Regulation of Limbic Information Outflow by the Subthalamic Nucleus: Excitatory Amino Acid Projections to the Ventral Pallidum

Abstract

The subthalamic nucleus (STN), a component of the basal ganglia motor system, sends an excitatory amino acid (EAA)-containing projection to the ventral pallidum (VP), a major limbic system output region. The VP contains both NMDA and AMPA subtypes of EAA receptors. To characterize the physiology of the subthalamic pathway to the VP, and to determine the influence of EAA receptor subtypes, in vivo intracellular recordings, and in vivo extracellular recordings combined with microiontophoresis, were made from VP neurons in anesthetized rats. Of the intracellularly recorded neurons, 86% responded to STN stimulation, and these displayed EPSPs with an onset of 8.7 msec, consistent with a monosynaptic input. The EPSPs evoked in spontaneously firing neurons were nearly twice the amplitude of those in nonfiring cells (13.1 vs 6.8 mV, respectively). As neurons were depolarized by current injection, the latency for spiking decreased from 24.2 to 14.2 msec, although EPSP latency was unaffected. Eighty-seven percent of the extracellularly recorded VP neurons responded to STN stimulation with a rapid and robust enhancement of spiking; the response onset, like the EPSP onset, equaled 8.7 msec. Firing rate was enhanced by NMDA in 94% of the STN-excited cells, and AMPA increased firing in 94% as well. The NMDA-selective antagonist AP-5 attenuated 67% of the STN-evoked excitatory responses, and the AMPA-selective antagonist CNQX attenuated 52%. Both antagonists attenuated 33% of responses, and 78% were attenuated by at least one. This evidence suggests that a great majority of VP neurons are directly influenced by STN activation and that both NMDA and non-NMDA receptors are involved. Moreover, the VP response to STN stimulation appears to be strongly dependent on the depolarization state of the neuron.Key words: ventral pallidum; subthalamic nucleus; basal ganglia; NMDA; AMPA; CNQX; electrophysiology The subthalamic nucleus (STN) is a basal ganglia structure involved in processes related to both normal motor function and movement disorders (Albin et al., 1989;Graybiel et al., 1994). Damage to the STN can result in hemiballismus (Whittier and Mettler, 1949;Guridi and Obeso, 1997), whereas surgically placed lesions of the STN in Parkinson's disease patients reverse many of the motor symptoms associated with this pathology (Guridi and Obeso, 1997). STN function may not be limited to effects on motor systems, for excitotoxic lesions of the STN in rats induce deficits in attentional tasks (Baunez and Robbins, 1997). It is plausible that the nonmotor functions of the STN reflect its reciprocal projections with the ventral pallidum (VP) (Groenewegen and Berendse, 1990), a brain region that serves as a major output from the limbic (ventral) striatum (Heimer et al., 1985). As suggested by the anatomy of the VP and its associated structures, recent electrophysiological studies have demonstrated that the VP integrates information flow among distinct limbic and basal ganglia systems (Chrobak and Napier, 1993;Maslowski-Cobuzzi and Napier,...

Changes in Accumbal and Pallidal pCREB and ΔFosB in Morphine-Sensitized Rats: Correlations with Receptor-Evoked Electrophysiological Measures in the Ventral Pallidum

“…Thus, for the pooled data, 85% of the recorded action potentials were biphasic, the initial deflection for the action potential was in the negative direction for 73% and the average action potential peak to peak amplitude was 426 ± 24 μV, with a duration of 1.30 ± 0.03 ms. These action potential characteristics are consistent with prior studies (Mitrovic and Napier, 1996;Turner et al, 2001Turner et al, ,2002 and they suggest that we have been sampling from similar populations of VP neurons. Also similar to prior studies, the spontaneous firing rate for VP neurons recorded from saline-pretreated rats was 15 ± 1 spikes/s.…”

Alcohol Preference Influences the Subthalamic Nucleus Control on Motivation for Alcohol in Rats

“…The limbic areas of the prefrontal cortex activate the STN, either by a direct excitatory input (the so-called 'hyperdirect pathway') or by an indirect disinhibitory pathway linking the NAC core and the STN via the VP (Maurice et al, 1998a, b). In return, the VP receives glutamatergic projections from the STN (Groenewegen and Berendse, 1990), thus allowing the STN to regulate the limbic output of the basal ganglia (Turner et al, 2001). Electrophysiological data have demonstrated that various neuronal populations in the NAC and the ventral striatum are activated by natural reward and drugs of abuse in behaving rats and monkeys (Bowman et al, 1996;Carelli, 2002;Carelli et al, 2000).…”

“…Within the cortico-basal ganglia-thalamocortical limbic loop, the subthalamic nucleus (STN) is a key structure that may modulate the basal ganglia outflow (Turner et al, 2001). The STN is, above all, currently known as a therapeutic target for Parkinson's disease treatment with high-frequency stimulation (HFS) (Limousin et al, 1995).…”

The Reward Circuit: Linking Primate Anatomy and Human Imaging

“…It demonstrated that a large part of the area referred to as 'substantia innominata' is actually an extension of the reward-related striatopallidal complex (Alheid and Heimer, 1988). In addition to receiving VS input, the VP also receives a glutamatergic input from the subthalamic nucleus (STN) and a dopaminergic input from the midbrain (Klitenick et al, 1992;Turner et al, 2001) (Figure 8).…”