Localization of preproenkephalin mRNA-expressing cells in rat auditory brainstem with in situ hybridization

Immunocytochemical distribution of Met-enkephalin-Arg6-Gly7-Leu8 (Met-8) in the auditory system of the rat

Methionine-enkephalin-Arg(6)-Gly(7)-Leu(8) (Met(8)) is known to act as a neurotransmitter or neuromodulator and it has been implicated in pain, cardiovascular and motor mechanisms, but its role in audition is currently unknown. In the present study we have applied an immunocytochemical technique and describe the distribution of cell bodies and fibers containing Met(8) in the auditory pathway of the rat. The main finding is that we found either Met(8)-immunoreactive fibers or cell bodies or both in virtually all nuclei of the rat auditory system except for the medial superior olive and the ventral division of the medial geniculate body in which we did not find any immunoreactivity for Met(8). This suggests that the neuropeptide Met(8) is widely distributed throughout the auditory system of the rat. Our results suggest that Met(8) could play at least two roles in hearing. It seems to be involved in the processing of the descending auditory pathway, and it may be implicated in the multisensory integration of auditory information that takes place in the non-lemniscal auditory pathway.

Morphological relationships of peptidergic and noradrenergic nerve terminals to olivocochlear neurones in the rat

In the rat, the outer hair cells in the cochlea receive direct synaptic input from neurones in the ventral nucleus of the trapezoid body. These so-called medial olivocochlear neurones exert an inhibitory influence on the cochlear neural output. Electrophysiological in vitro studies suggest that the activity of medial olivocochlear neurones may be affected by a variety of neuropeptides as well as noradrenaline, but anatomical confirmation of direct synaptic input is still lacking. We have investigated, at the light microscopical level, the morphological relationships between terminals containing noradrenaline, substance P, cholecystokinin and leu-enkephalin, and medial olivocochlear neurones in the rat. A retrograde tracer was injected into the cochlea to label medial olivocochlear neurones and a double labelling immunocytochemical method was used to visualise the retrograde tracer as well as the neurotransmitters within each brain section. Light microscopical analysis revealed nerve endings containing substance P, cholecystokinin and leu-enkephalin in close apposition to the dendrites of medial olivocochlear neurones, and nerve endings containing dopamine-beta-hydroxylase, a marker for noradrenaline, in close contact with the somata as well as dendrites of medial olivocochlear neurones. Although the technique cannot prove the existence of functional synaptic contacts, the results are broadly consistent with electrophysiological data and suggest a direct input to medial olivocochlear neurones from substance P, cholecystokinin, leu-enkephalin and noradrenaline-containing neural pathways. Differences in the densities and spatial distribution of the various neuropharmacological inputs suggest differences in the relative strengths and possible roles of these diverse inputs to the olivocochlear system.

Effects of bioamines and peptides on neurones in the ventral nucleus of trapezoid body and rostral periolivary regions of the rat superior olivary complex: an in vitro investigation

Intracellular microelectrode recordings were made from single neurones of the ventral nucleus of trapezoid body and rostral periolivary regions in the rat auditory brainstem, using in vitro slic techniques. Bath application was used to examine the effects of putative neurotransmitters and neuromodulators on cell responses to constant depolarizing current pulse. Noraderaline exerted excitatory effects (increased firing rate) that were probably mediated by alpha-receptors, whereas inhibitory effects (decreased firing rate) were probably mediated by beta-receptors. Serotonin also produced either excitatory effects in different cells. Of the neuroactive peptides, substance P and enkephalin were especially potent. Substance P was found to be exclusively excitatory and enkephalin was exclusively inhibitory. Choleycystokinin exerted either inhibitory or excitatory effects in a small percentage of cells. Somatostatin had only very weak or non-existent effects. These effects were able to be elicited under conditions of synaptic blockade, indicating they were mediated by direct action on the cells in question. Most effects on firing rate were accompanied by either depolarization or hyperpolarization of the resting membrane potential although in many cases this change in membrane potential was small. Changes in cell access resistance were also relatively difficult to detect, but in the case of both noradrenaline and substance P, clear increases in cell access resistance were recorded in a number of cells. These could be obtained in the presence of tetrodotoxin, again indicating a direct action of these substances rather than an indirect action mediated via synaptic connections. Although the exact mechanisms of action remain to be investigated in each case, it is clear that neurones in this region of the auditory brainstem are potentially subject to a wide variety of modulatory influences that could be important in auditory processing.

Choline acetyltransferase, glutamate decarboxylase, tyrosine hydroxylase, calcitonin gene-related peptide and opioid peptides coexist in lateral efferent neurons of rat and guinea-pig

The lateral efferent (olivocochlear) innervation of the cochlea originates in the brainstem lateral superior olive. It is likely to use acetylcholine, gamma-aminobutyric acid, dopamine and various neuropeptides as neurotransmitters and/or neuromodulators. In order to determine the different coexistence patterns of these molecules in lateral efferent perikarya, we have used double and triple immunofluorescence co-localization techniques to colocalize choline acetyltransferase, glutamate decarboxylase, tyrosine hydroxylase, calcitonin gene-related peptide and enkephalins in single sections of the lateral superior olive. We also used a non-radioactive in situ hybridization technique onto serial sections of this nucleus to confirm the immunofluorescence co-localization data at the mRNA level. Whatever the pair or triplet of primary antibodies tested was, a high ratio of coexistence was observed in the immunofluorescence experiments. In triple co-localization experiments, 90-93% of the choline acetyltransferase-like immunoreactive neurons were also immunoreactive to the two other antigens investigated. The in situ hybridization co-localization data, based on the use of biotin-labelled oligoprobes, qualitatively confirmed these immunofluorescence data. In conclusion, it can be postulated that acetylcholine, gamma-aminobutyric acid, dopamine, calcitonin gene-related peptide, enkephalins and dynorphins (whose coexistence with choline acetyltransferase and enkephalins has been previously described immunocytochemically) coexist in lateral efferent neurons. Based on these results, it is tempting to propose the lateral efferent innervation as a useful model with which the functional implications of the coexistence of neurotransmitters/neuromodulators can be investigated in vivo.

Blockade of opioid-induced changes in auditory function at the level of the cochlea

OBJECTIVE

We have previously investigated the auditory neural effects of the kappa-opioid receptor agonist, (-)pentazocine. When administered intravenously (i.v.), this drug temporarily alters auditory nerve compound action potential (CAP) amplitudes. To test the hypothesis that the observed neural effects of i.v. (-)pentazocine occur via kappa-receptor interactions within the cochlea, we attempted to block these effects by employing a specific kappa-opioid receptor antagonist applied directly to the cochlear round window (RW) membrane.

DESIGN

In 31 normal-hearing, male pigmented chinchillas, amplitude changes in the click-evoked auditory CAP (N1) were tracked at six stimulus intensities during a baseline and a postbaseline period in which i.v. (-)pentazocine (8 mg/kg) was administered. (-)Pentazocine administration was preceded by the delivery to the cochlear RW membrane of an artificial perilymph solution given alone or containing the kappa-opioid receptor selective antagonist, norbinaltorphimine (Nor-BNI), which was administered at two concentrations in separate groups of animals.

RESULTS

The amplitude increase in the CAP after (-)pentazocine was significantly reduced when i.v. (-)pentazocine was preceded by RW-administered Nor-BNI (4 mM).

CONCLUSIONS

The reversibility of agonist effects by Nor-BNI indicates direct or indirect opioid kappa-receptor-mediated auditory neural effects at the level of the cochlea and suggests a connection between kappa-receptors and auditory neural function.

Naloxone blockade of (-)pentazocine-induced changes in auditory function

OBJECTIVE

In a previous report, we found that intravenous (i.v.) (-)pentazocine improved auditory sensitivity and significantly altered compound action potential (CAP) amplitudes. Its sigma (sigma)-receptor-selective optical isomer (+)pentazocine administered at the same dose was without effect, suggesting that the observed auditory neural effects might be mediated by an opioid receptor. To directly test this hypothesis, in the present investigation we attempted to antagonize the auditory neural effects of (-)pentazocine using the pure, nonspecific drug antagonist naloxone.

DESIGN

In 25 normal-hearing, male, pigmented chinchillas, amplitude and latency changes in the click-evoked auditory nerve CAP (N1) and cochlear microphonic (CM) were tracked at six stimulus intensities during a baseline period and after the postbaseline administration of the opioid drug agonist (-)pentazocine (16 mg/kg; i.v.). In separate groups of chinchillas, (-)pentazocine was given alone or administered in combination with the standard opioid receptor antagonist naloxone administered at two doses.

RESULTS

Robust changes in CAP amplitudes after (-)pentazocine occurred in the absence of measurable alterations in CAP response latencies, CM amplitudes, or blood chemistries and were significantly antagonized when naloxone (5 mg/kg) was added to the i.v. infusion.

CONCLUSIONS

The observed blockade clearly indicates that the agonist effects of (-)pentazocine are opioid receptor-mediated and suggests a connection between opioid receptors and auditory neural function. Mechanisms of action and the connection between an opioid modulation of auditory function and stress, hyperacusis, and tinnitus are discussed.

Neurotransmitter and neuromodulator systems of the rat inferior colliculus and auditory brainstem studied by in situ hybridization

This study was concerned with the distribution of a variety of putative neuromodulator and neurotransmitter systems in auditory regions of the rat brainstem using in situ hybridization histochemistry. Serial brain sections were screened for the presence of mRNAs for (i) precursors of the neuroactive substances cholecystokinin, somatostatin, proenkephalin and substance P (preprotachykinin), (ii) glutamic acid decarboxylase, the key synthesizing enzyme for GABA, or (iii) subunits alpha 1, alpha 2 and alpha 3 of the GABAA receptor. Detectable message for all of these probes was found in at least one auditory brainstem area. There were clear differences in the distribution of the various mRNAs in subregions of the inferior colliculus, superior olivary complex, lateral lemniscus and cochlear nucleus. Cells expressing mRNA for glutamic acid decarboxylase were most prominent in the inferior colliculus, but were also present in all lower auditory brainstem nuclei, except the medial superior olivary nucleus and medial nucleus of trapezoid body. The mRNA for GABAA alpha 1 receptor subunits was detectable in all auditory regions investigated, although at different levels of expression. GABAA alpha 2 and alpha 3 mRNA signals were seen in inferior colliculus, lateral lemniscus and in almost all superior olivary complex regions, but in fewer cells and at lower levels than the GABAA alpha 1 subtype. Moderate to high levels of preprocholecystokinin mRNA expression were seen in all subregions of the inferior colliculus. In other auditory brainstem areas, preprocholecystokinin mRNA levels were either low or absent. With regard to mRNAs for the neuroactive peptides somatostatin, preprotachykinin and preproenkephalin, all were expressed in the inferior colliculus but there were differences in their cellular distribution. For example, there were almost no preprotachykinin mRNA expressing cells in the central nucleus of inferior colliculus and levels of somatostatin mRNA were especially high in the dorsal cortex and in layer 3 of the external cortex of inferior colliculus. There were also differences in the pattern of expression of these mRNAs in the various brainstem auditory nuclei; there was no preprotachykinin mRNA in any part of the superior olivary complex, only somatostatin mRNA was found in the ventral cochlear nucleus, and expression of preproenkephalin mRNA was pronounced in the ventral nucleus of the trapezoid body and the rostral periolivary zone. The data are considered in light of the connectivity and functional organization of the auditory brainstem.

Enkephalin-like immunoreactivity in the chick brainstem: possible relation to the cochlear efferent system

Mammalian lateral olivocochlear (LOC) neurons that are immunoreactive for choline acetyltransferase (ChAT) are also immunoreactive for enkephalin (Enk). To determine whether cochlear efferent neurons in birds might also contain Enk-like immunoreactivity (Enk-LI), we studied the auditory brainstem of the domestic chicken using antisera to ChAT, leucine-enkephalin (L-Enk) and methionine-enkephalin (M-Enk). Enk-LI terminals are found around, but not within, the superior olivary nucleus (SO) and the nucleus of the lateral lemniscus, pars intermedia (LLi). A moderate concentration of Enk-LI terminals is found ventromedial to the ventral facial nucleus (VIIv) where the ventrolateral group of ChAT-I cochlear efferent neurons is located. After colchicine injections into the lateral ventricle, a population of intensely stained Enk-LI perikarya was found in the nucleus of the lateral lemniscus, pars ventralis (LLv) with scattered cells in the LLi and the nucleus subceruleus ventralis (SCv). The distribution of Enk-LI and ChAT-I somata, however, never overlapped, even in adjacent sections. Thus, in the chick, Enk-LI perikarya are not distributed in areas where cochlear efferent neurons are found. Instead, a dense concentration of Enk-I terminals can be found in areas containing ChAT-I cochlear efferent neurons. The source of these enkephalinergic terminals may be a population of Enk-LI cells in the LLv.