The role of noradrenergic modulation for auditory discrimination learning in rats: development of a behavioral paradigm

The brain noradrenergic (NA) system is of great importance for animal vital activities as it modulates a variety of cognitive processes, such as vigilance, arousal, attention, memory and learning (Berridge Waterhouse, 2003). Earlier studies showed a relation between NA systems activity and attention. For example, pharmacological activation of the NA system with 2-adrenergic receptor antagonist (idazoxan) resulted in improved performance in cognitive tasks involving exploration of novel stimuli (Devauges Sara, 1990). In contrast, 2-agonist (clonidine) administration led to increase of the reaction time in a visuospatial target detection task (Mair et al. 2005). We aim to investigate the effects of NA on performance of an auditory discrimination task in rats. To manipulate the activity of NA system we will apply electrical stimulation of the Locus Coeruleus (LC), a small brainstem nucleus that contains the majority of brain NA neurons. This nucleus presents a very widespread net of efferent projections in the central nervous system and its activation leads to facilitation of sensoryevoked neuronal firing in different brain regions involved in information processing, including sensory cortices and thalamus. In all of these structures, the NA systemâs neurotransmitter norepinephrine (NE) optimizes neuronal signaling by increasing the signal-to-noise ratio of the neuronal responses and enhancing the response selectivity. The LC neurons display two types of activity: tonic and phasic discharges, depending on the behavioral state. The tonic discharge defines the general arousal state and phasic discharges are closely linked to processing of salient stimuli and focused attention, e.g. attendance to novel stimuli or target detection (Aston-Jones et al. 1997). In our experiments, we will use the LC stimulation parameters to mimic natural phasic discharges of the LC neurons and phasically activate LC at presentation of auditory stimuli. First, we developed a behavioral paradigm for auditory discrimination task in a fully-automated operant box. The task consisted in the discrimination of two sounds of distinct frequencies, 4 and 10 kHz. The rats had to initiate the sound presentation by nose-poking to an aperture in the center of the box wall. Each sound was associated with the reward (chocolate milk) delivery on the right or left port located on the opposite side of the box. The sounds were presented in a pseudo-random order. In case of incorrect choice, the trial was terminated and the rat had to initiate the next sound presentation. In order to achieve a reliable above-chance performance, the rats were trained using a multiple-step conditioning paradigm, which gradually led to the final performance of the discrimination task: (1) habituation - familiarization with the box and substance of reward; (2) sound-reward association â each sound presentation was paired with reward (3) reward delivery only when nose-poking during sound presentation (4) introduction of the trial initiation by nose-poking; and (5) introduction of the auditory discrimination task with repetition of the same sound in case of incorrect choice. The latter step was crucial for the effective discrimination learning as it prevented the rats from using a place preference strategy and forced them to pay attention to the properties of the sounds presented instead of responding randomly and inattentively and performing at a 50 success rate. Using this paradigm the rats needed 10-15 daily training sessions for steps 1-4; and 30 additional sessions to achieve a stable performance at 70 accuracy (step 5 and the full task). All the rats trained with this paradigm (n=6) achieved the above-chance performance and were qualified for implantation of the chronic stimulation electrode into LC. All rats were able to perform the task with the same accuracy when tested 1 week after surgery. The majority of rats continued performing satisfactorily after the cable plugging procedure to the electrode socket on the rat head was introduced. Thus, the implemented behavioral paradigm results in a stable discrimination learning that was resistant to such manipulations as anesthesia during surgery, 1 week postsurgery recovery period, and the cable plugging procedure. We are currently collecting data on the effects of electrical stimulation of LC on performance of auditory discrimination task in the trained rats. The stimulation parameters are based on results obtained in our laboratory in anesthetized animals, which induce a transient desynchronization of the local field potentials in the prefrontal cortex. We will stimulate LC with 500ms trains of pulses (0.4 ms, 50 μA, 50Hz) delivered immediately after trial initiation. We expect that such phasic activation of LC will increase NE release in the target cortical regions involved in the sensory discrimination and decision making and therefore affect the behavioral performance