Water Drinking Behavior Associated with Aversive Arousal in Rats: An Integrative Approach.

Cholinergic muscarinic stimulation of vast areas of the limbic brain induced a well-documented polydipsia in laboratory rats. This excessive water-drinking behavior has not received any convincing biological and physiological interpretation for the last 50 years. This review offers such an interpretation and suggests that cholinergically induced drinking response, mostly by carbachol, is associated with activation of the ascending mesolimbic cholinergic system that serves for initiation of emotional aversive arousal of the organism. The ascending cholinergic system originates from the laterodorsal tegmental nucleus, has a diffuse nature, and affects numerous subcortical limbic structures. It is proposed that the carbachol-induced drinking response is related to the state of anxiety and does not serve the regulation of thirst. Instead, the response is anxiety-induced polydipsia that might occur as a soothing procedure that decreases the aversiveness of the negative emotional state induced by carbachol. It is concluded that carbachol-induced water-drinking behavior is a rewarding process that contributes to alleviating the feeling of anxiety by bringing some relief from the cholinergically induced aversive state, and it is a homologue to anxiety-driven polydipsia in humans.

Recent Studies of Rat Ultrasonic Vocalizations-Editorial.

Since the realization that human emotional experiences and behavior evolved from mammalian ancestors and are evolutionary continuations of animal emotional behavior [...].

Biological Functions of Rat Ultrasonic Vocalizations, Arousal Mechanisms, and Call Initiation.

This review summarizes all reported and suspected functions of ultrasonic vocalizations in infant and adult rats. The review leads to the conclusion that all types of ultrasonic vocalizations subserving all functions are vocal expressions of emotional arousal initiated by the activity of the reticular core of the brainstem. The emotional arousal is dichotomic in nature and is initiated by two opposite-in-function ascending reticular systems that are separate from the cognitive reticular activating system. The mesolimbic cholinergic system initiates the aversive state of anxiety with concomitant emission of 22 kHz calls, while the mesolimbic dopaminergic system initiates the appetitive state of hedonia with concomitant emission of 50 kHz vocalizations. These two mutually exclusive arousal systems prepare the animal for two different behavioral outcomes. The transition from broadband infant isolation calls to the well-structured adult types of vocalizations is explained, and the social importance of adult rat vocal communication is emphasized. The association of 22 kHz and 50 kHz vocalizations with aversive and appetitive states, respectively, was utilized in numerous quantitatively measured preclinical models of physiological, psychological, neurological, neuropsychiatric, and neurodevelopmental investigations. The present review should help in understanding and the interpretation of these models in biomedical research.

Association of medial corticostriatal regions with amphetamine-induced emission of 50 kHz vocalizations as studied by Zif-268 expression in the rat brain.

Behavioural sensitization of locomotion and 50 kHz ultrasonic vocalizations (USVs) following repeated amphetamine (AMPH) injections in rats has been extensively demonstrated. These two behaviours appear dissociable in their sensitization patterns and are thought to be reflective of underlying emotional states of the organism. Although AMPH is often used to induce 50 kHz USVs there is little research to date on the extent of cortical and subcortical forebrain region involvement in 50 kHz call production associated with the drug. Nor has general ergometric activity (a measure that in addition to locomotor activity includes all major muscular activity of the body) been investigated in such a framework. The present study sought to address this by performing a minimal sensitization protocol, utilizing only two injections, to investigate expression of the inducible transcription factor Zif-268 (Zif) among brain regions thought to be associated with 50 kHz USV emission. It was found that animals that spent a longer time emitting 50 kHz calls after a second AMPH injection showed statistically significant correlative patterns of Zif expression in medial prefrontal and striatal regions. These associations were not significant in animals that spent a shorter period of time calling after AMPH. There was also no significant correlation between any ergometric activity and time spent calling. The results provide evidence that the medial prefrontal cortices (prelimbic and infralimbic regions) of the rat may be involved with 50 kHz USV emission induced by AMPH in association with medial portions of the ventral and dorsal striatum.

Dissimilar interaction between dopaminergic and cholinergic systems in the initiation of emission of 50-kHz and 22-kHz vocalizations.

Rats emit 22-kHz or 50-kHz ultrasonic vocalizations (USVs) to signal their emotional state to other conspecifics. The 22-kHz USVs signal a negative emotional state while 50-kHz USVs reflect a positive affective state. The initiation of 22-kHz USVs is dependent on the activity of cholinergic neurons within the laterodorsal tegmental nucleus that release acetylcholine along the medial cholinoceptive vocalization strip. Emission of 50-kHz USVs is dependent upon the activation of dopaminergic neurons located within the ventral tegmental area that release dopamine into the medial shell of the nucleus accumbens. There have been reports that showed an antagonistic interaction between acetylcholine and dopamine during the expression of emotional states, and dopamine agonists decreased carbachol-induced emission of 22-kHz USVs. The current study tests the hypothesis that initial antagonism of dopamine receptors by systemic haloperidol or intraacumbens raclopride should increase the subsequent emission of 22 kHz USVs induced by carbachol from the lateral septum. Our findings showed that antagonism of dopaminergic signaling either via systemic haloperidol or via intracerebral raclopride did not alter the number of emitted 22-kHz USVs. Thus, inhibition of the mesolimbic dopamine system did not increase the magnitude of a negative emotional state. It was found, however, that prolonged emission of 22-kHz USVs initiated by carbachol caused a delayed rebound emission (R) of 50-kHz USVs appearing after 300 s of emission of 22-kHz USVs, i.e., when the response was subsiding. The R-50-kHz USVs were predominantly frequency modulated (FM) USVs and their number was directly proportional to the number of recorded 22-kHz USVs. The emission of R-50-kHz USVs was reversed by systemic pretreatment with haloperidol or intraacumbens injection of raclopride. It is argued that the R-50-kHz USVs represent a rebound emotional state that is opposite in valence and arousal induced by carbachol. Importantly, prolonged emission of amphetamine-induced 50 kHz USVs failed to show any vocalization rebound effect.

The antagonistic relationship between aversive and appetitive emotional states in rats as studied by pharmacologically-induced ultrasonic vocalization from the nucleus accumbens and lateral septum.

Rats can emit 22-kHz or 50-kHz ultrasonic vocalizations (USVs) in negative, as well as positive contexts which index their emotional state. 22-kHz USVs are emitted during aversive contexts and can be initiated by activation of the ascending cholinergic pathways originating from the laterodorsal tegmental nucleus or initiated pharmacologically by injection of cholinergic agonists into target areas of these pathways (medial cholinoceptive vocalization strip). Conversely, 50-kHz USVs are emitted during positive pro-social contexts and can be initiated by stimulation of ascending dopaminergic pathways originating from the ventral tegmental area or by injection of dopamine agonists into target areas of these pathways (nucleus accumbens shell). Recently, we have shown an inhibitory effect a positive emotional state has on the emission of carbachol-induced 22-kHz USVs from the anterior hypothalamic/medial preoptic area. However, this structure is a fragment of that cholinoceptive vocalization strip. We wanted to examine if we could observe similar effect when the aversive state is induced from the lateral septum, the most rostral division of the cholinoceptive vocalization strip. The results supported previous findings. First, microinjection of the dopamine agonist R-(-)-apomorphine into the nucleus accumbens shell resulted in increased emission of frequency modulated (FM) 50-kHz USVs that are regarded as signals expressing a positive emotional state in rats. Second, FM 50-kHz USVs and not flat (F) 50-kHz USVs were able to decrease 22-kHz USVs induced by microinjections of carbachol into the lateral septum. This research provides further support to the hypothesis that the initiation of a positive emotional state functionally antagonizes initiation of a negative emotional state in rats.

Intracerebral injection of R-(-)-Apomorphine into the nucleus accumbens decreased carbachol-induced 22-kHz ultrasonic vocalizations in rats.

Rats can produce ultrasonic vocalizations (USVs) in a variety of different contexts that signal their emotional state to conspecifics. Under distress, rats can emit 22-kHz USVs, while during positive pro-social interactions rats can emit frequency-modulated (FM) 50-kHz USVs. It has been previously reported that rats with increasing emission of FM 50-kHz USVs in anticipation of rewarding electrical stimulation or positive pro-social interaction decrease the number of emitted 22-kHz USVs. The purpose of the present investigation was to determine, in a pharmacological-behavioural experiment, if the positive emotional arousal of the rat indexed by the number of emitted FM 50-kHz USVs can decrease the magnitude of a subsequent negative emotional state indexed by the emission of 22-kHz USVs. To induce a positive emotional state, an intracerebral injection of a known D1/D2 agonist R-(-)-apomorphine (3.0 µg/0.3 µl) into the medial nucleus accumbens shell was used, while a negative emotional state was induced by intracerebral injection of carbachol (1.0 µg/0.3 µl), a known broad-spectrum muscarinic agonist, into the anterior hypothalamic-medial preoptic area. Our results demonstrated that initiation of a positive emotional state was able to significantly decrease the magnitude of subsequently expressed negative emotional state measured by the number of emitted 22-kHz USVs. The results suggest the neurobiological substrates that initiate positive emotional state indirectly antagonize the brain regions that initiate negative emotional states.

Emission of 22 kHz vocalizations in rats as an evolutionary equivalent of human crying: Relationship to depression.

There is no clear relationship between crying and depression based on human neuropsychiatric observations. This situation originates from lack of suitable animal models of human crying. In the present article, an attempt will be made to answer the question whether emission of rat aversive vocalizations (22 kHz calls) may be regarded as an evolutionary equivalent of adult human crying. Using this comparison, the symptom of crying in depressed human patients will be reanalyzed. Numerous features and characteristics of rat 22 kHz aversive vocalizations and human crying vocalizations are equivalent. Comparing evolutionary, biological, physiological, neurophysiological, social, pharmacological, and pathological aspects have shown vast majority of common features. It is concluded that emission of rat 22 kHz vocalizations may be treated as an evolutionary vocal homolog of human crying, although emission of 22 kHz calls is not exactly the same phenomenon because of significant differences in cognitive processes between these species. It is further concluded that rat 22 kHz vocalizations and human crying vocalizations are both expressing anxiety and not depression. Analysis of the relationship between anxiety and depression reported in clinical studies supports this conclusion regardless of the nature and extent of comorbidity between these pathological states.

Effect of microinjections of dopamine into the nucleus accumbens shell on emission of 50 kHz USV: Comparison with effects of d-amphetamine.

Systemic pharmacological manipulation of dopamine (DA) signaling has been central to many investigations of 50 kHz ultrasonic vocalizations (USVs) in the rat. In particular, the indirect DA releaser d-amphetamine (AMPH) has been used extensively in many such investigations. The possible unique character of the native transmitter relative to DA-stimulating drugs such as AMPH in inducing and modulating emission of 50 kHz USVs has not been investigated. Adult male Long Evans rats were tested with intracerebral application of DA into the nucleus accumbens shell at several doses (3.75 µg-120 µg) to determine its capacity to induce 50 kHz USV emission. Additionally, the call profile characteristics of intracerebral DA injections were compared with those of intracerebral application of AMPH. Results indicated that local increases in DA signaling within the nucleus accumbens shell are sufficient to increase 50 kHz call rate, reduce latency to call, and increase the degree of frequency modulation of emitted USVs. However, our results found that microinjections of DA were not as efficacious in either inducing 50 kHz USVs or increasing frequency modulation without antagonism of the dopamine reuptake transporter when compared with AMPH. In summary, these results support the notion that the native transmitter DA is driving the increase in frequency modulation seen after administration of DA stimulating drugs. These results also suggest that drugs affecting dopamine may be altering the 50 kHz call profile in a distinct manner from the native transmitter and thus caution should be used in interpreting their effects.

Non-pharmacological induction of rat 50 kHz ultrasonic vocalization: Social and non-social contexts differentially induce 50 kHz call subtypes.

The emission of 50 kHz ultrasonic vocalizations (USVs) by rats is thought to represent a measurable expression of the individuals underlying emotional state. These calls are also posited as fulfilling important communicative functions among conspecifics. In addition to social situations, 50 kHz USVs are recorded in a variety of reward-related contexts including sugary foods and drink, consumable ethanol, and drugs of abuse. The current study sought to directly compare several of these behavioural contexts in their capability to induce and modulate 50 kHz USV emission in adult male rats. Using two social conditions (exposure of naïve rat to a naturally cycling female and reuniting with a same-sex cage partner) and two non-social conditions (access to consumables as Fruit Loops or 2% ethanol v/v), we analyzed USVs recorded in 6 stimulus-presentation sessions. Only the female-exposure condition was found to increase 50 kHz call rate significantly over baseline, and this induction sensitized across 4 standard recording sessions. The use of a same-sex cage-mate and the two consumable food rewards did not elicit higher than baseline 50 kHz calling. None of the behavioural contexts altered the acoustic parameters (peak frequency, duration, and bandwidth) of emitted 50 kHz calls. In counter-balanced recording sessions, calling across all groups was significantly reduced by pre-treatment with the dopamine antagonist haloperidol compared with vehicle. Non-social conditions appeared to induce a greater proportion of flat calls at the expense of non-trill FM calls, while the reverse was seen for social conditions. However, the type of food reward and the type of social context mattered for proportion of flat and trill calls respectively. When compared with a control, access to sugary food but not ethanol induced a greater proportion of flat calls, and the female but not the cage-mate stimulus induced a greater proportion of trill calls.

Rat 50-kHz ultrasonic vocalizations as a tool in studying neurochemical mechanisms that regulate positive emotional states.

BACKGROUND: Adolescent and adult rats emit 50-kHz ultrasonic vocalizations (USVs) to communicate the appetitive arousal and the presence of positive emotional states to conspecifics. NEW METHOD: Based on its communicative function, emission of 50-kHz USVs is increasingly being evaluated in preclinical studies of affective behavior, motivation and social behavior. RESULTS: Emission of 50-kHz USVs is initiated by the activation of dopamine receptors in the shell subregion of the nucleus accumbens. However, several lines of evidence show that non-dopaminergic receptors may influence the numbers of 50-kHz USVs that are emitted, as well as the acoustic parameters of calls. COMPARISON WITH EXISTING METHODS: Emission of 50-kHz USVs is a non-invasive method that may be used to study reward and motivation without the need for extensive training and complex animal manipulations. Moreover, emission of 50-kHz USVs can be used alone or combined with other well-standardized behavioral paradigms (e.g., conditioned place preference, self-administration). CONCLUSIONS: This review summarizes the current evidence concerning molecular mechanisms that regulate the emission of 50-kHz USVs. Moreover, the review discusses the usefulness of 50-kHz USVs as an experimental tool to investigate how different neurotransmitter systems regulate the manifestations of positive emotional states, and also use of this tool in preclinical modeling of psychiatric diseases.

Individual behavioural predictors of amphetamine-induced emission of 50 kHz vocalization in rats.

Measurement of ultrasonic vocalizations (USVs) produced by adult rats represents a highly useful index of emotional arousal. The associations found between 50 kHz USV production and a variety of behavioural and pharmacological protocols increasingly suggests they serve as a marker of positive motivational states. This study used a powerful within-subjects design to investigate the relationships among individual differences in approach to a sweet-food reward, predisposition to emit 50 kHz USVs spontaneously, and 50 kHz USVs emission following acute systemic administration of amphetamine. Both approach motivation and predisposition to call were found to not correlate with each other but did predict 50 kHz USV response to acute amphetamine. These two behavioural phenotypes appear to represent dissociable predictors of acute amphetamine-induced emission of 50 kHz USVs in a non-sensitization paradigm. In contrast to that, a measure of sucrose preference was not found to predict 50 kHz USV emission following amphetamine. Acute amphetamine was also found to increase average sound frequency of emitted USVs and selectively increase the proportion of Trill subtype 50 kHz USVs. Together, these data demonstrate that acute amphetamine-induced 50 kHz USVs in the adult rat represent more than just a univariate motivational state and may represent the product of dissociable subsystems of emotional behavior.

Pharmacology of Ultrasonic Vocalizations in adult Rats: Significance, Call Classification and Neural Substrate.

Pharmacological studies of emotional arousal and initiation of emotional states in rats measured by their ultrasonic vocalizations are reviewed. It is postulated that emission of vocalizations is an inseparable feature of emotional states and it evolved from mother-infant interaction. Positive emotional states are associated with emission of 50 kHz vocalizations that could be induced by rewarding situations and dopaminergic activation of the nucleus accumbens and are mediated by D1, D2, and partially D3 dopamine receptors. Three biologically significant subtypes of 50 kHz vocalizations have been identified, all expressing positive emotional states: (1) flat calls without frequency modulation that serve as contact calls during social interactions; (2) frequencymodulated calls without trills that signal rewarding and significantly motivated situation; and (3) frequency-modulated calls with trills or trills themselves that are emitted in highly emotional situations associated with intensive affective state. Negative emotional states are associated with emission of 22 kHz vocalizations that could be induced by aversive situations, muscarinic cholinergic activation of limbic areas of medial diencephalon and forebrain, and are mediated by M2 muscarinic receptors. Two biologically significant subtypes of 22 kHz vocalizations have been identified, both expressing negative emotional sates: (1) long calls that serve as alarm calls and signal external danger; and (2) short calls that express a state of discomfort without external danger. The positive and negative states with emission of vocalizations are initiated by two ascending reticular activating subsystems: the mesolimbic dopaminergic subsystem as a specific positive arousal system, and the mesolimbic cholinergic subsystem as a specific negative arousal system.

The ascending mesolimbic cholinergic system--a specific division of the reticular activating system involved in the initiation of negative emotional states.

The review summarizes evidences from extensive studies suggesting that ascending mesolimbic cholinergic system (AMCS) that terminates in vast areas of forebrain and diencephalic limbic areas is responsible for specific generation of aversive arousal and aversive emotional state. This state is accompanied by emission of threatening and/or alarming vocalizations that served as a quantitative measure of the emotional response. The AMCS originates from the cholinergic neurons within the laterodorsal tegmental nucleus that have widespread and diffuse ascending connections. Activity of the AMCS induced by activation of the muscarinic cholinergic receptors in the terminal fields of this system, or by glutamate stimulation of neurons of the laterodorsal tegmental nucleus, brings about aversive state with alarming vocalizations. It is postulated that release of acetylcholine from the terminals of the AMCS in the vast areas of the forebrain and diencephalon serves as the initiator of the aversive emotional state with concomitant manifestations and alarming vocal signaling. It is concluded that the AMCS serves as a specific physiological, psychological, and social arousing and alarming system.

Rats selectively bred for low levels of play-induced 50 kHz vocalizations as a model for autism spectrum disorders: a role for NMDA receptors.

Early childhood autism is characterized by deficits in social approach and play behaviors, socio-emotional relatedness, and communication/speech abnormalities, as well as repetitive behaviors. These core neuropsychological features of autism can be modeled in laboratory rats, and the results may be useful for drug discovery and therapeutic development. We review data that show that rats selectively bred for low rates of play-related pro-social ultrasonic vocalizations (USVs) can be used to model social deficit symptoms of autism. Low-line animals engage in less social contact time with conspecifics, show lower rates of play induced pro-social USVs, and show an increased proportion of non-frequency modulated (i.e. monotonous) ultrasonic vocalizations compared to non-selectively bred random-line animals. Gene expression patterns in the low-line animals show significant enrichment in autism-associated genes, and the NMDA receptor family was identified as a significant hub. Treatment of low-line animals with the NMDAR functional glycine site partial agonist, GLYX-13, rescued the deficits in play-induced pro-social 50-kHz USVs and reduced monotonous USVs. Since the NMDA receptor has been implicated in the genesis of autistic symptoms, it is possible that GLYX-13 may be of therapeutic value in the treatment of autism.

Ethotransmission: communication of emotional states through ultrasonic vocalization in rats.

Adult rats emit two categories of ultrasonic vocalizations, 22 kHz calls and 50 kHz calls. These vocalizations communicate animal's emotional state to other members of the social group. Production of social vocalizations is an evolutionary old activity in vertebrates, and is regulated by well-preserved brain circuitries. The 22 kHz calls express negative, aversive state and are initiated by activity of the mesolimbic cholinergic system originating from laterodorsal tegmental nucleus. The 50 kHz calls express positive, appetitive state and are initiated by activity of the mesolimbic dopaminergic system originating from the ventral tegmental area. The 22 kHz calls serve as warning and alarm calls, while the 50 kHz calls serve as affiliative and social-cooperating calls. These specie-specific vocalizations play role of ethological transmitters, termed ethotransmitters, that is, they are species-specific signals that are selectively recognized by receivers and have capability of changing emotional state of the receivers.

Quinpirole-induced 50 kHz ultrasonic vocalization in the rat: role of D2 and D3 dopamine receptors.

The goal of the present investigation was to study a full dose-response of quinpirole in production of species-specific 50 kHz ultrasonic vocalizations in rats, and to study involvement of D2 and D3 dopamine receptors in this response. Quinpirole, a D2/D3 dopamine agonist with high selectivity for D2 dopamine receptors, was injected into the shell of the nucleus accumbens. Quinpirole induced species-specific 50 kHz ultrasonic vocalizations at a wide range of doses as compared to saline. The dose-response study showed a triphasic effect of quinpirole and reached two comparable peak responses in the number of emitted vocalizations at 0.25 µg and 6 µg, respectively (a 24-fold dose difference). These two peaks were separated by a decreased phase. A medium dose range (0.5-1.0 µg) of quinpirole consistently depressed production of calls to the control level. Application of antagonists of D2 dopamine receptors (raclopride) and D3 dopamine receptors (U-99194A) before quinpirole revealed that quinpirole activates differentially the D2 and D3 dopamine receptors at different doses. The vocalization response induced by the low dose of quinpirole (0.25 µg) was antagonized by local pretreatment with the D3 receptor antagonist but not by the D2 receptor antagonist. On the other hand, the response induced by the high dose of quinpirole (6 µg) was antagonized by a similar local pretreatment with the D2 receptor antagonist but not by the D3 receptor antagonist. In conclusion, the results indicated that quinpirole can induce 50 kHz vocalizations after its direct intra-accumbens application in rats, and both D2 and D3 dopamine receptors are involved in the response. They play, however, different functional roles, as revealed by the triphasic effect of increasing doses of quinpirole.

Activity of cholinergic neurons in the laterodorsal tegmental nucleus during emission of 22kHz vocalization in rats.

It has been postulated that the ascending cholinergic tegmental system is responsible for the initiation of the aversive emotional state with a concomitant alarm vocalization in the rat. It is assumed that the activity of cholinergic neurons of the laterodorsal tegmental nucleus (LDT) will cause release of acetylcholine in the target areas and will initiate the emission of 22kHz vocalizations. The goal of the present study was to test the hypothesis that the cholinergic neurons of the LDT increase their activity during emission of 22kHz alarm calls. Vocalizations were induced by an air puff or by intrahypothalamic-preoptic injection of carbachol. The activity of the LDT cholinergic neurons was studied by a double histochemical labelling for choline acetyltransferase, as a marker of cholinergic somata, and for c-Fos protein, as a marker of cells with heighten metabolic activity. Both air puff stimulation and intracerebral carbachol induced comparable 22kHz alarm vocalizations. The activity of neurons in the LDT was significantly higher during prolonged emission of 22kHz alarm calls induced by air puff or injection of carbachol than in the non-vocalizing or low-vocalizing controls. There were approximately two times more of all c-Fos-labelled cells in the LDT of vocalizing animals and 2.5 times more active cholinergic neurons during prolonged 22kHz vocalization than in the control conditions without vocalization. However, the active cholinergic neurons constituted only a small proportion of all active LDT cells (2.3%). At the same time, there were no significant increases in the number of c-Fos-labelled cells in the neighbouring pedunculopontine nucleus (PPT). These findings lead to the conclusion that the neurons of the LDT, including cholinergic neurons, but not those of the PPT, significantly increased their activity during prolonged emission of alarm vocalizations, as evidenced by the c-Fos immunoreactivity.

Motor and locomotor responses to systemic amphetamine in three lines of selectively bred Long-Evans rats.

The goal of the study was to measure spontaneous and amphetamine-induced motor and locomotor activity in three selectively bred lines of male Long-Evans rats. The number of 50 kHz ultrasonic vocalizations (USVs) emitted in response to heterospecific play with human hand ("tickling") had been measured daily in these lines of rats from 21 to 24 days of age, as a criterion for dividing them into high vocalizing line, low vocalizing line, and random breeding and testing lines. This study sought to determine whether selection of rats based on their affective social-vocalizations also had effects on their locomotor performance and sensitivity to amphetamine. In this study adult animals from the 25th generation (with no further selection) were tested. The results showed that rats, which were selectively bred to emit high numbers of 50 kHz vocalizations, also exhibited elevated levels of spontaneous locomotor activity. After systemic injection of d-amphetamine (1.5mg/kg), the level of motor and locomotor activity significantly increased further in all the lines as compared to saline controls. The horizontal and vertical activities and the distance covered by rats of the high line, both at the baseline and after amphetamine challenge, were significantly higher than those of the low line animals in absolute scores but not as proportion of relevant saline controls. Since appetitive 50 kHz USVs and locomotor activity are both dependent on the activity of the dopamine system, it is concluded that selection of rats based on the expression of their positive emotional state is also selecting other features than vocalization, namely locomotor behavior. This may help explain why these animals are relatively resistant to depressogenic manipulations.

A novel NMDA receptor glycine-site partial agonist, GLYX-13, has therapeutic potential for the treatment of autism.

Deficits in social approach behavior, rough-and-tumble play, and speech abnormalities are core features of autism that can be modeled in laboratory rats. Human twin studies show that autism has a strong genetic component, and a recent review has identified 99 genes that are dysregulated in human autism. Bioinformatic analysis of these 99 genes identified the NMDA receptor complex as a significant interaction hub based on protein-protein interactions. The NMDA receptor glycine site partial agonist d-cycloserine has been shown to treat the core symptom of social withdrawal in autistic children. Here, we show that rats selectively bred for low rates of play-induced pro-social ultrasonic vocalizations (USVs) can be used to model certain core symptoms of autism. Low-line animals engage in less social contact time with conspecifics, show lower rates of play induced pro-social USVs, and show an increased proportion of non-frequency modulated (i.e. monotonous) ultrasonic vocalizations, compared to non-selectively bred random-line animals. Gene expression patterns in the low-line animals show significant enrichment in autism-associated genes and the NMDA receptor family was identified as a significant hub. Treatment of low-line animals with the NMDAR glycine site partial agonist GLYX-13 rescued the deficits in play-induced pro-social 50-kHz and reduced monotonous USVs. Thus, the NMDA receptor has been shown to play a functional role in autism, and GLYX-13 shows promise for the treatment of autism. We dedicate this paper to Ole Ivar Lovaas (May 8, 1927-August 2, 2010), a pioneer in the field of autism.