Discharge Profiles of Abducens, Accessory Abducens, and Orbicularis Oculi Motoneurons During Reflex and Conditioned Blinks in Alert Cats

Abstract

The discharge profiles of identified abducens, accessory abducens, and orbicularis oculi motoneurons have been recorded extra- and intracellularly in alert behaving cats during spontaneous, reflexively evoked, and classically conditioned eyelid responses. The movement of the upper lid and the electromyographic activity of the orbicularis oculi muscle also were recorded. Animals were conditioned by short, weak air puffs or 350-ms tones as conditioned stimuli (CS) and long, strong air puffs as unconditioned stimulus (US) using both trace and delayed conditioning paradigms. Motoneurons were identified by antidromic activation from their respective cranial nerves. Orbicularis oculi and accessory abducens motoneurons fired an early, double burst of action potentials (at 4-6 and 10-16 ms) in response to air puffs or to the electrical stimulation of the supraorbital nerve. Orbicularis oculi, but not accessory abducens, motoneurons fired in response to flash and tone presentations. Only 10-15% of recorded abducens motoneurons fired a late, weak burst after air puff, supraorbital nerve, and flash stimulations. Spontaneous fasciculations of the orbicularis oculi muscle and the activity of single orbicularis oculi motoneurons that generated them also were recorded. The activation of orbicularis oculi motoneurons during the acquisition of classically conditioned eyelid responses happened in a gradual, sequential manner. Initially, some putative excitatory synaptic potentials were observed in the time window corresponding to the CS-US interval; by the second to the fourth conditioning session, some isolated action potentials appeared that increased in number until some small movements were noticed in eyelid position traces. No accessory abducens motoneuron fired and no abducens motoneuron modified their discharge rate for conditioned eyelid responses. The firing of orbicularis oculi motoneurons was related linearly to lid velocity during reflex blinks but to lid position during conditioned responses, a fact indicating the different neural origin and coding of both types of motor commands. The power spectra of both reflex and conditioned lid responses showed a dominant peak at approximately 20 Hz. The wavy appearance of both reflex and conditioned eyelid responses was clearly the result of the high phasic activity of orbicularis oculi motor units. Orbicularis oculi motoneuron membrane potentials oscillated at approximately 20 Hz after supraorbital nerve stimulation and during other reflex and conditioned eyelid movements. The oscillation seemed to be the result of both intrinsic (spike afterhyperpolarization lasting approximately 50 ms, and late depolarizations) and extrinsic properties of the motoneuronal pool and of the circuits involved in eye blinks.

The Cerebellar Interpositus Nucleus and the Dynamic Control of Learned Motor Responses

“…According to the present results, the maximum instantaneous frequency reached by type A (Gruart et al, 2000) interpositus neurons increased during the successive conditioning sessions, reaching mean amplitude values of Ϸ150 spikes/s during the first (C01) conditioning session and up to Ϸ320 spikes/s during the 10th (C10) session. The slope of this increase (0.14 spikes/s per trial) is similar to values reported for orbicularis oculi motoneurons (Trigo et al, 1999), interpositus neurons (Gruart et al, 2000), and hippocampal CA3 and CA1 pyramidal neurons (Mú nera et al, 2001) during associative learning using both delay and trace conditioning procedures in alert behaving cats. This important finding reinforces the hypothesis that associative learning using classical conditioning procedures evokes a functional state in many different neuronal centers, characterized by a common increase in firing rate, and not necessarily related with the specific function fulfilled by each neural site (Delgado-García and Gruart, 2002).…”

“…Wires were soldered to a socket fixed to the holding system. A more detailed description of these chronic preparations have been published previously (Trigo et al, 1999;Gruart et al, 2000;Jiménez-Díaz et al, 2004).…”

“…Only antidromically identified interpositus neurons (n ϭ 174) and orbicularis oculi motoneurons (n ϭ 110) were stored and analyzed. Site location and identification procedures have been described in detail for both interpositus neurons (Gruart et al, 2000) and facial motoneurons (Trigo et al, 1999). Tones were applied from a loudspeaker located 80 cm below the animal's head.…”

“…In this regard, the eyelid motor system appears a suitable model for the study of cerebellar function in the acquisition of new motor abilities, because the brainstem and cerebellar circuits involved have been described precisely (Krupa et al, 1993;Llinás and Welsh, 1993;Mauk, 1997;Bracha et al, 2001;Carey and Lisberger, 2002;Delgado-García and Gruart, 2002;Morcuende et al, 2002;Christian and Thompson, 2003), the inputs and outputs of the system can be presented and/or measured quantitatively (Evinger et al, 1991;Gruart et al, 1995), and the neuronal activity of the relevant centers can be recorded in vivo during the actual performance of associative learning tasks (Trigo et al, 1999;Gruart et al, 2000;Chen and Evinger, 2006). Moreover, this motor system can be easily trained using classical conditioning paradigms (Gormezano et al, 1983).…”

Central cannabinoid receptors modulate acquisition of eyeblink conditioning

“…Disruption of auditory system function could also impair acquisition as well (Halverson and Freeman 2006). Low levels of CB1Rs are found in the red nucleus (Herkenham et al 1991), but alterations in magnocellular neuronal function could impair CR performance by affecting excitatory input to the facial motor nucleus, which produces the blink response (Trigo et al 1999). However, previous studies of eyeblink conditioning in human cannabis users, knockout mice, and mice administered a CB1R antagonist suggest that the effects of CB1R manipulations on conditioning are selective for cerebellar cortical function (reviewed in Edwards and Skosnik 2007).…”