Thalamic relay cells fire in two distinct modes, burst or tonic, and the operative mode is dictated by the inactivation state of low-threshold, voltage-gated, transient (or T-type) Ca 2ϩ channels. Tonic firing is seen when the T channels are inactivated via membrane depolarization, and burst firing is seen when the T channels are activated from a hyperpolarized state. These response modes have very different effects on the relay of information to the cortex. It had been thought that only tonic firing is seen in the awake, alert animal, but recent evidence from several species suggests that bursting may also occur. We have begun to explore this issue in macaque monkeys by recording from thalamic relay cells of unanesthetized, behaving animals. In the lateral geniculate nucleus, the thalamic relay for retinal information, we found that tonic mode dominated responses both during alert behavior as well as during sleep. We nonetheless found burst firing present during the vigilant, waking state. There was, however, considerably more burst mode firing during sleep than wakefulness. Surprisingly, we did not find the bursting during sleep to be rhythmic. We also recorded from relay cells of the somatosensory thalamus. Interestingly, not only did these somatosensory neurons exhibit much more burst mode activity than did geniculate cells, but bursting during sleep was highly rhythmic. It thus appears that the level and nature of relay cell bursting may not be constant across all thalamic nuclei.

There is a strong correlation between the behavior of an animal and the firing mode (burst or tonic) of thalamic relay neurons. Certain differences between first-and higher-order thalamic relays (which relay peripheral information to the cortex versus information from one cortical area to another, respectively) suggest that more bursting might occur in the higher-order relays. Accordingly, we recorded bursting behavior in single cells from awake, behaving rhesus monkeys in first-order (the lateral geniculate nucleus, the ventral posterior nucleus, and the ventral portion of the medial geniculate nucleus) and higher-order (pulvinar and the medial dorsal nucleus) thalamic relays. We found that the extent of bursting was dramatically greater in the higher-order than in the first-order relays, and this increased bursting correlated with lower spontaneous activity in the higher-order relays. If bursting effectively signals the introduction of new information to a cortical area, as suggested, this increased bursting may be more important in corticocortical transmission than in transmission of primary information to cortex. pulvinar ͉ lateral geniculate nucleus ͉ medial dorsal nucleus ͉ medial geniculate nucleus ͉ ventral posterior nucleus A key feature of the thalamus is the ability of its relay cells to fire in two distinct modes (called tonic and burst), and the firing mode is determined by the inactivation state of voltagegated, T-type Ca 2ϩ channels in the membranes of the soma and dendrites (1-3). This firing mode strongly affects the nature of the signal that is relayed to the cortex (4). For example, compared with tonic mode, burst mode produces much more nonlinear distortion in the relay of information, but the information relayed has greater detectability because of a greater signal-to-noise ratio and stronger activation of postsynaptic cortical targets (4-9). From these properties, the hypothesis has been forwarded that burst firing, with its greater detectability and cortical activation, serves as a ''wake-up call'' to the cortex that there has been a change in the outside world (e.g., a novel stimulus within the receptive field of a relay cell for one of the sensory thalamic nuclei); tonic mode, with its more linear relay of information, is then better suited for a more faithful analysis of the relayed information (4, 7). Thus, burst mode would be more effective for cells dealing with information that is not fully attended to, and the burst would help to redirect attention to the novel stimulus and, ultimately, lead to a shift in firing to tonic mode.Evidence of burst and tonic firing has been reported in various species during waking behavior, including cats (10-12), rats (13-16), guinea pigs (17, 18), rabbits (5, 6), monkeys (19), and humans (20)(21)(22). In general, bursting is relatively rare during full wakefulness and more common during periods of inattention or drowsiness (6). However, nearly all of these data were obtained from cells in first-order thalamic relays, namely, the lateral geniculate nu...

The lateral geniculate nucleus (LGN) is the thalamic relay of retinal information to cortex. An extensive complement of nonretinal inputs to the LGN combine to modulate the responsiveness of relay cells to their retinal inputs, and thus control the transfer of visual information to cortex. These inputs have been studied in the most detail in the cat. The goal of the present study was to determine whether the neurotransmitters used by nonretinal afferents to the monkey LGN are similar to those identified in the cat. By combining the retrograde transport of tracers injected into the monkey LGN with immunocytochemical labeling for choline acetyl transferase, brain nitric oxide synthase, glutamic acid decarboxylase, tyrosine hydroxylase, or the histochemical nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase reaction, we determined that the organization of neurotransmitter inputs to the monkey LGN is strikingly similar to the patterns occurring in the cat. In particular, we found that the monkey LGN receives a significant cholinergic/nitrergic projection from the pedunculopontine tegmentum, gamma-aminobutyric acid (GABA)ergic projections from the thalamic reticular nucleus and pretectum, and a cholinergic projection from the parabigeminal nucleus. The major difference between the innervation of the LGN in the cat and the monkey is the absence of a noradrenergic projection to the monkey LGN. The segregation of the noradrenergic cells and cholinergic cells in the monkey brainstem also differs from the intermingled arrangement found in the cat brainstem. Our findings suggest that studies of basic mechanisms underlying the control of visual information flow through the LGN of the cat may relate directly to similar issues in primates, and ultimately, humans.

We show for the first time with in vitro recording that burst firing in thalamic relay cells of the monkey is evoked by activation of voltage-dependent, low threshold Ca(2+) spikes (LTSs), as has been described in other mammals. Due to variations in LTS amplitude, the number of action potentials evoked by an LTS could vary between 1 and 8. These data confirm the presence of two modes of firing in the monkey for thalamic relay cells, tonic and burst, the latter related to the activation of LTSs. With these details of the cellular processes underlying burst firing, we could account for many of the firing patterns we recorded from the lateral geniculate nucleus of the thalamus in behaving monkeys. In particular, we found clear evidence of burst firing during alert wakefulness, which had been thought to occur only during sleep or certain pathological states. This makes it likely that the burst firing seen in awake humans has the same cellular basis of LTSs, and this supports previous suggestions that burst firing represents an important relay mode for visual processing.

Although the hippocampal theta rhythm is thought to be linked to memory processes, its mechanism of action is unknown. Furthermore, the hippocampus forms strong connections with a functionally similar structure, the medial prefrontal cortex (mPFC). The midline thalamus appears to be an intermediate between these two structures. We recorded neurons of a midline nucleus (nucleus reuniens, RE) during theta and non-theta states. Additionally, we recorded hippocampal CA1 population responses to RE stimulation. RE cell firing patterns are classified as (i) spike rate response to stimulation (ii) determination of bursting events (iii) coherence estimation between hippocampal EEG and RE response to stimulation (within the theta frequency band of 5 - 12 Hz). The present data suggests an increase in RE spike rate due to tail pinch elicited theta activity, with no evidence of bursting activity and a weak coherence within the theta band. Furthermore, we evaluated evoked excitatory post-synaptic potentials (EPSPs) in the hippocampal CA1 to RE stimulation, as well as entorhinal cortex (EC) stimulation. We demonstrated a consistent reduction in evoked potential (EP) latency at CA1 to RE and EC stimulation during theta compared to non-theta states.

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We present a case of spontaneously occurring irrepressible saccades in an experimental Rhesus monkey. Though eye jerks are sometimes associated with cerebellar disease, central demyelination or brainstem lesions, there is little consensus on their neurological mechanisms. From neurological and anatomical investigation we report that these irrepressible saccades were caused by a discrete cerebrovascular accident that involved the rostral superior colliculus along with its commissure, and with minor invasion of periaqueductal gray and adjacent mesencephalic reticular formation. Other suspected structures, like the raphe interpositus, substantia nigra and the cerebellum, were unaffected.

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