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 m otor response that can be elicited by sudden and loud acoustic stimuli . The ASR shows several forms of plasticity, such as habituation, sens itization, and prepulse inhibition, thereby making it an interesting m odel for studying the underlying neuronal mechanisms. Among the neuron s 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 ste p of this study, we employed retrograde and anterograde tracing techni ques to identify the possible sources of input and the efferent target s of these neurons. In a second step, we performed intracellular recor dings in vivo, followed by subsequent injections of HRP for morphologi cal identification, thereby investigating whether characteristic featu res 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 originati ng from neurons in the cochlear nuclear complex and the superior oliva ry complex. Almost no input neurons were found in the nuclei of the la teral lemniscus. As the relatively long neuronal response latencies in several of these auditory nuclei appear to be incompatible with the p rimary ASR, we conclude that neurons in the cochlear root nuclei most likely provide the auditory input to PnC neurons that is required to e licit the ASR. The giant PnC neurons have a remarkable number of physi ological 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) th ey are sensitive to changes in stimulus rise time and to paired-pulse stimulation, (4) repetitive acoustic stimulation results in habituatio n of their response, and (5) amygdaloid activity enhances their respon se to acoustic stimuli. Anterograde tracing showed that most giant PnC neurons are reticulospinal cells. Axon collaterals and terminal arbor s were found in the reticular formation as well as in cranial and spin al motoneuron pools. The results of this study indicate that giant PnC neurons form a sensorimotor interface between the cochlear nuclear co mplex and cranial and spinal motoneurons. This neuronal pathway implie s that the elementary acoustic startle circuit is composed of only thr ee central relay stations and thus appears to be organized more simply than assumed in the past.