The metabotropic glutamate receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) blocks fear conditioning in rats.

Glutamate receptors play an essential role in fear-related learning and memory. The present study was designed to assess the role of the group I metabotropic glutamate receptor (mGluR) subtype 5 in the acquisition and retrieval of conditioned fear in rats. The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) was applied systemically (0.0, 0.3, 3.0, 30.0 mg/kg per os) 60 min before the acquisition training and before the expression of conditioned fear, respectively, in the fear-potentiated startle paradigm. MPEP dose-dependently blocked the acquisition of fear. This effect was not due to state-dependent learning. MPEP also prevented the expression of fear at a dose of 30.0 mg/kg. As a positive control for these effects, we showed that the benzodiazepine anxiolytic compound diazepam (1.25 mg/kg intraperitoneally) also blocked acquisition and expression of fear potentiated startle. MPEP did not affect the baseline startle magnitude, short-term habituation of startle, sensitisation of startle by footshocks or prepulse inhibition of startle. These data indicate a crucial role for mGluR5 in the regulation of fear conditioning. In the highest dose MPEP might exert anxiolytic properties.

Giant neurons in the rat reticular formation: a sensorimotor interface in the elementary acoustic startle circuit?

The mammalian acoustic startle response (ASR) is a relatively simple motor response that can be elicited by sudden and loud acoustic stimuli. The ASR shows several forms of plasticity, such as habituation, sensitization, and prepulse inhibition, thereby making it an interesting model for studying the underlying neuronal mechanisms. Among the neurons that compose the elementary startle circuit are giant neurons in the caudal pontine reticular nucleus (PnC), which may be good candidates for analyzing the neuronal basis of mammalian behavior. In a first step of this study, we employed retrograde and anterograde tracing techniques to identify the possible sources of input and the efferent targets of these neurons. In a second step, we performed intracellular recordings in vivo, followed by subsequent injections of HRP for morphological identification, thereby investigating whether characteristic features of the ASR are reflected by physiological properties of giant PnC neurons. Our observations demonstrate convergent, bilateral input from several auditory brainstem nuclei to the PnC, predominantly originating from neurons in the cochlear nuclear complex and the superior olivary complex. Almost no input neurons were found in the nuclei of the lateral lemniscus. As the relatively long neuronal response latencies in several of these auditory nuclei appear to be incompatible with the primary ASR, we conclude that neurons in the cochlear root nuclei most likely provide the auditory input to PnC neurons that is required to elicit the ASR. The giant PnC neurons have a remarkable number of physiological features supporting the hypothesis that they may be a neural correlate of the ASR: (1) they receive short-latency auditory input, (2) they have high firing thresholds and broad frequency tuning, (3) they are sensitive to changes in stimulus rise time and to paired-pulse stimulation, (4) repetitive acoustic stimulation results in habituation of their response, and (5) amygdaloid activity enhances their response to acoustic stimuli. Anterograde tracing showed that most giant PnC neurons are reticulospinal cells. Axon collaterals and terminal arbors were found in the reticular formation as well as in cranial and spinal motoneuron pools. The results of this study indicate that giant PnC neurons form a sensorimotor interface between the cochlear nuclear complex and cranial and spinal motoneurons. This neuronal pathway implies that the elementary acoustic startle circuit is composed of only three central relay stations and thus appears to be organized more simply than assumed in the past.

Principal cells of the rat medial nucleus of the trapezoid body: an intracellular in vivo study of their physiology and morphology.

The medial nucleus of the trapezoid body (MNTB) is one of several principal nuclei in the superior olivary complex (SOC) of mammals. It is classically thought to function as a relay station between the contralateral ventral cochlear nucleus and the lateral superior olive (LSO), playing a role among those brainstem nuclei that are involved in binaural hearing. In order to characterise the physiology and morphology at the cellular level of the major neuronal component of the MNTB, the principal cells, we have analysed these neurons in rats in vivo using intracellular recordings and horseradish peroxidase-labelling. Our data demonstrate that MNTB principal cells, when being stimulated acoustically via the contralateral ear, show a phasic-tonic response with an onset latency of 3.5 ms and a suppression of their spontaneous activity following stimulus offset. These neurons have an axonal morphology whose complexity has not yet been described. All cells (n = 10) projected exclusively ipsilaterally and had terminal axonal arbors in a variety of auditory brainstem nuclei. At least two and maximally seven auditory targets were innervated by an individual cell. Each cell projected into the LSO and the superior paraolivary nucleus (SPN). Additional projections that were intrinsic to the SOC were often observed in the lateral nucleus of the trapezoid body and in periolivary regions, with only one cell projecting into the medial superior olive. Most, if not all, MNTB principal cells also had projections that were extrinsic to the SOC, as their axons ascended into the lateral lemniscus. In two neurons the ascending axon formed terminal arbors in the ventral nucleus of the lateral lemniscus, and the dorsal nucleus of the lateral lemniscus could be identified as a target of one neuron. The location of the cell bodies of the MNTB principal cells correlated with the neurons' best frequencies, thereby demonstrating a tonotopic organisation of the MNTB, with high frequencies being represented medially and low frequencies laterally. The axonal projections into the LSO and the SPN were also tonotopically organised and the alignment of the tonotopically organised and the alignment of the tonotopic axes was similar to that in the MNTB. Our results confirm previous data from other species and suggest that MNTB principal cells have a great amount of physiological and morphological similarities across mammalian species. Furthermore, the complexity of the axonal projections indicates that these neurons play a role in auditory information processing which goes far beyond their previously described classical role.

Giant neurons in the caudal pontine reticular formation receive short latency acoustic input: an intracellular recording and HRP-study in the rat.

The reticular formation is composed of heterogeneous cell populations with multiple functions. Among these multiple functions is the processing of sensory information in the context of behavior. The purpose of the present study was to identify and characterize neurons in the reticular formation of the rat that receive auditory input. In order to do so, we combined intracellular electrophysiology in vivo with intracellular injection of horseradish peroxidase, enabling us to correlate electrophysiology unequivocally with anatomy at the single cell level. We found that many neurons in the caudal pontine reticular nucleus (PnC), which we analyzed intracellularly, responded to acoustic stimuli and were excited at short latency (mean EPSP latency: 2.6 ms; mean spike latency: 5.2 ms). This short latency suggests a direct input from the cochlear nucleus, the first central nucleus of the auditory pathway. The morphology revealed that the acoustically driven PnC neurons have very large somata (mean diameter: 44.0 microns). They can therefore be referred to as "giant PnC neurons." Complex dendritic arbors extended from these neurons into the reticular formation and thus formed a large membrane surface for the integration of multimodal inputs. Most of the giant PnC neurons sent their axons caudally into the medial longitudinal fasciculus and can therefore be regarded as reticulospinal neurons. The results demonstrate that the giant reticulospinal PnC neurons are in a position to transmit acoustic information very quickly to spinal cord neurons and to receive converging input from other parts of the brain. They are thus good candidates for participation in the mediation and modulation of acoustically elicited behaviors, such as the short latency acoustic startle response.

Loss of the acoustic startle response following neurotoxic lesions of the caudal pontine reticular formation: possible role of giant neurons.

The effect of the excitotoxic N-methyl-D-aspartate agonist quinolinic acid in the caudal pontine reticular formation on the acoustic startle response was investigated in rats. Bilateral injections of 90 nmol of quinolinic acid led to large lesions in the reticular formation characterized by the loss of all neurons and a marked reduction or even abolition of the acoustic startle response; 18 nmol of quinolinic acid led to smaller lesions characterized by a selective loss of giant neurons within the caudal pontine reticular formation and a reduction of the startle amplitude. The partial correlation analysis revealed that the reduction of the amplitude of the acoustic startle response can be correlated with the loss of the giant neurons (r = 0.575; d.f. = 29; P less than 0.001) but not with the reduction of the number of all neurons (r = 0.207; d.f. = 29; P greater than 0.2) in the caudal pontine reticular formation. These findings were reconciled with electrophysiological and anatomical data indicating that the giant neurons in the caudal pontine reticular formation receive acoustic input and project to motoneurons of the spinal cord. It is concluded that the caudal pontine reticular formation is an important element of the startle pathway and that the giant reticulospinal neurons constitute an important part of the sensorimotor interface mediating this response.