Hyperbaric effects on sensorimotor reactivity studied with acoustic startle in the rat

High-pressure neurological syndrome (HPNS)

High-pressure neurological syndrome (HPNS) is a condition encountered in diving beyond a depth of 100 m. Manifestations include headache, tremor, myoclonus, neuropsychiatric disturbances and EEG changes. Convulsions are seen only in experimental animals. Most of the changes are reversible on surfacing but some such as memory disturbances may linger on for long periods. Excessive atmospheric pressure is the most important factor in the pathogenesis of HPNS. Neurotransmitter changes occur of which serotonin appears to be a more likely mediator because of the resemblance of HPNS to serotonin syndrome. Anesthetics and anticonvulsants have been used in experimental animals but are unsuitable for use in human divers. Breathing gas mixtures such as heliox have enabled the extension of depth of diving without HPNS. Use of 5-HT1A receptor antagonists may provide an interesting approach to prevention of HPNS.

The mesencephalic locomotor region is activated during the auditory startle response of the unrestrained rat

We describe an acoustically evoked potential in the midbrain of the rat which occurred in conjunction with the auditory startle response, 'startle correlated potential'. This potential had a variable latency to the onset of the startle-eliciting acoustic stimuli, but was precisely coupled to the startle response in the electromyogram (EMG) of the temporal muscle which was simultaneously recorded. We tried to localize the source of this potential by recording evoked potentials at different recording sites in individual awake and unrestrained rats using a specially constructed microdrive. The potential may be generated in part by neurons in the region of the pedunculopontine tegmental nucleus, lying within the electrophysiologically defined mesencephalic locomotor region (MLR). We suggest, therefore, that the startle correlated potential reflects the activation of the MLR during the startle response. Timing calculations make it unlikely that the startle correlated potential is generated by a sensorimotor relay within the primary startle circuit which produced a fast startle twitch in the temporal muscle. Instead, the startle correlated potential probably reflects the involvement of the MLR in a later secondary startle response or in response modulation, e.g. habituation. In our opinion the most interesting possibility is that the MLR could be activated during a startle response to inhibit and reset the current motor program.

Neural organization in the brainstem circuit mediating the primary acoustic head startle: an electrophysiological study in the rat

In awake rats the latency of auditory startle recorded electromyographically in the neck is about 5 ms, suggesting that the primary component of this brainstem reflex is mediated by a neural circuit with only a few synapses. In the present work, neural relays on acoustic head startle circuit are studied in alpha-chloralose-anesthetized rats by means of precise measurements, at putative brainstem relays, of the click-evoked potential latency and of the latency of nuchal EMG startle-like response elicited electrically from each recorded site using the same bipolar electrode. Allowing 0.6 ms for total synaptic transmission time in every relay nuclei, systematic comparisons of the shortest latencies of evoked potentials and shock-elicited startle, as well as estimations of conduction velocities in pathways from cochlea to C1-C5 spinal cord, suggest that one primary acoustic head startle circuit consists of ventral cochlear nucleus (postsynaptic evoked potential: 1.4 ms; startle: 3.6-4.0 ms), ventral nucleus of the lateral lemniscus (evoked potential: 2.3 ms; startle: 2.7-2.8 ms), medial bulbar reticular formation (evoked potential: 3.2-3.6 ms; startle: 2.1 ms), spinal interneuron and motoneuron. The nucleus reticularis pontis caudalis (NRPC) cannot be considered as an head startle relay intercalated between the ventral nucleus of the lateral lemniscus (VNLL) and the medial bulbar reticular formation (MBRF) because mean latencies of field potentials in the pontine RF and the LL nucleus are the same (2.3 ms). Moreover, startle-like responses in neck muscles are elicited through the three brainstem regions with latency differentials which exclude the possibility of a classical synaptic delay either between VNLL (2.8 ms) and NRPC (2.5 ms) or between NRPC and bulbar RF (2.1 ms). Nevertheless, NRPC probably remains a main primary relay on the acoustic startle circuitry; very short latency auditory responses (2.3 ms) are evoked in NRPC by clicks, and low current stimulations of this reticular region produce startle-like activity in neck muscles with a latency of only 2.5 ms. Two other alternative paths consisting of the VCN and NRPC which then would project directly, or through an unknown bulbar site, upon the spinal motor center are hypothetically proposed in conclusion.

Effects of various direct or indirect dopamine agonists on the latency of the acoustic startle response in rats

The effects of dopamine agonists were investigated on the latency of the acoustic startle response in male Wistar rats. Four indirect dopamine agonist were tested: GBR 12783 (5-20 mg/kg), BTCP (5-20 mg/kg), dexamphetamine (3-6 mg/kg) and L-DOPA 100 mg/kg associated with benserazide 25 mg/kg; they induced an increase in startle latency. Apomorphine at a dose (50 micrograms/kg) known to decrease dopaminergic transmissions, was ineffective on the startle response. On the contrary, at 0.6 or 2 mg/kg, apomorphine induced an increase in the startle latency. A similar effect was observed with bromocriptine at 10 mg/kg from the 10th min up to at least the 9th hour after treatment. The specific agonist of D2 receptors Ru 24926 (0.45 mg/kg) enhanced the startle latency as well as the specific agonist of D1 receptors SKF 38393 (10 mg/kg). The association of these drugs resulted in an apparent additivity of their individual effects. The effect of apomorphine (0.6 mg/kg) was only partially reduced by a high dose of the specific D2 antagonist amisulpride (80 mg/kg) and more clearly antagonized by the specific D1 antagonist SCH 23390 (50 micrograms/kg). It is concluded that D2 and D1 receptors contribute to the increase in startle latency elicited by direct or indirect dopamine agonists.