Discussion, © Callie Clark, 2008

CHAPTER FIVE

DISCUSSION




DISCUSSION

Hypothesis One: Orienting in the Haloperidol-treated Rat
Our first study examined the orienting responses of saline- and haloperidol-treated rats while in a quiet resting state. We observed a characteristic three-stage orient in normal rats following sounds that were activating, including key jingle, crinkling chip bag, and ultrasonic vocalization. The presence of Orient, Attend, and Disengage phases were absent in haloperidol-treated rats, a finding consistent with studies suggesting that syntactic grooming chains are often incomplete in rats with striatal damage (Whishaw and Berridge, 1992), and that the expression of normal behaviour, whether due to movement or reward processing deficits, is abnormal in haloperidol-treated rats (Teitelbaum, Schallert, and Whishaw, 1983). We did not find a cephalocaudal orient to single frequencies in the normal or cataleptic rat. We also did not document larger orienting responses of the body to certain frequencies more often than others. A general conclusion that can be made is that cataleptic rats were more responsive to auditory stimuli comprised of multiple frequencies with complex timbre than to simple and less rhythmic tones. These findings support the theory that the intact BG maintain the expression of motor chains through internal cueing, and that although haloperidol is thought to be “selective” for the indirect pathway, it has a range of behavioural and physiological effects throughout the central nervous system.

The current research made use of a cephalocaudal score that was applied during both the orienting and righting tasks. A cephalocaudal principle of motor recovery can be seen in infant rats as they explore their surroundings and also in recovery following LH€ lesions (Golani et al., 1981; Teitelbaum, Schallert, and Whishaw, 1983). Lesions to the hypothalamus can also produce a period of acute catalepsy before eating and locomotion resume (Golani et al., 1981; Teitelbaum, Schallert, and Whishaw, 1983). Additionally, cephalocaudal recovery can be seen during haloperidol-induced catalepsy, as documented in the current experiments. By applying a cephalocaudal scale to the responses observed, we gained insight not only into the presence or absence of movement, but could demonstrate subtle differences in movement that would otherwise be missed by temporal measures of orienting or righting alone. To complement the cephalocaudal five-point score used in this thesis, a modified Eshkol-Wachman Movement Notation (EWMN; Eshkol and Wachman, 1958) score was used to describe movement components during orienting and righting tasks. Of special interest is the assigning of “heavy” and “light” limbs in EWMN. The orienting and righting responses, in particular, used in this thesis were adaptable to an EWMN-like movement analysis system, as the elicited cephalocaudal movements were characterized by lateral rotation at the shoulder or hip and followed by independent movements of the lighter limbs.

The current study differentiated movement associated with the acoustic startle response from that of sound-induced orienting and release from catalepsy. The acoustic startle reflex relies on low-level auditory processing and alerts the animal to an impending object within the environment (Sokolov, 1963). Conversely, the neural and motor responses to key jingle, for example, are more robust and complex. At the level of the inferior and superior colliculi, sensorimotor integration allows for a directed behaviour to localize a stimulus. Additionally, the activating stimuli used in the current study can be thought of as providing sensory stimulation in place of proprioception that allows for automatic behaviours to be released. It is possible that the motor experience given during the grid-climbing training reinforced cortical areas to respond with motor activation to salient auditory cues within the grid-box task. Thus, we postulate that we are looking at two different behaviours, one characterized by simple spinal reflexes, and the other characterized by the postural, proprioceptive, and auditory guidance of movement.

Hypothesis Two: Sound-induced Release from Catalepsy
Catalepsy, a model of PD akinesia in which there is an absence of spontaneous or voluntary movement, can be produced experimentally in the animal by the administration of opiates or classical neuroleptics. Further, morphine- and haloperidol-induced catalepsy produce different forms of akinesia/catalepsy (De Ryck, Schallert, and Teitelbaum, 1980). Conversely, haloperidol rats will remain immobile until physical destabilization elicits a brief jump (Teitelbaum, Schallert, and Whishaw, 1983; De Ryck, Schallert, and Teitelbaum, 1980); postural mechanisms responsible for static support are left intact. In addition to vestibular stimuli, sensory stimuli including heat and neck bandaging temporarily release animals from catalepsy (Teitelbaum et al., 1976; Teitelbaum, Schallert, and Whishaw, 1983). Haloperidol was chosen for this series of experiments because it provides an established model of parkinsonism that allows for collection of data on isolated postural systems and associated subsystems.

There is a large literature on neuroleptic-induced catalepsy. Previously, experimental catalepsy was measured in terms of latency to move from an imposed posture. Haloperidol-induced catalepsy is a behavioural state in which postural support systems remain active, to provide static equilibrium for the animal. Correspondingly, rats displaying haloperidol-induced may exhibit spontaneous release after a period of time, or movement if the allied reflexes for postural support are activated through physical displacement. Established paradigms to test spontaneous or induced release from catalepsy included the classical “bar test” and the “jump task.” First, the bar test consists of placing the rat’s forepaws upon a raised block that may be varied according to body size of the rat (Kuschinsky and Hornykiewicz, 1972). Sanberg, Ossenkopp, and Kavaliers (1996) note that handling can significantly influence sensitization to catalepsy by activating the immobility reflex with pressure at the nape. Second, the jump task involves placing a rat on a textured surface that can slowly be tilted along the anterior-posterior plane of the rat, to produce an all-or-none jump (i.e., “release from catalepsy”). When the rat is no longer able to brace against the increasing displacement, postural reflexes prompt a quick jump. The rat then resumes immobility once landed (Field, Whishaw, and Pellis, 2000). The present experiments provided a modified version of the jump task in which lateral tilt of the animal revealed a graded cephalocaudal righting response, made possible by an apparatus that supports the entire body of the animal. The current research is the first to document a graded release from haloperidol-induced catalepsy. Future work could compare the amount of displacement needed to prompt righting in the current grid-climbing task with the angle of tilt needed to elicit a jump during the jump task (Field, Whishaw, and Pellis, 2000).

We developed a modified version of the jump task that could be used to show graded vestibular- and auditory-induced release from catalepsy. Correlation analysis of vestibular- and auditory-induced righting indicated that movements prompted by key jingle were more “normal” appearing than those produced by the vestibular stimulus; the sound stimulus produced a continuous movement that was not dependent on a progressive physical stimulus. Second, movement notation analysis indicated that multiple brief presentations of key jingle (perhaps more analogous to a prolonged musical stimulus) was not more effective than a single key jingle for release from catalepsy. Third, our grid box experiments revealed that certain sounds are more effective for eliciting righting responses in haloperidol-treated rats. Namely, the key jingle was more effective than ultrasonic vocalization and sounds produced by a crumpling chip bag. The 55-kHz frequency was least effective during the righting task. Thus, this research demonstrates that central mechanisms responsible for paradoxical movement in parkinsonism respond preferentially to complex sound stimuli.

Hypothesis Three: Effect of Experience on Sound-induced Release from Catalepsy
We aimed to investigate the contribution of cued grid box experience on release from catalepsy. As noted, multiple presentations of key jingle stimulus were not more effective than a single presentation, and previous experience improved only the righting induced by the single key jingle stimulus. A second experiment indicated that rats trained and tested with chip bag righted with smaller movements than those trained and tested with key jingle. Conversely, rats trained with crumpling chip bag sounds but then tested with key jingle performed at levels equivalent to rats trained and tested with key jingle. Rats naïve to key jingle righted with smaller movements than rats pre-trained with chip bag sounds. Therefore, grid-climbing experience with a less potent sound stimulus (i.e., chip bag) facilitated a level of activation that could be utilized for testing with a more effective auditory cue (i.e., key jingle). As well, rats that were familiar with the key jingle stimulus but tested with chip bag sounds performed worse than rats that were entirely naïve to the task. Release from catalepsy induced by the ultrasonic vocalization stimulus was not improved by experience, an observation that could be expanded on in future work. One surprising finding included diminished performance with experience following 55-kHz cued trials. Lastly, A sex difference was found when key jingle was used to induce righting. Specifically, the performance of females did not reach male performance levels and training did not improve righting during the task, despite no group differences in naïve animals. It is suggested that estrogen is neuroprotective, as estrogen replacement therapy serves as a treatment for early PD (Leranth et al., 2000; Saunders-Pullman et al., 1999). As well, estrogen serves to increase DAergic tone (McDermott, Liu, and Dluzen, 1994), and thus may provide a situation in which females receive increased reinforcement of stimuli compared to males.

“Paradoxical kinesis” in haloperidol rats has previously been demonstrated in experiments using physical displacement, changes in temperature, tail pinch, and neck bandaging (Whishaw, Mittleman, and Evenden, 1989). The current findings suggest that the movement systems left active during DAergic blockade function to protect not only static stance and respond to defensive stimuli but also to regulate safety and arousal. As noted by Miklyaeva, Martens, and Whishaw (1995), rats with unilateral 6-OHDA lesions show deficits of the contralateral limbs that are likely due to an impaired ability to apply sufficient force to propel the body. Correspondingly, reflexes that are active during parkinsonian catalepsy may be a compensatory mechanism for movement initiation and due to external triggers or templates for movement. In addition, the demonstration that sensory stimuli other than vestibular displacement alone are able to elicit movement in the cataleptic rat suggest that PD akinesia functions to do more than preserve static equilibrium alone. Thus, dedicated neural systems might exist that are resistant to dopaminergic depletion and function to activate the animal in response to salient, well-trained environmental cues. Experimental factors such as previous drug administration, handling, and movements made while under haloperidol, can influence the expression of catalepsy (Sanberg, Ossenkopp, and Kavaliers, 1980).

A related aim of this research was to address the question, “What is the most potent sound for inducing movement in the normal and parkinsonian rat?” Results from our orienting and righting experiments revealed six main findings. First, normal rats do not respond with an equivalent orienting response and change in behavioural arousal to all frequencies within their hearing range. Second, haloperidol-treated rats exhibit a smaller orienting response (both “detection” and “localization”) to frequencies that were salient for normal rats. Third, the presentation of all salient sounds and frequencies used in this thesis to untreated rats while climbing a grid (i.e., during training sessions) appeared to prolong behavioural activation beyond that of animals not presented with sound stimuli. A general observation that could be explored in future work is the finding that sound has the ability to bring about behavioural arousal that otherwise would not be present. For example, once in a quiet resting state in the orienting chamber, we found that rats left untouched would fall asleep, whereas rats given auditory stimuli would become alert and exhibit walking and rearing for a period of time. Fourth, a single key jingle was sufficient to prompt a quick righting response in haloperidol-treated animals naïve or familiar with the task. The potency of key jingle for righting, as well as for audiogenic seizures, highlights the ability of this stimulus to induce widespread, synchronous activity within the central nervous system. Fifth, sound stimuli or frequency bands that may be important for social communication were not potent enough to release haloperidol-treated from catalepsy during a righting task. Sixth, females did not improve their righting performance beyond naïve levels when given motor experience, providing an observation for future study. Although it was expected that females might respond more strongly to activating sounds, as it is the females who respond to the attracting calls of males, this was not the case. Taken together, these six observations provide a starting point for future research into sound-induced movement in the rat, and also suggest that an “intrinsically” activating cue that may not be directly relevant to a species may be potent enough to be used in acute motor recovery.

Revisiting Music Therapy
The processing of musical stimuli in the brain, and the performing of music serve as models of human cognition and memory (Thaut et al., 2001). Within the study of music and motor behaviour, it has been shown that auditory cues can be used as discrete cues or as entrainment stimuli that can influence movement over time (McIntosh et al., 1998; Thaut et al., 1996). Briefly, the present experiments are consistent with emerging approaches to neurologic music therapy that encourage in-depth and standardized clinical assessments prior to application of music or rhythmic stimulation programs (Thaut, 2005). Currently, a cognitive neuroscience model is proposed (Thaut et al., 1999) in which external rhythmic cues work through coupled oscillator mechanisms to entrain ongoing movement: 1) EMG patterns synchronize to the felt rhythm (underlying meter) within two phases of the auditory stimulus (Thaut, 2005); 2) this immediate synchronization is also seen in controls who do not lack the integrity of internal movement generation provided by the basal ganglia; 3) the coupled oscillatory model is economical in terms of movement speed, time, and energy (Thaut, 2005); and 4) work by Ballanger (2005) suggests that “paradoxical kinesis” may be a general property of the motor system and not tied to deficient basal ganglia. Ballanger (2005) demonstrated that controls and PD patients increased their reaction speed during a signal detection task when provided with urgent or external cues. Thus, the body of literature on music-induced activation in movement disorders highlights the following possibilities for why certain music and auditory cues can improve movement in PD: 1) redirects attention to the task at hand; 2) acts to “prime” the motor cortex for movement and increase tonic activity; 3) activates the remaining connections of the striatum that play a role in movement initiation and force; 4) bypass deficient areas by various compensatory mechanisms (e.g., regulation of timing is a distributed function in the brain; Cunnington et al., 2005; Thaut, Rathbun, and Miller, 1997); or 5) provide a sensory cue that helps the brain to compensate for impaired voluntary movement and the proprioceptive guidance of movement (Escola et al., 2002), and thus provide an external timekeeper or “template” (Ma et al., 2004).

CONCLUSION

In conclusion, the current research demonstrated that rats exhibiting haloperidol catalepsy could be released from catalepsy with activating sound, that auditory-motor experience influences the expression of sound-induced movement, and that key jingle is the most potent sound for inducing movement in the rat. These findings challenge the contemporary understanding of parkinsonian akinesia and suggest that the postural support systems left intact constitute a compensatory mechanism that allows environmental stimuli to guide the release of movement.