Neonatal hearing structures suffer loss of neurons when exposed to monosodium glutamate, Pennsylvania study shows



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Widespread damage to the cochlea and auditory brainstem, and drastic loss of neurons in the midbrain. These were among hypotheses confirmed by US researchers investigating the effects of neonatal exposure in lab rats to monosodium glutamate (MSG).

The recent study, published in the journal Hearing Research, and carried out by scientists from the Department of Anatomy, Lake Erie College of Osteopathic Medicine, Pennsylvania, followed years of previous findings by these scientists and many others of neurodegenerative changes resulting from excitotoxic injury through MSG exposure.

Outlined in this study was evidence produced over many years prior to these Lake Erie experiments that: neonatal MSG exposure results in degeneration of cochlear hair cells (Hyodo et al., 2009), fewer neurons in the spiral ganglion (Janssen et al., 1991, Carricondo et al., 2002), reduced amplitude of the compound action potential in the auditory nerve (Carricondo et al., 2002), and increased thresholds to clicks and tones (Janssen et al., 1991), among other studies that show glutamate or kainic acid exposure resulting in reduced auditory brainstem responses.

Breakthrough evidence

These new studies looked at neural morphology outcomes, analysis of tissue sections per nucleus per animal, estimates of neuronal number, and brainstem auditory evoked potentials, among other data.

In what the authors claim as the "first evidence that neonatal excitotoxic injury results in loss of neurons throughout the major nuclei of the ascending auditory brainstem pathways extending into the mid-brain", results showed that after MSG exposure, there was a 71% decrease in the number of globular bushy cells; a 36% decrease in the number of MNTB (medial nucleus of the trapezoid body) neurons; fewer and smaller neurons in the central nucleus of the inferior colliculus. Neonatal MSG exposure also results in abnormal ABR (auditory brainstem responses).

"Our results are consistent with widespread damage to the cochlea and auditory brainstem following neonatal excitotoxic injury and abnormal evoked potentials in response to click stimuli. Surprisingly, the loss of neurons was drastic in the midbrain, several synapses away from inner hair cells and terminals for SG [spiral ganglion] neurons, where the initial insult is believed to be focused," explain the authors in their conclusions.

MSG, a controversial food additive in use generally around the world and approved by both the EU and the US Food and Drug Administration (FDA), can be found in fast food, snacks, seasoning blends, frozen meals, soups, processed meats, condiments, and instant noodle products. Although many Chinese restaurants have now stopped using MSG, others add MSG to dishes, among them fried rice. And it occurs naturally in certain foods, cheese and tomatoes for example.

Source: Elsevier Hearing Research