Rapid shifts in receptive fields of cells in trigeminal subnucleus interpolaris following infraorbital nerve transection in adult rats

Cortical influences on rapid brainstem plasticity

Cortical contributions to brainstem plasticity in the somatosensory system are poorly understood. Tactile receptive fields (RFs) of brainstem dorsal column nuclei (DCN) neurons rapidly enlarge when peripheral inputs are disrupted by local anesthetic blocks with lidocaine (LID). Cortical inputs appear to influence this plasticity because enlargements have been shown to be greater when cortical inputs are disrupted. Like disruptions of peripheral inputs, disruptions of DCN inhibition by DCN administration of the GABAA receptor antagonist bicuculline methiodide (BMI) also cause rapid enlargements of DCN RFs when cortical inputs are intact. These findings leave questions about interactions between cortical inputs, DCN inhibition, and DCN RF plasticity. To study potential interactions, the present experiments evaluated RF sizes of DCN tactilely responsive neurons in anesthetized rats following DCN microinjection of BMI when cortical inputs were acutely disrupted or intact. These tests were also supplemented by subsequent LID tests to directly compare post-BMI and post-LID effects on the same RF. BMI caused DCN RF enlargements when cortical inputs were disrupted or intact; however, enlargements after cortical input disruption were greater than when cortical inputs were intact. Following RF enlargement and retraction after BMI, LID often caused a second enlargement of the same RF, across skin that partially matched skin involved in the enlargement after BMI. This occurred when cortical inputs were disrupted or intact. We hypothesize that cortical inputs are not required for BMI and LID to initiate partially matching enlargements in individual DCN tactile RFs, however, cortical inputs constrain magnitudes of these enlargements.

Mechanisms underlying reorganization of fractured tactile cerebellar maps after deafferentation in developing and adult rats

Our previous studies showed that fractured tactile cerebellar maps in rats reorganize after deafferentation during development and in adulthood while maintaining a fractured somatotopy. Several months after deafferentation of the infraorbital branch of the trigeminal nerve, the missing upper lip innervation is replaced in the tactile maps in the granule cell layer of crus IIa. The predominant input into the denervated area is always the upper incisor representation. This study examined whether this reorganization was caused by mechanisms intrinsic to the cerebellum or extrinsic, i.e., occurring in somatosensory structures afferent to the cerebellum. We first compared normal and deafferented maps and found that the expansion of the upper incisor is not caused by a preexisting bias in the strength or abundance of upper incisor input in normal animals. We then mapped tactile representations before and immediately after denervation. We found that the pattern of reorganization observed in the cerebellum several months later is not caused by unmasking of a silent or weaker upper incisor representation. Both results indicate that the reorganization is not a result of subsequent growth or sprouting mechanism within the cerebellum itself. Finally, we compared postlesion maps in the cerebellum and the somatosensory cortex. We found that the upper incisor representation significantly expands in both regions and that this expansion is correlated, suggesting that reorganization in the cerebellum is a passive consequence of reorganization in afferent cerebellar pathways. This result has important developmental and functional implications.

Simultaneous multi-site recordings and iontophoretic drug and dye applications along the trigeminal system of anesthetized rats

A multi-electrode system that permits simultaneous recordings from multiple neurons and iontophoretic applications at two or three different brain sites during acute experiments is described. This system consists of two or three microdrive terminals, each of which includes four electrodes that can be moved independently and used for both extracellular recordings and microiontophoretic drug administration. Drug applications were performed during standard extracellular recordings of multiple single-units via specialized combined electrodes (CEs), which enable ejection of neuroactive substances and recording of neuronal activity from the same electrode. With this system, we were able to successfully record simultaneously from different levels (brainstem, thalamus, and cortex) of the vibrissal ascending pathway of the anesthetized rat. Herein, examples of simultaneous recordings from the brainstem and thalamus and from the thalamus and cortex are presented. An effect of iontophoretic applications of agonists and antagonists of metabotropic glutamate receptors (mGluRs) in the thalamus is demonstrated, and the extent of drug diffusion in the barrel cortex is demonstrated with biocytin. This new multi-electrode system will facilitate the study of transformations of sensory information acquired by the whiskers into cortical representations.