Claudia F. Plappert

Lack of the metabotropic glutamate receptor subtype 7 selectively impairs short-term working memory but not long-term memory.

Metabotropic glutamate receptors (mGluRs), and in particular the mGluR group III receptors (subtypes 4, 6, 7, 8) are known to play a role in synaptic plasticity and learning. Here, we report the effect of mGluR7 gene ablation in different learning paradigms. In the acoustic startle response (ASR), no differences were seen between knockout (KO) mice and wildtype (WT) littermates in parameters including prepulse inhibition and habituation. In an open field test, no differences were seen between genotypes in motor activity, exploratory behaviour, and fearful behaviour. In a T-maze reinforced alternation working memory (WM) task, again no difference was seen between groups. However, when increasing the demands on working-memory in a 4-arm and 8-arm maze task, KO mice committed more WM errors than WT littermates thereby uncovering a highly significant difference between the two groups that persisted every day for the whole 9 days of the experiment. In a 4-arm maze with 2 arms baited, KO and wildtype mice committed the same number of LTM errors, whereas KOs committed more WM errors. Altogether, these findings suggest that a lack of mGluR7 mainly impairs short-term working but not long-term memory performance while having no effect on sensorimotor processing, non-associative learning, motor activity and spatial orientation. The effects on WM are task-dependent and become apparent in more complex but not simple learning tasks. We discuss how mGluR7 could influence WM.

Synaptic depression and short-term habituation are located in the sensory part of the mammalian startle pathway

BackgroundShort-term habituation of the startle response represents an elementary form of learning in mammals. The underlying mechanism is located within the primary startle pathway, presumably at sensory synapses on giant neurons in the caudal pontine reticular nucleus (PnC). Short trains of action potentials in sensory afferent fibers induce depression of synaptic responses in PnC giant neurons, a phenomenon that has been proposed to be the cellular correlate for short-term habituation. We address here the question whether both this synaptic depression and the short-term habituation of the startle response are localized at the presynaptic terminals of sensory afferents. If this is confirmed, it would imply that these processes take place prior to multimodal signal integration, rather than occurring at postsynaptic sites on PnC giant neurons that directly drive motor neurons.ResultsPatch-clamp recordings in vitro were combined with behavioral experiments; synaptic depression was specific for the input pathway stimulated and did not affect signals elicited by other sensory afferents. Concordant with this, short-term habituation of the acoustic startle response in behavioral experiments did not influence tactile startle response amplitudes and vice versa. Further electrophysiological analysis showed that the passive properties of the postsynaptic neuron were unchanged but revealed some alterations in short-term plasticity during depression. Moreover, depression was induced only by trains of presynaptic action potentials and not by single pulses. There was no evidence for transmitter receptor desensitization. In summary, the data indicates that the synaptic depression mechanism is located presynaptically.ConclusionOur electrophysiological and behavioral data strongly indicate that synaptic depression in the PnC as well as short-term habituation are located in the sensory part of the startle pathway, namely at the axon terminals of sensory afferents in the PnC. Our results further corroborate the link between synaptic depression and short-term habituation of the startle response.

Effects of sex and estrous cycle on modulation of the acoustic startle response in mice.

Potential sex differences in amplitude, habituation, prepulse inhibition (PPI) and prepulse facilitation (PPF) of the acoustic startle response (ASR) were investigated using male and female mice from the two different inbred mouse strains C57BL/6J (C57) and C3H. Furthermore, the effects of the estrous cycle were tested. The estrous cycle appeared to have no effect on ASR amplitude, habituation, PPF and PPI, the latter being in contrast to results in rats and humans. While sex had no effect on PPI or PPF, males exhibited higher startle amplitudes than females, irrespective of strain, which we discuss to be due to increased male anxiety. In addition, long-term habituation was stronger in C57 males and short-term habituation was weaker in C3H males with respect to females. These results provide evidence for influence of the reproductive hormones on startle reactivity and startle habituation; we therefore conclude that future studies involving genetic influences on behavior using inbred strains are only complete if both sexes are included.

Factors governing prepulse inhibition and prepulse facilitation of the acoustic startle response in mice.

The influence of prepulses on the acoustic startle response (ASR) was measured in three inbred mouse strains, C57BL/6J, 129/SvHsd, and AKR/OlaHsd, and one hybrid strain produced by crossing wild mice and NMRI mice. Prepulse inhibition (PPI), i.e. reduction of ASR by prepulses, was maximal when the interval between prepulses and startle stimuli was in the range of 37.5-100 ms. Prepulse facilitation (PPF), i.e. increase of ASR by prepulses, was maximal when the prepulse preceded the startle stimulus by 12.5 ms. PPI increased with increasing prepulse SPL, PPF first increased then decreased when prepulse SPL was increased. Percent PPI was independent from startle stimulus SPL. All strains showed a long-term increase of PPI when tested for several days; one strain (129) also showed an increase of PPF over days. The present results clearly show that PPI and PPF are independent processes, which add to yield the final response change. PPF and the observed long-term changes of PPI and PPF are stronger expressed in mice than have been observed in rats under similar conditions. Since there were significant differences between the strains of mice with respect to PPI and PPF, genetically different strains of mice are a promising tool to study these two processes.

Acoustic startle response and habituation in freezing and nonfreezing rats

Rats can be divided according to their responses to startle-eliciting stimuli into 2 groups with different emotional states. About half of the 54 female Sprague-Dawley rats showed long-lasting freezing behavior after 1-8 stimuli (10 kHz, 110 dB spl). In freezing rats the startle amplitude was higher than in nonfreezing rats, even on the very first startle response. This finding demonstrates that the anxiety state of these animals before the first startle-eliciting stimulus, and not just the aversiveness of the stimulus, contributes to freezing behavior. In addition, in freezing rats there was no influence of spontaneous motor activity or of adaptation time on startle amplitude. Only in nonfreezing rats were high motor activities correlated with lowered startle amplitudes, and only in these rats did the course of startle habituation depend on adaptation time.

The acoustic startle response as an effective model for elucidating the effect of genes on the neural mechanism of behavior in mice

One current approach in investigating the neural basis of behavior is to use mutant mice with specific genetic alterations which affect neural functions. We are convinced that this approach is only effective if a behavioral model with sufficiently known underlying neuronal mechanisms is used. We present a model system which is well-suited for the above approach. Because the neural basis is known in great detail, in the startle system behavioral results can be very well interpreted. This is demonstrated here by using footshock sensitization of the acoustic startle response (ASR) as an example. Sensitization is elicited by aversive stimuli such as electric footshocks and causes an increase in ASR amplitude. The present experiment showed that this ASR increase is not due to a drop in the startle threshold but to increased gain in the response to suprathreshold stimuli. This makes it possible to draw conclusions about the neuronal site of the startle threshold in the startle pathway and the synapse at which the gain shift during sensitization occurs. The possibility of interpreting behavioral output on a well known neural basis (as demonstrated here) makes the ASR a promising model system for investigating (neuro-) genetic influences of behavior.

Long-term habituation of the startle response in mice evoked by acoustic and tactile stimuli.

The present study shows that repetitive presentation of tactile and acoustic stimuli evoke long-term habituation (LTH) of the startle response in C57BL/6J mice. This was indicated by a decrease in response strength over several days. For the LTH of the acoustic startle response two controls were included: first, developing hearing loss during the time of testing did not account for the startle decrease--only 7 days of acoustic stimulation but not 7 days of adaptation led to a decrease in the startle. Second, repetitive presentation of loud acoustic startle stimuli did not raise the auditory threshold, which might otherwise have accounted for the startle decrease: prepulse inhibition (used here as a hearing test) was identical after both 7 days of acoustic startle stimulation and 7 days without stimulation. This proves that LTH to tactile and acoustic stimuli is present and fully functional in mice.

Longterm-habituation of the startle response in mice is stimulus modality, but not context specific

In mice, the specificity of longterm-habituation (LTH) of startle was tested in two experiments. In two strains of mice (C57Bl/6 and C3H) there was pronounced LTH over 10 days of acoustic stimulation in two different contexts of startle measurement. (We found LTH to be greater after stimulation with 14 kHz sine stimuli compared to noise or tactile stimuli). A change of context showed LTH to be independent of context, i.e., startle LTH in mice is a non-associative learning process. In the second experiment, 9 days of acoustic or tactile stimulation were given to C57B/6 mice. Both stimulus modalities produced LTH. When on the 10th day stimuli of the other modality were given, in both cases the long term habituated group showed no lower startle amplitude than a non-stimulated control group. This indicates LTH is stimulus-modality specific. Altogether, our results show that in mice—very similar to rats—LTH of startle is stimulus modality, but not context specific. In addition we found two indications that the LTH action site is on the sensory branch of the startle circuit.

Neural cell adhesion molecule-null mice are not deficient in prepulse inhibition of the startle response.

Mice constitutively deficient in the neural cell adhesion molecule have morphological changes in the brain, which are hallmarks of schizophrenia. Schizophrenic patients are impaired in sensorimotor processing indicated by a deficit in prepulse inhibition of the acoustic startle response. Here we tested whether prepulse inhibition and prepulse facilitation are changed in neural cell adhesion molecule-deficient mice compared with their wild-type littermates. Neither prepulse inhibition nor prepulse facilitation (which occurred only at the lowest prepulse intensity used and was weak) was altered. This result is discussed in the light of the ‘two-hit’ hypothesis of schizophrenia, suggesting that in neural cell adhesion molecule-deficient mice, a prepulse inhibition deficit may become apparent only after treatment with a ‘second hit’ (such as increased stress).

Accumbal dopamine D2 receptors are important for sensorimotor gating in C3H mice.

One operational measure of sensorimotor gating that is deficient in many psychiatric disorders is prepulse inhibition (PPI) of the startle response. To investigate the role of dopamine D1 and D2 receptors within the nucleus accumbens (NAC) in sensorimotor gating in mice, we infused dopamine D1 and D2 receptor agonists (dihydrexidine and quinpirole respectively) directly into the NAC and measured the effects on PPI and on prepulse facilitation. Quinpirole infusions increased PPI and attenuated prepulse facilitation, whereas dihydrexidine had no effects. These results stand in contrast to data after systemic injections in mice and rats and intra-accumbal infusions in rats, suggesting that the role of dopamine D2 receptors within the NAC in mice differs from their role in rats.