Effects of noradrenaline and serotonin on activity of single lateral and medial geniculate neurons

Expression and localization of α2A-adrenergic receptor in the rat post-natal developing cochlea

Lots of adrenergic receptors (ARs) are widely present across the auditory pathways and are positioned to affect auditory and vestibular functions. However, noradrenergic regulation in the cochlea has not been well characterized. In this study, a rat model of noise-induced hearing loss was developed to investigate the expression of α2A-adrenergic receptor (AR) after acoustic trauma, then, we investigated the expression of α2A-AR in the developing rat cochlea using immunofluorescence, qRT-PCR, and Western blotting. We found that the expression of α2A-AR significantly increased in rats exposed to noise compared with controls. Immunofluorescence analysis demonstrated that α2A-AR is localized on hair cells (HCs), spiral ganglion neurons (SGNs), and the stria vascularis (SV) in the postnatal developing cochlea from post-natal day (P) 0 to P28. Furthermore, we observed α2A-AR mRNA reached a maximum level at P14 and P28 when compared with P0, while no significant differences in α2A-AR protein levels at the various stages when compared with P0. This study provides direct evidence for the expression of α2A-AR in HCs, SGNs, and the SV of the cochlea, indicating that norepinephrine might play a vital role in hearing function within the cochlea through α2A-AR.

Distribution and co-expression of adrenergic receptor-encoding mRNA in the mouse inferior colliculus

Adrenergic receptors are mediators of adrenergic and noradrenergic modulation throughout the brain. Previous studies have provided evidence for the expression of adrenergic receptors in the midbrain auditory nucleus, the inferior colliculus (IC), but have not examined the cellular patterns of expression in detail. Here, we utilize multichannel fluorescent in situ hybridization to detect the expression of adrenergic receptor-encoding mRNA in the inferior colliculus of male and female mice. We found expression of α₁ , α_2A , and β₂ receptor-encoding mRNA throughout all areas of the IC. While we observed similar levels of expression of α₁ receptor-encoding mRNA across the subregions of the IC, α_2A and β₂ receptor-encoding mRNA was expressed differentially. To account for developmental changes in noradrenergic receptor expression, we measured expression levels in mice aged P15, P20, and P60. We observed little change in levels of expression across these ages. To ascertain the modulatory potential of multiple adrenergic receptor subtypes in a single IC cell, we measured co-expression of α₁ , α_2A , and β₂ receptor-encoding mRNA. We found greater proportions of cells in the IC that expressed no adrenergic receptor-encoding mRNA, α1 and α2A adrenergic receptor-encoding mRNA, and α1, α2A, and β2 receptor-encoding mRNA than would be predicted by independent expression of each receptor subtype. These data suggest a coordinated pattern of adrenergic receptor expression in the IC and provide the first evidence for adrenergic receptor expression and co-expression in the subregions of the mouse auditory midbrain.

Iontophoresis of norepinephrine onto neurons of the pigeon's lateral geniculate nucleus: characterization of an inhibitory response

A group of neurons in the pigeon's lateral geniculate equivalent nucleus (LGNe) shows associative enhancement of their response to light during visually conditioned heart rate change. The source of the relevant unconditioned stimulus input to LGNe for this enhancement has been identified as the locus coeruleus (LC). Thus, we have begun to examine neurotransmitters synthesized in LC for possible involvement in associative modification of neuronal discharge in LGNe. As a first step, we have examined the responses of LGNe neurons to iontophoretic application of norepinephrine (NE) and identified the receptor mediating one response class. The majority of neurons in LGNe show inhibition of maintained activity in response to iontophoretic application of NE or its agonists. The potency of the NE agonists is alpha-methyl NE greater than epinephrine greater than NE greater than phenylephrine greater than isoproterenol. This profile is characteristic of an alpha 2-adrenoceptor. The alpha 2-agonist clonidine also induces inhibition of maintained activity. The alpha 2-antagonists WB-4101 and yohimbine block the NE-inhibition while the alpha 1-antagonist prazosin and beta-antagonist sotalol do not. It is thus suggested that the receptor mediating the NE-inhibition of maintained activity has the characteristics of an alpha 2-adrenoceptor.

Illumination dependent hyperpolarization of the photoreceptor outer segment membrane (role of calcium, cyclic GMP and calmodulin)

Calmodulin has been isolated from the outer segment of toad (Bufo marinus) photoreceptors. Bioelectric processes have been recorded from the same preparation during various experimental conditions. The results led to the conclusion that during illumination the rotation of the chemoreceptors opens the Ca channels, and free Ca is versed into the intermembrane space of the outer segment. Changes of this free Ca concentrations depend on fast influx and slower removal (mainly dependent on Ca binding on calmodulin and back-diffusion into the disc membrane). The changes of the free Ca concentrations generate the changing hyperpolarization, a Ca-dependent process of first phases of encoding the photic stimulus. The effectiveness of Ca to generate the oscillating hyperpolarization depends on optimum state of bioelectric potentials. This potential seems to be regulated by the effective decrease of cGMP concentrations by the increased activity of the cGMP phosphodiesterase during illumination (a sudden and gross regulation). A finer regulation is exerted by Ca during its release and removal (e.g., Ca-dependent decrease of Na channels, decreased release of cGMP by Ca-dependent inhibition of guanylate cyclase activity and increased phosphodiesterase activity by both free Ca and Ca-bound calmodulin).

Activation of lateral geniculate neurons by norepinephrine: mediation by an alpha-adrenergic receptor

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1-15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic alpha-adrenergic receptors: epinephrine greater than NE greater than phenylephrine greater than or equal to alpha-methylnorepinephrine greater than dopamine greater than isoproterenol. The alpha-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (less than 10 nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The beta-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic alpha-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brain alpha1-adrenoceptors labeled with [3H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic or alpha1-adrenergic receptor.

Relationship of geniculate and occipital PGO waves and the effects of nonvisual sensory activity

The nature of occipital PGO waves (REM) was addressed by comparing the occipital PGO waves measured at the marginal gyrus with the geniculate PGO waves and the occipital PGO waves "directly" transmitted to the occipital cortex. The directly transmitted PGO waves lack the first short positive-going spike that precedes the negative phase of the occipital PGO waves. The "directly" transmitted PGO wave was recorded from cats with either deafferented or excised LG nucleus. The "directly" transmitted PGO wave resembled pontine negative-going PGO wave more regarding amplitude and wave shape changes than the occipital PGO waves of intact cats. The ascending negativity of pontine PGO waves is indirectly transmitted through phasic increase of the local excitatory processes, more in the LG than in the occipital cortex. The PGO waves seem to be affected by endogenous and exogenous nonvisual processes, more in the LG than the occipital cells alone. The occipital PGO waves seem to result from the vectorial sum of geniculate and "directly" transmitted occipital PGO waves.

Attenuation in central neurons: its relationship to increased intracytoplasmic free calcium, calcium binding and memory storage

A mechanism responsible for attenuation of dendritic spikes of nucleus reticularis gigantocellularis (NRG) origin was addressed. Stimulation of afferent neurons with pulses of 100 per sec frequency generated attenuation in specific neurons. Intrasomatic infusion of EGTA reversed the attenuation. The results suggest that attenuation results from intracytoplasmic accumulation of free Ca2+ due to differences in rate of Ca influx and Ca binding. This accumulation generates a concentration dependent prevention of Ca2+ (and Na+) influx that causes the attenuation. The free intracytoplasmic Ca2+ is removed mainly by binding on proteins and/or polypeptides, a mechanism that promotes memory storage.