GABAergic axons in the ventral forebrain of the rat: an electron microscopic study

Ventral pallidal regulation of motivated behaviors and reinforcement

The interconnected nuclei of the ventral basal ganglia have long been identified as key regulators of motivated behavior, and dysfunction of this circuit is strongly implicated in mood and substance use disorders. The ventral pallidum (VP) is a central node of the ventral basal ganglia, and recent studies have revealed complex VP cellular heterogeneity and cell- and circuit-specific regulation of reward, aversion, motivation, and drug-seeking behaviors. Although the VP is canonically considered a relay and output structure for this circuit, emerging data indicate that the VP is a central hub in an extensive network for reward processing and the regulation of motivation that extends beyond classically defined basal ganglia borders. VP neurons respond temporally faster and show more advanced reward coding and prediction error processing than neurons in the upstream nucleus accumbens, and regulate the activity of the ventral mesencephalon dopamine system. This review will summarize recent findings in the literature and provide an update on the complex cellular heterogeneity and cell- and circuit-specific regulation of motivated behaviors and reinforcement by the VP with a specific focus on mood and substance use disorders. In addition, we will discuss mechanisms by which stress and drug exposure alter the functioning of the VP and produce susceptibility to neuropsychiatric disorders. Lastly, we will outline unanswered questions and identify future directions for studies necessary to further clarify the central role of VP neurons in the regulation of motivated behaviors. Significance: Research in the last decade has revealed a complex cell- and circuit-specific role for the VP in reward processing and the regulation of motivated behaviors. Novel insights obtained using cell- and circuit-specific interrogation strategies have led to a major shift in our understanding of this region. Here, we provide a comprehensive review of the VP in which we integrate novel findings with the existing literature and highlight the emerging role of the VP as a linchpin of the neural systems that regulate motivation, reward, and aversion. In addition, we discuss the dysfunction of the VP in animal models of neuropsychiatric disorders.

Characterization of age-related changes in synaptic transmission onto F344 rat basal forebrain cholinergic neurons using a reduced synaptic preparation

Basal forebrain (BF) cholinergic neurons participate in a number of cognitive processes that become impaired during aging. We previously found that age-related enhancement of Ca(2+) buffering in rat cholinergic BF neurons was associated with impaired performance in the water maze spatial learning task (Murchison D, McDermott AN, Lasarge CL, Peebles KA, Bizon JL, and Griffith WH. J Neurophysiol 102: 2194-2207, 2009). One way that altered Ca(2+) buffering could contribute to cognitive impairment involves synaptic function. In this report we show that synaptic transmission in the BF is altered with age and cognitive status. We have examined the properties of spontaneous postsynaptic currents (sPSCs) in cholinergic BF neurons that have been mechanically dissociated without enzymes from behaviorally characterized F344 rats. These isolated neurons retain functional presynaptic terminals on their somata and proximal dendrites. Using whole cell patch-clamp recording, we show that sPSCs and miniature PSCs are predominately GABAergic (bicuculline sensitive) and in all ways closely resemble PSCs recorded in a BF in vitro slice preparation. Adult (4-7 mo) and aged (22-24 mo) male rats were cognitively assessed using the water maze. Neuronal phenotype was identified post hoc using single-cell RT-PCR. The frequency of sPSCs was reduced during aging, and this was most pronounced in cognitively impaired subjects. This is the same population that demonstrated increased intracellular Ca(2+) buffering. We also show that increasing Ca(2+) buffering in the synaptic terminals of young BF neurons can mimic the reduced frequency of sPSCs observed in aged BF neurons.

Vesicular glutamate transporter 1 and vesicular glutamate transporter 2 synapses on cholinergic neurons in the sublenticular gray of the rat basal forebrain: a double-label electron microscopic study

The basal forebrain (BF) comprises morphologically and functionally heterogeneous cell populations, including cholinergic and non-cholinergic corticopetal neurons that are implicated in sleep-wake modulation, learning, memory and attention. Several studies suggest that glutamate may be among inputs affecting cholinergic corticopetal neurons but such inputs have not been demonstrated unequivocally. We examined glutamatergic axon terminals in the sublenticular substantia innominata in rats using double-immunolabeling for vesicular glutamate transporters (Vglut1 and Vglut2) and choline acetyltransferase (ChAT) at the electron microscopic level. In a total surface area of 30,000 microm(2), we classified the pre- and postsynaptic elements of 813 synaptic boutons. Vglut1 and Vglut2 boutons synapsed with cholinergic dendrites, and occasionally Vglut2 axon terminals also synapsed with cholinergic cell bodies. Vglut1 terminals formed synapses with unlabeled dendrites and spines with equal frequency, while Vglut2 boutons were mainly in synaptic contact with unlabeled dendritic shafts and occasionally with unlabeled spines. In general, Vglut1 boutons contacted more distal dendritic compartments than Vglut2 boutons. About 21% of all synaptic boutons (n=347) detected in tissue that was stained for Vglut1 and ChAT were positive for Vglut1, and 14% of the Vglut1 synapses were made on cholinergic profiles. From separate cases stained for Vglut2 and ChAT, 35% of all synaptic boutons (n=466) were positive for Vglut2, and 23% of the Vglut2 synapses were made on cholinergic profiles. On average, Vglut1 boutons were significantly smaller than Vglut2 synaptic boutons. The Vglut2 boutons that synapsed cholinergic profiles tended to be larger than the Vglut2 boutons that contacted unlabeled, non-cholinergic postsynaptic profiles. The presence of two different subtypes of Vgluts, the size differences of the Vglut synaptic boutons, and their preference for different postsynaptic targets suggest that the action of glutamate on BF neurons is complex and may arise from multiple afferent sources.

Forebrain circuitry involved in effort-related choice: Injections of the GABAA agonist muscimol into ventral pallidum alter response allocation in food-seeking behavior

Organisms often make effort-related choices based upon assessments of motivational value and work requirements. Nucleus accumbens dopamine is a critical component of the brain circuitry regulating work output in reinforcement-seeking behavior. Rats with accumbens dopamine depletions reallocate their instrumental behavior away from food-reinforced tasks that have high response requirements, and instead they select a less-effortful type of food-seeking behavior. The ventral pallidum is a brain area that receives substantial GABAergic input from nucleus accumbens. It was hypothesized that stimulation of GABA(A) receptors in the ventral pallidum would result in behavioral effects that resemble those produced by interference with accumbens dopamine transmission. The present studies employed a concurrent choice lever pressing/chow intake procedure; with this task, interference with accumbens dopamine transmission shifts choice behavior such that lever pressing for food is decreased but chow intake is increased. In the present experiments, infusions of the GABA(A) agonist muscimol (5.0-10.0 ng) into the ventral pallidum decreased lever pressing for preferred food, but increased consumption of the less preferred chow. In contrast, ventral pallidal infusions of muscimol (10.0 ng) had no significant effect on preference for the palatable food in free-feeding choice tests. Furthermore, injections of muscimol into a control site dorsal to the ventral pallidum produced no significant effects on lever pressing and chow intake. These data indicate that stimulation of GABA receptors in ventral pallidum produces behavioral effects similar to those produced by accumbens dopamine depletions. Ventral pallidum appears to be a component of the brain circuitry regulating response allocation and effort-related choice behavior, and may act to convey information from nucleus accumbens to other parts of this circuitry. This research may have implications for understanding the brain mechanisms involved in energy-related psychiatric dysfunctions such as psychomotor retardation in depression, anergia, and apathy.

Neurokinin-1 receptors in cholinergic neurons of the rat ventral pallidum have a predominantly dendritic distribution that is affected by apomorphine when combined with startle-evoking auditory stimulation

Cholinergic neurons of the basal forebrain are implicated in startle reflex inhibition by a prior weak stimulus often referred to as prepulse inhibition (PPI) and used as an index of sensorimotor gating deficits in schizophrenia. Gating deficits can be produced in rodent models by acute systemic administration of apomorphine, a non-selective dopamine D1 and D2 receptor agonist that also affects trafficking of neurokinin-1 (NK(1)) receptors induced by startle evoking auditory stimulation (AS) in midbrain neurons. We used electron microscopic immunolabeling of NK(1) receptors and the vesicular acetylcholine transporter (VAchT) to test the hypothesis that the subcellular distributions of these receptors in cholinergic neurons of the rat ventral pallidum are subject to a similar regulation. In vehicle controls, NK(1) immunogold was often seen near cytoplasmic endomembranes in somata and large dendrites, but was more equally distributed in cytoplasmic and plasmalemmal compartments of medium dendrites, and principally located on the plasma membrane of small dendrites. These labeling patterns appeared to be largely independent of whether the NK(1) receptor was co-expressed with VAchT, however only the medium and small VAchT-labeled dendrites showed significant treatment-specific differences in NK(1) immunogold distributions. The NK(1) receptor immunogold particle density on the plasma membrane of medium cholinergic dendrites was significantly enhanced by combined apomorphine and AS, while neither alone affected either the plasmalemmal density or the equality of the plasmalemmal and cytoplasmic distributions of NK(1) receptors in these dendrites. Small cholinergic dendrites showed a significant AS-induced increase in both the plasmalemmal and cytoplasmic density of NK(1) gold particles, and an apomorphine-induced disruption of the preferential plasmalemmal targeting of the NK(1) receptors. These results provide ultrastructural evidence that NK(1) receptors in cholinergic neurons of the ventral pallidum have subcellular locations and plasticity conducive to active involvement in dopamine-dependent sensorimotor processing.

Projections from the amygdaloid complex to the magnocellular cholinergic basal forebrain in rat

The amygdaloid complex has a key role in the modulation of behavioral responses in life-threatening situations, including the direction of attentional responses to sensory stimuli. The pathways from the amygdala to the basal forebrain cholinergic system, which projects to the cortex, are proposed to contribute to the modulation. To further explore the topography and postsynaptic targets of these pathways, we investigated the projections from the different divisions of the lateral, basal, accessory basal, and central nuclei of the amygdala to the cholinergic basal forebrain in rat using a sensitive anterograde tracer, Phaseolus vulgaris leucoagglutinin. The most substantial projections from the amygdala to the basal forebrain are directed to the ventrolateral and dorsomedial aspects of the substantia innominata and the fundus of the striatum. The heaviest projections originate in the capsular, lateral, and intermediate divisions of the central nucleus as well as in the magnocellular and parvicellular divisions of the basal nucleus. Light microscopic analysis of double-stained preparations revealed that the distribution of amygdaloid efferents and cholinergic neurons overlaps most prominently in the ventrolateral substantia innominata. Despite the fact that the central nucleus efferents and cholinergic elements overlap in the ventrolateral substantia innominata, electron microscopic analysis revealed, first, that the postsynaptic targets of the central nucleus efferents are non-cholinergic, probably GABAergic, neurons. Second, 80% of the synaptic contacts were symmetric. The present data extend previous observations showing that the different amygdaloid nuclei provide projections to the selective basal forebrain areas. Further, the central nucleus efferents modulate cholinergic neurons in the basal forebrain indirectly via the GABAergic interneurons.

GABAergic input to cholinergic nucleus basalis neurons

The potential influence of GABAergic input to cholinergic basalis neurons was studied in guinea-pig basal forebrain slices. GABA and its agonists were applied to electrophysiologically-identified cholinergic neurons, of which some were labelled with biocytin and confirmed to be choline acetyltransferase-immunoreactive. Immunohistochemistry for glutamate decarboxylase was also performed in some slices and revealed GABAergic varicosities in the vicinity of the biocytin-filled soma and dendrites of electrophysiologically-identified cholinergic cells. From rest (average - 63 mV), the cholinergic cells were depolarized by GABA. The depolarization was associated with a decrease in membrane resistance and diminution in firing. The effect was mimicked by muscimol, the specific agonist for GABA(A) receptors, and not by baclofen, the specific agonist for GABA(B) receptors, which had no discernible effect. The GABA- and muscimol-evoked depolarization and decrease in resistance were found to be postsynaptic since they persisted in the presence of solutions containing either high Mg2+/low Ca2+ or tetrodotoxin. They were confirmed as being mediated by a GABA(A) receptor, since they were antagonized by bicuculline. The reversal potential for the muscimol effect was estimated to be approximately -45 mV, which was -15 mV above the resting membrane potential. Finally, in some cholinergic cells, spontaneous subthreshold depolarizing synaptic potentials (average 5 mV in amplitude), which were rarely associated with action potentials, were recorded and found to persist in the presence of glutamate receptor antagonists but to be eliminated by bicuculline. These results suggest that GABAergic input may be depolarizing, yet predominantly inhibitory to cholinergic basalis neurons.

Neurosteroids: deficient cognitive performance in aged rats depends on low pregnenolone sulfate levels in the hippocampus

Pregnenolone sulfate (PREG S) is synthesized in the nervous system and is a major neurosteroid in the rat brain. Its concentrations were measured in the hippocampus and other brain areas of single adult and aged (22-24 month-old) male Sprague-Dawley rats. Significantly lower levels were found in aged rats, although the values were widely scattered and reached, in about half the animals, the same range as those of young ones. The spatial memory performances of aged rats were investigated in two different spatial memory tasks, the Morris water maze and Y-maze. Performances in both tests were significantly correlated and, accompanied by appropriate controls, likely evaluated genuine memory function. Importantly, individual hippocampal PREG S and distance to reach the platform in the water maze were linked by a significant correlation, i.e., those rats with lower memory deficit had the highest PREG S levels, whereas no relationship was found with the PREG S content in other brain areas (amygdala, prefrontal cortex, parietal cortex, striatum). Moreover, the memory deficit of cognitively impaired aged rats was transiently corrected after either intraperitoneal or bilateral intrahippocampal injection of PREG S. PREG S is both a gamma-aminobutyric acid antagonist and a positive allosteric modulator at the N-methyl-D-aspartate receptor, and may reinforce neurotransmitter system(s) that decline with age. Indeed, intracerebroventricular injection of PREG S was shown to stimulate acetylcholine release in the adult rat hippocampus. In conclusion, it is proposed that the hippocampal content of PREG S plays a physiological role in preserving and/or enhancing cognitive abilities in old animals, possibly via an interaction with central cholinergic systems. Thus, neurosteroids should be further studied in the context of prevention and/or treatment of age-related memory disorders.

Modulation of inhibitory transmission by dopamine in rat basal forebrain nuclei: activation of presynaptic D1-like dopaminergic receptors

The effects of dopamine (DA) on inhibitory transmission onto identified magnocellular neurons were examined in rat basal forebrain slices using whole-cell recording. IPSCs evoked by focal stimulation within basal forebrain nuclei were reversibly blocked by 10 microM bicuculline and had a decay time constant of 20.1 +/- 0.77 msec in the presence of 6-cyano-7-nitroquinoxalline-2,3-dione (5 mM). Bath application of DA reduced the amplitude of IPSCs up to 71.1 +/- 1.49% in a concentration-dependent manner between 0.003 and 1 mM (the IC50 value being 6.6 microM), without any effect on the holding current at -70 mV. DA (10 microM) reduced the frequency of miniature IPSCs (mIPSCs) recorded in the presence of TTX (0.5 microM), without affecting their mean amplitude, rise time, and decay time constant. Furthermore, the DA-induced effect on mIPSCs remained unaffected by 100 microM cadmium, suggesting a presynaptic mechanism independent of calcium influx. SKF 81297, a D1-like agonist, mimicked DA-induced effect on evoked IPSCs (IC50, 10.9 microM), whereas R(-)-TNPA or (-)-quinpirole, D2-like agonists (30 microM), had little or no effect on the amplitude of evoked IPSCs. R(+)-SCH 23390, a D1-like antagonist, antagonized the DA-induced effect on IPSCs (K(B) 0.82 microM), whereas S(-)-eticlopride, a D2-like antagonist, showed slight antagonism (K(B) 7.8 microM). Forskolin (10 microM) reduced the amplitude of evoked IPSCs to approximately 58% of the control and occluded the inhibitory effect of DA. These findings indicate that DA reduces inhibitory transmission onto magnocellular basal forebrain neurons by activating presynaptic D1-like receptors.