Cholinergic and monoaminergic substrates of startle habituation

Sensorimotor gating and spatial learning in α7-nicotinic receptor knockout mice

The role of acetylcholine and specific nicotinic receptors in sensorimotor gating and higher cognitive function has been controversial. Here, we used a commercially available mouse with a null mutation in the Chrna7(tm1Bay) gene [α7-nicotinic acetylcholine receptor (nAChR) knockout (KO) mouse] in order to assess the role of the α7-nAChR in sensorimotor gating and spatial learning. We examined prepulse inhibition (PPI) of startle and nicotine-induced enhancement of PPI. We also tested short- and long-term habituation of the startle response as well as of locomotor behaviour in order to differentiate the role of this receptor in the habituation of evoked behaviour (startle) vs. motivated behaviour (locomotion). To address higher cognition, mice were also tested in a spatial learning task. Our results showed a mild but consistent PPI deficit in α7-nAChR KO mice. Furthermore, they did not show nicotine-induced enhancement of startle or PPI. Short- and long-term habituation was normal in KO mice for both types of behaviours, evoked or motivated, and they also showed normal learning and memory in the Barnes maze. Thorough analysis of the behavioural data indicated a slightly higher degree of anxiety in α7-nAChR KO mice; however, this could only be partially confirmed in an elevated plus maze test. In summary, our data suggest that α7-nAChRs play a minor role in PPI, but seem to mediate nicotine-induced PPI enhancement. We found no evidence to suggest that they are important for habituation or spatial learning.

Neotic preferences in laboratory rodents: Issues, assessment and substrates

Neotic preference refers to the extent to which animals prefer stimuli of differing novelty value. Degree of novelty is determined by within- and between-trials habituation and amount of temporal (novelty) and spatial change (complexity) in stimulation which in turn will determine the amount of curiosity-based approach (neophilia) or fear-based avoidance (neophobia) of novel stimuli. Tests of genuine neotic preferences enable direct assessments of responsiveness to temporal and spatial changes and include measurements of novel versus familiar locations (such as novelty-related location preferences), responsiveness to stimulus complexity (such as object exploration) and learning for exploratory rewards (such as light-contingent bar-pressing). Effects of brain lesions and peripherally administered drugs have implicated several brain areas and neurotransmitters that subserve memory, fear and reward in neotic preferences namely the hippocampus and ACh (memory), the amygdala, GABA and 5-HT (fear), and the mesolimbic DA reward system. However, more attention should be paid to the complexity of interactions between different brain and neurotransmitter systems and improvements in methodology before conclusions should be drawn about the neurobiological basis of neotic preferences.

Prenatal heroin exposure. Effects on development, acoustic startle response, and locomotion in weanling rats

The purpose of this study was to investigate the effects of prenatal heroin exposure on the offspring in postnatal behavioral development. Pregnant Sprague-Dawley rats were injected daily (s.c.) with 10mg/kg of heroin from gestational day 8 to 20. The control dam received saline injections and the pair-fed dam received saline and was yoked to a weight-matched heroin-treated dam. Litters were culled to eight to ten pups and weighed at postnatal day (PND) 1, 8, 15, and 22. Acoustic prepulse inhibition and habituation were parameters used for evaluating the sensorimotor gating and simple form of learning respectively. Locomotor activity and rearing were assessed using the photobeam activity system. All behavioral tests were performed on the offspring at PND 21 to 23. Results showed that heroin treatment significantly reduced maternal food intake, water consumption, and weight gain. Both heroin-exposed and pair-fed groups showed a marked reduction in birth weight in both male and female pups when compared with controls; however the postnatal weight gain in heroin-exposed pups was significantly lower than the pair-fed group by 3 weeks postnatally, particularly in the female pups. These female pups also showed a significant increase in ambulation and rearing when compared to the pair-fed pups. The habituation rate in both types of behavioral tests was also decreased in these female pups as compared to control and pair-fed groups. The present study indicated that prenatal heroin exposure could result in a marked retardation of postnatal development and learning. These effects are sex related.

Relating acoustic startle reactivity and plasticity to alcohol consumption in male Wistar rats

The purpose of this study was to investigate the relationship between the startle response and ethanol. Aspects of the startle response, including initial and average startle, habituation, and prepulse inhibition (PPI) were studied. The startle response was measured to detect potential predictors of voluntary ethanol consumption and to observe whether ethanol ingestion would affect startle in subsequent tests. Rats were tested three times in a standard startle chamber. After the initial startle test, rats categorized as showing high or low PPI were allocated in a balanced way to a free-choice ethanol-water regime or to the water-regime control group. At the end of the ethanol period (lasting for 16 days, including access to ethanol for 10 days), the rats were tested again in the startle chamber 24 h after ethanol removal. After 5 weeks of ethanol abstinence, rats were exposed to a final startle test. The response to the first 120-dB stimulus showed an inverted U-shaped, curvilinear relationship to later ethanol consumption. Startle habituation appeared to have predictive value regarding ethanol consumption, with rats showing the most efficient habituation drinking most. Data showed no relationship between PPI and ethanol intake. Rats given access to ethanol showed greater habituation in the post-ethanol test than did the water controls. After 5 weeks of abstinence, low ethanol-consuming rats showed lower startle responses to the first 120-dB stimulus than did high ethanol-consuming rats. The present data suggest a two-way relationship between startle response characteristics and alcohol.

Personality and response to repeated visual stimulation in migraine and tension-type headaches

Migraine sufferers potentiate their visual evoked potentials (VEPs) from a short period of 2 min to a longer period of 15 min. As a lack of habituation is linked to higher level arousal, we thus hypothesized that short-term VEP potentiation might be correlated with an arousal-related personality trait. We therefore carried out short-term VEPs, Plutchik-van Praag's Depression Inventory, Zuckerman's Sensation-Seeking Scales (Form V), and Zuckerman-Kuhlman's Personality Questionnaire in 26 healthy subjects, 22 patients suffering from migraine without aura between attacks, 13 episodic and 20 chronic tension-type headaches. The chronic tension-type headache sufferers showed increased depression compared with other groups, which might be a consequence of the headache itself. Migraines, however, showed steeper habituation slopes of N1-P1 and P1-N2, decreased thrill and adventure-seeking, and general sensation-seeking than healthy controls; in addition, the habituation slope of P1-N2 was positively correlated with experience-seeking in migraine. The short-term VEP potentiation and the decreased thrill and adventure-seeking and general sensation-seeking in migraine might be related to a high level of cortical arousal and a low 5HT neurotransmission. In compliance with the long-term VEP study, the positive correlation between the P1-N2 habituation slope and experience-seeking in migraine suggests a continuous metabolic overload for the brain interictally, which can trigger the activation of a migraine attack.

Cholinergic modulation of the acoustic startle response in the caudal pontine reticular nucleus of the rat

The startle response is a useful behavioural model to assess drug effects on sensorimotor information processing in the mammalian central nervous system. Prepulse inhibition of the acoustic startle response in rats is an operational measure for sensorimotor gating mechanisms which may be necessary for attention and response selection. The caudal pontine reticular nucleus is a key element of the pathway that mediates the acoustic startle response and receives an inhibitory cholinergic projection that might be important for prepulse inhibition. The present study tested whether prepulse inhibition of acoustic startle is modulated by microinfusions of the muscarinic/nicotinic acetylcholine receptor agonist carbachol and of the muscarinic acetylcholine receptor antagonist scopolamine. Carbachol (0-40 nmol/0.5 microl) dose dependently attenuated startle and enhanced prepulse inhibition. Scopolamine (0-40 nmol/0.5 microl) dose-dependently enhanced startle and reduced prepulse inhibition at a dose of 10 nmol. Scopolamine (40 nmol) also increased the spontaneous motor activity of the rats. These findings lend support to the hypothesis that muscarinic acetylcholine receptors in the caudal pontine reticular nucleus inhibit the acoustic startle response and are involved in the mediation of prepulse inhibition of startle.

Clinical neurophysiology and neurotransmitters

Clinical neurophysiology allows non-invasive assessment of neurotransmitter function in various regions of the central and peripheral nervous system. In this review, we describe examples of functional evaluation of neurotransmission at the neuromuscular junction, in some spinal interneurons and intracortical circuits as well as evaluation of pharmacological modulation of some electrophysiological tests. These investigations are carried out to help our understanding of the pathophysiology of brain diseases. Finally, we discuss possible relationships between electrophysiological tests (evoked/event-related potentials and exteroceptive suppression of temporalis muscle activity) and neurotransmitter function in headache.

Behavioral evidence implicating dopamine in sensorimotor arousal and norepinephrine in the sedative effects of antidepressant drugs

The effects of acute and chronic antidepressant treatment on acoustic startle were evaluated in three experiments. Administration of 2.5-10.0 mg/kg desipramine, amitriptyline, and nortriptyline depressed acoustic startle responding after repeated sensory stimulation. In contrast to the tricyclic drugs, the serotonin reuptake inhibitor zimelidine increased acoustic startle, and inhibition of dopamine reuptake following acute nomifensine and bupropion administration did not influence startle reactivity in the doses examined. The response reducing effects of desipramine and amitriptyline persisted following chronic exposure to these drugs, and these findings were discussed in relation to the inhibitory actions of the tricyclics on locus coeruleus neurons. A second major finding in this study was that animals challenged with d-amphetamine during desipramine and amitriptyline withdrawal showed a facilitated startle response. Enhanced startle reactivity to amphetamine was also observed following long-term exposure to iprindole, and a withdrawal hyperactivity of acoustic startle was evident after chronic treatment with amoxapine, bupropion, and nomifensine. These results agree with evidence that repeated administration of antidepressants increases dopamine neurotransmission which modulates sensorimotor arousal.

Motor habituation in the DHT model: bin analysis of daytime and nocturnal locomotor activity

We evaluated the usefulness of locomotor bin analysis as a measure of motor habituation, and the effects on habituation of serotonergic manipulations. Spontaneous and L-5-hydroxytryptophan (5-HTP)-evoked locomotor activity (LMA) was measured by photocells and computer-tabulated at 10 minute intervals (bins) during the day and at night in 150 adult rats treated intracisternally with 5,7-dihydroxytryptamine (DHT) or vehicle. Bins were analyzed by visual inspection, which differentiated decremental, incremental, continuous, and discontinuous patterns, and by calculating the ratio of the first to last bin of an hour. In controls, bin ratios were high due to rapid decline (decremental bin pattern) of daytime LMA. After DHT lesions, in contrast, ratios were low for one week. This was due to a failure of normal motor habituation, as reflected by an increase in continuous, discontinuous, and incremental bin patterns. 5-HTP evoked similar dose-related bin abnormalities in DHT-treated rats, after spontaneous patterns had returned to normal, and to a lesser extent in controls. However, no consistent drug or lesion effects on nocturnal LMA were seen. In comparison to daytime LMA, nocturnal LMA showed less habituation, and bin patterns were predominantly of the discontinuous type and more varied. These data suggest that bin analysis contributes useful information on motor habituation and drug and lesion effects and is easily incorporated into automated recording of LMA.