Anesthetics eliminate somatosensory-evoked discharges of neurons in the somatotopically organized sensorimotor striatum of the rat

The somatotopic organization of the lateral striatum has been demonstrated by anatomical studies of corticostriatal projections from somatosensory and motor cortices and by single-cell recordings in awake animals. The functional organization in the rat, characterized thus far in the freely moving rat preparation, could be mapped more precisely if a stereotaxic, and possibly an anesthetized, preparation could be used. Because striatal discharges evoked by innocuous somatosensory stimulation are used in mapping, this study tested whether such discharges can be observed during anesthesia, encouraged by responsiveness during anesthesia in somatosensory cortical layers projecting to the striatum. Electrode tracks through lateral striatum of anesthetized rats (pentobarbital or ketamine) revealed spontaneously discharging neurons but no discharges evoked by somatosensory examination (passive manipulation and cutaneous stimulation of 14 body parts). Similar tracks in chronically implanted rats showed evoked firing at numerous sites during wakefulness but not during anesthesia (pentobarbital or urethane). Comparisons of the activity of individual neurons between wakefulness and anesthesia showed that pentobarbital, ketamine, chloral hydrate, urethane, or metofane eliminated evoked firing and suppressed spontaneous firing. Recovery time was greater for neural than for behavioral measures. Thus, mapping as proposed is ruled out, and more importantly, the data show that somatotopically organized lateral striatal neurons stop discharging in response to natural stimulation during anesthesia. Available data indicate they do not reach threshold in response to depolarizations produced by glutamatergic corticostriatal synaptic transmission projected from the somatosensory cortex. These data and demonstrations of anesthetic- induced imbalances in most striatal neurotransmitters emphasize that many results regarding striatal physiology and pharmacology during anesthesia cannot be extrapolated to behavioral conditions, thus indicating the need for more empirical testing in conscious animals.