Chronic Morphine Exposure Affects Visual Response Latency of the Lateral Geniculate Nucleus in Cats

Abstract

Chronic morphine exposure results in degradation of the functional properties of cortical cells. However, little evidence has been reported about the effect of morphine on the temporal properties of the visual system. We compared the visual response latency of different cells of the lateral geniculate nucleus (LGN) in morphine (10 mg/mL)- and saline-treated cats. Morphine-treated cats were given morphine sulphate by cervical subcutaneous injection twice daily for 10 days, whereas saline-treated cats received injections of 0.9% saline instead. We found that LGN neurons in morphine-treated cats exhibited significantly longer response latency than those in saline-treated cats (P < 0.001). To investigate whether different types of neurons exhibited similar changes in response to morphine, we classified LGN neurons as on- and off-centre, X and Y, and layer A, A1 and C neurons. There was a tendency for prolonged latency in layer C neurons and a significantly longer latency for the other neurons in morphine-treated cats (P < 0.05). These findings suggest that chronic morphine administration (10 mg/kg by cervical subcutaneous injection twice daily for 10 days) delays information transfer earlier in the visual pathway.

Chronic morphine exposure affects contrast response functions of V1 neurons in cats

“…Additionally, signal-to-noise ratios were found to be lower in MCs, consistent with the previous study (He et al, 2005a). These findings support the idea that the neural functions are substantially influenced by chronic morphine exposure in the visual system (He et al, 2005a,b,c;Li et al, 2007a;Hu et al, 2008;Long et al, 2008).…”

“…Morphological studies in our lab also reveal that chronic morphine administration leads to the prominent structural modification in the primary visual cortex (Li et al, 2007c;Hu et al, 2008). Additionally, many visual response properties of the neurons in LGN and V1, such as response modulation (He et al, 2005b), orientation and direction selectivity (He et al, 2005a,c) and the visual response latency (Long et al, 2008), are found to be significantly affected by chronic morphine exposure. Taken together, these results indicate a substantial influence of opiates on the visual system.…”

“…Though research on opiate abuse suggests that the decline of inhibitory neurotransmission actually happens in many brain regions, there is also a study showing that chronic opiate exposure affects excitatory neurotransmission such as glutamatergic (Martin et al, 1999). Additionally, our previous study has shown that chronic morphine exposure causes morphological changes in pyramidal cells of V1 (Li et al, 2007c;Hu et al, 2008) and impacts the LGN (Long et al, 2008), suggesting that excitatory inputs to V1 and excitatory V1 neurons themselves are directly influenced by morphine. It is tempting to speculate that despite the effect of intracortical suppression, the decreased excitatory input from LGN and/or other cortical cells may have a role in decreasing the contrast sensitivity of V1 neurons.…”

GABA increases stimulus selectivity of neurons in primary visual cortices of cats chronically treated with morphine

“…These findings suggested that excitatory neurotransmission can be changed by opiate exposure. Consistent with this idea, our previous studies have shown that chronic morphine exposure results in morphological changes in pyramidal cells of V1 (Li et al, 2007c;Hu et al, 2008) and impacts the neurons in LGN (Long et al, 2008), which suggests that excitatory inputs to V1 and excitatory V1 neurons themselves are directly influenced by morphine.…”

“…A degraded signal-to-noise ratio leads to decreased ability of the neural system to discriminate signal from background, which therefore may have some relationship with the disrupted visual functions, such as the reduced visual sensitivity, described above. Additionally, decreased response modulation, extended time course of response and visual response latencies were also found in the visual pathway of cats given chronic morphine treatment (He et al, 2005c;Long et al, 2008). Most notably, orientation and direction selectivity, which play important roles in the perception of form (Hubel, 1988) and motion (Albright and Stoner, 1995),were also found to be impaired in the LGN (He et al, 2005a) and V1 (He et al, 2005b).…”