Altered, Neurophysiology

Discrete parts of nerve cells, single neurons, simple circuits, and complex brain systems have been used to evaluate the impact of thyroid disruption on brain function. The nervous system functions as a highly integrated and organized communication and information processing network. Neurons transmit and receive information from sensory and motor organs, but the largest number of neuronal connections is with other neurons. This is largely accomplished by chemical transmission across the synaptic cleft, the space where the specialized ending of the presynaptic axon terminus of the transmitting neuron meets the specialized postsynaptic region of the neuron that is receiving that information. Activation of the presynaptic neuron to produce an action potential causes the release of neurotransmitter substances into the synaptic cleft. Most of the released neurotransmitters bind with molecules at receptors on the dendrites of the postsynaptic neuron. The chemical signal is then transduced back into an electrical impulse which travels in the receiving neuron and if of sufficient magnitude, fires the neuron, and the signal is transduced once again to a chemical signal in the next neuron along the pathway. The scale of neurophysiological techniques that can be used ranges along a broad continuum from the very small (e.g., individual ion channel fluxes) to the very large anatomical pathways (e.g., electroencephalograms) in both in vitro and in vivo preparations.

A number of studies on TH and brain function have incorporated field potentials in the hippocampus, an area critical for certain forms of learning and memory, to probe functional integrity after TH insufficiency (e.g., Dong et al., 2005; Gilbert and Sui, 2006; Opazo et al., 2008; Vara et al., 2002). Field potentials have been recorded following thyroid disruption both in vivo and in vitro, at two discrete hippocampal synaptic regions, area CA1 and the dentate gyrus. Excitatory and inhibitory synaptic transmission are assessed by recording electrical field potentials (voltage changes across large populations of neurons) across a monosynaptic circuit (e.g. axons from cortical neurons that synapse on dentate gyrus granule cells) in response to electrical pulses delivered to the incoming pathway (Gilbert and Burdette, 1995).

The synapses in this region of the brain are also intensely studied as they are imbued with a capacity for use-dependent plasticity, the best studied model being long-term potentiation (LTP). LTP is a model of information acquisition (learning) and storage (memory) at the synaptic level. It is induced by applying trains of stimulus pulses at high frequencies to the incoming pathway of the monosynaptic circuit and measuring the amplitude of the induced change in synaptic responsiveness that persists for hours, and in some instances days to weeks (Malenka and Nicoll, 1999; Martinez and Derrick, 1996; Gilbert and Burdette, 1995). The induction of LTP is believed to emulate, both at the synaptic and molecular level, the coincident firing of large numbers of neurons that are engaged during a learning event. The persistence of LTP emulates the duration of the memory of that learning event.

Field potentials are recorded from slices of hippocampus taken from exposed animals, or from indwelling electrodes placed within the appropriate hippocampal field. One electrode is placed in the afferent fiber pathway (e.g., perforant path for dentate gyrus, Schaeffer collaterals for CA1 region) and a brief electrical pulse applied to stimulate these axons. A second electrode is placed in the synaptic or cell body region to record the activity evoked by the incoming pulse. Electrodes are placed visually according to established landmarks in in vitro preparations, and acccording to stereotaxic coordinates for in vivo preparations. Once accurately placed, a series of stimulus pulses at increasing stimulus current intensities are applied to the input pathway, and the response evoked in the receiving neuronal population is recorded.

Excitatory Synaptic Transmission: Two measures, the excitatory postsynaptic potential (EPSP) and the population spike are derived from the compound field potential in response to a series of single pulse stimulations applied at increasing stimulus strengths. The function described by the relationship of current strength (input) and evoked response (output), the I-O curve is the measure of excitatory synaptic transmission.

Inhibitory Synaptic Transmission: Pairs of stimulus pulses delivered in close temporal proximity is used to probe the integrity of inhibitory synaptic transmission. The response evoked by the second pulse of the pair at brief intervals (<30msec) arrives during the activation of feedback inhibitory loops in the hippocampus. An alteration in the degree of suppression to the 2nd pulse of the pair reflects altered inhibitory synaptic function.

Long Term Potentiation (LTP): Synaptic plasticity in the form of LTP is assessed by delivering trains of high frequency stimulation to induce a prolonged augmentation of synaptic responsivity. Probe stimuli at mid-range stimulus strenghts are delivered before and after application of LTP-inducing trains. The degree of increase in EPSP and PS amplitude to the probe stimulus after train application, and the longevity of that induced enhancement are metrics of LTP. Additionally, contrasting I-O functions of excitatory synaptic transmission before and after (1-5 hours) LTP is induced is also a common measure of induced LTP.

Assays of this type are fit for purpose, have been well accepted in the literature, and are reproducible across laboratories. The assay directly measures the key event of altered neurophysiological function.

The majority of evidence for this key event come from work in rodent species (i.e., rat, mouse). there is a moderate amount of evidence from other species.

Dong, J., H. Yin, et al. (2005). "Congenital iodine deficiency and hypothyroidism impair LTP and decrease C-fos and C-jun expression in rat hippocampus." Neurotoxicology 26(3): 417-26.

Gilbert, M.E. and Burdette, L.J. (1995). Hippocampal Field Potentials: A Model System to Characterize Neurotoxicity. In Neurotoxicology: Approaches and Methods. L.W Chang and W. Slikker (Eds). Academic Press:New York, 183-204.

Gilbert, M. E. and L. Sui (2006). "Dose-dependent reductions in spatial learning and synaptic function in the dentate gyrus of adult rats following developmental thyroid hormone insufficiency." Brain Res 1069(1): 10-22.

Malenka RC, Nicoll RA (1999) Long-term potentiation--a decade of progress? Science, 285:1870-1874.

Martinez, J.L. and Derrick, B.E. (1996). Long term potentiation and learning. Annual Review of Psychology, 47,173-203.

Opazo MC, Gianini A, Pancetti F, Azkcona G, Alarcón L, Lizana R, Noches V, Gonzalez PA, Marassi MP, Mora S, Rosenthal D, Eugenin E, Naranjo D, Bueno SM, Kalergis AM, Riedel CA (2008), Maternal hypothyroxinemia impairs spatial learning and synaptic nature and function in the offspring. Endocrinology 149:5097-5106.

Vara H, et al. Thyroid hormone regulates neurotransmitter release in neonatal rat hippocampus. Neuroscience 2002, 110(1), 19-28.