By Shahriar SheikhBahaei, Ph.D., Neuron-Glia Signaling and Circuits Unit, National Institute of Neurological Disorders and Stroke (NINDS), National Institutes of Health (NIH), Bethesda, MD

It is not feasible to non-invasively study complex behaviors at the molecular, cellular, and circuit levels in humans during development, and animal research is crucial in understanding the developmental intricacy of brain functions. These models offer a controlled environment to study the impact of changes in genetics, epigenetics, internal states, or external environments, providing insights into the fundamental mechanisms of brain function and the pathophysiology of neuropsychiatric conditions.

Historically, animal models have played a vital role in numerous groundbreaking discoveries in neuroscience. The use of animals has been essential in understanding neural circuits, synaptic transmission, and neuroplasticity. For instance, Alan Hodgkin and Andrew Huxley's research on the giant squid axon elucidated the ionic mechanisms underlying action potentials, the fundamental mechanism underlying signal transduction along brain cells, earning them the Nobel Prize in Physiology or Medicine in 1963. Similarly, Eric Kandel's work with sea slugs (Aplysia) uncovered the molecular mechanisms of learning and memory, leading to his Nobel Prize in 2000. In fact, approximately 85% of Nobel Prize recipients in the Physiology or Medicine category used animal models in their research, underscoring the indispensable role of animal research in contributing to significant scientific and medical advancements.

Similarly, animal research is essential for enhancing our understanding of human speech. Speaking is one of the most complex behaviors, involving the coordinated effort of various brain regions and over 100 muscles in the body. In a simplistic view, key brain areas involved include Broca's area, responsible for language production and constructing grammatically correct sentences; Wernicke's area, essential for language comprehension and coherent speech formulation; the motor cortex, which plans and executes voluntary movements necessary for speech; the basal ganglia, which coordinate the timing and smoothness of speech movements; and the cerebellum, which ensures smooth and coordinated speech by integrating sensory inputs and fine-tuning motor activities. The process of speech production begins with conceptualization, where the intention to communicate is formed, followed by formulation, where linguistic forms are selected, and sentences are constructed, and articulation, where the motor cortex signals muscles to produce sounds. Auditory feedback helps monitor and adjust speech in real time. Neural pathways like the arcuate fasciculus connect Broca's and Wernicke's areas, facilitating speech production and comprehension coordination. The basal ganglia's motor loop refines articulatory movements by promoting the correct motor action and impeding competing drives. As speech development progresses, the basal ganglia motor loop assumes control over the sequencing of individual phonemes, allowing higher cortical regions to handle more complex tasks. Speech disorders such as aphasia and stuttering can result from disruptions in the coordination of these brain pathways/regions, and understanding these mechanisms at the molecular, cellular, and circuit levels certainly offers critical insights into speech disorders and potential therapies.

Although human speech fundamentally differs from most animal vocalizations, animal models have been invaluable in studying the motor circuits that control vocal production. Various animal models, including vocal learners and non-vocal learners, have been used to explore the genetic, cellular, and circuit mechanisms underlying speech production. Some species, like humans, whales, dolphins, elephants, and certain birds (songbirds, hummingbirds, parrots), learn to produce novel vocalizations. This vocal production learning depends on specialized brain motor circuits and sensory feedback from the auditory system. These species are valuable models for studying vocal learning and the brain pathways involved. While not typically considered vocal learners, rodents show some degree of plasticity in their vocalization behaviors.

Like bats (and a few other species), rodents produce ultrasonic vocalizations (USVs) that vary with their emotional state and social interactions. Although we cannot hear USVs and special microphones and software are required to record and analyze these types of vocalizations, studies on rodent vocalizations have provided significant insights into the neural circuits involved in vocal production and how these might relate to human speech disorders, including stuttering.

A significant breakthrough in stuttering research in recent years has been the development of the first transgenic mouse model carrying a variant in the Gnptab gene, which is associated with stuttering in humans, by Dr. Dennis Drayna and his colleagues at the National Institute on Deafness and Other Communication Disorders (NIDCD). Previous data from Drayna's lab, together with our data, showed that the vocal behaviors of the mutant mice are different from their normal siblings. It is important to note that since human speech and mouse vocalizations are fundamentally different (audible speech vs. USVs), we should not expect that stuttering phenotypes in mice would mirror those observed in humans. The generation of the mouse model significantly helped researchers explore the genetic basis of stuttering and discover that stuttering might be related to a dysfunction of astrocytes, star-shaped cells in the brain.

Another significant contribution of the Gnptab-mutant mice involves the discovery of iron accumulation. Recent research has highlighted the potential role of iron accumulation in the brain in people who stutter. Iron is essential for normal brain function, but its dysregulation can lead to neurodegenerative and neurodevelopmental disorders. A recent study by Cler and colleagues, using advanced MRI techniques, found that individuals who stutter exhibit abnormal iron accumulation in the basal ganglia. This finding supports and expands on an earlier study by Liam and colleagues that used transcranial ultrasound to reveal increased brain iron level in adults who stutter. Surprisingly, we also found iron accumulated in the Gnptab-mutant mice's basal ganglia and other brain circuits involved in stuttering. The fact that 'stuttering mice' also showed iron accumulation strengthens the hypothesis that iron dysregulation might disrupt the normal function of brain circuits involved in speech, contributing to the development of stuttering. For the first time, these mice provide a platform to study stuttering at the molecular and cellular levels in the brain. 

Developmental stuttering affects over 80 million people globally, and its root causes remain largely unknown. Basic and clinical studies in individuals who stutter often employ non-invasive techniques to study brain regions linked to stuttering. Still, these methods lack the resolution to explore the molecular, cellular, and circuit mechanisms involved. Animal models, however, are crucial for advancing our understanding of stuttering. They offer a great opportunity to explore the basic science of vocal production at the cellular and circuits levels and investigate the genetic and molecular mechanisms underlying stuttering.

From the Fall 2024 Magazine