Harmaline effects on the sensory-motor reactivity: modifications of the acoustic startle pattern

Azimuthal sensitivity of rat pinna reflex: EMG recordings from cervicoauricular muscles

Electromyographic (EMG) responses of the cervicoauricular muscles (CAM) to free-field sounds were recorded in two groups of rats whose brainstems were dissected transversely either at a pretectal or transtectal level. After the rat recovered from anesthesia, wide-band noise pulses were presented and speaker positions were varied systematically in azimuth. Sound levels were set at 10-15 dB above empirically determined threshold for an EMG response to a sound from 0 degree azimuth. In both animal groups, transient CAM EMGs with short latency were produced and three main types of azimuthal sensitivity of CAM EMG response were observed. (1) For the majority of the cases, an inverted "U' type of azimuthal sensitivity was identified: the maximum activity occurred around 0 degree azimuth, but as the speaker was moved toward either the ipsilateral or contralateral fields, the sound-evoked activity declined systematically. This directional tuning is quite different from the passive pinna directionality which is very lateral in the resting positions used in this study. (2) In a small number of cases, the spatial sensitivity curves were not symmetrical about the midline (0 degree azimuth): the EMG response was vigorous in one hemifield and dropped off systematically as the speaker was moved toward extreme positions of the other hemifield. Regardless of shapes of EMG spatial tuning curves, obstruction of either the ipsilateral or contralateral meatus reduced the sound-elicited response dramatically and eliminated the spatial sensitivity. (3) Some cases exhibited an omnidirectional function: the EMG spike rate had no or minor systematical variation as the speaker position was changed in azimuth. The results of this study indicate that with either pretectal or transtectal decerebrate preparations, the acoustically evoked CAM EMG can exhibit an azimuthal sensitivity which is based on binaural processing.

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.

Comparison of vestibular and auditory startle responses in the rat and cat

Cats, humans, and many other animals show stereotyped EMG responses in limb and axial muscles if suddenly dropped into free-fall. In cats, these free-fall responses (FFR) consist of highly synchronized bursts in most limb and axial muscles at 18-22 ms. We have used FFR to evaluate descending motor function and recovery in chronic spinal injured cats. Here FFR are compared with auditory evoked startle reflexes (ASR) in the hindlimb muscles of the rat and cat to determine whether they are related, and whether they could be used to evaluate descending motor function in the rat. ASR and FFR in the two species were similar except that the earliest components for both responses occurred around 9 ms in the rat versus 18-20 ms in the cat. Also, FFR in cats were usually more consistent and greater in amplitude during repeated drops than in rats, while the converse was true for ASR. Rat FFR amplitudes increased significantly after administering ketamine or 4-aminopyridine (4-AP), especially with both drugs together, while ASR amplitudes did not. FFR in cats recorded under ketamine analgesia were not normally improved by 4-AP. Finally, both FFR and ASR were suppressed by pentobarbital, chloralose, or motor activity. These data suggest that: (1) FFR appears to be a vestibular evoked startle reflex; (2) in the rat, ASR should be useful as a test of descending motor function following spinal injury, and (3) the combination of ketamine and 4-AP may be useful in revealing the presence of functional spinal pathways after CNS trauma.

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

In three experiments, startle responses to brief intense tone-bursts (30 msec, 110 dB, 6000 Hz) and single pulse (0.1 msec) stimulation of the cochlear nucleus and the nucleus reticularis pontis caudalis are studied under high pressures of heliox (from 0 bar to 50 bars) in the rat. For each rat (N = 12), mean amplitude and latency changes in startle responses (nuchal electromyography and whole-body accelerometry) are compared at normobaric pressure and during compression, at a speed of 100 bar/H. The results indicate that high pressures decrease (50% of control size) tone evoked startle by acting on the peripheral auditory organ, probably through middle and/or inner ear barotrauma. The large increases in electrically-elicited startle (250% of control size) from the cochlear and reticular nuclei, under hyperbaric conditions, suggest that high pressures affect sensorimotor reactivity by excitatory action on synaptic transmission in the relays of the acoustic startle reflex arc at the lower brainstem and spinal levels. Startle latencies remain unaffected by the heliox high pressures studied here.

Harmala alkaloids and related beta-carbolines: a myoclonic model and antimyoclonic drugs

The neuropharmacologic profile of intraperitoneally injected harmala alkaloids and related beta-carbolines was evaluated in the rat. All drugs induced central effects (convulsions, catalepsy, or altered startle), but only harmaline and harmine were tremorogenic at low doses. Methoxylation of the carboline 7 position was requisite for this effect. Coadministration of harmaline (but not harmine) and 5-hydroxytryptophan (tryptamine or m-chlorophenyl-piperazine) induced a lethal convulsive myoclonic syndrome which could not be evoked by either drug separately. Compared with the myoclonic-serotonergic syndrome evoked by 5-hydroxytryptophan in rats with 5.7-dihydroxytryptamine lesions, harmaline+5-hydroxytryptophan-treated rats displayed more continuous and greater axial myoclonic jerks and some postural differences. Clorgyline or tranylcypromine but not pargyline could be substituted for harmaline. The harmaline syndrome was blocked by the benzodiazepine agonists, physostigmine and verapamil. The harmaline+5-hydroxytryptophan syndrome was blocked by drugs acting at benzodiazepine receptors (CL 218,872 greater than ethyl-beta-carboline-3-carboxylate, clonazepam, diazepam, Ro 15-1788, pentobarbital), and baclofen. Naloxone, benztropine, quipazine, and apomorphine had partial effects, and calcium channel blockers prevented death but did not prevent convulsions. 5-Hydroxytryptamine antagonists were poor blockers, although cyproheptadine and ketanserin significantly reduced mortality. Phenobarbital was more effective than other anticonvulsants. Lesion studies suggested a role for monoaminergic neurons and the inferior olive in the expression of the harmaline+5-hydroxytryptophan syndrome. These data describe a complex convulsive myoclonic syndrome that is behaviorally related to but pharmacologically distinct from the serotonin syndrome, which may be useful in studying serotonergic-benzodiazepine interactions in the pathophysiology of myoclonus.