disorders of voice ( dysphonia ) and speech production ( dysarthria ) can be related to developmental issues , neuro - degeneration , structural defects , and/or advanced aging . neurodegeneration related to disease and aging causes significant negative changes to vocal production and thus affects communication ability and quality of life . for example , dysarthria is an early - onset sign in individuals with parkinson disease typically starting with changes to vocal loudness and quality [ 1 - 6 ] . as the disease progresses vocal acoustic and auditory - perceptual features degrade and cause reduced amplitude ( loudness ) , decreased variation of intonation ( monopitch and monoloudness ) , and increased signal to noise ratio ( breathiness / harshness in vocal quality ) , which significantly impair voice production and , therefore , speech intelligibility and communication efficacy [ 7 - 11 ] . although the primary pathology of parkinson disease is depletion of nigrostriatal dopamine , there are many signs and symptoms that do not fit with this model of the disease , including dysarthria . unfortunately , pharmacologic and surgical inter - ventions aimed at modulating the nigrostriatal dopamine system / circuits ( deep brain stimulation , levodopa ) do not markedly improve and may even worsen voice production [ 12 - 20 ] . further , it is now understood that parkinson disease encompasses a widespread pathology , with changes to the autonomic nervous , peripheral nerves and muscles [ 23 - 25 ] and dysregulation of norepinephrine and degeneration of the locus coeruleus [ 26 - 29 ] . as such , how the complex pathology of parkinson disease contributes to vocal dysfunction is unclear . to address this problem , a logical first step in understanding how parkinson disease affects voice is to determine the effects of depleting nigrostriatal dopamine on vocal production , as this is the primary feature of disease pathology . once this is defined , then systematically studying degeneration of other neural structures and neurotransmitters as they pertain to dysarthria will lead to a more complete picture of vocal dysfunction in parkinson disease . for the purposes of this review , we will focus on work that has been done in depleting nigrostriatal dopamine . age - related voice disorders in older adults negatively impact quality of life , present both physical and mental health risks , and are common ; the prevalence of voice disorders in older adults has been estimated to be between 20 - 30% [ 30 , 31 ] . laryngeal neuromuscular decline is associated with age - related voice problems and may also contribute to difficulty swallowing through compromised laryngeal closure [ 32 - 35 ] . therefore , understanding the effects of advanced age on laryngeal neuromuscular mechanisms has important clinical implications . although considerable work has been done in humans , much of our understanding of these problems comes from animal models . rat ultrasonic vocalizations ( usvs ) have been extensively studied with regard to basic neurophysiologic mechanisms underlying production as well as affect and reward / addiction [ 36 - 49 ] . usvs can also be useful to study sensorimotor control for actions of the larynx , such as vocalization , since usvs and human vocalizations share similar mechanisms of production ; specifically , both are produced by rapidly constricting the laryngeal muscles to modulate an egressive airflow [ 50 , 51 ] . the primary difference between human vocalizations and rat usvs is that human vocalizations are created by vibrations of the vocal folds , while usvs are thought to be produced using a whistle mechanism [ 50 , 52 , 53 ] . it has been shown that rat usvs share certain features with human speech communication in that they have semiotic value   serve to establish and maintain social contact , and consist of modular vocal behavior . that is not to say that usvs are equivalent to human speech ; rather , usvs , like speech , require fine sensorimotor control of the larynx , have meaning , and can change the behavior of the signal recipient . thus , usvs can be an appropriate model for studying sensorimotor control in the context of communication . the purpose of this paper for the special issue is to review the literature related to rat 50-khz usvs as it pertains to dopamine denervation as a model for parkinson disease and aging . dopamine denervation has been modeled extensively in rats by infusing the neurotoxin 6-hydroxydopamine ( 6-ohda ) into the medial forebrain bundle or striatum , where it is taken up by the neuron causing a cascade of events leading to dopaminergic and noradrenergic cell death ( for examples of literature out of thousands of papers see ungerstedt , 1968   and more recently johnson et al . , 1999 ) . this has also been modeled less commonly with pesticides / herbicides such as rotenone and paraquat . regardless of the mechanism , these methods attempt to model the cardinal features of parkinson disease ; tremor , postural instability , bradykinesia by causing striatal dopamine depletion ( for a review see blesa , phani , jackson - lewis , przedborski , 2012 ) . however , the important clinical features of parkinson disease have expanded to include other sensorimotor deficits such as freezing , dysarthria and dysphagia ( disordered swallowing ) ( for a review see ciucci et al . , 2013 ) and non - motor deficits such as hyposmia , sleep and mood disturbances , cognition and autonomic dysfunction ( see natale et al . , 2013 , and as an example in the 6-ohda rat model see tadaiesky , et al . , 2008 ) . thus , efforts have been made to define which signs are attributed to dopamine depletion in nigrostriatal pathways or other more recently identified extranigral and non - dopaminergic mechanisms [ 21 , 26 , 62 ] . in our review of the literature , only two papers addressed the effects of unilateral dopamine denervation on 50-khz vocalizations with 6-ohda ( [ 63 , 64 ] ) ; no studies were found using other neurotoxin or pesticide / herbicide models . the first study ( ciucci et al . , 2007 ) was preliminary , thus results from the 2009 study are reported here . using a mating paradigm , the authors elicited 50-khz usvs in male long - evans rats by exposing sexually - experienced males to receptive females . usv complexity ( trill - like frequency modulated usvs vs. flat usvs ) was decreased relative to controls without a decrease in the total number of usvs produced . amplitude ( acoustic intensity measured in decibels ) and bandwidth ( the frequency range of a vocalization , measured in hertz as the difference between the highest and lowest frequency ) were also significantly reduced compared to controls ( for representative spectrogram , see fig . although it has been established in adult usvs that acoustic features , such as number of calls , intensity , and frequency range , carry communicative value , it is unknown how changes in these parameters change the semiotic content and/or the response of the recipient . based on recent advances in understanding the physiology of usv production [ 50 , 56 , 67 ] , however , these acoustic changes suggest that rats , like humans , have decreased control at the level of the larynx with dopamine denervation . however , although denervation with unilateral 6-ohda infusion alters the acoustic structure of rat usvs , certain features do not change or change to a small degree . we calculated effect sizes from the ciucci 2009 paper and there was a large effect on amplitude ( cohen s d 8.0 , r=0.97 ) but a smaller effect on maximum frequency ( cohen s d=2.6 , r=0.79 ) , while there was no statistically significant difference in usv duration .   in another study , neurodegeneration was induced by unilaterally injecting recombinant adeno - associated virus ( raav ) serotype 2/5-expressing human wildtype -synuclein , a protein implicated in the neuropathology of parkinson disease , in the rat substantia nigra . functional impairment in measures of forelimb use were observed in rats with 8 week duration of -synuclien ( -syn ) expression and these deficits were highly correlated with striatal tyrosine hydroxylase loss . rats in this cohort also demonstrated decreased usv amplitude and rate , but there were no significant differences in usv type , duration , bandwidth , peak frequency or latency to call between nave control and raav2/5--syn rats . this suggests that while dopamine denervation affects acoustic properties of the usv , it does not affect all aspects of the usv , pointing toward other mechanisms . this in part has been recently addressed in a mouse model over - expressing human wildtype -syn . mice showed a significant reduction in the duration and amplitude of usvs with an altered usv profile . these acoustic deficits occur in the absence of nigrostriatal dopamine loss , although -syn pathology was found in the periaqueductal gray , a region important for vocal production [ 70 - 72 ] . thus , we can tentatively conclude that dopamine denervation as well as other pathological mechanisms associated with parkinson disease contribute to vocal dysfunction . in other words , like humans , vocal communication in rats is susceptible to pathologies related to parkinson disease , exhibit acoustic changes that mimic those seen in the human manifestation of the disease ( reductions in frequency bandwidth and amplitude ) , and can serve as useful tool in understanding the behavioral and underlying neurological complexities of parkinson disease . although it is beyond the scope of the current paper to review all effects of pharmacologic manipulation on usvs , a considerable amount of work has been done to characterize dopaminergic mechanisms in the production of usvs [ 42 , 73 - 75 ] . specifically , manipulations of dopamine with drugs such as amphetamine 40 , 43 , 48 , 76 , 77 , cocaine [ 78 , 79 ] , apomorphine , and methylphenidate   result in an increase in the quantity of 50-khz usvs . likewise , agonizing or antagonizing dopaminergic receptors affects both the quantity and quality of 50-khz vocalizations [ 81 - 86 ] . for example , selective antagonism of the dopamine d2 receptor sub - type with haloperidol results in decreased bandwidth , amplitude ( loudness ) , and complexity of usvs , similar to what is observed following unilateral infusions of 6-ohda [ 63 , 64 ] . likewise , selective d1 , d2 , or combined d1 + d2 antagonism results in not only decreased call rate and altered call profile [ 81 , 82 ] , but also reductions in latency to call , duration , intensity , bandwidth and peak frequency . given the role of dopamine in rewarding and appetitive behaviors , there is also a body of literature devoted to characterizing both direct manipulations of the nucleus accumbens [ 40 , 43 , 46 , 76 , 87 ] and the 50-khz usv as an indicator of positive affect ( for examples , see [ 54 , 88 , 89 ] ) . while not as well characterized as dopaminergic mechanisms in usvs , manipulations of other neurotransmitters such as norepinephrine 75 , 90 - 96 and serotonin   have also been shown to play a role in the production of 50-khz usvs . although pharmacological manipulations do not recapitulate dopaminergic denervation , as it relates to parkinson disease , these studies offer valuable insight into the relationship between specific dopaminergic mechanisms and qualitative aspects of usvs . this , in turn , may help to better understand how and why voice deficits manifest in parkinson disease and inform future treatment approaches . only a handful of studies have looked at the effects of advanced age on rat usvs [ 98 - 100 ] . overall , age - related acoustic changes in 50-khz usvs are similar to those found with unilateral dopamine denervation with 6-ohda , with aged rats demonstrating decreases in measures of frequency ( smaller bandwidth and decreased maximum and peak frequency ) and decreased amplitude of vocalizations relative to young adult rats [ 98 - 100 ] . different than the findings in the 6-ohda model , however , vocalization complexity and rate do not appear to be influenced by advanced age . furthermore , aging does appear to impact the duration of complex vocalizations , with longer mean durations of step   and frequency modulated   usvs in old adult rats compared to usvs of young adult rats . although few in number , these studies indicate aging does have an impact on the acoustics of rat 50-khz usvs , and that the acoustic deficits have some similarities with those found with dopamine denervation . these acoustic changes in usvs may be due to age - related changes in laryngeal structure and/or central and peripheral denervation - like neuromuscular adaptations . in the aging rat larynx , senescence is associated with neuromuscular deficits , including decreased laryngeal kinematics during resting breathing , reduced capillary surface area and branch points in the thyroarytenoid muscle ( the primary muscle of the vocal fold ) , and a shift to slower contracting muscle fiber types in the thyroarytenoid muscle [ 101 - 103 ] . in both the human and rat larynx , aging impacts laryngeal neuromuscular junction ( nmj ) morphology in a manner similar to denervation , but in ways unique from nmjs found in the limb musculature [ 32 , 104 - 106 ] . peterson et al . ( 2013 )   reported age - related acoustic deficits of usvs were correlated with structural changes in the microarchitecture of the vocal folds , including an increase in connective tissue and collagen and a decline in elastin and hyaluronic acid , all of which may result in increased stiffness of the vocal folds . peripheral neuromuscular changes , such as dispersion of the motor endplate within the nmj , and central changes , such as a decreased number of motoneurons in the nucleus ambiguus , have also been associated with acoustic deficits in 50-khz usvs . furthermore , other unexplored mechanisms underlying usv production may be impacted by advanced age and cause changes in acoustic parameters . for example , frequency modulation of usvs is related to respiratory patterns ; therefore , age - related changes in respiratory capacity and function may also play a role in acoustic deficits . no direct studies have been performed , however , investigating usv production and respiratory function in an aging model . overall , these results suggest advanced age negatively impacts the physiology of the rat laryngeal neuromuscular system , and therefore , the acoustic signal of 50-khz usvs . age - related acoustic deficits in usvs correspond to those found in the aging human voice . typical age - related acoustic changes in the human voice include alterations in average fundamental frequency , decreased fundamental frequency range , decreased vocal stability ( increased perturbation ) , and decreased volume ( amplitude ) ( for review , see baken ( 2005 ) ) . as with the rat , these acoustic changes in the human voice are likely due to underlying age - related changes in vocal fold microarchitecture and biomechanics [ 108 - 110 ] and neuromuscular deficits . therefore , study of age - related changes in rat usvs and laryngeal neuromuscular system can provide insight into age - related changes in the human voice . this paper reviewed the currently published literature on male rat usvs as it pertains to dopamine denervation and aging . there were only 2 papers published on dopamine denervation with the neurotoxin 6-ohda and one paper using viral transfection of -syn to deplete dopamine in the substantia nigra ; no other neurotoxin or herbicide / pesticide studies examining usvs were found . findings from these papers show an altered usv profile ( less complex and more simple usvs ) along with reduced bandwidth and amplitude of usvs . thus , unilateral denervation of nigrostriatal fibers in the medial forebrain bundle or substantia nigra is associated with acoustic deficits in male rat usvs . however , as vocal deficits appear early in humans and are refractory to treatments aimed at nigrostriatal dopamine depletion , other mechanisms in the complex pathology of parkinson disease are indicated with potential association to dopamine pathways . midbrain dopamine cells that die in the disease appear particularly susceptible to abnormal alpha - synuclein protein deposits . transgenic mouse models that over - express human wild type -syn under the thy-1 promoter demonstrate early and progressive usv deficits accompanied by -syn pathology in the periaqueductal gray ( pag ) which happens prior to substantial dopamine cell loss .   as described in the seminal works by braak [ 21 , 26 , 62 ] , dopamine neurons appear to be vulnerable to degeneration and alpha - synuclein aggregation and the association between alpha - synuclein and dopamine signaling pathways may be mediated by molecular intermediaries . knockout mice with -syn deficiency and altered usvs show downregulation of foxp1 mrna in the striatum during developmental and adult periods . recent evidence from the zebra finch songbird suggests that lentiviral - mediated knockdown of the speech - related gene foxp2 in the basal ganglia song nucleus , area x , affects dopamine modulation in cortico - striatal circuitry and modification of the song . in addition to these foxp molecules , a whole cascade of other behaviorally - driven genes in zebra finches associate with dopamine - modulated pathways in the basal ganglia [ 114 , 115 ] . these molecular interactions may be key mediators of vocal plasticity necessary for song learning and on - going maintenance . these examples from multiple species highlight the need to leverage the advantages of genetic tools in mice and rats with the well - characterized neural circuitry of songbirds in order to move forward in the investigation of the onset , progression , and pathology of vocalization deficits in motor disorders , particularly ones associated with dopamine denervation . the results of the aging studies may indicate an age - related decrease in fine motor control needed to accurately produce these relatively short ( < 50 ms ) , rapidly - modulated vocalizations . loss of fine motor control may result from an increase in the size of the motor unit seen with aging . indeed , age - related acoustic changes in rats are associated with a decrease in the number of primary motoneurons in the nucleus ambiguus   and pre - synaptic remodeling of the nmj , suggesting motor unit remodeling with age . this interpretation is supported by findings using electro - myography in the human larynx that show longer motor unit durations in older adults . additionally , age - related changes at the nmj may be a primary cause of motor unit remodeling . evidence is limited , however , by the few studies on age - related changes in usvs and related changes in underlying neuromuscular mechanisms . age - related changes both within the larynx and throughout the body likely impact usvs . for example , usvs are produced with an egressive airflow and , therefore , age - related changes in breathing and pulmonary function likely contribute to changes in usvs . studies of human speech breathing have shown older adults use different respiratory mechanisms when producing speech , likely due to changes in the strength of respiratory musculature , compliance of the chest and lungs , and lung volume [ 119 , 120 ] . changes in rat pulmonary function with aging has been identified , although subglottal pressure during usv production has only been studied in young rats . these multiple neurological and physiological mechanisms involved with usv production make usvs a unique behavioral biomarker of aging , and offer a relatively untapped area of research . one of the primary acoustic usv variables affected by both dopamine denervation and aging is usv amplitude . when measuring changes in amplitude , variations in distance from the sound source to the microphone will result in variations of amplitude . in the reviewed studies , variability of individual rats was accounted for by taking an average measurement of many vocalizations for each rat . therefore , the observed differences in usv amplitude were likely due to true differences between groups and not variations in mouth - to - microphone distance . a potential solution to control for this variation in microphone distance could be to use an array of microphones , allowing for localizing of the animal and subsequent calculation of a calibrated amplitude measure . future investigations may consider this or another controlled method of measuring amplitude to provide further insight into usv amplitude changes . many of the reported changes in usv acoustics differed by vocalization subtype ; however , there is no standard method for classifying usvs and , therefore , different methods of classification were used across studies . although one study analyzed all 50-khz usvs together without further classification , most of the studies discussed in this review manually classified 50-khz usvs into at least 2 subtypes by frequency , either  flat the frequency modulated usvs are often further divided into 2 , 4 , or as many 12 different sub - classification [ 64 , 77 , 88 ] . although automatic classification schemes have been proposed for both rat [ 122 , 123 ] and mouse   usvs , none have been widely accepted . evidence for a reliable , automated , meaningful classification scheme is critical for comparing results across studies .   despite some methodological limitations , rat 50-khz usvs have been a useful tool for studying changes to vocal production with regard to dopamine denervation and aging . additionally , usvs also offer an opportunity to study behavioral interventions targeted at preventing and/or reversing these changes in vocal production and the underlying neurological and sensorimotor deficits [ 99 , 125 ] . although only a limited number of studies were identified by this review , it is clear rat 50-khz usvs are an emerging tool to investigate many disorders of the nervous system and genetic conditions that cause communication deficits .