PSPs) and local field potentials (LFPs) from HVC neurons recorded in a brain slice preparation made from adult male zebra finches. These rhythmic events were sustained for seconds in the absence of ongoing, phasic stimulation, and they had temporal properties similar to those of syllable occurrence within zebra finch song. Paired recordings revealed synchrony between repetitive PSP and LFP occurrence, indicating that a population of cells participates in this patterned activity. The PSPs resulted from multiple, coordinated, fast-glutamatergic, synaptic inputs. Moreover, their occurrence and timing relied on inhibitory synaptic transmission. Thus, HVC itself has rhythmic abilities that could influence the timing of neural activity over relatively long time windows. These rhythmic properties may contribute to song production or perception in vivo.

Noradrenergic modulation of activity in a vocal control nucleus in vitro

Norepinephrine (NE) can profoundly modulate sensory processing, but its effect on motor function is less well understood. Birdsong is a learned behavior involving sensory and motor processes that are influenced by NE. A potential site of NE action is the robust nucleus of the arcopallium (RA): RA receives noradrenergic inputs and has adrenergic receptors, and it is a sensorimotor area instrumental to song production. We hypothesized that NE modulates RA neurons, and as a first test, we examined the effect of NE on RA activity in vitro. We recorded spontaneous activity extracellularly from isolated RA neurons in brain slices made from adult male zebra finches. These neurons exhibited regular tonic activity with firing rates averaging 5.5 Hz. Bath application of NE rapidly and reversibly decreased firing for the majority of neurons, to the extent that spontaneous activity was often abolished. This was likely a direct effect on the cell recorded, because it occurred with blockade of fast excitatory and inhibitory synaptic transmission or of all synaptic transmission. The NE-induced suppression involved alpha2-adrenergic receptors: yohimbine, an antagonist, completely reversed the suppression, and clonidine, an agonist, partially mimicked it. Perforated patch recordings revealed that NE induced a conductance increase in RA neurons; however, this did not prevent cells from firing when stimulated by afferents in HVC. For some neurons, NE application resulted in an increase in signal-to-noise ratio for spikes evoked by HVC stimulation. Thus NE could strongly modulate the spontaneous activity of RA cells, potentially enhancing signals relayed through RA.

Birdbrains could teach basal ganglia research a new song

Recent advances in anatomical, physiological and histochemical characterization of avian basal ganglia neurons and circuitry have revealed remarkable similarities to mammalian basal ganglia. A modern revision of the avian anatomical nomenclature has now provided a common language for studying the function of the cortical-basal-ganglia-cortical loop, enabling neuroscientists to take advantage of the specialization of basal ganglia areas in various avian species. For instance, songbirds, which learn their vocal motor behavior using sensory feedback, have specialized a portion of their cortical-basal ganglia circuitry for song learning and production. This discrete circuit dedicated to a specific sensorimotor task could be especially tractable for elucidating the interwoven sensory, motor and reward signals carried by basal ganglia, and the function of these signals in task learning and execution.

Unitary IPSPs drive precise thalamic spiking in a circuit required for learning

Song learning in birds requires a basal ganglia-thalamo-pallial loop that contains a calyceal GABAergic synapse in the thalamus. Information processing within this circuit is critical for proper song development; however, it is unclear whether activation of the inhibitory output of the basal ganglia structure Area X can drive sustained activity in its thalamic target, the medial portion of the dorsolateral thalamic nucleus (DLM). We show that high-frequency, random activation of this GABAergic synapse can drive precisely timed firing in DLM neurons in brain slices in the absence of excitatory input. Complex IPSP trains, including spike trains recorded in vivo, drive spiking in slices with high reproducibility, even between animals. Using a simple model, we can predict much of DLM's response to natural stimulus trains. These data elucidate basic rules by which thalamic relay neurons translate IPSPs into suprathreshold output and demonstrate extrathalamic GABAergic activation of thalamus.

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain - in particular the neocortex-like cognitive functions of the avian pallium - requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

Electrophysiological properties of neurons in the basal ganglia of the domestic chick: conservation and divergence in the evolution of the avian basal ganglia

Although the basal ganglia of birds and mammals share an enormous number of anatomical, histochemical, and electrophysiological characteristics, studies in songbirds have revealed some important differences. Specifically, a specialized region of songbird striatum (the input structure of the basal ganglia) has an anatomical projection and a physiologically defined cell type that are characteristic of the globus pallidus. At present, it is not clear if these differences result from adaptations specific to songbirds and perhaps a few other avian taxa or are common to all birds. We shed some light on this issue by characterizing the morphology and electrophysiological properties of basal ganglia neurons in an avian species that is only distantly related to songbirds: the domestic chick. We recorded neurons in chick basal ganglia in a brain slice preparation, using the whole cell technique. We found that chick striatum, like songbird striatum, contains a pallidum-like cell type never reported in mammalian striatum, supporting the hypothesis that this feature is common to all birds. We also discovered that spiny neurons, the most common cell type in the striatum of all amniotes, possess a diverse set of physiological properties in chicks that distinguish them from both mammals and songbirds. This study revealed an unexpectedly complex pattern of conservation and divergence in the properties of neurons recorded in avian striatum.

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain - in particular the neocortex-like cognitive functions of the avian pallium - requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

Seasonal-like plasticity of spontaneous firing rate in a songbird pre-motor nucleus

Many animals exhibit seasonal changes in behavior and its underlying neural substrates. In seasonally breeding songbirds, the brain nuclei that control song learning and production undergo substantial structural changes at the onset of each breeding season, in association with changes in song behavior. These changes are largely mediated by photoperiod-dependent changes in circulating concentrations of gonadal steroid hormones. Little is known, however, about whether changes in the electrophysiological activity of neurons accompany the dramatic morphological changes in the song nuclei. Here we induced seasonal-like changes in the song systems of adult white-crowned sparrows and used extracellular recording in acute brain slices from those individuals to study physiological properties of neurons in the robust nucleus of the arcopallium (RA), a pre-motor nucleus necessary for song production. We report that: RA neurons from birds in breeding condition show a more than twofold increase in spontaneous firing rate compared to those from nonbreeding condition; this change appears to require both androgenic and estrogenic actions; and this change is intrinsic to the RA neurons. Thus, neurons in the song circuit exhibit both morphological and physiological adult seasonal plasticity.

Evidence for ?direct? and ?indirect? pathways through the song system basal ganglia

Song learning in oscine birds relies on a circuit known as the "anterior forebrain pathway," which includes a specialized region of the avian basal ganglia. This region, area X, is embedded within a telencephalic structure considered homologous to the striatum, the input structure of the mammalian basal ganglia. Area X has many features in common with the mammalian striatum, yet has distinctive traits, including largely aspiny projection neurons that directly innervate the thalamus and a cell type that physiologically resembles neurons recorded in the mammalian globus pallidus. We have proposed that area X is a mixture of striatum and globus pallidus and has the same functional organization as circuits in the mammalian basal ganglia. Using electrophysiological and anatomical approaches, we found that area X contains a functional analog of the "direct" striatopallidothalamic pathway of mammals: axons of the striatal spiny neurons make close contacts on the somata and dendrites of pallidal cells. A subset of pallidal neurons project directly to the thalamus. Surprisingly, we found evidence that many pallidal cells may not project to the thalamus, but rather participate in a functional analog of the mammalian "indirect" pathway, which may oppose the effects of the direct pathway. Our results deepen our understanding of how information flows through area X and provide more support for the notion that song learning in oscines employs physiological mechanisms similar to basal ganglia-dependent forms of motor learning in mammals.

Properties of dopamine release and uptake in the songbird basal ganglia

Vocal learning in songbirds requires a basal ganglia circuit termed the anterior forebrain pathway (AFP). The AFP is not required for song production, and its role in song learning is not well understood. Like the mammalian striatum, the striatal component of the AFP, Area X, receives dense dopaminergic innervation from the midbrain. Since dopamine (DA) clearly plays a crucial role in basal ganglia-mediated motor control and learning in mammals, it seems likely that DA signaling contributes importantly to the functions of Area X as well. In this study, we used voltammetric methods to detect subsecond changes in extracellular DA concentration to gain better understanding of the properties and regulation of DA release and uptake in Area X. We electrically stimulated Ca(2+)- and action potential-dependent release of an electroactive substance in Area X brain slices and identified the substance as DA by the voltammetric waveform, electrode selectivity, and neurochemical and pharmacological evidence. As in the mammalian striatum, DA release in Area X is depressed by autoinhibition, and the lifetime of extracellular DA is strongly constrained by monoamine transporters. These results add to the known physiological similarities of the mammalian and songbird striatum and support further use of voltammetry in songbirds to investigate the role of basal ganglia DA in motor learning.

Songbirds and the revised avian brain nomenclature

It has become increasingly clear that the standard nomenclature for many telencephalic and related brainstem structures of the avian brain is based on flawed once-held assumptions of homology to mammalian brain structures, greatly hindering functional comparisons between avian and mammalian brains. This has become especially problematic for those researchers studying the neurobiology of birdsong, the largest single group within the avian neuroscience community. To deal with the many communication problems this has caused among researchers specializing in different vertebrate classes, the Avian Brain Nomenclature Forum, held at Duke University from July 18-20, 2002, set out to develop a new terminology for the avian telencephalon and some allied brainstem cell groups. In one major step, the erroneous conception that the avian telencephalon consists mainly of a hypertrophied basal ganglia has been purged from the telencephalic terminology, and the actual parts of the basal ganglia and its brainstem afferent cell groups have been given new names to reflect their now-evident homologies. The telencephalic regions that were incorrectly named to reflect presumed homology to mammalian basal ganglia have been renamed as parts of the pallium. The prefixes used for the new names for the pallial subdivisions have retained most established abbreviations, in an effort to maintain continuity with the pre-existing nomenclature. Here we present a brief synopsis of the inaccuracies in the old nomenclature, a summary of the nomenclature changes, and details of changes for specific songbird vocal and auditory nuclei. We believe this new terminology will promote more accurate understanding of the broader neurobiological implications of song control mechanisms and facilitate the productive exchange of information between researchers studying avian and mammalian systems.

Origin of the anterior forebrain pathway

The brain nuclei and pathways comprising the song system of oscine songbirds bear many similarities with circuits in other bird species and in mammals. This suggests that the song system evolved as a specialization of pre-existing circuits and may retain fundamental properties in common with those of other taxa. Here we review evidence for these similarities, including electrophysiological, morphological, and neurochemical data for identifying specific cell types. In addition, we discuss connectional data, addressing similarities in axonal projections among nuclei across taxa. We focus primarily on the anterior forebrain pathway, a circuit essential for song learning and vocal plasticity, because the evidence is strongest that this circuit is homologous to mammalian circuits. These fundamental similarities highlight the importance of comparative approaches; for example, understanding the role the anterior forebrain pathway plays in song plasticity may shed light on general principles of basal ganglia function. In addition, understanding specializations of such circuits in songbirds may illuminate specific innovations critical for vocal learning.

Revised nomenclature for avian telencephalon and some related brainstem nuclei

The standard nomenclature that has been used for many telencephalic and related brainstem structures in birds is based on flawed assumptions of homology to mammals. In particular, the outdated terminology implies that most of the avian telencephalon is a hypertrophied basal ganglia, when it is now clear that most of the avian telencephalon is neurochemically, hodologically, and functionally comparable to the mammalian neocortex, claustrum, and pallial amygdala (all of which derive from the pallial sector of the developing telencephalon). Recognizing that this promotes misunderstanding of the functional organization of avian brains and their evolutionary relationship to mammalian brains, avian brain specialists began discussions to rectify this problem, culminating in the Avian Brain Nomenclature Forum held at Duke University in July 2002, which approved a new terminology for avian telencephalon and some allied brainstem cell groups. Details of this new terminology are presented here, as is a rationale for each name change and evidence for any homologies implied by the new names. Revisions for the brainstem focused on vocal control, catecholaminergic, cholinergic, and basal ganglia-related nuclei. For example, the Forum recognized that the hypoglossal nucleus had been incorrectly identified as the nucleus intermedius in the Karten and Hodos (1967) pigeon brain atlas, and what was identified as the hypoglossal nucleus in that atlas should instead be called the supraspinal nucleus. The locus ceruleus of this and other avian atlases was noted to consist of a caudal noradrenergic part homologous to the mammalian locus coeruleus and a rostral region corresponding to the mammalian A8 dopaminergic cell group. The midbrain dopaminergic cell group in birds known as the nucleus tegmenti pedunculopontinus pars compacta was recognized as homologous to the mammalian substantia nigra pars compacta and was renamed accordingly; a group of gamma-aminobutyric acid (GABA)ergic neurons at the lateral edge of this region was identified as homologous to the mammalian substantia nigra pars reticulata and was also renamed accordingly. A field of cholinergic neurons in the rostral avian hindbrain was named the nucleus pedunculopontinus tegmenti, whereas the anterior nucleus of the ansa lenticularis in the avian diencephalon was renamed the subthalamic nucleus, both for their evident mammalian homologues. For the basal (i.e., subpallial) telencephalon, the actual parts of the basal ganglia were given names reflecting their now evident homologues. For example, the lobus parolfactorius and paleostriatum augmentatum were acknowledged to make up the dorsal subdivision of the striatal part of the basal ganglia and were renamed as the medial and lateral striatum. The paleostriatum primitivum was recognized as homologous to the mammalian globus pallidus and renamed as such. Additionally, the rostroventral part of what was called the lobus parolfactorius was acknowledged as comparable to the mammalian nucleus accumbens, which, together with the olfactory tubercle, was noted to be part of the ventral striatum in birds. A ventral pallidum, a basal cholinergic cell group, and medial and lateral bed nuclei of the stria terminalis were also recognized. The dorsal (i.e., pallial) telencephalic regions that had been erroneously named to reflect presumed homology to striatal parts of mammalian basal ganglia were renamed as part of the pallium, using prefixes that retain most established abbreviations, to maintain continuity with the outdated nomenclature. We concluded, however, that one-to-one (i.e., discrete) homologies with mammals are still uncertain for most of the telencephalic pallium in birds and thus the new pallial terminology is largely devoid of assumptions of one-to-one homologies with mammals. The sectors of the hyperstriatum composing the Wulst (i.e., the hyperstriatum accessorium intermedium, and dorsale), the hyperstriatum ventrale, the neostriatum, and the archistriatum have been renamed (respectively) the hyperpallium (hypertrophied pallium), the mesopallium (middle pallium), the nidopallium (nest pallium), and the arcopallium (arched pallium). The posterior part of the archistriatum has been renamed the posterior pallial amygdala, the nucleus taeniae recognized as part of the avian amygdala, and a region inferior to the posterior paleostriatum primitivum included as a subpallial part of the avian amygdala. The names of some of the laminae and fiber tracts were also changed to reflect current understanding of the location of pallial and subpallial sectors of the avian telencephalon. Notably, the lamina medularis dorsalis has been renamed the pallial-subpallial lamina. We urge all to use this new terminology, because we believe it will promote better communication among neuroscientists. Further information is available at http://avianbrain.org

Long-term potentiation in an avian basal ganglia nucleus essential for vocal learning

Vocal learning in songbirds provides an excellent model for sensorimotor learning in vertebrates, with an accessible, well-defined behavior and discrete neural substrate. The rich behavioral plasticity exhibited by songbirds, however, contrasts starkly with the scarcity of candidate cellular mechanisms. Here, we report for the first time on an activity-dependent form of synaptic plasticity in area X, a component of the song system required for song learning and song maintenance. In slice preparations of zebra finch area X, pairing of high-frequency presynaptic stimulation with postsynaptic depolarization induces Hebbian long-term potentiation (LTP) of the glutamatergic inputs to spiny neurons. This form of LTP requires activation of NMDA receptors and D1-like dopamine receptors. In addition, LTP is observed in birds as young as 47 d after hatching and also in adult birds but not in younger birds, providing evidence of developmental regulation of the onset of synaptic plasticity. These properties make this form of LTP the best known candidate mechanism for reinforcement-based vocal learning in juveniles and song maintenance in adult birds.

Presynaptic depression of glutamatergic synaptic transmission by D1-like dopamine receptor activation in the avian basal ganglia

Vocal behavior in songbirds exemplifies a rich integration of motor, cognitive, and social functions that are shared among vertebrates. As a part of the underlying neural substrate, the song system, the anterior forebrain pathway (AFP) is required for song learning and maintenance. The AFP resembles the mammalian basal ganglia-thalamocortical loop in its macroscopic organization, neuronal intrinsic properties, and microcircuitry. Area X, the first station in the AFP, is a part of the basal ganglia essential for vocal learning. It receives glutamatergic inputs from pallial structures and sends GABAergic outputs to thalamic structures. It also receives dense dopaminergic innervation from the midbrain. The role of this innervation is essentially unknown. Here we provide evidence that dopamine (DA) can modulate the glutamatergic inputs to spiny neurons in area X. In whole-cell voltage-clamp recordings from neurons in brain slices of adult zebra finches, we found that activation of D1-like DA receptors depresses ionotropic glutamatergic synaptic current in area X spiny neurons. This effect is mediated by a presynaptic site of action, mimicked by activation of adenylyl cyclase, and blocked by protein kinase A inhibitor and an adenosine A1 receptor antagonist. These results suggest that, in addition to altering the input-output function of spiny neurons by modulating their excitability, as we have shown previously, DA can directly influence the excitatory inputs to these neurons as well. Thus, DA can exert fine control over information processing through spiny neurons in area X, the dynamics of the AFP output, and ultimately song learning and maintenance.

Intrinsic and synaptic properties of neurons in an avian thalamic nucleus during song learning

The anterior forebrain pathway (AFP) of the avian song system is a circuit essential for song learning but not for song production. This pathway consists of a loop serially connecting area X in the basal ganglia, the medial portion of the dorsolateral nucleus of thalamus (DLM), and the pallial lateral magnocellular nucleus of the anterior neostriatum (lMAN). The majority of DLM neurons in adult male zebra finches closely resemble mammalian thalamocortical neurons in both their intrinsic properties and the strong GABAergic inhibitory input they receive from the basal ganglia. These observations support the hypothesis that the AFP and the mammalian basal ganglia-thalamocortical pathway use similar information-processing mechanisms during sensorimotor learning. Our goal was to determine whether the cellular properties of DLM neurons are already established in juvenile birds in the sensorimotor phase of song learning when the AFP is essential. Current- and voltage-clamp recording in DLM of juvenile male zebra finches showed that juvenile DLM has two distinct cell types with intrinsic properties largely similar to those of their respective adult counterparts. Immunostaining for glutamic acid decarboxylase (GAD) in juvenile zebra finches revealed that, as in adults, most area X somata are large and strongly GAD+ and that their terminals in DLM form dense GAD+ baskets around somata. GAD immunoreactivity in DLM was depleted by lesions of area X, indicating that a strong GABAergic projection from area X to DLM is already established in juveniles. Some of the DLM neurons exhibited large, spontaneous GABAergic synaptic events. Stimulation of the afferent pathway evoked an inhibitory postsynaptic potential or current that was blocked by the GABA(A) receptor antagonist bicuculline methiodide. The decay of the GABA(A) receptor-mediated currents was slower in juvenile neurons than in adults. In addition, the reversal potential for these currents in juveniles was significantly more depolarized both than that in adults and than the Cl(-) equilibrium potential; yet the reversal potential was still well below the firing threshold and thus inhibitory in the slice preparation. Our findings suggest that the signal-processing role of DLM during sensorimotor learning is generally similar to that in adulthood but that quantitative changes in synaptic transmission accompany the development of stereotyped song.

Dopamine modulates excitability of spiny neurons in the avian basal ganglia

The neural substrate of vocal learning in songbirds is an accessible system for studying motor learning and motor control in vertebrates. In the so-called song system, the anterior forebrain pathway (AFP), which is essential for song learning, resembles the mammalian basal ganglia-thalamocortical loop in its macroscopic organization, neuronal intrinsic properties, and microcircuitry. Area X, the first station in the AFP, and the surrounding lobus parolfactorius (LPO), are both parts of the avian basal ganglia. Like their mammalian counterparts, they receive dense dopaminergic innervation from the midbrain, but the physiological functions of this projection remain unclear. In this study, we investigated the effect of dopamine (DA) on excitability of spiny neurons in area X and LPO. We recorded from neurons in brain slices of adult zebra finches and Bengalese finches, using whole-cell and perforated-patch recording techniques in current-clamp configuration. We found that DA modulates excitability in spiny neurons; activation of D1- and D2-like DA receptors enhances and reduces excitability, respectively. These effects are similar to those observed in the mammalian neostriatum, with the main difference being that D1-like DA receptor activation enhances excitability in avian spiny neurons at hyperpolarized states. Our findings also indicate that some spiny neurons express both receptor types and suggest that receptor colocalization in the entire population can account for the spectrum of DA actions. The diversity of DA actions enables the DA system to fine-tune the dynamics of the song system and allows flexible control over song learning and production.

Electrophysiological analysis of a songbird basal ganglia circuit essential for vocal plasticity

The discrete, interconnected nuclei of the songbird brain, collectively termed the song system, underlie the learning and production of song. Two main forebrain pathways have been identified that contribute to song production, learning, and adult plasticity. A posterior "motor pathway" including nucleus HVc (used as the proper name), the robust nucleus of the archistriatum (RA) and descending projections to the brainstem, is essential for song production. An "anterior forebrain pathway," arising from HVc, passing through area X of the lobus parolfactorius, the medial portion of the dorsolateral nucleus of the anterior thalamus and the lateral magnocellular nucleus of the anterior neostriatum, and finally terminating in RA, is essential for song learning and adult plasticity. The fact that the lobus parolfactorius is thought to form a part of the avian striatum implies several predictions for the connections of area X and for the properties of its neurons. Here, we review the existing anatomical and electrophysiological data bearing on the nature of area X as a striatal structure. In general, the data strongly favor the notion that area X is striatal. One set of observations, however, is at odds with that idea, and we provide and partially test the hypothesis that area X also contains a pallidal component. We discuss further tests of this idea and implications for thinking of the song system as a basal ganglia loop similar to that described in mammals.

An avian basal ganglia pathway essential for vocal learning forms a closed topographic loop

The mammalian basal ganglia-thalamocortical pathway is important for motor control, motor learning, and cognitive functions. It contains parallel, closed loops, at least some of which are organized topographically and in a modular manner. Songbirds have a circuit specialized for vocal learning, the anterior forebrain pathway (AFP), forming a basal ganglia loop with only three stations: the pallial ("cortex-like") lateral magnocellular nucleus of the anterior neostriatum (lMAN), the basal ganglia structure area X, and the medial portion of the dorsolateral thalamic nucleus (DLM). Several properties of this pathway resemble those of its mammalian counterpart, but it is unknown whether all projections in the loop are topographically organized, and if so, whether topography is maintained through the entire loop. After small single- or dual-tracer injections into area X and/or the lMAN of adult zebra finches, we found that the area X to DLM projection is topographically organized, and we confirmed the topography for all other AFP projections. Quantitative analysis suggests maintained topography throughout the loop. To test this directly, we injected different tracers into corresponding areas in lMAN and area X. We found somata retrogradely labeled from lMAN and terminals anterogradely labeled from area X occupying the same region of DLM. Many labeled somata were tightly surrounded by tracer-labeled terminals, indicating the microscopically closed nature of the AFP loop. Thus, like mammals, birds have at least one closed, topographic loop traversing the basal ganglia, thalamus, and pallium. Each such loop could serve as a computational unit for motor or cognitive functions.

Disparity in neurotransmitter release probability among competing inputs during neuromuscular synapse elimination

Competition among the several motor axons transiently innervating neonatal muscle fibers results in an increasing disparity in the quantal content and synaptic territory of each competitor, culminating in the permanent loss of all but one axon from neuromuscular junctions. We asked whether differences in the probability of neurotransmitter release also contribute to the increasing disparity in quantal content among competing inputs, and when in the process of competition changes in release probability become apparent. To address these questions, intracellular recordings were made from dually innervated neonatal mouse soleus muscle fibers, and quantal content and paired-pulse facilitation were evaluated for each input. At short interpulse intervals, paired-pulse facilitation was significantly higher for the weaker input with the smaller quantal content than the stronger input with the larger quantal content. Because neurotransmitter release probability across all release sites is inversely related to the extent of facilitation observed after paired-pulse stimulation, this result suggests that release probability is lower for weak compared with strong inputs innervating the same junction. A disparity in the probability of neurotransmitter release thus contributes to the disparity in quantal content that occurs during synaptic competition. Together, this work suggests that an input incapable of sustaining a high release probability may be at a competitive disadvantage for synaptic maintenance.

Complementary 'bottom-up' and 'top-down' approaches to basal ganglia function

Recently, two quite different approaches exemplifying 'bottom-up' and 'top-down' philosophies have shed new light on basal ganglia function. In vitro work using organotypic co-cultures has implicated the subthalamic nucleus (STN) and the external segment of the globus pallidus (GP(e)) as pacemakers for low-frequency bursting that is reminiscent of the activity produced in Parkinsonian tremor. A circuit essential for avian song learning has been identified as part of the basal ganglia with surprisingly well conserved cellular details; investigation of this system may help to address general issues of basal ganglia function.

Electrophysiological properties of avian basal ganglia neurons recorded in vitro

The forebrains of mammals and birds appear quite different in their gross morphology, making it difficult to identify homologies between them and to assess how far they have diverged in organization. Nevertheless one set of forebrain structures, the basal ganglia, has been successfully compared in mammals and birds. Anatomical, histochemical, and molecular data have identified the avian homologues of the mammalian basal ganglia and indicate that they are very similar in organization, suggesting that they perform similar functions in the two classes. However, the physiological properties of the avian basal ganglia have not been studied, and these properties are critical for inferring functional similarity. We have used a zebra finch brain slice preparation to characterize the intrinsic physiological properties of neurons in the avian basal ganglia, particularly in the input structure of the basal ganglia, the striatum. We found that avian striatum contains a cell type that closely resembles the medium spiny neuron, the principal cell type of mammalian striatum. Avian striatum also contains a rare cell type that is very similar to an interneuron class found in mammalian striatum, the low-threshold spike cell. On the other hand, we found an aspiny, fast-firing cell type in avian striatum that is distinct from all known classes of mammalian striatal neuron. These neurons usually fired spontaneously at 10 Hz or more and were capable of sustained firing at very high rates when injected with depolarizing current. The existence of this cell type represents an important difference between avian striatum and mammalian dorsal striatum. Our data support the general idea that the organization and functional properties of the basal ganglia have been largely conserved in mammals and birds, but they imply that avian striatum is not identical to mammalian dorsal striatum.

Slow synaptic inhibition mediated by metabotropic glutamate receptor activation of GIRK channels

Glutamate is the predominant excitatory neurotransmitter in the vertebrate CNS. Ionotropic glutamate receptors mediate fast excitatory actions whereas metabotropic glutamate receptors (mGluRs) mediate a variety of slower effects. For example, mGluRs can mediate presynaptic inhibition, postsynaptic excitation, or, more rarely, postsynaptic inhibition. We previously described an unusually slow form of postsynaptic inhibition in one class of projection neuron in the song-control nucleus HVc of the songbird forebrain. These neurons, which participate in a circuit that is essential for vocal learning, exhibit an inhibitory postsynaptic potential (IPSP) that lasts several seconds. Only a portion of this slow IPSP is mediated by GABA(B) receptors. Since these cells are strongly hyperpolarized by agonists of mGluRs, we used intracellular recording from brain slices to investigate the mechanism of this hyperpolarization and to determine whether mGluRs contribute to the slow synaptic inhibition. We report that mGluRs hyperpolarize these HVc neurons by activating G protein-coupled, inwardly-rectifying potassium (GIRK) channels. MGluR antagonists blocked this response and the slow synaptic inhibition. Thus, glutamate can combine with GABA to mediate slow synaptic inhibition by activating GIRK channels in the CNS.