The results of this study demonstrate that schizophrenia patients produce abnormally large EOG responses following H2
S stimulation, which are unrelated to acute symptomatology, antipsychotic medication dosage or smoking history. This suggests that disruptions of neural physiology in schizophrenia are not limited to the cortex and subcortical limbic structures, but rather include even the most peripheral sensory neurons. It is thus consistent with an increasing number of reports of abnormalities in primary sensory areas of the cortex (Sweet et al, 2007
; Turetsky et al, 2003b
) and suggests that the pathological processes of schizophrenia are more ubiquitous than cognition-based models of hypofrontality (Snitz et al, 2005
) or frontal–temporal dysregulation (Weiss et al, 2006
) might imply.
Two aspects of the data require specific comment. First, as indicated in , there was a large degree of intersubject variability in EOG responses. This is consistent with in vitro
findings of large response variability even at the cellular level. For example, among a group of 329 ORNs, all of which exhibited active responses to the odorant cineole, the magnitudes of responses following exposure ranged from 9 to 724 pA (mean±SD = 215±163 pA; Takeuchi et al, 2003
). However, this may also reflect, to some extent, differences in placement of the recording electrode on the olfactory epithelium. ORNs are scattered throughout the epithelium in a diffuse and patchy manner, and there are inevitably some between-subject differences in the proximity of the EOG electrode to the olfactory neurons. As comparable variability was seen in the responses of both patients and controls, it is unlikely that this would reflect any sort of systematic bias across the diagnostic groups.
Second, although lateralized cerebral findings are commonplace in schizophrenia research, it is surprising to see such lateralized effects extend to the level of primary sensory receptors. However, it is well known that the right hemisphere is better adapted to processing olfactory inputs than the left (Doty et al, 1997
). We previously observed a similar lateralized right-sided abnormality when we assessed olfactory bulb volumes in unaffected first-degree relatives of schizophrenia patients (Turetsky et al, 2003a
). Other studies have demonstrated larger right olfactory bulbs, as a consequence of normal development, in diverse species (Heine and Galaburda, 1986
; Prasada Rao and Finger, 1984
). There is also evidence that the two bulbs contain different levels of modulating neurotransmitters and enzymes (Dluzen and Kreutzberg, 1996
; Rodriguez-Gomez et al, 2000
). If the right and left olfactory bulbs, which are the axonal targets for the peripheral ORNs, are structurally and functionally distinct then the dysregulation that gives rise to elevated ORN responses in patients could also be one that is manifested primarily on the right side. If this is the case, then an understanding of the normal structural and functional asymmetry of the peripheral olfactory system could provide an important clue to the etiology of this abnormality in schizophrenia.
At this point, though, the pathophysiological mechanisms that might underlie this abnormality are not entirely clear. There appear to be at least three different possibilities: (1) the absolute number of olfactory neurons is greater in schizophrenia patients, hence the observed EOG response is more robust; (2) there is a loss of specificity of olfactory receptor expression in schizophrenia, such that the number of neurons that respond to a particular odorant is increased even if the absolute number of neurons is not; (3) the magnitude of the membrane depolarization current of individual ORNs is increased, so that the recorded EOG response is greater even if the number of responding neurons is unchanged. These alternative mechanisms should not be considered mutually exclusive and there is, in fact, some indirect evidence to support each of them.
With regard to the total number of olfactory neurons, Arnold et al (2001)
reported that olfactory epithelial tissue obtained at autopsy from schizophrenia patients had increased numbers of immature GAP43 + neurons relative to p75NGFR + precursor stem cells, compared to epithelial tissue obtained from healthy individuals. Similarly, cultures of olfactory neuroepithelial tissue biopsied from living schizophrenia patients exhibited increased mitosis and greater cell proliferation than olfactory tissue cultures derived from healthy subjects (Féron et al, 1999
; McCurdy et al, 2006
). Finally, gene expression profiling of olfactory epithelial tissue found increased expression of multiple genes related to cell proliferation, differentiation and neurogenesis in schizophrenia (McCurdy et al, 2006
). Collectively, these findings indicate that there is increased neuronal proliferation associated with dysregulated olfactory receptor development in schizophrenia. The observation of increased EOG amplitude is consistent with, and may be a marker of, this increased cell proliferation.
This process may also be exacerbated by a loss of selectivity of olfactory neurons. Normally, a given olfactory neuron expresses only one olfactory receptor on its membrane surface, restricting its response to a specific odorant molecule configuration (Ronnett and Moon, 2002
). There is evidence, however, that such selectivity can be altered, as in the case of the alteration of olfactory function with normal human aging (Rawson et al, 1998
). Perhaps more importantly, electrophysiological studies of olfactory development also indicate that the olfactory epithelium of immature animals is highly nonselective, with individual olfactory neurons responding to many different odorants (Gesteland et al, 1982
). Selectivity appears to be acquired only later in development. As the basic finding of Arnold et al (2001)
was an increased density of immature rather than mature neurons in schizophrenia, it is possible that this would translate into an increased number of neurons that respond nonselectively to H2
S. Although this is a plausible hypothesis, specific evidence remains lacking.
Another possibility is that the magnitude of the EOG response is increased at the level of the individual neuron—ie, that there are alterations in the intracellular signal transduction pathways that lead to increased membrane depolarization. The binding of an odorant to an olfactory receptor results in increased levels of intracellular cyclic AMP (cAMP). cAMP functions as a second messenger, causing cyclic nucleotide-gated ion channels to open and cations to enter the cell. There is a strong correlation between the magnitude of this transmembrane current, which produces the observed EOG response, and the levels of adenylyl cyclase activation and cAMP accumulation within olfactory neurons (Lowe et al, 1989
). There is increasing evidence to suggest that this intracellular signaling cascade may be dysregulated in schizophrenia. An early study, using B lymphocytes, found increased adenylyl cyclase activity and cAMP accumulation in cells from schizophrenia patients following stimulation with forskolin, which binds to a high affinity site on the catalytic subunit of adenylyl cyclase (Natsukari et al, 1997
). More recently it has been shown that DISC1, the schizophrenia susceptibility gene located on chromosome 1q42, acts intracellularly to sequester phosphodiesterase, the enzyme responsible for the degradation of cAMP, and to release it in response to elevated levels of cAMP (Millar et al, 2005
). Alterations of the quantity or function of the DISC1 protein will therefore necessarily alter the regulation of cAMP levels. Similarly, a polymorphism of the GNAS1
gene on chromosome 20q13, which codes for the α-subunit of the G protein that stimulates adenylyl cyclase, has been associated with deficit syndrome schizophrenia (Minoretti et al, 2006
). Alterations of this protein would affect the production, rather than the degradation, of cAMP following stimulation. Alterations in cAMP levels could also be a secondary effect of either glutamatergic (Chetkovich and Sweatt, 1993
) or dopaminergic (Neves et al, 2002
) dysregulation, both of which have been implicated in schizophrenia pathophysiology. Although these associations support the idea that cAMP signaling is disrupted in schizophrenia, the status of olfactory signal transduction in this disorder has yet to be examined.
We cannot, at this time, delineate the relative contribution of each of these potential mechanisms to the abnormalities we have observed. Any one of these disturbances at the level of the epithelial receptor could, presumably, lead to the olfactory sensory perceptual deficits that are observed in behavioral studies of odor identification and threshold detection sensitivity. Future studies using olfactory epithelial biopsy material are required to clarify the relationship between the electrophysiological and biochemical responses of ORNs. The extent to which these abnormalities generalize to other odorants may also shed light on their underlying mechanisms, because odorants differ substantially in the level of adenylyl cyclase excitation that they produce (Sklar et al, 1986
). The extent to which this finding is specific to certain groups of odorants needs to be determined, as does its specificity to schizophrenia. Olfactory neurons from bipolar patients exhibit abnormal responses to odor stimulation in culture (Hahn et al, 2005
), but it is not clear if their EOG responses are similar to or different from those of schizophrenia patients. The specificity of this finding to schizophrenia, its relationship to observed psychophysical olfactory deficits, and its status in unaffected individuals at genetic risk for the disorder are all questions that have yet to be investigated.
Finally, although we observed no association between EOG amplitude and either the dosage or class (typical vs
atypical) of antipsychotic medication, we cannot entirely rule out the possibility that these findings are a consequence of patients’ use of antipsychotic medications, which may have altered dopaminergic activity in the nasal mucosa in a manner that is not dose-dependent. Recent in vitro
evidence from slice preparations of the mouse olfactory epithelium (Hegg and Lucero, 2004
) indicates that exogenous dopamine can decrease the odor-induced responses of ORNs through inhibition of L-type voltage-gated Ca2+
dopamine receptor antagonism can completely reverse this response inhibition and return neuronal excitability to normal. Importantly, though, a D2
receptor antagonist alone does not appear to amplify the odor-induced ORN response. Also, previous studies have demonstrated odor-induced cortical evoked potential abnormalities in schizophrenia patients independent of medication status (Turetsky et al, 2003b
), as well as in healthy unmedicated first-degree relatives (Turetsky et al, 2008
). Nevertheless, altered mucosal dopaminergic activity in patients, either as a primary dysfunction of the illness or as a secondary response to pharmacological treatment, could have contributed to our findings. It will therefore be very important to determine, in future studies, whether new-onset unmedicated schizophrenia patients, individuals with prodromal symptoms, or unaffected family members exhibit similar EOG abnormalities.