Role of medullary cholinoceptors in baroreflex bradycardia

Suppression of noise-induced hyperactivity in the dorsal cochlear nucleus following application of the cholinergic agonist, carbachol

Increased spontaneous firing (hyperactivity) is induced in fusiform cells of the dorsal cochlear nucleus (DCN) following intense sound exposure and is implicated as a possible neural correlate of noise-induced tinnitus. Previous studies have shown that in normal hearing animals, fusiform cell activity can be modulated by activation of parallel fibers, which represent the axons of granule cells. The modulation consists of a transient excitation followed by a more prolonged period of inhibition, presumably reflecting direct excitatory inputs to fusiform cells and an indirect inhibitory input to fusiform cells from the granule cell-cartwheel cell system. We hypothesized that since granule cells can be activated by cholinergic inputs, it might be possible to suppress tinnitus-related hyperactivity of fusiform cells using the cholinergic agonist, carbachol. To test this hypothesis, we recorded multiunit spontaneous activity in the fusiform soma layer (FSL) of the DCN in control and tone-exposed hamsters (10 kHz, 115 dB SPL, 4h) before and after application of carbachol to the DCN surface. In both exposed and control animals, 100 μM carbachol had a transient excitatory effect on spontaneous activity followed by a rapid weakening of activity to near or below normal levels. In exposed animals, the weakening of activity was powerful enough to completely abolish the hyperactivity induced by intense sound exposure. This suppressive effect was partially reversed by application of atropine and was usually not associated with significant changes in neural best frequencies (BF) or BF thresholds. These findings demonstrate that noise-induced hyperactivity can be pharmacologically controlled and raise the possibility that attenuation of tinnitus may be achievable by using an agonist of the cholinergic system.

Cardiac dysrhythmias complicating maxillofacial surgery

Cardiac asystole, bradycardia and other dysrhythmias are complications associated with surgery of the craniofacial and maxillofacial structures. These dysrhythmias have been discussed frequently in ophthalmology but less attention has been paid to this problem in the maxillofacial surgery literature. Eight patients, 6 men and 2 women, who underwent maxillofacial surgery during a 16-month period at the Victoria General Hospital, Halifax, Canada demonstrated either asystole or bradycardia. This group of patients represents 1.6% of the total number of patients treated during the 16-month period. Six patients demonstrated either asystole or bradycardia during stretching of the soft tissues associated with advancement of the maxilla after bony mobilization. Two patients had either asystole or bradycardia during manipulation of the temporalis muscle fibres during surgery to correct total bony ankylosis of the temporomandibular joint. The etiology of this relatively rare but troublesome complication is discussed.