Auditory-evoked response of the cortex after yohimbine administration: phase advance effect of central noradrenergic activation

Psychiatric symptoms of noradrenergic dysfunction: a pathophysiological view

What psychiatric symptoms are caused by central noradrenergic dysfunction? The hypothesis considered in this review is that noradrenergic dysfunction causes the abnormalities in arousal level observed in functional psychoses. In this review, the psychiatric symptoms of noradrenergic dysfunction were inferred pathophysiologically from the neuroscience literature. This inference was examined based on the literature on the biology of psychiatric disorders and psychotropics. Additionally, hypotheses were generated as to the cause of the noradrenergic dysfunction. The central noradrenaline system, like the peripheral system, mediates the alarm reaction during stress. Overactivity of the system increases the arousal level and amplifies the emotional reaction to stress, which could manifest as a cluster of symptoms, such as insomnia, anxiety, irritability, emotional instability and exaggerated fear or aggressiveness (hyperarousal symptoms). Underactivity of the system lowers the arousal level and attenuates the alarm reaction, which could result in hypersomnia and insensitivity to stress (hypoarousal symptoms). Clinical data support the hypothesis that, in functional psychoses, the noradrenergic dysfunction is in fact associated with the arousal symptoms described above. The anti-noradrenergic action of anxiolytics and antipsychotics can explain their sedative effects on the hyperarousal symptoms of these disorders. The results of animal experiments suggest that excessive stress can be a cause of long-term noradrenergic dysfunction.

Age-related temporal processing speed deterioration in auditory cortex

A common problem among the elderly is a difficulty in discriminating speech sounds. One factor that may contribute to this is a deterioration in the ability to process dynamic aspects of speech such as formant transitions. For the aging auditory system, this deterioration in temporal processing speed may be manifest as a deficit in encoding time-varying sounds that contain rapidly changing frequencies such as formant transitions. The primary goal of this study was to explore the neural basis of the effects of aging on temporal processing speed. To this end, single units were recorded from the auditory cortex of young and aged rats in response to frequency-modulated (FM) sweeps that changed from trial to trial in both direction and speed. Results showed that the majority of cells recorded from young rats responded most vigorously to fast and medium speeds. By contrast, the majority of units recorded from aged animals responded best to slow speeds. For preferred direction of FM sweep, similar results were observed for both age groups, namely, approximately half of the units exhibited a direction-selective response. The results of the present study demonstrate an age-related decrease in the rate of change of frequency that can be processed by the auditory cortex.

Substance P-induced inward current in identified auditory efferent neurons in rat brain stem slices

The effects of substance P (SP) on whole cell currents were studied in neurons of the medial olivocochlear efferent system (MOCS) in the ventral nucleus of the trapezoid body (VNTB) of brain stem slices from neonatal rats. Each neuron was identified by retrograde labeling with Fast Blue injected into the cochlea. Bath application of SP (0.1-10 microM) reversibly induced an apparent inward current in 49 of 63 labeled neurons when voltage clamped at near resting voltages. This apparent inward current was consistent with the SP-induced membrane depolarization observed in current-clamp mode. The SP-induced change in current was dose dependent with a half-maximal response dose of 200 nM. It was mimicked by [Cys3,6, Tyr8, Pro9]-SP, a neurokinin (NK1) receptor selective agonist, whereas [Succinyl-Asp6, MePhe8]-SP 6-11 (Senktide), a NK3 receptor agonist, had no detectable effect. The SP effect was not blocked by 10(-6) M tetrodotoxin (TTX) and persisted when the perfusate contained 30 mM tetraethylammonium (TEA) or 100 microM Cd2+ or was in a 0-Ca solution. In a TTX-containing solution, SP caused a voltage-dependent decrease of membrane conductance, and the SP-evoked current reversed at a potential at around -105 mV. The predicted K+ equilibrium potential was -93.8 mV under the experimental conditions. The SP-induced inward current was attenuated by 66% when the perfusate contained 3 mM Cs+. We conclude that the apparent inward current is partly caused by SP decreasing an outward current normally maintained by the inward rectifier K+ channels in these cells. In the presence of Cs solution in the recording pipette and with a perfusate containing 3 mM Cs+, 0.1 mM Cd2+ and 10(-6) M TTX, a residual SP-induced inward current was observed at test voltages ranging from -120 to 40 mV. This subcomponent reversed its polarity at approximately 20 mV. This inward current was reduced substantially (but not abolished) when all NaCl in the external solution was replaced by TEA-Cl. The results indicate that SP also opens an unknown cation channel, which the available data suggests may be relatively nonselective. The results suggest that MOCS neurons are subject to modulation by SP, which depolarizes the cell membrane by decreasing the activity of inward rectifier K+ channels as well as concurrently activating a separate cation conductance. It also was found that in MOCS neurons responsive to both SP and norepinephrine, the norepinephrine effect was abolished by TTX, suggesting that an interneuronal population excited by norepinephrine converges selectively onto SP-sensitive MOCS neurons in the VNTB.

Specific sound-induced noradrenergic and serotonergic activation in central auditory structures

We have studied the noradrenergic and serotonergic changes induced by white noise stimulation at 70, 90 or 110 dB SPL for 45 min, in cochlear nuclei, inferior colliculus (IC), primary auditory cortex (PAC) and as a comparison in locus coeruleus (LC) and raphe dorsalis using HPLC. Both noradrenergic and serotonergic pathways were activated in the dorsal+posteroventral cochlear nuclei (DCN+PVCN) without changes in the anteroventral cochlear nucleus (AVCN) and IC. In the DCN+PVCN the noradrenergic activation was restricted to animals exposed to 70 dB SPL whereas the increase of serotonin content was intensity-dependent. In PAC serotonergic activation was observed only after 70 dB SPL exposure. These data suggest that in physiological conditions (70 dB SPL) noradrenergic and serotonergic regulation of the processing of auditory information occurs specifically in the dorsal cochlear nucleus where the control of incoming information to higher auditory structures takes place (i.e. IC and PAC). We suggest that the serotonergic activation in the primary auditory cortex for 70 dB SPL sound stimulation could be related to the fact that low-intensity white noise stimulation could be the most plastic-demanding processing in the auditory cortex.

Two types of actions of norepinephrine on identified auditory efferent neurons in rat brain stem slices

Whole cell voltage-clamp recordings were performed on auditory olivocochlear neurons in the ventral nucleus of the trapezoid body (VNTB) of brain stem slices from neonatal rats. Each neuron was identified by retrograde labeling with Fast Blue injected into the cochlea. Bath application of norepinephrine (NE; 1-10 microM) reversibly induced an inward current in 26 of 38 labeled neurons that were voltage clamped at -75 mV. This was responsible for the membrane depolarization to NE observed in current-clamp mode. The NE-induced inward current appeared to be more prominent at -55 mV than at -75 mV and reversed at around -100 mV. It was attenuated but not prevented by 20 mM tetraethylammonium, and it persisted when the perfusate contained 2 mM Cs+ or 100 microM Cd2+. However, the NE-induced inward current was attenuated to varying degrees in a zero-Ca2+ solution. Current-voltage plots revealed that NE caused a decrease in membrane K+ conductance. A suppression of voltage-gated Ca2+ currents by NE was also observed. The excitatory action of NE was blocked by the alpha-adrenoreceptor antagonist phentolamine. The alpha1-adrenoreceptor agonist phenylephrine had an effect similar to that of NE. In 6 of 38 labeled neurons, an inhibitory action of NE (1-10 microM) was observed that appeared to be due to an activation of an inwardly rectified K+ current, which caused hyperpolarization of resting membrane potentials in current-clamp mode. This inhibitory response was independent of external Ca2+ and was abolished by 2-5 mM Cs+ or 0.5 mM Ba2+ applied in the perfusate. The receptors involved in the inhibitory actions of NE are not clear. The effect was partially and reversibly blocked by propranolol (10 microM), a beta-adrenoreceptor antagonist. However, isoprenaline (10 microM), a beta-adrenoreceptor agonist, failed to induce any effect. On the other hand, the inhibitory effect was irreversibly blocked by pretreatment with phentolamine (5-10 microM). Phenylephrine (5-10 microM) had no effect.

Effects of metamfetamine on auditory P3 component of event-related potentials in rats

Three rats were trained with an oddball paradigm, in which 5 kHz frequent and 10 kHz target rare tones were presented with a probability of 0.8 and 0.2. Pressing the lever within 2 s following the onset of the target tones was rewarded with a drop of food paste. Long latency event-related potentials (ERPs) corresponding to those in humans could be recorded in rats. After the administration of metamfetamine (0.2 mg/kg), the amplitude of surface and intracortical N1 increased, indicating an augmentation of the primary cortical response. Surface P3 showed a decrease of amplitude, accompanied by an increase of intracortical negativity. These alterations of ERPs may be caused by the dysregulation of central noradrenergic system.

Increased neuronal firing in the rat auditory cortex associated with preparatory set

Extracellular single neuronal firings were recorded in the auditory cortex of rats (n = 4) performing a visual reaction-time task with a warning tone (10 kHz, 10 ms duration), which preceded the imperative light stimulus by an interstimulus interval (ISI) of 1.4 s. Thirty-six neuronal firings were evoked by the warning tone, with the peak latency being between 15 and 55 ms. Among them, nine neurons (25%) showed an increased firing frequency following the evoked response during the ISI, which was, in average, 2.5 times as high as the firing frequency during the baseline period. When the tones were presented independent of the imperative stimulus, such sustained increase in neuronal firing was not observed. Activation of the sensory cortex during the ISI may constitute one of the neuronal modulations related to preparatory set.

Event-related potentials during an auditory discrimination task in rats

Event-related potentials (ERPs) during an auditory discrimination task were recorded both on the surface and at a depth of the auditory cortex in rats. Ten-kilohertz rare and 5-kHz frequent tones were used with the probabilities of 0.2 and 0.8. Lever pressing within 2 s, following the onset of the rare tone, was rewarded with food paste. In the performing condition, the surface ERPs for the rare tones consisted of P30, N50, P80, N130, and P290 components. Only the surface P30 and N50 showed a polarity reversal at the intracortical electrode, suggesting that these components are generated in the auditory cortex. The P290 was elicited in the performing condition but not in the resting condition, showing its task-relevancy. These results suggest that auditory ERPs similar to those in humans can be recorded in rats during an active discrimination task.