What is the only one of the senses that does not have to pass through the thalamus?

J Neurophysiol. 2014 Mar 15; 111[6]: 1274–1285.

Abstract

Thalamus is a key crossroad structure involved in various functions relative to visual, auditory, gustatory, and somatosensory senses. Because of the specific organization of the olfactory pathway [i.e., no direct thalamic relay between sensory neurons and primary cortex], relatively little attention has been directed toward the thalamus in olfaction. However, an olfactory thalamus exists: the mediodorsal nucleus of the thalamus [MDT] receives input from various olfactory structures including the piriform cortex. How the MDT contributes to olfactory perception remains unanswered. The present study is a first step to gain insight into the function of the MDT in olfactory processing. Spontaneous and odor-evoked activities were recorded in both the MDT [single unit and local field potential] and the piriform cortex [local field potential] of urethane-anesthetized rats. We demonstrate that: 1] odorant presentation induces a conjoint, coherent emergence of beta-frequency-band oscillations in both the MDT and the piriform cortex; 2] 51% of MDT single units were odor-responsive with narrow-tuning characteristics across an odorant set, which included biological, monomolecular, and mixture stimuli. In fact, a majority of MDT units responded to only one odor within the set; 3] the MDT and the piriform cortex showed tightly related activities with, for example, nearly 20% of MDT firing in phase with piriform cortical beta-frequency oscillations; and 4] MDT-piriform cortex coherence was state-dependent with enhanced coupling during slow-wave activity. These data are discussed in the context of the hypothesized role of MDT in olfactory perception and attention.

Keywords: olfaction, mediodorsal thalamus, dorsomedial thalamus, single unit, LFP, slow-wave sleep, piriform cortex

olfaction is a unique sensory modality in terms of its anatomic organization. In fact, for all senses except olfaction, the information from the sensory neurons necessarily passes through a thalamic nucleus before reaching the primary sensory cortex. In olfaction, sensory neurons project directly to the olfactory bulb, which, in turn, projects to the olfactory cortex, including the piriform cortex [PCX; Haberly and Price 1977], the cortical nucleus of the amygdala, the olfactory tubercle, the lateral entorhinal cortex, and the anterior olfactory nucleus [Haberly and Price 1978; Price and Powell 1971]. Although there is no thalamic relay between the olfactory sensory neurons and the olfactory cortex, an olfactory thalamic nucleus exists. The mediodorsal thalamic nucleus [MDT] receives direct input from the PCX [Cornwall and Phillipson 1988; Heimer 1968; Kuroda and Price 1991; Powell et al. 1963; Price 1985; Price and Slotnick 1983] and, in turn, projects to the orbitofrontal cortex [OFC; Krettek and Price 1977], forming a transthalamic PCX-MDT-OFC pathway. The OFC also has direct reciprocal connections with the PCX [Illig 2005; Schoenbaum and Eichenbaum 1995]. The MDT additionally receives direct input from other olfactory structures, such as the olfactory tubercle, the cortical nucleus of the amygdala, and the entorhinal cortex [Bay and Cavdar 2013; Krettek and Price 1974; Price 1985; Price and Slotnick 1983; Ray and Price 1992].

In other sensory systems, the thalamus is involved in many functions ranging from basic sensory information processing [McCormick and Bal 1994; Saalmann and Kastner 2009] to more complex functions, such as sensory gating and attention modulation [Coull 1998; Newman 1995], control of sleep states [Steriade 1992], and memory processing [Jankowski et al. 2013]. Despite the unusual anatomic arrangement of the olfactory pathway, does the MDT play similar roles in olfaction? Some evidence does suggest different roles for the MDT in olfaction [Tham et al. 2009]. MDT single units have been shown to respond to both lateral olfactory tract and odorant stimulation [Benjamin and Jackson 1974; Imamura et al. 1984; Jackson and Benjamin 1974; Motokizawa 1974; Price 1985; Price and Slotnick 1983; Takagi 1986; Yarita et al. 1980]. In addition, evidence from lesion studies has supported a role for the MDT in odor learning and memory by demonstrating, for example, that MDT lesions do not lead to anosmia but impair odor reversal learning [Eichenbaum et al. 1980; Koger and Mair 1994; Sapolsky and Eichenbaum 1980; Slotnick and Kaneko 1981; Slotnick and Risser 1990; Staubli et al. 1987]. Finally, several groups have proposed a role for the MDT in olfactory attention [Plailly et al. 2008; Tham et al. 2009, 2011a,b; Veldhuizen and Small 2011]. In humans, using functional magnetic resonance imaging [fMRI], Plailly et al. [2008] showed specific changes in functional connectivity in the PCX-MDT-OFC transthalamic pathway during olfactory attention processing.

To examine more closely the contributions of the MDT to olfactory perception, the present study begins a more detailed analysis of how the MDT contributes to olfaction in the rodent and how its activity is shaped by its primary olfactory afferent, the PCX. As a first step, we characterized MDT single-unit and local field potential [LFP] spontaneous and odor-evoked activity and examined the relationship between MDT activity and PCX activity in urethane-anesthetized rats.

MATERIALS AND METHODS

Subjects

A total of 34 male Long-Evans rats [>200 g] obtained from Charles River Laboratories were used in the present study. All animals were group-housed ranging in groups of 3–4 animals in polypropylene cages. Food and water were available ad libitum. Experimental procedures were developed in accordance with, and reviewed and approved by, the Institutional Animal Care and Use Committee at Nathan Kline Institute for Psychiatric Research and National Institutes of Health guidelines for the proper treatment of animals.

Experimental Design

Animal preparation.

Rats were anesthetized with urethane [1.25 g/kg ip with additional supplements as needed] and placed in a stereotaxic apparatus. The animals were placed on a heating pad to maintain constant body temperature.

Electrophysiological recordings.

Single-unit recording procedures for the MDT were performed similar to previous reports [Wilson 1998; Xu and Wilson 2012]. Single units were recorded using a tungsten microelectrode [1–5 MΩ], and signals were acquired [sampling rate: 10 kHz] and analyzed with Spike2 [CED]. MDT units were identified by histological confirmation [Fig. 1] with coordinates ranging from −2.64 to −3.36 mm in the anteroposterior axis and 0.3 to 1.3 mm in the mediolateral axis relative to bregma. MDT LFPs [filtered at 0.1–300 Hz] were recorded with the same electrode simultaneously with the single units. Another tungsten microelectrode was used to record LFPs either in the PCX [+0.72 mm in the anteroposterior axis and 4.6 mm in the mediolateral axis relative to bregma] or the visual neocortex [coordinates ranging from −5 to −6.5 mm in the anteroposterior axis, from 3 to 5 mm in the mediolateral axis, and 1-mm depth relative to bregma]. Lateral olfactory tract stimulations were used to determine the electrode position in the PCX. Respiration was monitored throughout the recording session with a piezoelectric device placed under the animal's chest.

Electrode placement in the mediodorsal thalamic nucleus [MDT]. A: example of 1 frontal section [cresyl violet]. The 2 asterisks represent 2 units recorded in the MDT relative to the position of the tip of the electrode. B: frontal stereotaxic images showing the approximate placement of the tip location of recording electrode tracks [black dots]. Images were adapted with permission from Paxinos and Watson [2009].

Odorant stimulation.

Odors were delivered with a flow-dilution olfactometer that was positioned 2 cm from the animal's nose. Odor vapor was added with a computer-controlled solenoid at a rate of 0.1 l/min to a constant flow of nitrogen gas [N2] at 1 l/min. Odors were administered for 2 s per trial with at least a 30-s interstimulus interval.

We used 3 categories of odorants: mixtures and monomolecular and biological odorants. The 3 mixtures were 10c, 10c-1, and 10cR1. Components of the mixture have been detailed elsewhere [Barnes et al. 2008; Chapuis and Wilson 2012; Lovitz et al. 2012]. Mixture 10c comprised 10 components: isoamyl acetate, nonane, ethyl valerate, 5-methyl-2-hexanone, isopropylbenzene, 1-pentanol, 1,7-octadiene, 2-heptanone, heptanal, and 4-methyl-3-penten-2-one. Isoamyl acetate was removed from the mixture to produce 10c-1, and it was replaced by limonene in 10cR1. Mixtures were created by adding odorant components to mineral oil in amounts that provided concentrations of 100 parts per million [ppm] for all components except 1,7-octadiene, which, due to a calculation error, was at 442 ppm. Two monomolecular odorants were also used: ethyl valerate and isoamyl acetate. Ethyl valerate was used with an identical concentration as in the mixture. Isoamyl acetate was used at 2 concentrations: 1 with an identical concentration as in the mixture [ISO1] and 1 with a ppm 65-fold higher than ISO1 [ISO2]. Finally, 2 biological odorants were used: fresh dirty Litter and rat Feces. These components were used pure and obtained from the animal's own cage.

Data Analysis

Data analysis was performed using Spike2 and Excel.

LFPs.

For all odorant stimulations combined, power spectra [fast Fourier transform [FFT] size, 0.2048 s; Hanning window] were calculated for the 0- to 3-s period after stimulus onset and compared with the power of the 3-s prestimulus baseline. FFTs were also calculated in three respiratory cycles before odor onset and in six cycles after odor onset for two representative odors [10cR1 and dirty Litter].

To quantify the phase-locking of MDT units relative to beta cycle, raw LFPs [MDT and PCX] were band-pass filtered between 15 and 35 Hz. Beta cycles [negative troughs] were then detected using a threshold of −2 SD of the filtered signal in both PCX and MDT.

To measure the coherence between MDT and PCX LFP activities, we used a coherence script within Spike2. Values of coherence were either determined on the entire duration of the file [including both spontaneous and odor-evoked activity] or calculated for two representative odors [10cR1 and dirty Litter] on the 3-s period before the stimulus onset and the 0- to 3-s period after the stimulus onset. A ratio of the 0- to 3-s period after stimulus onset on the 3-s prestimulus period was then determined for statistical comparisons.

To quantify the troughs of slow waves [SW; MDT and visual neocortex] or sharp waves [SPWs; PCX], raw LFPs [MDT, PCX, and visual neocortex] were low-pass filtered to