Effects of mediodorsal thalamic lesions on medial prefrontal cortex neurons during dDNMTP

Medial thalamus has reciprocal connections with prefrontal cortex (PFC) that are thought to coordinate its activity with other areas of the telencephalon that support flexible goal-directed behavior. The mediodorsal nucleus is the primary source of specific thalamo-cortical projections to middle layers of PFC while midline and intralaminar nuclei provide modulatory inputs to deep and superficial layers of PFC and to anatomically-related areas of the basal ganglia and hippocampus. Each of these nuclei receive direct cortico-thalamic and indirect cortico- striato-pallidal input from PFC.  To understand the nature of this interaction we are comparing the effects of mediodorsal lesions made before or after the rats have learned the dDNMTP task on the activity of medial prefrontal neurons.

As an example of this work, we recently examined the effects of temporary thalamic inactivation on the response properties of prefrontal neurons in rats performing a dynamic DNMTP task. Cellular activity was recorded throughout medial PFC using a drivable array of tetrodes. Unilateral inactivation had no significant effect on DNMTP performance. Day 2, inactivation was consistently associated with disruption of spatial and event-related activity of prefrontal neurons.  Some neurons exhibited increased and others and decreased levels of activity, suggesting both inhibitory and excitatory modulation PFC by thalamus. These data indicate a critical role for medial thalamus in shaping the activity of cortical neurons in relation to actions, outcomes, and context.

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