On the basis of our initial neurochemical studies (Mittleman et al. 2008), we identified two potential neuronal circuits originating in the DN by which Purkinje cell activity in the cerebellum may modulate dopamine release in the mPFC (Fig. 11.2). With the exception of the flocculonodular lobe which projects to vestibular nuclei in the brainstem, GABA-containing Purkinje cells in the cerebellar cortex project ipsilaterally to three deep cerebellar nuclei (dentate, interpositus, and fastigial) where they make on the order of 1000 contacts each with several types of nuclear cells (Teune et al. 1998, 2000; Llinas et al. 2004; Ruigrok 2011). Excitatory inputs entering the cerebellum as mossy fibers excite neurons in deep cerebellar nuclei via collaterals and then excite granule cells that stimulate Purkinje cells. The inhibitory output of Purkinje cells on deep cerebellar nuclei, in turn, modulates spatial and temporal patterns of activity of neurons within these nuclei, and therefore the output to various brain regions.

Deep cerebellar nuclei projections are generally very widespread and may be found throughout the entire brain stem, including the thalamus (Teune et al. 2000; Ruigrok 2011). With respect to the first cerebellar-mPFC circuit, the DN and the interpositus nucleus both have in common excitatory glutamatergic neurons that send contralateral projections to reticulo-tegmental nuclei of the pons (RTN) in a topographical manner (also known as the nucleus reticularis tegmenti pontis) (Angaut et al. 1985; Torigoe et al. 1986; Teune et al. 2000; Schwarz and Schmitz 1997). In addition to the RTN providing reciprocal excitatory glutamatergic projections back to the DN and interpositus nucleus (Cicirata et al. 2005; Mihailoff 1993; Gerrits and Voogd 1987), the RTN also sends rostral neuronal projections to pedunculopontine tegmental nuclei (PPT), which provide reciprocal projections back to the RTN (Garcia-Rill et al. 2001; Reese et al. 1995; Vertes et al. 1986). Excitatory cholinergic and glutamatergic neurons, including neurons co-containing glutamate and acetylcholine , within the rostral and caudal aspects of the PPT project to dopamine-containing cells in the ventral tegmental area (VTA) and substantia nigra compacta (SNc) (Oakman et al. 1995, 1999; Lavoie and Parent 1994). Electrophysiological studies have shown that neurons in the RTN respond with prolonged spiking activity (up to 12 s) to stimulation of the PPT (Garcia-Rill et al. 2001). This sustained activity of RTN neurons provides reciprocally sustained activation of PPT glutamatergic/cholinergic afferents to dopaminergic cells in the VTA and SNc. These findings are consistent with those showing in rodents that a similar period of electrical stimulation of the PPT effectively enhances neurotransmission in both midbrain mesocortical and nigrostriatal dopaminergic systems (Blaha and Winn 1993; Blaha et al. 1996; Forster et al. 2002; Forster and Blaha 2003). Dopaminergic neurons in the VTA comprising the mesocortical dopaminergic system project mainly to limbic associative cortices such as anterior cingulate, insular, piriform, perirhinal, and entorhinal cortices, including the mPFC (Björklund and Lindvall 1984; Berger et al. 1988, 1991; Gaspar et al. 1989). Thus, it is noteworthy that cortical dopamine neurotransmission, particularly in the mPFC, has been shown to be associated with autism (Ernst et al. 1997), schizophrenia (Bennett 1998; Laruell et al. 2003), emotional processing (Morrow et al. 1999), and a variety of cognitive functions including attention, working memory, and planning (Aalto et al. 2005; Gamo et al. 2010; Jackson and Moghaddam 2004; Rose et al. 2010).

It is also worth noting that, in addition to a DN-RTN-PPT-VTA circuit mediating mPFC dopamine release, injections of the anterograde tracer biocytin into deep cerebellar nuclei of squirrel monkeys reveal fine collaterals from superior cerebellar peduncle fibers terminating onto dendrites and cell bodies in the PPT that contain choline acetyltransferase (Hazrati and Parent 1992). As well, Perciavalle et al. (1989) have shown in the rat that the DN, and to a lesser extent the interpositus nucleus, may provide direct contralateral projections to the VTA and dorsal region of the SNc, midbrain regions containing a high density of dopaminergic cells. Thus, it is possible that more direct circuits, such as DN-PPT-VTA or DN-VTA, may participate in modulating mesocortical dopaminergic cell activity.

A second cerebellar-mPFC neuronal circuit that may mediate dopamine release in the mPFC involves deep cerebellar nuclei glutamatergic projections to thalamic nuclei that in turn project to the cortex. Middleton and Strick (1997, 2001) have shown in cebus monkeys that cerebellar projections to prefrontal, oculomotor, and skeletomotor areas of cortex via contralateral thalamic relay nuclei are derived from topographically distinct regions of the ventral DN comprising 60 % of the volume of this cerebellar nuclei. In particular, using retrograde labelling, these investigators demonstrated that the DN to mPFC projections, via mediodorsal and ventrolateral thalamic nuclei (ThN md and ThN vl, respectively), target exclusively prefrontal cortex (PFC) areas 9 and 46-dorsal, while primarily avoiding ventral PFC areas 12 and 46-ventral. Interestingly, the majority of labelled neurons (90–100 %) were found in the DN, and only a small number (0–6 %) were located in the interpositus and fastigial nuclei. Similar findings of near exclusive projections to thalamic nuclei from DN, compared to fastigial and interpositus nuclei, have been reported in cynomolgus monkeys (Erickson et al. 2004).

In the rodent there are four structural divisions of the mPFC: the medial agranular (AGm), anterior cingulate (AC), prelimbic (PL), and infralimbic (IL) cortices. The dorsal mPFC (AGm and AC) receive mainly sensorimotor input, whereas ventral mPFC (PL and IL) receive primarily limbic input. Thus, integration of information from the thalamus , including other subcortical structures, in the dorsal mPFC is thought to mediate goal directed actions. In contrast, the PL region of the ventral mPFC is positioned to serve a direct role in cognitive functions homologous to dorsolateral PFC of primates, whereas IL region of the ventral mPFC appears to represent a visceromotor center homologous to the orbitomedial PFC of primates (Hoover and Vertes 2007). In this regard, a number of tract tracing studies in rats have shown that thalamic nuclei, primarily ThN md compared to ThN vl, send glutamate-containing projections (Pirot et al. 1994; Pinto et al. 2003) predominantly to the limbic PL region of the ventral mPFC, compared to sensorimotor AGm or AC regions of the dorsal mPFC (Condé et al. 1990; Hoover and Vertes 2007). The PFC then projects back to the cerebellum via pontine nuclei completing a cerebellum-cortical- cerebellum loop (Kelly and Strick 2003; Middleton and Strick 2000).

At the cellular level, in rodents, primates, and humans, VTA dopaminergic afferents provide dense input to the deep layers V–VI in the mPFC, while the density of dopamine innervation in the superficial layers I–III varies across species (relatively sparser in rodents). Dopamine terminals in the mPFC form symmetric synapses on dendritic spines and shafts of pyramidal neurons in layers V–VI. Many of the postsynaptic spines innervated by dopamine terminals also receive asymmetric excitatory amino acid (glutamate) terminals forming a synaptic “triad” whereby pre- and postsynaptic modulation between dopamine and excitatory afferents to pyramidal cells, including GABAergic interneurons, may occur (Goldman-Rakic et al. 1989; Verney et al. 1990; Smiley and Goldman-Rakic et al. 1989; Carr and Sesack 1996; Paul et al. 2013). Indeed, morphological studies have shown that glutamate-containing terminals are in close apposition to dopamine-containing terminals in the mPFC (Pinto et al. 2003; Del Arco and Mora 2005), suggesting that glutamate exerts a local modulation of mPFC dopamine release. In support of this notion, reverse microdialysis of glutamate agonists and antagonists into the PFC has been shown to increase and decrease mesocortical dopaminergic transmission by activation and inhibition of ionotropic glutamatergic receptors in the PFC (Feenstra et al. 1995; Jedema and Moghaddam 1994). For example, antagonism of AMPA glutamate receptors in the PFC profoundly reduces dopamine release in the PFC suggesting that basal output of mesocortical dopamine is under tonic excitatory control from glutamatergic input to the PFC (Takahata and Moghaddam 1998). Although unlikely, an additional mechanism by which the cerebellum mediates dopamine release in the mPFC may involve thalamic activation of descending mPFC glutamatergic projections to the PPT and/or VTA that, in turn, stimulate mesocortical dopaminergic neurons to release dopamine in the mPFC (Overton and Clark 1997; Del Arco and Mora 2009).

As noted above, the mPFC receives dense dopaminergic projections from the VTA that are essential for optimal cognitive function . A number of studies have suggested that dopamine inputs and GABAergic interneuron interactions with cortical pyramidal neurons serve to fine-tune recurrent excitation in mPFC networks that may ultimately underlie working memory and executive function (Sawaguchi and Goldman-Rakic 1991; Rao et al. 2000; Constantinidis et al. 2002). Electrophysiological studies in rat brain slices have shown that bath applied dopamine evokes a complex, temporally biphasic effect on inhibitory synaptic responses in the mPFC, as exhibited by an initial suppression, followed by a long-lasting facilitation of evoked IPSCs (Seamans et al. 2001a; Seamans and Yang 2004; Trantham-Davidson et al. 2004). The initial suppression of GABA interneuron IPSCs in the mPFC is thought to occur via activation of postsynaptic D2 receptors. The longer latency enhancement of IPSCs has been shown to be mediated by D1 receptors that increase the excitability of presynaptic GABAergic interneurons that ultimately modulate postsynaptic activity of cortical pyramidal cells (Trantham-Davidson et al. 2004). A similar D2-mediated initial suppression followed by a D1-mediated long-lasting facilitation has also been observed for excitability of cortical pyramidal cells and thus may be a common feature of dopaminergic modulation of mPFC neurons (Yang and Seamans 1996; Gulledge and Jaffe 1998; Gorelova and Yang 2000; Henze et al. 2000). In addition, D2 agonists have been shown to reduce NMDA glutamate receptor EPSCs, whereas D1 agonists increase them in mPFC neurons (Zheng et al. 1999; Seamans et al. 2001b) .

Yang and co-workers have termed the D2-mediated initial suppression as the D2-dominated network dynamic “state 1” (Seamans et al. 2001a; Seamans and Yang 2004) and have suggested that state 1 may be important in situations requiring response flexibility, during which many options for action must be held in memory and compared. Synaptically located D2 receptors may respond first to the high phasic dopamine levels evoked by a particularly novel or salient stimulus. This would create a state 1 (transient) dynamic before dopamine diffuses into the extrasynaptic space, activating D1 receptors to initiate a state 2 (long-lasting) dynamic (Seamans and Yang 2004). Thus, the initial D2-mediated transient attenuation of IPSCs may function to reset PFC networks so that the robustness-enhancing effects of D1 receptor activation are reversed momentarily, allowing new information easy access to working memory buffers and the establishment of new goal-state representations. Under these conditions, state 1 would favor weak activation of a widespread cortical area, whereas state 2 would favor heightened “tuned” activation of a smaller and more focused cortical region (Durstewitz et al. 2000; Seamans et al. 2001a; Seamans and Yang 2004) .

As such, dopamine does not provide PFC networks with specific information but rather sets them into one of two processing states. In either state, working memory buffers remain able to retain and use information transiently. Glutamatergic and GABAergic systems appear to be critical for the actual generation of persistent activity patterns (Seamans et al. 2003). Dopamine only biases the expression of this activity, making glutamate- and GABA-mediated persistent activity more robust (state 2) or weaker yet more receptive to subsequent inputs (state 1). In this way dopamine is a true neuromodulator, acting on glutamate and GABA systems, biasing the way they process and maintain information in working memory, although providing no information itself to be held in working memory .

Our initial neurochemical results indicating a dysregulation of cerebellar mediated dopamine transmission in the mPFC of lurcher mutant mice (Mittleman et al. 2008) provided a potential mechanism whereby developmental cerebellar damage and accompanying alterations in cerebellar-mPFC neuronal circuitry could lead to some of the behavioral deficits typically observed in ASD (Nieoullon 2002). In order to further investigate the hypothesis that developmental cerebellar damage could result in behavioral deficits we specifically targeted two behaviors in which patients with ASD have prominent defects; repetitive behavior and executive function. Specifically, it has been reported that repetitive and restrictive behaviors are a major behavioral component of ASD (e.g. Hutt and Hutt 1968; Bodfish et al. 2000; Militerni et al. 2002). In autism, repetitive behavior may function to control behavior-environment relations such as access to reinforcers, or to gain access to or eliminate particular types of stimulation such as hyperarousal (Ridley 1994; Kennedy et al. 2000). Hyperactivity often accompanies repetitive behavior in developmental disorders with approximately 30 % of ASD children comorbid for attention deficit/hyperactivity disorder (ADHD; Simonoff et al. 2008). Thus, the common goal of the behavioral experiments was to determine if mice showed deficits similar to those observed in ASD, and more importantly to determine if these deficits related to Purkinje cell number .

In order to explore the relationship between repetitive behavior and cerebellar Purkinje cell number we used chimeric mice (Lc/+ ↔ +/+) with varying proportions of cells from the wildtype +/+ and heterozygous Lc/+ genotypes (Goldowitz et al. 1992). Depending on incorporation of the wildtype lineage, 50 % of the chimeric mice would be expected to be truly chimeric, or of a mixed genotype (Lc/+ ↔ +/+). Thus total cerebellar Purkinje cell numbers in chimeric animals would be expected to vary, while very few Purkinje cells (< 200) would be expected in Lc/+ mice. In contrast, +/+ mice would be expected to have a full complement of Purkinje cells (> 150,000). Total numbers of cerebellar Purkinje cells were determined at the conclusion of the experiment by histological analysis of cerebellar Purkinje cell counts (Martin et al. 2010).

Chimeric mice were tested in two behavioral paradigms involving unconditioned ambulatory activity during exposure to a novel open field, and conditioned lever pressing in progressive ratio (PR) responding. The PR task is a commonly used lever-press task that programs a systematic increase in the value of the fixed-ratio (FR) requirement following reinforcer delivery. Subjects are free to lever-press as frequently or infrequently as they choose, but must make progressively more lever-presses to obtain successive reinforcers. In this case, subjects were reinforced for lever-pressing under an arithmetically increasing FR 5 schedule of reinforcement (i.e. 5, 10, 15, 20, etc lever-presses were necessary to obtain reinforcement), using a procedure similar to that described by Hodos and Kalman (1963). An experimental session terminated when the subject failed to lever-press for 5 consecutive minutes. The last ratio completed (reinforced) is known as the subject’s breakpoint and is typically followed by a pause in responding known as the post-reinforcer pause (PRP). As repetitive behavior can take any form and occur in virtually any environment (Kennedy et al. 2000), we reasoned that this task would allow us to explore the role of cerebellar pathology in repetitive behaviors similar to those observed in autism.

Figure 11.3 shows the amount of ambulatory activity that occurred during a 1 h exposure to a novel open field as a function of Purkinje cell numbers in chimeric mice. A significant negative relationship between Purkinje cell numbers and ambulation was observed in that chimeric mice with low numbers of Purkinje cells were progressively more active than mice with higher numbers of these cells.

Figure 11.4a and b describes the relationship between Purkinje cells and repetitive lever-pressing in the PR task in Lc/+ ↔ +/+ chimeras. Within the group of chimeric mice breakpoint was significantly and negatively correlated with the number of Purkinje cells in the cerebellum . We believe that the increases in lever-pressing that resulted in higher breakpoints in chimeric mice represent a repetitive and stereotyped behavior. As defined by Ridley (1994), stereotypy, in contrast to perseveration, refers to “the excessive production of one type of motor act, or mental state, which necessarily results in repetition.” In our case, Purkinje cell deficient mice were trained in a repetitive task to associate lever pressing with delivery of a primary reward. The significant positive relationship between Purkinje cell number and post-reinforcement-pause (Fig. 11.4b) demonstrates the increased focus of these mice on lever-pressing, as those chimeras with the fewest Purkinje cells, and therefore the highest breakpoints, paused for the shortest amount of time following reinforcement. This experiment supported the hypothesis that developmental cerebellar damage could result in behavioral deficits, specifically by demonstrating similar relationships between cerebellar Purkinje cell number, hyperactivity and repetitive behavior.

In the next experiment, we investigated the possibility that Purkinje cell number was also related to the degree of deficits in executive function. Executive function is an umbrella term for the group of closely linked, high level cognitive skills which enable the effective execution of goal-directed behaviors (Hughes et al. 1994; Pennington and Ozonoff 1996). These skills, which include working memory, inhibition, and behavioral flexibility, have consistently been shown to be dependent on the PFC in humans, non-human primates, and rodents (Dalley et al. 2004; Robbins and Arnsten 2009; Robbins and Roberts 2007). Cognitive theories of autism suggest that fundamental deficits in executive function may underlie the clinically significant symptoms of this disorder (Pennington and Ozonoff 1996; Hill 2004).

In this study, we assessed the impact of variable developmental cerebellar Purkinje cell loss on behavioral flexibility using a chimeric mouse model (Lc/+ ↔ +/+). Specifically we investigated the acquisition and serial reversals of a conditional visual discrimination task, one type of reversal learning task used to assess behavioral flexibility in rodents. Reversal learning tasks measure behavioral flexibility by assessing the ability of the subject to adapt its behavior following a reversal of stimulus-reward or stimulus-response contingencies, and have been shown to depend heavily on the PFC (Clark et al. 2004; Ragozzino 2007). Two stages are involved in reversal learning; inhibition of a previously rewarded strategy and acquisition of a new strategy (Mackintosh 1974). Normal animals and humans typically show improvement in reversal learning across reversals. Thus, as perseverative responses decline, a faster rate of switching to the new response occurs. Essentially the subject learns that if the current response is incorrect, the other response must be correct. In this experiment, mice initially were trained in a simultaneous (simple) visual discrimination using procedures of Jones et al. (1991) of the type “respond where the light is” (cue light = S+) or “respond where the light is not” (cue light = S−). This task can be learned simply by approaching cues predictive of reward and thus requires no response-rule learning. Once the criterion for acquisition was met, mice were then tested on a series of reversals of the discrimination contingency (e.g. if they learned, “respond where the light is,” they then must learn, “respond where the light is not”). The acquisition of reversals of stimulus-reward reversals is captured in two types of errors. Errors committed during sessions in which performance was below chance levels (≤ 40 % correct) were classified as Perseverative errors, suggesting that during these sessions mice continued to respond according to the response rules of the prior stage. Errors committed during sessions in which performance was above chance levels (41–85 % correct) were classified as Learning errors, suggesting that mice had shifted away from the use of the response rules of the prior stage.

All mice learned the initial visual discrimination and were then tested over 4 stimulus-reward reversals. Figure 11.5 shows the relationship between learning errors and Purkinje cell numbers in chimeric mice during reversals 3 and 4. Comparison of Purkinje cell number and performance in individual mice revealed that chimeras with fewer Purkinje cells committed more Learning errors than those with higher numbers of these cells. These data suggest that developmental cerebellar Purkinje cell loss can affect higher level cognitive processes which have previously been shown to be mediated by the mPFC, are disrupted in patients bearing cerebellar lesions (Bellebaum and Daum 2007), and are commonly deficient in ASD (Hill 2004; Pennington and Ozonoff 1996).

As our behavioral experiments, together with the neurochemical results of Mittleman et al. (2008), strongly suggested that mPFC function was altered in direct relationship to the number of cerebellar Purkinje cells, and that Purkinje cell loss could disrupt dopamine transmission in the mPFC, we next investigated the relative contribution of each of the two candidate neuronal circuits in modulating mPFC dopamine release (Rogers et al. 2011) . We again used in vivo fixed potential amperometry in combination with carbon-fiber microelectrodes (as in Mittleman et al. 2008) to monitor mPFC dopamine release evoked by DN electrical stimulation (100 cathodic monophasic pulses, 400 μA intensity, 0.5 ms pulse duration) at 50 Hz every 60 s before and during individual microinfusions of the local anesthetic lidocaine, the ionotropic glutamate receptor antagonist kynurenate, and the control (drug vehicle) solution phosphate buffered saline (PBS) into the ThN md, ThN vl, or VTA of urethane anesthetized lurcher wildtype (+/+) mice.

As shown in Fig. 11.6, the DN to mPFC pathway via dopaminergic neuronal cell bodies in the VTA was decreased by either lidocaine or kynurenate by ~ 50 %, while the DN to mPFC via thalamic nuclei was also decreased similarly by either lidocaine or kynurenate by ~ 50 % (35 % in the ThN md and 16 % in the ThN vl). Thus, the sum of the percent decreases occurring in each of these two pathways accounted for 100 % of DN stimulation-evoked dopamine release recorded in the mPFC. PBS infusions resulted in relatively minimal site-specific average percent decreases of 2.8 % ± 1.3 %, 2.0 % ± 1.1 % and 0.8 % ± 1.3 % for VTA, ThN md, and ThN vl, respectively. The results of this study, therefore showed that in lurcher wildtype mice, the pathway through the thalamus (ThN md and ThN vl) and the pathway through the VTA each contribute 50 % of the modulatory influence on mPFC dopamine neurotransmission. Additionally, reductions in dopamine release following lidocaine or kynurenate infusions were not significantly different which indicates that neuronal cells in the VTA and ThN were activated primarily if not entirely by glutamatergic inputs .

As these results indicated an equal contribution of the thalamic and VTA pathways to mPFC dopamine release, and those of Mittleman et al. (2008) showed that mPFC dopamine release was reduced in lurcher (Lc/+) mutant mice, the next experiment investigated the relative contributions of these pathways in Lc/+ mice (Rogers et al. 2013a). The same methods as Rogers et al. (2011) were used. Figure 11.7 (top panel) shows that Lc/+ mice exhibit a significant reduction in DN stimulation evoked dopamine release in the mPFC. Figure 11.7 (bottom panel) also shows that in Lc/+ mice there was a functional reorganization of the VTA and thalamic pathways mediating cerebellar modulation of mPFC dopamine release. Following electrical stimulation of the DN, inactivation of the VTA pathway by intra-VTA lidocaine or kynurenate infusions decreased dopamine release by 50 % in wildtype and 30 % in Lc/+ mutant mice. Intra-ThN vl infusions of either drug decreased dopamine release by 15 % in wildtype and 40 % in mutant mice, while dopamine release remained relatively unchanged following intra-ThN md drug infusions. These results indicated a shift in strength towards the thalamic vl projection, away from the VTA. These results were consistent with the idea that developmental loss of cerebellar Purkinje cells results in a reorganization of the neuronal pathways mediating mPFC dopamine release that ultimately contributes to a reduction in mPFC dopamine release .

One limitation of these results is that, although lurcher mutant mice represent a relatively good and reasonably selective model of cerebellar Purkinje cell loss, there is no known human equivalent to this genetic condition (i.e. 100 % Purkinje cell loss). This observation necessarily limits the applicability of this mouse model to ASD, although both share the common feature of cerebellar Purkinje cell loss. In order to overcome this limitation we replicated the previous experiment using Fmr1 mutant and wildtype mice . Fragile X syndrome is one of the forms of ASD with a known monogenetic cause, the loss of function of the Fmr1 gene. Fmr1-KO (knockout) mice have an Fmr1 ^tm1Cgr targeted mutation and are widely used as a mouse model of Fragile X syndrome (Goodrich-Hunsaker et al. 2011; Verkerk et al. 1991; Rogers et al. 2013b). These KO “mutant”mice display several behavioral deficits similar to those seen in ASD such as hyperactivity, perseverative behavior, reduced spatial learning abilities, memory deficits, and reduced fear conditioning (Bernardet and Crusio 2006; Ey et al. 2011; Olmos-Serrano and Corbin 2011). Fmr1 mutant mice display cerebellar abnormalities such as elongated Purkinje cell spines and decreased volume of deep cerebellar nuclei, which may also be indicative of reduced cerebellar output (Koekkoek et al. 2005; Ellegood et al. 2010). Abnormalities also exist in the PFC of Fmr1 mutant mice. For example, layer 5 pyramidal neurons display hyperconnectivity in the mPFC, and synapses between these neurons are not able to recover from long term depression as rapidly as the same neurons in control mice (Testa-Silva et al. 2011).

The electrical stimulation and neurochemical recording methods of Mittleman et al. (2008) and Rogers et al. (2011) were used. We initially compared the magnitude of dentate stimulation-evoked dopamine release in Fmr1 mutant and wildtype mice. Figure 11.8 (top panel) shows that, like Lc/+ mice, Fmr1 mutants showed a significant reduction in mPFC dopamine release evoked by stimulation of the DN. Figure 11.8 (bottom panel) also indicates that in Fmr1 mutant mice there was a functional reorganization of the VTA and thalamic pathways mediating cerebellar modulation of mPFC dopamine release that was very similar to that observed in Lc/+ mice (see Fig. 11.7). Inactivation of the VTA pathway by intra-VTA lidocaine or kynurenate infusions decreased dopamine release by 50 % in wildtype and 20 % in Fmr1 mutant mice. Intra-ThN vl infusions of either drug decreased dopamine release by 15 % in wildtype and 40 % in Fmr1 mutants, while evoked dopamine release remained relatively unchanged following intra-ThN md drug infusions. As in Lc/+ mice these results indicated a shift in strength towards the thalamic vl projection, away from the VTA in Fmr1 mutant mice. These results further indicated that the results observed in Lc/+ mice could be generalized to another mouse strain that exhibited developmental cerebellar neuropathology of a less severe nature. Both showed a similar reduction in mPFC evoked dopamine release suggesting a similar reorganization of the neuronal pathways mediating DN stimulation-evoked mPFC dopamine release (Rogers et al. 2011, 2013a). In this regard it should be additionally noted that Fmr1 KO mice also show deficits in serial reversal learning that are similar to those observed in lurcher chimeras (Dickson et al. 2013).