John S. Yeomans

The acoustic startle reflex: neurons and connections.

The startle reflex protects animals from blows or predatory attacks by quickly stiffening the limbs, body wall and dorsal neck in the brief time period before directed evasive or defensive action can be performed. The acoustic startle reflex in rats and cats is mediated primarily by a small cluster of giant neurons in the ventrocaudal part of the nucleus reticularis pontis caudalis (RPC) of the reticular formation. Activation of these RPC neurons occurs 3-8 ms after the acoustic stimulus reaches the ear. Undetermined neurons of the cochlear nuclei activate RPC via weak monosynaptic and strong disynaptic connections. The strong disynaptic input occurs via neurons of the contralateral ventrolateral pons, including large neurons of the ventrolateral tegmental nucleus that integrate auditory, tactile and vestibular information. RPC giant neurons, in turn, activate hundreds of motoneurons in the brain stem and the length of the spinal cord via large reticulospinal axons near the medial longitudinal fasciculus. To hindlimb motoneurons, monosynaptic connections from the reticulospinal tract are weak, but disynaptic connections via spinal cord interneurons are stronger and show temporal facilitation, like the startle response itself.

Tactile, acoustic and vestibular systems sum to elicit the startle reflex.

The startle reflex is elicited by intense tactile, acoustic or vestibular stimuli. Fast mechanoreceptors in each modality can respond to skin or head displacement. In each modality, stimulation of cranial nerves or primary sensory nuclei evokes startle-like responses. The most sensitive sites in rats are found in the ventral spinal trigeminal pathway, corresponding to inputs from the dorsal face. Cross-modal summation is stronger than intramodal temporal summation, suggesting that the convergence of acoustic, vestibular and tactile information is important for eliciting startle. This summation declines sharply if the cross-modal stimuli are not synchronous. Head impact stimuli activate trigeminal, acoustic and vestibular systems together, suggesting that the startle response protects the body from impact stimuli. In each primary sensory nucleus, large, second-order neurons project to pontine reticular formation giant neurons critical for the acoustic startle reflex. In vestibular nucleus sites, startle-like responses appear to be mediated mainly via the vestibulospinal tract, not the reticulospinal tract. Summation between vestibulospinal and reticulospinal pathways mediating startle is proposed to occur in the ventral spinal cord.

Role of Tegmental Cholinergic Neurons in Dopaminergic Activation, Antimuscarinic Psychosis and Schizophrenia

Cholinergic neurons of the pedunculopontine nucleus (Ch5) and laterodorsal tegmental nucleus (Ch6) monosynaptically activate dopamine neurons of the substantia nigra, zona compacta (A9), and ventral tegmental area (A10) via muscarinic and nicotinic receptors. Ch5 cells and Ch6 cells are inhibited by local injections of muscarinic agonists, suggesting the presence of autoreceptors. This review advances the hypothesis that the psychotogenic effects of antimuscarinics are triggered by disinhibition of Ch5 and Ch6 cells via their autoreceptors, and that these effects are distinct from the memory-blocking effects of antimuscarinics mediated through the Ch1–Ch4 projections to the forebrain. Neuroleptic and antiparkinson agents with antimuscarinic effects selectively block m1 muscarinic receptors, whereas psychotogenic antimuscarinics are nonselective. In rats, scopolamine injected near Ch5 cells facilitates rewarding brain stimulation and induces locomotion and stereotypy, apparently via activation of dopaminergic systems. Systemically administered scopolamine induces locomotion and stereotypy via muscarinic receptors near Ch5 cells. Ch5 activation and Ch6 activation may be a causal factor in some forms of schizophrenia. Some schizophrenics show early-onset REM sleep, a condition that can result from Ch5 and Ch6 cholinergic activation of the pontine reticular formation. Schizophrenics with early-onset REM, or visual hallucinations, show more severe positive symptoms and negative symptoms. Ch5 cells and Ch6 cells have been found in twice-normal numbers in a few brains of schizophrenics. Several genetic and onset factors for schizophrenia that may be linked to Ch5 cells are considered, as well as treatment strategies based on inhibition of Ch5 cells and Ch6 cells, or blockade of their terminals.

Rewarding brain stimulation: role of tegmental cholinergic neurons that activate dopamine neurons.

Cholinergic neurons of the pedunculopontine nucleus (Ch5) are believed to monosynaptically excite ventral tegmental dopamine neurons. Muscarinic blockers injected near dopamine cells block the rewarding effect of hypothalamic or dorsal tegmental rewarding brain stimulation (RBS) in rats. Because Ch5 cells are inhibited by muscarinic agonists, we injected muscarinic drugs unilaterally near Ch5 neurons to inhibit or disinhibit them. Carbachol raised thresholds for hypothalamic self-stimulation bilaterally by over 400%, whereas scopolamine reduced thresholds by 20%-80%. Pretreatment with either carbachol or scopolamine blocked the effect of the other drug, which suggests that both acted through the same receptors near Ch5 cells. Therefore, activation of Ch5 neurons is critical for hypothalamic RBS. A mechanism for the involvement of Ch5 neurons in drug rewards and antimuscarinic psychosis is also proposed.

Both nicotinic and muscarinic receptors in ventral tegmental area contribute to brain-stimulation reward.

Cholinergic neurons of the pedunculopontine tegmental nucleus (Ch5) and laterodorsal tegmental nucleus (Ch6) monosynaptically activate dopamine neurons of the substantia nigra and ventral tegmental area (VTA) via nicotinic and muscarinic receptors. The nicotinic receptors near the VTA have been proposed to be important for nicotine self-administration in rats and for tobacco smoking in humans. Nicotinic and muscarinic blockers were microinjected into the VTA of rats trained to lever-press for lateral hypothalamic stimulation via an ipsilateral electrode. The competitive nicotinic blocker dihydro-beta-erythroidine (DH beta E; 5-60 micrograms) shifted rate-frequency curves to the right by a mean of 6-27% in a dose-related manner; the noncompetitive nicotinic blocker mecamylamine (10-300 micrograms) produced similar shifts of 7-21%. Atropine (30 micrograms) shifted the curves to the right by a mean of 82% in three of the sites tested with DH beta E. All blockers decreased maximum bar-pressing rates significantly in some sites when the shifts were large. Therefore, nicotinic receptors in the VTA make small contributions to the maintained rewarding effect of brain-stimulation reward in rats, but muscarinic receptors in the VTA appear to be more important.

CBP Histone Acetyltransferase Activity Regulates Embryonic Neural Differentiation in the Normal and Rubinstein-Taybi Syndrome Brain

Increasing evidence indicates that epigenetic changes regulate cell genesis. Here, we ask about neural precursors, focusing on CREB binding protein (CBP), a histone acetyltransferase that, when haploinsufficient, causes Rubinstein-Taybi syndrome (RTS), a genetic disorder with cognitive dysfunction. We show that neonatal cbp(+/-) mice are behaviorally impaired, displaying perturbed vocalization behavior. cbp haploinsufficiency or genetic knockdown with siRNAs inhibited differentiation of embryonic cortical precursors into all three neural lineages, coincident with decreased CBP binding and histone acetylation at promoters of neuronal and glial genes. Inhibition of histone deacetylation rescued these deficits. Moreover, CBP phosphorylation by atypical protein kinase C zeta was necessary for histone acetylation at neural gene promoters and appropriate differentiation. These data support a model in which environmental cues regulate CBP activity and histone acetylation to control neural precursor competency to differentiate, and indicate that cbp haploinsufficiency disrupts this mechanism, thereby likely causing cognitive dysfunction in RTS.

Cholinergic involvement in lateral hypothalamic rewarding brain stimulation

Rats were implanted with stimulating electrodes in the lateral hypothalamus, and cannulae for chemical injections in the ventral tegmentum. Injections of atropine, a muscarinic antagonist, increased thresholds for self-stimulation in a dose-dependent fashion, without slowing bar pressing rates. Thresholds increased less for a self-stimulation site contralateral to the atropine injection. In a conditioned place preference test, the rats preferred compartments in which they received carbachol, a cholinergic agonist. Muscarinic receptors in ventral tegmentum therefore seem critical for medial forebrain bundle (MFB) reward. The possible cholinergic cells of origin are discussed.

M5 muscarinic receptors are needed for slow activation of dopamine neurons and for rewarding brain stimulation.

Mesopontine cholinergic neurons (Ch5 and Ch6 cell groups) activate the cerebral cortex via thalamic projections, and activate locomotion and reward via dopamine neurons in the substantia nigra and ventral tegmental area (VTA). Nicotinic receptors in VTA activate dopamine neurons quickly, and are needed for the stimulant and rewarding effects of nicotine in rats. Muscarinic receptors in VTA activate dopamine neurons slowly, and are needed for the rewarding effects of hypothalamic stimulation, but do not increase locomotion. Antisense oligonucleotides targetting M5 mRNA, when infused into the VTA, inhibited M5 receptor binding and rewarding hypothalamic stimulation. Mutant mice with truncated M5 muscarinic receptor genes drank more water than wild-type controls. Spontaneous locomotion and locomotor responses to amphetamine and scopolamine were unchanged. Electrical stimulation near Ch6 induced dopamine release in the nucleus accumbens in two phases, an early phase (0-2 min after stimulation) dependent on nicotinic and gluatamatergic receptors in VTA, and a late phase (8-50 min after stimulation) dependent on muscarinic receptors in VTA. The late phase was lost in M5 mutant mice, while the early phase was unchanged. M5 muscarinic receptors bind slowly to muscarinic ligands, and appear to mediate slow secretions.

Ultrasonic Vocalizations Induced by Sex and Amphetamine in M2, M4, M5 Muscarinic and D2 Dopamine Receptor Knockout Mice

Adult mice communicate by emitting ultrasonic vocalizations (USVs) during the appetitive phases of sexual behavior. However, little is known about the genes important in controlling call production. Here, we study the induction and regulation of USVs in muscarinic and dopaminergic receptor knockout (KO) mice as well as wild-type controls during sexual behavior. Female mouse urine, but not female rat or human urine, induced USVs in male mice, whereas male urine did not induce USVs in females. Direct contact of males with females is required for eliciting high level of USVs in males. USVs (25 to120 kHz) were emitted only by males, suggesting positive state; however human-audible squeaks were produced only by females, implying negative state during male-female pairing. USVs were divided into flat and frequency-modulated calls. Male USVs often changed from continuous to broken frequency-modulated calls after initiation of mounting. In M2 KO mice, USVs were lost in about 70–80% of the mice, correlating with a loss of sexual interaction. In M5 KO mice, mean USVs were reduced by almost 80% even though sexual interaction was vigorous. In D2 KOs, the duration of USVs was extended by 20%. In M4 KOs, no significant differences were observed. Amphetamine dose-dependently induced USVs in wild-type males (most at 0.5 mg/kg i.p.), but did not elicit USVs in M5 KO or female mice. These studies suggest that M2 and M5 muscarinic receptors are needed for male USV production during male-female interactions, likely via their roles in dopamine activation. These findings are important for the understanding of the neural substrates for positive affect.

Increased striatal dopamine efflux follows scopolamine administered systemically or to the tegmental pedunculopontine nucleus

The cholinergic cells of the tegmental pedunculopontine nucleus monosynaptically excite dopaminergic neurons of the substantia nigra. In vivo electrochemical methods were used to monitor dorsal striatal dopamine efflux in awake rats following intraperitoneal scopolamine injections and following the direct application of scopolamine to the tegmental pedunculopontine nucleus. Systemic injections of scopolamine (1.0, 3.0 or 10.0 mg/kg) resulted in dose-related increases in peak striatal dopamine oxidation currents of between 1.1 and 2.0 nA. Increases began within 10-20 min after injection and peaked after 40-90 min. Unilateral microinjections of scopolamine into the tegmental pedunculopontine nucleus (10, 50 or 100 micrograms/0.5 microliter) resulted in dose-related increases in dopamine oxidation currents that peaked 60-90 min postinjection (2.9-5.0 nA). Carbachol (4.0 micrograms/0.5 microliter) injected unilaterally into the tegmental pedunculopontine nucleus 20 min before 100 micrograms tegmental pedunculopontine nucleus scopolamine, or injected bilaterally 20 min before 3.0 mg/kg systemic scopolamine, attenuated the increases produced by scopolamine alone. The carbachol preinjection tests suggest that the effects of both systemic and tegmental pedunculopontine nucleus scopolamine treatments are mediated largely by muscarinic receptors near the tegmental pedunculopontine nucleus. These findings are consistent with the proposal that enhanced activation of substantia nigra dopamine cells results from scopolamine-induced disinhibition of the tegemental pedunculopontine nucleus cholinergic cell group via blockade of their inhibitory autoreceptors.