Habituation and prepulse inhibition of acoustic startle in rodents

The acoustic startle response is a protective response, elicited by a sudden and intense acoustic stimulus. Facial and skeletal muscles are activated within a few milliseconds, leading to a whole body flinch in rodents(1). Although startle responses are reflexive responses that can be reliably elicited, they are not stereotypic. They can be modulated by emotions such as fear (fear potentiated startle) and joy (joy attenuated startle), by non-associative learning processes such as habituation and sensitization, and by other sensory stimuli through sensory gating processes (prepulse inhibition), turning startle responses into an excellent tool for assessing emotions, learning, and sensory gating, for review see( 2, 3). The primary pathway mediating startle responses is very short and well described, qualifying startle also as an excellent model for studying the underlying mechanisms for behavioural plasticity on a cellular/molecular level(3). We here describe a method for assessing short-term habituation, long-term habituation and prepulse inhibition of acoustic startle responses in rodents. Habituation describes the decrease of the startle response magnitude upon repeated presentation of the same stimulus. Habituation within a testing session is called short-term habituation (STH) and is reversible upon a period of several minutes without stimulation. Habituation between testing sessions is called long-term habituation (LTH)(4). Habituation is stimulus specific(5). Prepulse inhibition is the attenuation of a startle response by a preceding non-startling sensory stimulus(6). The interval between prepulse and startle stimulus can vary from 6 to up to 2000 ms. The prepulse can be any modality, however, acoustic prepulses are the most commonly used. Habituation is a form of non-associative learning. It can also be viewed as a form of sensory filtering, since it reduces the organisms' response to a non-threatening stimulus. Prepulse inhibition (PPI) was originally developed in human neuropsychiatric research as an operational measure for sensory gating(7). PPI deficits may represent the interface of "psychosis and cognition" as they seem to predict cognitive impairment(8-10). Both habituation and PPI are disrupted in patients suffering from schizophrenia(11), and PPI disruptions have shown to be, at least in some cases, amenable to treatment with mostly atypical antipsychotics(12, 13). However, other mental and neurodegenerative diseases are also accompanied by disruption in habituation and/or PPI, such as autism spectrum disorders (slower habituation), obsessive compulsive disorder, Tourette's syndrome, Huntington's disease, Parkinson's disease, and Alzheimer's Disease (PPI)(11, 14, 15) Dopamine induced PPI deficits are a commonly used animal model for the screening of antipsychotic drugs(16), but PPI deficits can also be induced by many other psychomimetic drugs, environmental modifications and surgical procedures.